This work was partially supported by the Research Corporation for Science Advancement (Grant 10626), National Science Foundation (DMR-1308084), and the San Francisco State University internal award to W. M. We thank our collaborator Paul M. Chaikin from NYU for helpful discussions in experimental design and for providing the VNA system for us to use on site at SFSU. We thank our theoretical collaborators, the inventor of the HD PBG materials, Marian Florescu, Paul M. Steinhardt, and Sal Torquato for various discussions and for providing us the design of the HD point pattern and continuous discussions.

1. Design a 2D Hyperuniform Disordered Dielectric Structure11
<ol>
	<li>Chose a subclass of 2D hyperuniform disorder point pattern (blue circles in Figure 2) and partition it (blue lines in Figure 2) using Delaunay tessellation. A 2D Delaunay tessellation is a triangulation that maximizes the minimum angle for each triangle formed and guarantees there are no other points inside the circumcircle of each triangle11.</li>
	<li>Locate the centroids of each triangle (solid black circles in Figure 2); these centroids are the locations of the dielectric rods of radius r11.</li>
	<li>Connect the centroids of the neighboring triangles (thick red lines in Figure 2) to generate cells around each point11.</li>
	<li>Create the CAD design file for the 2 cm tall HD base template with holes and slots on which the rods and walls will be assembled14. Use an HD pattern with the average inner-rod spacing of a=1.33 cm and set the hole-radius to be 2.5 mm and slot-width to be 0.38 mm. Set the depth for holes and slots to be 1 cm deep to stabilize the inserted rods and walls.</li>
	<li>Create a similar CAD design file for the crystalline base template (a square lattice) for comparison14. Use the same lattice constant as the HD structure (1.33 cm) and the same hole-radius (2.5 mm) and slot-width (0.38 mm).</li>
</ol>
2. Sample Construction and Preparation
<ol>
	<li>Fabricate the template. Manufacture the HD and Square lattice plastic bases by using a stereolithography machine that produces a solid plastic model by ultraviolet laser photo-polymerization. Use a clear resin, for example polycarbonate-like plastic. The resolution is 0.1 mm in both lateral and vertical directions. (See Figure 3, center panel).</li>
	<li>Prepare the building blocks: Order commercially available Alumina rods and thin walls cut to precise dimensions (see Figure 3, left panel). Set the height to be no less than a few wavelengths, for example 10.0 cm. The diameter of all rods is 5.0 mm. Wall thickness is always 0.38 mm and widths vary from 1.0 mm to 5.3 mm, with 0.2 mm increments. &#160;&#160;</li>
	<li>Construct the defect-free test structure for bandgap measurements. Insert rods and walls into the base for the desired structure architecture. The side view of the constructed network of both rods and walls on the polymer base is shown in Figure 3, right panel.</li>
	<li>Design a waveguide or a cavity defect: Create various waveguides through the samples by directly removing or modifying rods and walls along the designed path, as shown in Figures 9A and 9C. The modular design of the samples allows fast and easy modification of point and line or curve defects.&#160;</li>
</ol>
3. Major Instruments
<ol>
	<li>Use a synthesized sweeper (microwave generator) to provide microwaves with frequency coverage of 45 MHz to 50 GHz with precise 1 Hz frequency resolution. Connect the generator to an S-parameter test set to measure transmission parameters between the two ports (terminals). Use General Purpose Interface Bus (GPIB) links and cables for communications between the sweeper and the test-set.</li>
	<li>Use a Microwave Vector Network analyzer (VNA) to process the signal received from the S-parameter test-set and to measure the signal&#8217;s magnitude and the phase. Set the S-parameter test set to S21 mode so that the VNA outputs a data file containing the real and imaginary components of the detected E-field at port 1 with respect to the source signal from port 2 as a function of frequency</li>
</ol>
4. Instrument Setup
<ol>
	<li>Start/End Frequency.&#160; Select the appropriate start and end values of the frequency range for the measurement using the VNA user menu. The relevant frequency range associated with PBG depends on the dielectric index of lattice spacing of the samples. Use 7 GHz to 15 GHz microwaves for the Alumina samples with lattice spacing a=1.33 cm.</li>
	<li>Averaging Factor. Vector analyzer computes each data point based on the average of multiple measurements to reduce random noise. Select an averaging factor from 512 to 4,096 by inputting the desired multiple on the VNA keypad. Choose a higher averaging factor to minimize noise and chose a lower averaging factor for a quicker scan.</li>
	<li>Number of Points. For measurements in the 7 GHz to 15 GHz range, chose the maximum number of data points (801), on the VNA on-screen menu, to achieving a frequency resolution of 10 MHz.</li>
	<li>Calibration. Calibrate the system by directly measuring the relative transmission ratio, and normalize it against the transmission of a pre-calibrated setting with the same background and without the sample between the horn antennas. By doing this, all the background loss due to the cables, adaptors, waveguides, and antennas can be eliminated, and the relative transmission ratio with and without the tested sample is directly recorded.&#160;&#160;
	<ol>
		<li>For bandgap measurements, measure the microwave transmission through free space between the horns facing each other at the distance of 28a and save the results as a calibration set in the VNA. Before taking data for the actual experiment with a structure between the horns, turn on the calibration set by selecting &#8220;CALIBRATION ON&#8221; on the VNA monitor. Data calculated by the VNA will be automatically normalized against the calibration set and return the ratio of transmission power with and without the sample in place.</li>
		<li>For waveguide measurements, a meaningful calibration is not well-defined, since the transmission through the sample waveguides can easily exceed the calibrated transmission between the two horns in free space. Turn off calibration on the VNA monitor and record the raw transmission, which is the detected signal over the source signal. Place the horns right next to the waveguide channel openings to achieve the best coupling efficiency.</li>
	</ol>
	</li>
</ol>
5. Experimental Setup
<ol>
	<li>Configure the experimental setup shown in Figure 4. Use high quality semi-flexible coaxial cables to connect the S-parameter test-set ports with input/output waveguides. Connect pyramidal horn antennas with the ports through rectangular single mode waveguides and adaptors to ensure the radiation to be linearly polarized, The E-field of the radiation from the horn is parallel to the short edge of the horn.</li>
	<li>For bandgap measurements: Adhere to the following steps to measure the transmission through the defect-free samples to characterize the PBG of the defect free samples.
	<ol>
		<li>Align the horns vertically and horizontally to face each other. Arrange the horns at a far enough distance, such as 20 times of the average wavelength, so that the far-field radiation reaching the sample can be approximated to plane waves. Calibrate the transmission between the facing horns in free space without the testing sample and store it in calibration memory.</li>
		<li>Place defect-free structures made of rods and walls on the rotating stage in between the two facing horns. Turn on the calibration set recorded into VNA memory during step 5.2.1. The system is now ready to measure the relative transmission ratio through the sample normalized against the transmission power of the calibrated memory.</li>
	</ol>
	</li>
	<li>For waveguides and cavity defects measurements: Adhere to the following steps to setup the experiments:
	<ol>
		<li>Construct various waveguides and cavities by removing or replacing rods and walls in the defect-free structures, as shown in Figures 9A and 9C.&#160;&#160;&#160;&#160;</li>
		<li>Arrange the horns as close to the channel openings as possible to ensure good coupling into the channel. For curved and bent channels center the horns in the middle of the channel with the edge parallel to the opening.</li>
		<li>Turn off calibration. Now the VNA system is ready to measure and record the raw transmission ratio of the detected power at port 2 over the source power at port 1.</li>
	</ol>
	</li>
</ol>
6. Data Acquisition and Analysis
<ol>
	<li>Characterize the angular dependence of the photonic properties of the samples:
	<ol>
		<li>Place structures made of rods and walls with a nearly circular boundary on a rotating stage in between the two facing horns.</li>
		<li>Make sure that the calibration saved in the VNA memory is turned on in step 5.2.2. Zero the angle scale on the rotating stage and measure transmission through the structure. After the initial measurement at zero incident angle, rotate the sample and measure the transmission in equal angle increments, such as every 2&#176;&#160;until 180&#176;&#160;rotation is reached.</li>
	</ol>
	</li>
	<li>Characterize the polarization dependence of the photonic properties for the samples: 
	Perform all measurements described above in two different polarizations respectively, by changing the horn opening orientations. For TM polarization, set the horns&#8217; short edge (the E-field direction) perpendicular to the horizontal plane of the sample base and parallel to the rods. For TE polarization, rotate the horns 90 degrees, so that their short edges (the E-field direction) are in the horizontal plane.</li>
	<li>Characterize various waveguides channels: Make sure that the calibration is turned off in step 5.3.3. Place the horns next to the sample for best coupling. Measure the transmission through various channels constructed by removing and/or replacing rods and walls along the channel path. While monitoring the transmission signal on the VNA in real time, modify the channel path by adding and removing extra rods and walls for optimized transmission power or desired filtering bandwidth.</li>
	<li>Perform analogous measurements similar to what are described above on a square lattice photonic crystal for comparison.</li>
	<li>Data analysis. Analyze and graph the data using a computer program, such as MATLAB.&#160; Plot measured transmission as a function frequency (line plot), such as Figure 5, Figure 2, and Figure 9B and 9D to study the stopgap through the samples or transmission pass though the waveguide channels.&#160; Plot transmission as a function of frequency and angle (color contour plot) to analyze the stop bands characteristics of the structures and their angular dependence, as shown in Figure 6 and Figure 7.</li>
	<li>This protocol suggests presenting the measured transmission through the samples as a function of frequency and incident angle in polar coordinates12, in order to directly visualize the rotational symmetries and angular dependence of photonic properties. Generate the polar-coordinate plots to directly show the Brillouin zone boundaries of crystalline structures and reveal the relationship between PBG formation and Bragg scattering planes (Brillouin zone boundaries) in crystals and quasicrystals.</li>
</ol>

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S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photon+management+in+two-dimensional+disordered+media.">Photon management in two-dimensional disordered media.</a> Nature Mater. 11, 1017-1022 (2012).</li><li>Ishizaki, K., Koumura, M., Suzuki, K., Gondaira, K., Noda, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Realization+of+three-dimensional+guiding+of+photons+in+photonic+crystals.">Realization of three-dimensional guiding of photons in photonic crystals.</a> Nature Photon. 7, 133-137 (2013).</li><li>Florescu, M., Torquato, S., Steinhardt, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Designer+disordered+materials+with+large,+complete+PBGs.">Designer disordered materials with large, complete PBGs.</a> Proc. Natl. Acad. Sci. 106, 20658-20663 (2009).</li><li>Man, W., Megens, M., Steinhardt, P. J., Chaikin, P. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+measurement+of+the+photonic+properties+of+icosahedral+quasicrystals.">Experimental measurement of the photonic properties of icosahedral quasicrystals.</a> Nature. 436, 993-996 (2005).</li><li>Torquato, S., Stillinger, F. 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The authors have nothing to disclose.

Starting from a hyperuniform disordered point pattern, 2D HD structures consisting rods and/or wall network can be designed to obtain a complete PBG for all polarization11. Based on the design, we constructed a template with holes and slots for assembling 2D Alumina rods and walls structures at cm-scale which could be tested with microwaves. We chose to work with microwaves, because cm-scale building blocks, such as Alumina rods and walls, are inexpensive, and easily handled. We have experimentally demonstrate...

The existence of a bandgap for photons has been the focus of many scientific works, starting from the earlier studies done by Lord Rayleigh on the one-dimensional stop-band, a range of frequencies that are forbidden from propagation through a periodic medium1. Research into electromagnetic wave (EM) propagation in periodic structures has really flourished in the last two decades after the seminal publications of E. Yablonovitch2,3 and S. John4. The term &#8220;photonic crystal&#8221; was coined by Yablonovitch to describe the periodic dielectric structures that possessed a photonic bandgap (PBG). &#160;

Photonic crystals are periodic dielectric structures possessing discrete translational symmetries, rendering them invariant under translations in directions of periodicity. When this periodicity is matched with the wavelengths of incoming electromagnetic (EM) waves, a band of frequencies becomes highly attenuated and may stop propagating. If wide enough, the ranges of the forbidden frequencies, also called stop bands, may overlap in all directions to create a PBG, forbidding the existence of photons of certain frequencies.

Conceptually, EM wave propagation in photonic crystals is similar to electron wave propagation in semiconductor materials, which have a forbidden region of electron energies, also known as a bandgap. Similar to the way engineers have employed semiconductors to control and modify the flow of electrons through semiconductors, PBG materials can be used for various applications requiring optical control. For example, PBG materials can confine light of certain frequencies in wavelength size cavities, and guide or filter light along line defects in them5. PBG materials are suggested to be used for controlling the flow of light for applications in telecommunication6, lasers7, optical circuits and optical computing8, and solar energy harvesting9.

A two-dimensional (2D) square lattice photonic crystal has 4-fold rotational symmetry. EM waves entering the crystal at different angles of incidence (for example, 0&#176; and 45&#176; with respect to the lattice planes) will face different periodicities. Bragg scattering in different directions leads to stop bands of different wavelengths that may not overlap in all directions to form a PBG, without very high refractive-index contrast of the materials. In addition, in 2D structures, two different EM wave polarizations, Transverse Electric (TE) and Transverse Magnetic (TM), often form bandgaps at differing frequencies, making it even harder to form a complete PBG in all directions for all polarizations5. In periodic structures, the limited choices of rotational symmetry lead to intrinsic anisotropy (angular dependence), which not only makes it hard to form a complete PBG, but also greatly limits the design freedom of functional defects. For example, waveguide designs are proven to be restricted along very limited choices of major symmetry directions in photonic crystals10.

Inspired to surpass these limitations due to periodicity, much research has been done in the past 20 years on unconventional PBG materials. Recently a new class of disordered materials was proposed to possess an isotropic complete PBG in the absence of periodicity or quasiperiodicity: the hyperuniform Disorder (HD) PBG structure11. The photonic bands do not have exact analytical solution in disorder structures. Theoretical study of the photonic properties of the disordered structures is limited to time&#8211;consuming numerical simulations. To calculate the bands, the simulation needs to employ a super-cell approximation method and the available computational power may limit the finite size of the super-cell. To calculate transmission through these structures, computer simulations often assume ideal conditions and thus neglect real world problems like the coupling between the source and detector, the actual incident EM wave profile, and alignment imperfections12. Furthermore, any modification (defect design) of the simulated structure would require another round of simulation. Due to the large size of the minimum meaning for super-cell, it is very tedious and impractical to systematically explore various defect design architectures for these disordered materials.

We can avert these computational problems by studying the disordered photonic structures experimentally. Through our experiments we are able to verify the existence of the complete PBG in HD structures. Using microwave experiments, we can also obtain phase information and reveal the field distribution and dispersion properties of existing photonic states in them. Using an easily modifiable and modular sample at cm-scale, we can test various waveguide and cavity (defect) designs in the disordered systems and analyze the robustness of the PBGs. This kind of analysis of complex disordered photonic structures is either impractical or impossible to obtain through numerical or theoretical studies.

The design process begins by selecting a &#8220;stealthy&#8221; hyperuniform point pattern13. Hyperuniform point patterns are systems in which the number variance of the points within a &#8220;spherical&#8221; sampling window of radius R, grows more slowly than the window volume for large R, that is, more slowly than Rd in d-dimensions. For example, in a 2D Poisson random distribution of point pattern, the variance of the number of points in domain R is proportional to R2. However, in a hyperuniform disorder point pattern, the variance of the points in a window of radius R, is proportional to R. Figure 1&#160;shows a comparison between a hyperuniform disordered point pattern and a Poisson point pattern11. We use a subclass of hyperuniform disordered point patterns called &#8220;stealthy&#8221;11.

Using the design protocol described in Florescu&#160;et al11, we construct a network of dielectric walls and rods, creating a 2D hyperuniform dielectric structure similar to a crystal, but without the limitations inherent to periodicity and isotropy. The wall networks are favorable for TE-polarization bandgap, while the rods are preferable for forming band gaps with TM-polarization.&#160; A modular design was developed, so that the samples can be easily modified for use with different polarizations and for introducing freeform waveguides and cavity defects. Due to the scale invariance of Maxwell&#8217;s equations, the electromagnetic properties observed in the microwave regime are directly applicable to the infrared and optical regimes, where the samples would be scaled to micron and submicron sizes.

<table><tbody><tr><td>Stereolithography machine</td><td>3D Systems</td><td>SLA-7000</td><td></td></tr><tr><td>Resin for base</td><td>3D Systems</td><td>Accura 60</td><td></td></tr><tr><td>Alumina rods</td><td></td><td></td><td>r=2.5 mm, cut to 10.0 cm height</td></tr><tr><td>Alumina sheets</td><td></td><td></td><td>Thickness 0.38 mm, various width: from 1.0 mm to 5.3 mm with 0.2 mm increments</td></tr><tr><td>Microwave generator</td><td>Agilent/HP</td><td>83651B</td><td></td></tr><tr><td>S-Parameter test set</td><td>Agilent/HP</td><td>8517B</td><td></td></tr><tr><td>Microwave Vector Network Analyzer</td><td>Agilent/HP</td><td>8510C</td><td></td></tr></tbody></table>

using microwave and macroscopic samples of dielectric solids to study the photonic properties of disordered photonic bandgap materials

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). In this article we will describe the methods for constructing and characterizing macroscopic disordered photonic structures using microwaves. The microwave regime offers the most convenient experimental sample size to build and test PBG media. Easily manipulated dielectric lattice components extend flexibility in building various 2D structures on top of pre-printed plastic templates. Once built, the structures could be quickly modified with point and line defects to make freeform waveguides and filters. Testing is done using a widely available Vector Network Analyzer and pairs of microwave horn antennas. Due to the scale invariance property of electromagnetic fields, the results we obtained in the microwave region can be directly applied to infrared and optical regions. Our approach is simple but delivers exciting new insight into the nature of light and disordered matter interaction.

Our representative results include the first experimental demonstration of the existence of a complete and isotropic PBG in a two-dimensional (2D) hyperuniform disordered dielectric structure. Additionally we demonstrate experimentally the ability of this novel photonic structure to guide electromagnetic waves (EM) through freeform waveguides of arbitrary shape.

We have achieved the first confirmation ever of an isotropic complete PBG present in hyperuniform disorder dielectric structures. Here, we present our HD structure results and compare them to that of a periodic square lattice photonic crystal.
Figure 5 shows a semi-log plot of TE polarization transmission (dB) vs. frequency (GHz) for a hyperuniform disorder structure at one incident angle. This plot shows that the stop band region is located approximately between 8.5 and 9.5 G...

Watch this Scientific Journal Video about Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials at JoVE.com

Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Disordered structures offer new mechanisms for forming photonic bandgaps and unprecedented freedom in functional-defect designs. To circumvent the computational challenges of disordered systems, we construct modular macroscopic samples of the new class of PBG materials and use microwaves to characterize their scale-invariant photonic properties, in an easy and inexpensive manner.

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). ...

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12.2K Views.  San Francisco State University. Disordered structures offer new mechanisms for forming photonic bandgaps and unprecedented freedom in functional-defect designs. To circumvent the computational challenges of disordered systems, we construct modular macroscopic samples of the new class of PBG materials and use microwaves to characterize their scale-invariant photonic properties, in an easy and inexpensive manner.

Video: Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various ...

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we demonstrate an optical spectroscopy method that is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots (QDs) using a standard, easy-to-use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser line-width limited resolution with multi-channel photoluminescence detection. Such a scheme allows for considerable improvement of resolution over that of a common single-stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonon&#39;s energy difference allows one to separate and filter out the laser light exciting the quantum dot. An advantageous feature of this method is its straight forward integration into standard spectroscopy setups, which are accessible to most researchers.

The manuscript describes a method of phonon-assisted quasi-resonant fluorescence spectroscopy that incorporates both laser-limited resolution and photoluminescence (PL) spectroscopy. This method utilizes optical phonons to provide linewidth-limited resolution spectra of atom-like semiconductor structures in the energy domain. The method is also easily realized with a single spectrometer optical spectroscopy setup.

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we ...

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

Dielectric RheoSANS &#8212; Simultaneous Interrogation of Impedance, Rheology and Small Angle Neutron Scattering of Complex Fluids

A procedure for the operation of a new dielectric RheoSANS instrument capable of simultaneous interrogation of the electrical, mechanical and microstructural properties of complex fluids is presented. The instrument consists of a Couette geometry contained within a modified forced convection oven mounted on a commercial rheometer. This instrument is available for use on the small angle neutron scattering (SANS) beamlines at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The Couette geometry is machined to be transparent to neutrons and provides for measurement of the electrical properties and microstructural properties of a sample confined between titanium cylinders while the sample undergoes arbitrary deformation. Synchronization of these measurements is enabled through the use of a customizable program that monitors and controls the execution of predetermined experimental protocols. Described here is a protocol to perform a flow sweep experiment where the shear rate is logarithmically stepped from a maximum value to a minimum value holding at each step for a specified period of time while frequency dependent dielectric measurements are made. Representative results are shown from a sample consisting of a gel composed of carbon black aggregates dispersed in propylene carbonate. As the gel undergoes steady shear, the carbon black network is mechanically deformed, which causes an initial decrease in conductivity associated with the breaking of bonds comprising the carbon black network. However, at higher shear rates, the conductivity recovers associated with the onset of shear thickening. Overall, these results demonstrate the utility of the simultaneous measurement of the rheo-electro-microstructural properties of these suspensions using the dielectric RheoSANS geometry.

Here, we present a procedure for the measurement of simultaneous impedance, rheology and neutron scattering from soft matter materials under shear flow.

A procedure for the operation of a new dielectric RheoSANS instrument capable of simultaneous interrogation of the electrical, mechanical and ...

Surface Properties of Synthesized Nanoporous Carbon and Silica Matrices

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) with a 4.6 nm pore size, and ordered silica porous matrix, SBA-15, with a 5.3 nm pore size. This work describes the surface properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the differently ordered porous materials with similar pore sizes. For this purpose, OMC and SBA-15 with highly ordered nanoporous structures are synthesized via impregnation of the silica matrix by applying a carbon precursor and by the sol-gel method, respectively. The porous structure of investigated systems is characterized by an N2 adsorption-desorption analysis at 77 K. To determine the electrochemical character of the surface of synthesized materials, potentiometric titration measurements are conducted; the obtained results for OMC shows a significant pHpzc shift toward the higher values of pH, relative to SBA-15. This suggests that investigated OMC has surface properties related to oxygen-based functional groups. To describe the surface properties of the materials, the contact angles of liquids penetrating the studied porous beds are also determined. The capillary rise method has confirmed the increased wettability of the silica walls relative to the carbon walls and an influence of the pore roughness on the fluid/wall interactions, which is much more pronounced for silica than for carbon mesopores. We have also studied the melting behavior of D2O confined in OMC and SBA-15 by applying the dielectric method. The results show that the depression of the melting temperature of D2O in the pores of OMC is about 15 K higher relative to the depression of the melting temperature in SBA-15 pores with a comparable 5 nm size. This is caused by the influence of adsorbate/adsorbent interactions of the studied matrices.

Here we report the synthesis and characterization of ordered nanoporous carbon (with a 4.6 nm pore size) and SBA-15 (with a 5.3 nm pore size). The work describes the surface and textural properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the materials.

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) ...

Chemistry

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method

This work presents a protocol employing the microwave photoconductivity decay (&#956;-PCD) for measurement of the carrier lifetime in semiconductor materials, especially SiC. In principle, excess carriers in the semiconductor generated via excitation recombine with time and, subsequently, return to the equilibrium state. The time constant of this recombination is known as the carrier lifetime, an important parameter in semiconductor materials and devices that requires a noncontact and nondestructive measurement ideally achieved by the &#956;-PCD. During irradiation of a sample, a part of the microwave is reflected by the semiconductor sample. Microwave reflectance depends on the sample conductivity, which is attributed to the carriers. Therefore, the time decay of excess carriers can be observed through detection of the reflected microwave intensity, whose decay curve can be analyzed for estimation of the carrier lifetime. Results confirm the suitability of the &#956;-PCD protocol in measuring the carrier lifetime in semiconductor materials and devices.

As one of the important physical parameters in semiconductors, carrier lifetime is measured herein via a protocol employing the microwave photoconductivity decay method.

This work presents a protocol employing the microwave photoconductivity decay (&#956;-PCD) for measurement of the carrier lifetime in semiconductor ...

Probe Type II Band Alignment in One-Dimensional Van Der Waals Heterostructures Using First-Principles Calculations

Computational tools based on density-functional theory (DFT) enable the exploration of the qualitatively new, experimentally attainable nanoscale compounds for a targeted application. Theoretical simulations provide a profound understanding of the intrinsic electronic properties of functional materials. The goal of this protocol is to search for photocatalyst candidates by computational dissection. Photocatalytic applications require suitable band gaps, appropriate band edge positions relative to the redox potentials. Hybrid functionals can provide accurate values of these properties but are computationally expensive, whereas the results at the Perdew-Burke-Ernzerhof (PBE) functional level could be effective for suggesting strategies for band structure engineering via electric field and tensile strain aiming to enhance the photocatalytic performance. To illustrate this, in the present manuscript, the DFT based simulation tool VASP is used to investigate the band alignment of nanocomposites in combinations of nanotubes and nanoribbons in the ground state. To address the lifetime of photogenerated holes and electrons in the excited state, nonadiabatic dynamics calculations are needed.

Calculations performed by the Vienna Ab initio Simulation Package can be used to identify the intrinsic electronic properties of nanoscale materials and predict the potential water-splitting photocatalysts.

Computational tools based on density-functional theory (DFT) enable the exploration of the qualitatively new, experimentally attainable nanoscale ...

Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures

Photonic crystals are periodic nanostructures that can support a variety of confined electromagnetic modes. Such confined modes are usually accompanied by local enhancement of electric field intensity that strengthens light-matter interactions, enabling applications such as surface-enhanced Raman scattering (SERS) and surface plasmon enhanced sensing. In the presence of magneto-optically active materials, the local field enhancement gives rise to anomalous magneto-optical activity. Typically, the confined modes of a given photonic crystal depend strongly on the wavelength and incidence angle of the incident electromagnetic radiation. Thus, spectral and angular-resolved measurements are needed to fully identify them as well as to establish their relationship with the magneto-optical activity of the crystal. In this article, we describe how to use a Fourier-plane (back focal plane) microscope to characterize magneto-optically active samples. As a model system, here we use a plasmonic grating built out of magneto-optically active Au/Co/Au multilayer. In the experiments, we apply a magnetic field on the grating in situ and measure its reciprocal space response, obtaining the magneto-optical response of the grating over a range of wavelengths and incident angles. This information enables us to build a complete map of the plasmonic band structure of the grating and the angle and wavelength dependent magneto-optical activity. These two images allow us to pinpoint the effect that the plasmon resonances have on the magneto-optical response of the grating. The relatively small magnitude of magneto-optical effects requires a careful treatment of the acquired optical signals. To this end, an image processing protocol for obtaining magneto-optical response from the acquired raw data is laid out.

Photonic band structure enables understanding how confined electromagnetic modes propagate within a photonic crystal. In photonic crystals that incorporate magnetic elements, such confined and resonant optical modes are accompanied by enhanced and modified magneto-optical activity. We describe a measurement procedure to extract the magneto-optical band structure by Fourier space microscopy.

Photonic crystals are periodic nanostructures that can support a variety of confined electromagnetic modes. Such confined modes are usually accompanied by ...

Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids

Rational design of disordered molecular aggregates and solids for optoelectronic applications relies on our ability to predict the properties of such materials using theoretical and computational methods. However, large molecular systems where disorder is too significant to be considered in the perturbative limit cannot be described using either first principles quantum chemistry or band theory. Multiscale modeling is a promising approach to understanding and optimizing the optoelectronic properties of such systems. It uses first-principles quantum chemical methods to calculate the properties of individual molecules, then constructs model Hamiltonians of molecular aggregates or bulk materials based on these calculations. In this paper, we present a protocol for constructing a tight-binding Hamiltonian that represents the excited states of a molecular material in the basis of Frenckel excitons: electron-hole pairs that are localized on individual molecules that make up the material. The Hamiltonian parametrization proposed here accounts for excitonic couplings between molecules, as well as for electrostatic polarization of the electron density on a molecule by the charge distribution on surrounding molecules. Such model Hamiltonians can be used to calculate optical absorption spectra and other optoelectronic properties of molecular aggregates and solids.

Here, we present a protocol for parametrizing a tight-binding excitonic Hamiltonian for calculating optical absorption spectra and optoelectronic properties of molecular materials from first-principles quantum chemical calculations.

Rational design of disordered molecular aggregates and solids for optoelectronic applications relies on our ability to predict the properties of such ...