The funding received by the authors within the scope of Horizon 2020 by the European Union is highly appreciated. This work was conducted as part of the Mid-TECH Marie Curie innovative training network [H2020-MSCA-ITN-2014-642661].

The setup of the upconversion detector is shown in Figure 1; mirrors, lenses, or other optics referenced in the protocol are identified here or in the diagram of the IR-DFWM setup shown in Figure 2. The protocol section mainly deals with aligning the optical setup used for this method, and the process can be paused at any point by turning off all running equipment. All mirrors are adjusted manually. The software used here to control the camera and LD was delivered together with the upconversion detector. The use of the software is described at the end of the protocol.
1. Upconversion
<ol>
	<li>Place the end mirror of the alignment cavity, UH, as indicated in Figure 1.</li>
	<li>Remove the PPLN crystal from the crystal mount.</li>
	<li>Place an IR-sensitive card (sensitive at 1,064 nm) at position A, see Figure 1.</li>
	<li>Turn the angle of the kinematic mount holding UH to the extreme position in both the horizontal and vertical direction. Then, turn on the LD at approximately 1/3 of the maximum output.</li>
	<li>Align the alignment cavity as follows.
	<ol>
		<li>Change the angle of UH by +0.2&#176; in the horizontal direction.</li>
		<li>Sweep the vertical angle of UH from one extreme to the other, while watching the IR card for a beam from the alignment cavity.</li>
		<li>Repeat steps 1.5.1 and 1.5.2 until the cavity starts lasing.</li>
		<li>When the alignment cavity is lasing, switch between adjusting the angle of UH for a higher power and reducing the LD drive current. The LD is dimensioned to drive the full cavity, which has much higher losses than the alignment cavity. Keep the power where the beam leaving UH is easily visible with the IR card but no more than that.</li>
	</ol>
	</li>
	<li>Remove the IR card.</li>
	<li>Adjust the angle of U2, so the alignment beam is reflected in the center of U3 (Figure 1). 
	NOTE: The beam from the alignment cavity should hit U2 in the center.</li>
	<li>Adjust the angle of U3 so the beam continues to U4, U5, and U6 and is reflected from U6 to U7.</li>
	<li>The beam must pass through the PPLN mount at the height of the middle of the channels of the PPLN crystal, and it must enter the crystal perpendicular to the surface. Use U2 to correct the height and angle, while adjusting U3 to keep the beam level and centered through the holes x and y.</li>
	<li>Remove the germanium window and place the IR card behind U7, so that an IR beam leaving the cavity will hit the card, and the fluorescence will be visible to the person aligning the cavity. 
	NOTE: The alignment beam will now be passing through the PPLN mount and hit U7.</li>
	<li>Adjust the angle of U7, so that the reflection from U7 passes back along the path of the alignment beam. While adjusting the angle of U7, watch for a beam on the IR card. When a beam is seen, adjust the angle of U7 to maximize the output.</li>
	<li>Mount the PPLN in the mount. Make sure the mount is placed so that the beam goes through one of the channels in the crystal.</li>
	<li>Continue with the substep (step 1.13.1, 1.13.2, or 1.13.3) matching the current situation.
	<ol>
		<li>If an IR beam is still visible exiting U7, adjust U7 to maximize the output, and continue with the next step.</li>
		<li>If the IR beam exiting U7 is no longer visible, increase the LD current to 1/3 of the maximum output, and check if the IR beam can be seen. If a beam is visible, go to step 1.13.1; otherwise, go to step 1.13.3.</li>
		<li>Reduce the LD current to the previous level and trace the guide beam to see if it passes through the PPLN in the center of one of the channels. If it does not, repeat from step 1.7, but with the PPLN in the mount.</li>
	</ol>
	</li>
	<li>Turn off the LD, remove UH, and attach the LP750 filter at position B (see Figure 1).</li>
	<li>Place the power meter behind U7 but leave space for checking the beam with an IR card. Then, turn on the LD at full power.</li>
	<li>If no signal is seen on the power meter, make small angle changes to U7, while watching for a signal on the power meter. If a signal is found, continue to the next step; otherwise, return to step 1.1.</li>
	<li>Optimize the cavity alignment by adjusting the angles of U2 and U7 to maximize power, while using a high-power IR card to check that the cavity is running in the fundamental Gaussian mode. 
	NOTE: While it might be possible to get higher power in a higher order mode, it is essential for the conversion efficiency that the laser is running in the fundamental mode.</li>
	<li>If the cavity is not running in the fundamental mode, it will be running in a higher order mode where multiple lobes are visible on the IR card. Turn U7 so that the lobes are brought closer together on the IR card, until they merge.</li>
	<li>Record the power output at U7. Use this and the transmission of U7 to calculate the intracavity field. Compare this value to the calibration curve in Figure 6.</li>
	<li>When the cavity has been optimized, remove the LP750 filter, and reattach the germanium window.</li>
</ol>
2. IR-DFWM Alignment
NOTE: See Figure 2 for a diagram of the DFWM setup.
<ol>
	<li>Align the HeNe laser beam (the guide beam) with M3 and M4 to hit L1 in the center, going horizontally from M4 to L1.</li>
	<li>Insert BOXCARS plate 1 at a 45&#176; angle to the beam (vertical direction) and ensure the beam goes through, producing two output beams.</li>
	<li>Insert BOXCARS plate 2 at a 45&#176; angle to the beams (horizontal direction) and ensure the beam goes through, producing four output beams. Adjust the angles of the plates so the beams are spaced like the corners in a square.</li>
	<li>Adjust the position of L1 until the beams are equally spaced around the center of the lens.</li>
	<li>Leave the signal beam, which will be generated along the path of the beam blocked by the beam block, unblocked for now, so it can be used to align the rest of the setup. Place the iris so it blocks the three pump beams but allows the fourth beam, the signal beam, to pass through.</li>
	<li>Align L2 so the signal beam is collimated. This must be done using the focal lengths at the wavelength of the pulsed laser and not by visual inspection, as the focal lengths will be different for the wavelength of the guide beam and the mid-IR.</li>
	<li>Place M5 and M6 so that the guide beam is centered on the input window of the upconversion detector and perpendicular to the input window.</li>
	<li>Place L3 one focal length optical distance from the center of the PPLN. Take into account the refraction of the germanium window, the cavity mirror, and the PPLN itself.</li>
	<li>Set up the upconversion module and turn it on (see section 1).</li>
	<li>Remove the germanium window of the upconversion detector. This will allow a 1064 beam to exit the upconversion module.</li>
	<li>Overlap the HeNe laser beam and the 1064 beam from the upconversion detector by using M6 to move the 1064 beam onto the signal beam so that they overlap at L2, and by using M5 to move the guide beam onto the 1064 beam at L3. Alternate between the two mirrors until the guide beam and the 1064 follow the same path.</li>
	<li>Reattach the germanium window.</li>
	<li>Place several ND filters in the beam path, in front of the upconversion detector. Take great care to never let an unattenuated beam from the pulsed laser into the upconversion detector, as the high energy will likely damage the detector.</li>
	<li>Turn on the pulsed laser and ensure it is running stable and at an appropriate energy per pulse.</li>
	<li>Overlap the pulsed laser and the guide beam as follows.
	<ol>
		<li>Adjust the angle of M1 till the pulsed laser overlaps the guide beam at the beam combiner (M2).</li>
		<li>Adjust the angle of the M2 so that the pulsed laser is reflected in the propagation direction of the guide beam.</li>
		<li>Check that the beams are overlapping at the beam combiner and at distances of 1 m, 2 m, and 3 m.</li>
	</ol>
	</li>
	<li>Find the focal point of the beams after L1. Place the gas flow or flame to be measured so that the measurement point is at the focal point of the beams.</li>
	<li>Connect the trigger signal from the pulsed laser to the upconversion detector to time gate the detection. If the time delay and gate time are not known, start with a long duration of the time gate and narrow it down when the signal is found.</li>
	<li>Search the setup, especially the BOXCARS plates, for errant reflections and ensure they are blocked.</li>
	<li>Optimize the alignment of the signal beam in the upconversion detector as follows.
	<ol>
		<li>If a signal is visible on the detector, adjust M5 and M6 to maximize the signal.</li>
		<li>If no signal is visible on the detector, reduce the ND filtering by one order of magnitude. Repeat until a signal is seen.</li>
		<li>If the signal on the detector is saturated, increase the ND filtering by one order of magnitude. Repeat until the signal is no longer saturated.</li>
		<li>Go through steps 2.19.1-2.19.3 until the signal can no longer be increased by adjusting M5 and M6.</li>
	</ol>
	</li>
	<li>Place the beam block so that it blocks the signal beam, as indicated in Figure 2. Then, remove the ND filters.</li>
	<li>Adjust the position of the beam block to reduce any scattering (background noise) seen on the detector. Take great care not to unblock the beam accidentally and expose the detector to direct light from the pulsed laser.</li>
	<li>Prepare the gas flow or flame to be measured. Then, scan the pulsed laser across the wavelength range of interest, while recording the signal from the detector. This will generate a spectrum matching the gas composition at the overlap of the beams.</li>
</ol>
3. Laser Diode Software
<ol>
	<li>Run the LabVIEW program AuroraOne control.vi.</li>
	<li>Click the Laser TEC enable button to the on position and click the RW/TW safety button off.</li>
	<li>Set the laser current by entering the desired value in microwatts in the TA set point field. Entering a new value while the laser is running will adjust the current.</li>
	<li>Click the TA enable button to the on position to turn the laser diode current on.</li>
	<li>Turn off the laser diode by clicking the TA enable and the Laser TEC enable to the off positions.</li>
</ol>
4. IDS Imaging Development Systems
<ol>
	<li>Run the LabVIEW program UpconversionControl.vi.</li>
	<li>Under the tab Settings, set the shutter speed to 8 &#181;s by typing the value in the field labeled Exposure Time (secs).</li>
	<li>Under the tab Settings, set the shutter type to Global in the field labeled IDS Shutter.</li>
	<li>Under the tab DBG, set the trigger type to Lo_Hi in the IDS trigger field.</li>
	<li>Under the tab DBG2, set the trigger delay in the field labeled IDS Trigger Delay (&#181;s). This will depend on the delay between the trigger pulse and the laser pulse from the laser.</li>
	<li>Under the tab Settings, set the off set x and off set y to 480 pixels and the Width and Height to 96 pixels.</li>
	<li>Under the tab Settings, set the frame rate to 0 in the Framerate field; this sets the camera to take one frame per trigger signal.</li>
	<li>Turn on the camera by pressing the Start Acquisition button.</li>
	<li>When a signal is entering the upconversion detector, the signal will be visible as a bright spot in the middle of the image shown on the right in the LabVIEW program. Use the Rect function on the left bar next to the image to draw a 6 x 6 pixel rectangle around the signal.</li>
	<li>View the average intensity from the selected pixels as a function of time under the History tab. If necessary, the graph can be cleared by right-clicking it and selecting clear.</li>
	<li>Press the Stop Acquisition button to stop the acquisition of new images from the camera.</li>
	<li>Export the data by right-clicking the intensity plot, select copy data to clipboard, and paste the data into a .txt file.</li>
	<li>Turn off the camera and control program by pressing the Shut down button.</li>
</ol>

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The authors have nothing to disclose.

The precision of the alignment of the pulsed laser beam is critical to the sensitivity of the method. Special care must be taken to ensure that the beams are separated by equal distance after the BOXCARS plates and that the beams are equally spaced around the center of L1. Deviation from this will lead to a significant drop in signal intensity and, therefore, sensitivity. Likewise, care must be taken that the upconversion module cavity is running in the fundamental mode and that the signal beam is aligned for an optimal overlap with the upconversion pump. The signal can easily be reduced by one or two orders of magnitude if the upconversion cavity is running in a wrong mode or the signal beam overlap with the cavity field is suboptimal. This includes placing L3 with millimeter precision so that the signal beam focal point is in the middle of the PPLN crystal. With an optimal overlap and 80 W of cavity power, a 6% quantum efficiency of the SFG stage is possible. With the detector and wavelength used here, the total detection efficiency is 3%. The maximum intracavity power that can be reached is 120 W, but 80 W can be achieved reliably. The conversion efficiency is proportional to the intracavity power, so signals recorded with a different intracavity power can be compared if the intracavity power is recorded.
The main limiting factor for the sensitivity of this method is the background scattering, which drowns weak signals. To limit this scattering, it is critical that the optics are kept dust-free, especially lens L1. Care must also be taken that the position of the beam block minimizes the background noise. The beam block should be placed on an xy-stage so that it can be moved in a controlled manner in both the horizontal and vertical plane, perpendicular to the direction of the beams.
The scanning discussed here is done with the PPLN at a constant temperature. The conversion efficiency is proportional to sinc(&#916;kL/2&#960;)2, where &#916;k is the phase mismatch and L is the crystal length. The full width half maximum (FWHM) of this function is the bandwidth of the detector at a constant PPLN crystal temperature. The FWHM of this function changes with the crystal temperature and wavelength but is generally on the order of 5 cm-1 in the mid-IR, for a 20 mm long crystal. The exception is near 4,200 nm, where the width increases greatly18.
No scaling optics have been included in the setup diagram in Figure 2, because there are a number of issues to consider before deciding what, if any, scaling is needed. For the setup described here, the pulsed laser beam is collimated at a beam diameter of ca. 2 mm when reaching L1. This gives a beam waist at the focal point of approximately 400 &#181;m, using a wavelength of 3 &#181;m. When implementing this technique, it might be desirable to change the focal length of L1, either because more space is needed between L1 and the focal point for practical reasons, or to shorten the measurement volume by increasing the convergence angles, which can be done by using a shorter focal length. In this case, the beam waist at the focal point should be kept at ca. 400 &#181;m and the collimated beam should be scaled to match. It should, however, be taken into account that increasing the beam diameter without increasing the spacing of the beams will increase the scattering from the beam block edges. The spatial resolution is given by the overlap of the pump beams. For the setup described here, the overlap is 6 mm long, so the measurement volume is a cylinder of 6 mm long, with a radius of 0.4 mm.
To achieve quasi-phase-matching in the PPLN crystal, both the mid-IR signal and the intracavity field of the upconversion cavity must be extraordinarily polarized in the PPLN crystal. The upconversion cavity should be built so that the polarization of the intracavity field is automatically right. If the mid-IR laser does not already match this, a waveplate can be inserted at the mid-IR laser output to turn the polarization.
IR-DFWM requires relatively high energy pulses, 1-4 mJ, combined with a narrow enough laser linewidth to resolve molecular lines, which are on the order of 0.1 cm-1. Lasers that match these criteria generally have low repetition rates, and as data acquisition with DFWM is generally done by scanning the wavelength of the laser, this limits measurements speed. This means the method is most readily applied to measurements where the subject does not change over time, although it has also been applied to temporally resolved measurements17. Another limitation is that, because of the sensitivity to scattered light, particles in or near the measurement volume will create scattering events that completely drown the signal17. The phase-match condition of the upconversion process is spectrally narrow, which helps eliminate noise from the thermal background radiation, but it does make scans over broad wavelength ranges more time-consuming as the PPLN temperature must be tuned to keep the signal wavelength phase-matched.
Future uses of IR-DFWM are planned for the detection of NH3 in flames, or to continue the work with HCN in more practical environments. The most obvious means for improving the method is to further reduce the background from scattered light. This might be done using spatial filtering of the signal beam after the signal is collected by L2.

IR-DFWM provides the capability of measuring concentrations of IR active species down to the ppm level1, with spatial resolution. IR-DFWM has several advantages that make it an attractive technique for combustion research. Flames can be changed drastically by the insertion of probes, but IR-DFWM is nonintrusive. It has a spatial resolution, so species&#39; concentrations at different points in the flame structure can be measured. It provides a coherent signal, which can be isolated from the thermal emission of the flame. Additionally, DFWM is less sensitive to the collision environment than, for instance, laser-induced fluorescence (LIF), which can be difficult to determine in a flame. The technique also provides access to molecular species that are IR active but lack visible or near-visible transitions that can be used to measure them with other techniques.
While DFWM has a number of advantages, alternative techniques might be preferable if one or more of these advantages are not needed. If spatial resolution is not needed, absorption-based techniques will be both simpler and more accurate. If the molecular species in question have transitions in the visible or near-IR region, LIF might be preferable, as LIF can provide spatially resolved information from a plane rather than just a single point. Under the right conditions, nonlinear methods, such as DFWM and PS, can also be used for single-shot 2D measurements2. The signal of these nonlinear methods is proportional to the probe beam intensity cubed, and as the pump beam must be expanded to cover the area of the 2D measurement, this requires either very high pulse energies or a combination of high third-order susceptibility, high concentrations, and low background noise to work. Therefore, it mostly depends on the molecular species whether this is a possibility.
In a more direct competition with DFWM, there are the other four-wave-mixing-based spectroscopic techniques: coherent anti-Stokes Raman spectroscopy (CARS), laser-induced grating spectroscopy (LIGS), and polarization spectroscopy (PS). CARS is a well-established technique for measuring the temperature and major species in combustion environments. However, it lacks the sensitivity to detect minor species, as the detection limit is usually approximately 1%2. PS and DFWM have previously been shown to have similar sensitivity and detection limits3; however, the signal-to-noise ratio of DFWM has been shown to increase by factor 500 when combined with upconversion detection4, while PS has shown only a 64-fold increase5. LIGS has the advantage of inducing a grating, using mid-IR light, but measures the effect by the refraction of a probe laser of this grating, and the wavelength of this probe laser can be chosen freely6. The wavelength of the probe laser can, therefore, be in the visible region, where fast, low-noise silicon-based detectors are available. This is the same advantage that is achieved by using upconversion. LIGS does have the drawback that it is very sensitive to collision2, which means that the concentration of major gas species must be known for precise concentration or temperature measurements with LIGS. If that issue is overcome, LIGS does have a similar sensitivity to both DFWM and PS at atmospheric pressure3, but where the LIGS signal increases with increasing pressure, the signal from DFWM and PS increases at lower pressures, which means the preferred technique will depend on the pressure environment.
Upconversion detection is the technique of converting a signal from long wavelengths to shorter ones by using sum-frequency generation. The advantage of this is that the detectors in the visible or near-infrared range have lower noise and a higher sensitivity than their counterparts in the mid-IR region. This was first investigated five decades ago7, but saw very little attention and use since then, due to the low conversion efficiencies. However, with advances in the production techniques for periodically poled lithium niobate (PPLN) and other materials with high nonlinear coefficients, as well the increased availability of high-power laser diodes (LDs), the technique has attracted increased attention in the last decade, with applications covering areas such as mid-IR single-photon detection8,9,10,11, IR lidar12,13, and hyperspectral imaging14,15 and microscopy16. The main advantage of combining upconversion detection with IR-DFWM is that the phase-match condition has a narrow angular and spectral acceptance band, which discriminates heavily against the thermal background, allowing the detection of weaker signals.

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	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:300px;">Nd:YAG laser, pulsed</td>
			<td style="width:183px;">Spectra Physics</td>
			<td style="width:165px;">Quantarau Pro-290-10</td>
			<td style="width:183px;">Quantity: 1 (For pumping the mid-IR laser)</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:300px;">Nd:YAG laser, injection seeding system</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:300px;">NIR Dye laser - OPA system</td>
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			<td height="23" style="height:23px;width:300px;">HeNe laser</td>
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		<tr height="64">
			<td height="64" style="height:64px;width:300px;">Dichroic mirror</td>
			<td style="width:183px;">LASEROPTIK</td>
			<td>NA</td>
			<td style="width:183px;">Quantity: 1, custom order: HR for the mid-IR, transparent for 632 nm</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Protected Gold Mirrors</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:300px;">BoxCars Plate</td>
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			<td style="width:183px;">Quantity: 2, Custom order</td>
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			<td height="21" style="height:21px;width:300px;">xy-stage</td>
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			<td height="21" style="height:21px;width:300px;">500 mm focal length CaF2 lens, &#216;2&#39;&#39;</td>
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		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">500 mm focal length CaF2 lens, &#216;1&#39;&#39;</td>
			<td style="width:183px;">Thorlabs</td>
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		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">100 mm focal length CaF2 lens, &#216;1&#39;&#39;</td>
			<td style="width:183px;">Thorlabs</td>
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			<td height="21" style="height:21px;width:300px;">ND1 IR filter (ZnSe)</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:300px;">ND2 IR filter (ZnSe)</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:300px;">ND3 IR filter (ZnSe)</td>
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		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:300px;">Upconversion Detector</td>
			<td style="width:183px;">NLIR</td>
			<td style="width:165px;">NA (closest equivalent: U3055 3.0-5.5&#181;m)</td>
			<td style="width:183px;">Quantity: 1, Custom order&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">VIS/NIR Detector Card</td>
			<td style="width:183px;">Thorlabs</td>
			<td>VRC2</td>
			<td style="width:183px;">Quantity: 1, (low intensity)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:300px;">NIR Detector Card</td>
			<td style="width:183px;">Thorlabs</td>
			<td>VRC4</td>
			<td style="width:183px;">Quantity: 1, (high intensity)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">MIR Detector Card</td>
			<td style="width:183px;">Thorlabs</td>
			<td>VRC6S</td>
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		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Thermal Power Sensor Head</td>
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			<td style="width:183px;">Quantity: 1</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Power meter console</td>
			<td style="width:183px;">Thorlabs</td>
			<td style="width:165px;">PM100D</td>
			<td style="width:183px;">Quantity: 1</td>
		</tr>
	</tbody>
</table>

infrared degenerate four-wave mixing with upconversion detection for quantitative gas sensing

We present a protocol for performing gas spectroscopy using infrared degenerated four-wave mixing (IR-DFWM), for the quantitative detection of gas species in the ppm-to-single-percent range. The main purpose of the method is the spatially resolved detection of low-concentration species, which have no transitions in the visible or near-IR spectral range that could be used for detection. IR-DFWM is a nonintrusive method, which is a great advantage in combustion research, as inserting a probe into a flame can change it drastically. The IR-DFWM is combined with upconversion detection. This detection scheme uses sum-frequency generation to move the IR-DFWM signal from the mid-IR to the near-IR region, to take advantage of the superior noise characteristics of silicon-based detectors. This process also rejects most of the thermal background radiation. The focus of the protocol presented here is on the proper alignment of the IR-DFWM optics and on how to align an intracavity upconversion detection system.

Figure 3 shows the signal from different concentrations of HCN in N2, averaged over three scans for each concentration. The mixture was prepared by mixing 300 ppm HCN in N2 with pure N2 using mass flow controllers and heating it to 843 K. The central peak is the P(20) line of the &#957;1 vibrational band of HCN. The inset in Figure 3 shows the peak value of the signal from this line for each concentration, with a second-degree polynomial fit. The concentration dependency of the signal can be described by S = ax2 + b, where S is the signal and a and b are fitting constants17. Absolute concentration measurements in a flame require a calibration measurement as shown here, at a known temperature, to determine the constant a. The temperature in the measurement volume in the flame must also be measured as the constant a scales with temperature; a full discussion of this has already been published17. The poling period used for this measurement was 21.5 &#181;m, with a crystal temperature of 104.5 &#176;C.
Figure 4 presents raw data from a premixed flame. It shows five consecutive scans over the range 3229.5-3232 cm-1, each scan taking approximately 65 s. These cover three groups of waterlines, used for temperature measurements. Ideally, when working with a stable system, each scan over the same range should be identical, as the concentration, pressure, and temperature should be unchanged. The intensity of the lines seen here changes drastically from scan to scan, which is because the laser pulse mode and energy is not stable from scan to scan. Results like these are unusable unless the laser pulse energy has been recorded and can be used to sort measurements with sufficient laser pulse energy from the rest. The poling period used for this measurement was 21.5 &#181;m, with a crystal temperature of 123 &#176;C.
In Figure 4, the background scattering is not seen because an ND2 filter was used to reduce the signal, to avoid saturating the detector. For weaker signals, it was found that the background scattering is on the order of 5 pJ per pulse, which corresponds to the signal generated from the P(20) line of the &#957;1 vibrational band of 100 ppm HCN at room temperature.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59040/59040fig1.jpg" /> 
Figure 1: Diagram of the upconversion detector. U1-U7 and UH are mirrors, highly reflective (HR)-coated for 1,064 nm. All the mirrors are planes, except for U3, which has a 200 mm radius of curvature. Mirrors U1-U5 were made to be transmissive at the wavelength of the laser diode, to ensure that the LD light does not reach the detector. U6 is transmissive for the upconverted signal, 650-1,050 nm. U7 is transmissive for the mid-IR signal. UH is 95% reflective for 1,064 nm and 5% transmissive. The path length from U1 to U3 is 156 mm, and the path length from U3 to U7 is 202 mm. L4 and L5 are achromatic lenses with 60 mm and 75 mm focal lengths, respectively. Both are transparent for 650-1,050 nm. The camera used as detector is placed 75 mm from L5. The cavity field is vertically polarized. The PPLN used here has poling periods of 21.0 &#181;m, 21.5 &#181;m, 22.0 &#181;m, 22.5 mm, and 23.0 &#181;m, and the crystal length is 20 mm. The visible and near-infrared detector used is a UI-5240CP-NIR-GL camera from IDS Imaging Development Systems. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59040/59040fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59040/59040fig2.jpg" /> 
Figure 2:&#160;Diagram of the DFWM setup. M1 is a dielectric mirror highly reflective (HR) at the wavelength of the pulsed laser. M2 is a dielectric mirror coated to be HR at the wavelength of the pulsed laser and transmissive for the HeNe guide beam. M3-M6 are protected gold mirrors. B.C.1 and B.C.2 are BOXCARS plates 1 and 2. L1 is a 500 mm focal length CaF2 lens with a 5.1 cm diameter. L2 is a 500 mm focal length CaF2 lens with a diameter of 2.54 cm. L3 is a 100 mm focal length CaF2 lens. The pulsed laser is vertically polarized. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59040/59040fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59040/59040fig3.jpg" /> 
Figure 3: Signal from different concentrations of HCN in N2. The central peak is the P(20) line of the &#957;1 vibrational band of HCN. The inset shows the peak signal from each concentration (diamond markers), with a second-order polynomial fit. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59040/59040fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59040/59040fig4.jpg" /> 
Figure 4: Five consecutive scans of ca. 65 s in duration per scan, done in a premixed flame. The laser was scanned over the range of 3229.5-3232 cm-1. The peaks seen here are the signal from several collections of H2O transition lines. The signal was reduced with an ND1 and an ND0.6 filter, to avoid saturating the detector. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59040/59040fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59040/59040fig5.jpg" /> 
Figure 5:&#160;A BOXCARS plate seen from the side. It is a block of transparent material. On the input side, it is coated with an antireflective coating on half of the surface. The laser beam enters here and reaches the output side, where half the surface is coated for a 50% transmission. The light reflected internally in the plate is then refracted to the part of the input side coated for high reflection and is reflected through the top half of the output side. This splits one beam into two parallel beams. The same effect could be achieved with a beam splitter and a mirror, but a beam splitter would have some reflection from the rear surface, which could increase the background noise. Also, the BOXCARS plate requires no alignment to ensure the two beams produced are parallel. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59040/59040fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59040/59040fig6.jpg" /> 
Figure 6:&#160;Intracavity power as a function of pump laser diode current for the upconversion module. Each point is an average of the power measured from three separate alignments of the cavity and the error bars indicate the spread between the separate alignments. The deviation from the ideal laser behavior is caused by thermal effects in the laser crystal and the PPLN crystal. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59040/59040fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing at JoVE.com

Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

Here, we present a protocol to perform sensitive, spatially resolved gas spectroscopy in the mid-infrared region, using degenerate four-wave mixing combined with upconversion detection.

We present a protocol for performing gas spectroscopy using infrared degenerated four-wave mixing (IR-DFWM), for the quantitative detection of gas species ...

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Universidad de Cartagena UDEC

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JoVE Journal

Engineering

6.1K Views.  Lund University. Here, we present a protocol to perform sensitive, spatially resolved gas spectroscopy in the mid-infrared region, using degenerate four-wave mixing combined with upconversion detection.

Video: Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing

This paper discusses fluorescent core microcavity-based sensors that can operate in a microfluidic analysis setup. These structures are based on the formation of a fluorescent quantum-dot (QD) coating on the channel surface of a conventional microcapillary. Silicon QDs are especially attractive for this application, owing in part to their negligible toxicity compared to the II-VI and II-VI compound QDs, which are legislatively controlled substances in many countries. While the ensemble emission spectrum is broad and featureless, an Si-QD film on the channel wall of a capillary features a set of sharp, narrow peaks in the fluorescence spectrum, corresponding to the electromagnetic resonances for light trapped within the film. The peak wavelength of these resonances is sensitive to the external medium, thus permitting the device to function as a refractometric sensor in which the QDs never come into physical contact with the analyte. The experimental methods associated with the fabrication of the fluorescent-core microcapillaries are discussed in detail, as well as the analysis methods. Finally, a comparison is made between these structures and the more widely investigated liquid-core optical ring resonators, in terms of microfluidic sensing capabilities.

Fluorescent-core microcavity sensors employ a high-index quantum-dot coating in the channel of silica microcapillaries. Changes in the refractive index of fluids pumped into the capillary channel cause shifts in the microcavity fluorescence spectrum that can be used to analyze the channel medium.

This paper discusses  fluorescent core microcavity-based sensors that can operate in a microfluidic  analysis setup. These structures are based on the ...

In-situ Tapering of Chalcogenide Fiber for Mid-infrared Supercontinuum Generation

Supercontinuum generation (SCG) in a tapered chalcogenide fiber is desirable for broadening mid-infrared (or mid-IR, roughly the 2-20 &mu;m wavelength range) frequency combs1, 2 for applications such as molecular fingerprinting, 3 trace gas detection, 4 laser-driven particle acceleration, 5 and x-ray production via high harmonic generation. 6 Achieving efficient SCG in a tapered optical fiber requires precise control of the group velocity dispersion (GVD) and the temporal properties of the optical pulses at the beginning of the fiber, 7 which depend strongly on the geometry of the taper. 8 Due to variations in the tapering setup and procedure for successive SCG experiments-such as fiber length, tapering environment temperature, or power coupled into the fiber, in-situ spectral monitoring of the SCG is necessary to optimize the output spectrum for a single experiment.
	In-situ fiber tapering for SCG consists of coupling the pump source through the fiber to be tapered to a spectral measurement device. The fiber is then tapered while the spectral measurement signal is observed in real-time. When the signal reaches its peak, the tapering is stopped. The in-situ tapering procedure allows for generation of a stable, octave-spanning, mid-IR frequency comb from the sub harmonic of a commercially available near-IR frequency comb. 9 This method lowers cost due to the reduction in time and materials required to fabricate an optimal taper with a waist length of only 2 mm.
	The in-situ tapering technique can be extended to optimizing microstructured optical fiber (MOF) for SCG10 or tuning of the passband of MOFs, 11 optimizing tapered fiber pairs for fused fiber couplers12 and wavelength division multiplexers (WDMs), 13 or modifying dispersion compensation for compression or stretching of optical pulses.14-16

In-situ Tapering of Chalcogenide Fiber for Mid-infrared Supercontinuum Generation

We describe a method for in-situ tapering of As2S3 fibers to achieve efficient mid-infrared supercontinuum generation. By tapering while monitoring the supercontinuum&#x2019;s spectrum, the spectral width can be maximized for a fiber taper. In-situ fiber tapering can be applied to optimize the performance of other fiber-based devices.

Supercontinuum  generation (SCG) in a tapered chalcogenide fiber is desirable for broadening mid-infrared  (or mid-IR, roughly the 2-20 &mu;m wavelength ...

Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system.&#160; Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization.&#160; The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted.&#160; The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism.&#160; We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques.&#160;&#160; This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.

The technique of femtosecond four-wave mixing is described, including spectrally-resolved and time-resolved configurations. We illustrate the utility of this technique for the investigation of crucial physical properties in the III-V diluted magnetic semiconductors, afforded by its nonlinearity and high temporal resolution.

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, ...

Frequency Mixing Magnetic Detection Scanner for Imaging Magnetic Particles in Planar Samples

The setup of a planar Frequency Mixing Magnetic Detection (p-FMMD) scanner for performing Magnetic Particles Imaging (MPI) of flat samples is presented. It consists of two magnetic measurement heads on both sides of the sample mounted on the legs of a u-shaped support. The sample is locally exposed to a magnetic excitation field consisting of two distinct frequencies, a stronger component at about 77 kHz and a weaker field at 61 Hz. The nonlinear magnetization characteristics of superparamagnetic particles give rise to the generation of intermodulation products. A selected sum-frequency component of the high and low frequency magnetic field incident on the magnetically nonlinear particles is recorded by a demodulation electronics. In contrast to a conventional MPI scanner, p-FMMD does not require the application of a strong magnetic field to the whole sample because mixing of the two frequencies occurs locally. Thus, the lateral dimensions of the sample are just limited by the scanning range and the supports. However, the sample height determines the spatial resolution. In the current setup it is limited to 2 mm. As examples, we present two 20 mm &#215; 25 mm p-FMMD images acquired from samples with 1 &#181;m diameter maghemite particles in silanol matrix and with 50 nm magnetite particles in aminosilane matrix. The results show that the novel MPI scanner can be applied for analysis of thin biological samples and for medical diagnostic purposes.

A scanner for imaging magnetic particles in planar samples was developed using the planar frequency mixing magnetic detection technique. The magnetic intermodulation product response from the nonlinear nonhysteretic magnetization of the particles is recorded upon a two-frequency excitation. It can be used to take 2D images of thin biological samples.

The setup of a planar Frequency Mixing Magnetic Detection (p-FMMD) scanner for performing Magnetic Particles Imaging (MPI) of flat samples is presented. ...

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are proposed to create the electrical network in this work: (a) the incorporation of the nanoplatelets into the epoxy matrix and (b) the coating of the glass fabric with a sizing filled with the same nanoplatelets. Both types of multiscale composite materials, with an in-plane electrical conductivity of ~10-3 S/m, showed an exponential growth of the electrical resistance as the strain increases due to distancing between adjacent functionalized graphene nanoplatelets and contact loss between overlying ones. The sensitivity of the materials analyzed during this research, using the described procedures, has been shown to be higher than commercially available strain gauges. The proposed procedures for self-sensing of the structural composite material would facilitate the structural health monitoring of components in difficult to access emplacements such as offshore wind power farms. Although the sensitivity of the multiscale composite materials was considerably higher than the sensitivity of metallic foils used as strain gauges, the value reached with NH2 functionalized graphene nanoplatelets coated fabrics was nearly an order of magnitude superior. This result elucidated their potential to be used as smart fabrics to monitor human movements such as bending of fingers or knees. By using the proposed method, the smart fabric could immediately detect the bending and recover instantly. This fact permits precise monitoring of the time of bending as well as the degree of bending.

The integration of conductive nanoparticles, such as graphene nanoplatelets, into glass fiber composite materials creates an intrinsic electrical network susceptible to strain. Here, different methods to obtain strain sensors based on the addition of graphene nanoplatelets into the epoxy matrix or as a coating on glass fabrics are proposed.

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are ...

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation ...

Scalable Quantum Integrated Circuits on Superconducting Two-Dimensional Electron Gas Platform

To form a coherent quantum transport in hybrid superconductor-semiconductor (S-Sm) junctions, the formation of a homogeneous and barrier-free interface between two different materials is necessary. The S-Sm junction with high interface transparency will then facilitate the observation of the induced hard superconducting gap, which is the key requirement to access the topological phases (TPs) and observation of exotic quasiparticles such as Majorana zero modes (MZM) in hybrid systems. A material platform that can support observation of TPs and allows the realization of complex and branched geometries is therefore highly demanding in quantum processing and computing science and technology. Here, we introduce a two-dimensional material system and study the proximity induced superconductivity in semiconducting two-dimensional electron gas (2DEG) that is the basis of a hybrid quantum integrated circuit (QIC). The 2DEG is a 30 nm thick In0.75Ga0.25As quantum well that is buried between two In0.75Al0.25As barriers in a heterostructure. Niobium (Nb) films are used as the superconducting electrodes to form Nb- In0.75Ga0.25As -Nb Josephson junctions (JJs) that are symmetric, planar and ballistic. Two different approaches were used to form the JJs and QICs. The long junctions were fabricated photolithographically, but e-beam lithography was used for short junctions&#8217; fabrication. The coherent quantum transport measurements as a function of temperature in the presence/absence of magnetic field B are discussed. In both device fabrication approaches, the proximity induced superconducting properties were observed in the In0.75Ga0.25As 2DEG. It was found that e-beam lithographically patterned JJs of shorter lengths result in observation of induced superconducting gap at much higher temperature ranges. The results that are reproducible and clean suggesting that the hybrid 2D JJs and QICs based on In0.75Ga0.25As quantum wells could be a promising material platform to realize the real complex and scalable electronic and photonic quantum circuitry and devices.

Quantum integrated circuits (QICs) consisting of array of planar and ballistic Josephson junctions (JJs) based on In0.75Ga0.25As two-dimensional electron gas (2DEG) is demonstrated. Two different methods for fabrication of the two-dimensional (2D) JJs and QICs are discussed followed by the demonstration of quantum transport measurements in sub-Kelvin temperatures.

To form a coherent quantum transport in hybrid superconductor-semiconductor (S-Sm) junctions, the formation of a homogeneous and barrier-free interface ...

Femtosecond Laser Filaments for Use in Sub-Diffraction-Limited Imaging and Remote Sensing

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode scanners. In classical optics, the intensity that can be brought to bear on a remote target is limited by the spot size of the laser at the distance of the target. This spot size has a lower bound determined by the diffraction limit of classical optics. However, amplified femtosecond laser pulses generate intensity sufficient to modify the refractive index of the ambient air and undergo self-focusing. This self-focusing effect leads to the generation of highly intense laser filaments which maintain their intensity and small sub-millimeter diameter size at distances well beyond the classical Rayleigh length. Such intensity provides the capability of remote scanning, imaging, sensing, and spectroscopy with enhanced spatial resolution. We describe a technique for generating filaments with a femtosecond regenerative chirped-pulse amplifier, and for using the resulting filament to conduct imaging and spectroscopic measurements at remote distances of at least several meters.

High-intensity femtosecond pulses of laser light can undergo cycles of Kerr self-focusing and plasma defocusing, propagating an intense sub-millimeter-diameter beam over long distances. We describe a technique for generating and using these filaments to perform remote imaging and sensing beyond the classical diffraction limits of linear optics.

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode ...

Quantitative Analysis by Thermogravimetry-Mass Spectrum Analysis for Reactions with Evolved Gases

During energy conversion, material production, and metallurgy processes, reactions often have the features of unsteadiness, multistep, and multi-intermediates. Thermogravimetry-mass spectrum (TG-MS) is seen as a powerful tool to study reaction features. However, reaction details and reaction mechanics have not been effectively obtained directly from the ion current of TG-MS. Here, we provide a method of an equivalent characteristic spectrum analysis (ECSA) for analyzing the mass spectrum and giving the mass flow rate of reaction gases as precise as possible. The ECSA can effectively separate overlapping ion peaks and then eliminate the mass discrimination and temperature-dependent effect. Two example experiments are presented: (1) the decomposition of CaCO3 with evolved gas of CO2 and the decomposition of hydromagnesite with evolved gas of CO2 and H2O, to evaluate the ECSA on single-component system measurement and (2) the thermal pyrolysis of Zhundong coal with evolved gases of inorganic gases CO, H2, and CO2, and organic gases C2H4, C2H6, C3H8, C6H14, etc., to evaluate the ECSA on multi-component system measurement. Based on the successful calibration of the characteristic spectrum and relative sensitivity of specific gas and the ECSA on mass spectrum, we demonstrate that the ECSA accurately gives the mass flow rates of each evolved gas, including organic or inorganic gases, for not only single but multi-component reactions, which cannot be implemented by the traditional measurements.

Precise determination of the evolved gases&#39; flow rate is key to study the details of reactions. We provide a novel quantitative analysis method of equivalent characteristic spectrum analysis for thermogravimetry-mass spectrum analysis by establishing the calibration system of the characteristic spectrum and relative sensitivity, for obtaining the flow rate.

During energy conversion, material production, and metallurgy processes, reactions often have the features of unsteadiness, multistep, and ...

A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This FOSP relies on a silicon Fabry-Perot interferometer (FPI) attached to the fiber end, referred to as Si-FOSP in this work. The Si-FOSP generates an interferogram determined by the optical path length (OPL) of the silicon cavity. Measurand alters the OPL and thus shifts the interferogram. Due to the unique optical and thermal properties of the silicon material, this Si-FOSP exhibits an advantageous performance in terms of sensitivity and speed. Furthermore, the mature silicon fabrication industry endows the Si-FOSP with excellent reproducibility and low cost toward practical applications. Depending on the specific applications, either a low-finesse or high-finesse version will be utilized, and two different data demodulation methods will be adopted accordingly. Detailed protocols for fabricating both versions of the Si-FOSP will be provided. Three representative applications and their according results will be shown. The first one is a prototype underwater thermometer for profiling the ocean thermoclines, the second one is a flow meter to measure flow speed in the ocean, and the last one is a bolometer used for monitoring exhaust radiation from magnetically confined high-temperature plasma.

This work reports an innovative silicon-tipped fiber-optic sensing platform (Si-FOSP) for high-resolution and fast-response measurement of a variety of physical parameters, such as temperature, flow, and radiation. Applications of this Si-FOSP span from oceanographic research, mechanical industry, to fusion energy research.

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This ...

Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

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Quantitative Analysis by Thermogravimetry-Mass Spectrum Analysis for Reactions with Evolved Gases

A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response