Thin Absorber Research Articles

Optical modeling of periodic nanostructures in ultra-thin CdTe solar cells with an electron reflector layer

CdTe solar cells have become a competitive player in the commercial photovoltaic market competing with CIGS and Si wafers. Considerable research attention has been devoted to increasing the overall conversion efficiency of CdTe which reached 22%. Among the challenges facing this technology is the scarcity of Te. In this paper, we investigated the use of an ultra-thin CdTe layer, which would result in a substantial decrease in Te consumption by almost 90%. However, this substantial decrease in the absorber thickness must be accompanied by some embedded nano-light-trapping structures. The purpose of these nanostructures is to increase the light absorption of the UV-VIS and NIR wavelengths which leads to an increase in the generated photo-current. Also, an electron reflector layer must be added between the metallic contact and the thin absorber layer to mitigate the high recombination rate. Additionally, three different metallic contacts were explored: Au, Ag, and Al. The absorption in the CdTe layer which is responsible for the photo-generated current and the parasitic absorption in the metallic back contact, as well as the short circuit current densities, were determined and analyzed. The numerical Wave optics finite element method and the analytical transmission line theory were utilized. It was noted that these nanostructures showed a substantial increase in the absorption in the CdTe absorber layer in the visible and NIR ranges. A net increase in the short circuit current density of 36% has been realized with the top nano-structured CdTe layer comparing to the flat reference cell of the same absorber volume. Furthermore, the Ag metallic back-contact outperforms both Al and Au and achieved the highest short circuit current density and the lowest parasitic absorption. These findings would open a new route to integrate optimized light trapping nano-structures into the commercial CdTe solar cells to further enhance their material utilization and efficiency. • Ultrathin cadmium telluride CdTe solar cells with electron reflector layer. • Light-trapping-structures in the thin-film solar cells. • Optical modeling of the ultra-thin solar cells. • Materials utilization of ultra-thin cadmium telluride solar cells. • Transmission line theory model for the thin-film solar cells.

Numerical study on the angular light trapping of the energy yield of organic solar cells with an optical cavity

A limiting factor in organic solar cells (OSCs) is the incomplete absorption in the thin absorber layer. One concept to enhance absorption is to apply an optical cavity design. In this study, the performance of an OSC with cavity is evaluated. By means of a comprehensive energy yield (EY) model, the improvement is demonstrated by applying realistic sky irradiance, covering a wide range of incidence angles. The relative enhancement in EY for different locations is found to be 11-14% compared to the reference device with an indium tin oxide front electrode. The study highlights the improved angular light absorption as well as the angular robustness of an OSC with cavity.

Equivalent Circuit Design Method for Wideband Nonmagnetic Absorbers at Low Microwave Frequencies

An accurate equivalent circuit (EC) design approach for wideband nonmagnetic absorbers operating at the low microwave frequency (1–10 GHz) is presented. Following the impedance matching approach, this communication introduces an EC model based on the simulated data and synthetic asymptotes for single- and double-layer frequency-selective surface (FSS)-based nonmagnetic absorbers. Two simple and commonly used resistive FSSs, i.e., square patch and single square loop, are considered in this communication. Compared to the full-wave simulations, the proposed EC model shows more than 95% accuracy. By employing the proposed model and genetic algorithm-based optimization, several designs of broadband absorbers are demonstrated. The presented single- and double-layer FSSs show 126% and 161% fractional bandwidth, respectively, with the total thickness close to the Rozanov limit. The results confirm that the proposed method is a simple and efficient way of designing thin wideband absorbers using single- or double-layer FSS configurations.

(Invited) Interfacial Engineering for High Voltage Performance of Perovskite Photovoltaic Cells

Since our first discovery of perovskite solar cell (PSC) in 2009, enormous efforts have been put into different aspects of PSCs and the progress has been incredibly fast on all fronts. The background, on-going R&Ds, and future direction of PSC research have been reported by our group as a comprehensive review.1 Although efficiency level has reached 25%, PSCs face serious challenges of practical stability and durability required for industrialization. Compositional engineering of lead halide perovskites by mixing different cations and anions, using modulator molecules and mixing 2D and 3D structures have improved the stability of perovskites against heat and moisture. However, organic cations in halide perovskites (methylammonium, etc.) and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120oC). In this respect, use of all-inorganic perovskite materials and dopant-free HTMs is highly desired.2 We have conducted the device fabrication in this direction using CsPbI3 and CsPbI2Br as solution-processed perovskite films. The perovskite absorbers were made into junction with dopant-free HTMs to develop thermally stable PSCs.By chemical engineering to improve the quality of interfacial structure at the hetero-junction, all-inorganic and dopant-free PSCs yielded power conversion efficiency up to 15%. Further we could enhance the open-circuit voltage (Voc) of this PSC up to 1.42V, which is the highest Voc ever achieved with a single perovskite cell.3 In preparation of all-inorganic perovskites, a big challenge should also be directed to development of lead-free perovskite materials for environmental safety in practical applications.2 Among extensive applications of PSCs for outdoor and indoor power generation, use of PSCs in space environments is promising because thin perovskite photovoltaic films demonstrate high stability and tolerance against exposure to high energy particle irradiations (proton and electron beams).4 Thin absorbers (<500 nm) avoid accumulation of particles and due to intrinsic defect tolerant nature of perovskites, radiation-induced collision damage is highly suppressed. Our current efforts in making PSCs based on both lead and lead-free perovskites, and future perspectives of perovskite photovoltaics will be presented.

Identification of surface and volume hot-carrier thermalization mechanisms in ultrathin GaAs layers

Hot-carrier solar cells offer the opportunity to harvest more energy than the limit set by the Shockley–Queisser model by reducing the losses due to the thermalization of photo-generated carriers. Previous reports have shown lower thermalization rates in thinner absorbers, but the origin of this phenomenon is not precisely understood. In this work, we investigate a series of ultrathin GaAs absorber layers sandwiched between AlGaAs barriers and transferred on host substrates with a gold back mirror. We perform power-dependent photoluminescence characterizations at different laser wavelengths from which we determine the carrier temperature in four absorber thicknesses between 20 and 200 nm. We observe a linear relationship between the absorbed power and the carrier temperature increase. By relating the absorbed and thermalized power, we extract a thermalization coefficient for all samples. It shows an affine dependence with the thickness, leading to the identification of distinct volume and surface contributions to thermalization. We confirm that volume thermalization is linked to LO phonon decay. We discuss the origin of the interface-related thermalization, showing that the effect of LO phonon transport is negligible. Overall, this work sheds new light on thermalization processes in ultrathin semiconductor layers and introduces a method to compare the performance of hot-carrier absorbers.

Design and simulation of a wideband, wide-angle and polarization-insensitive microwave absorber based on pattern optimization of resistive films

A wideband, thin, and multilayer (planar) microwave absorber is designed in this paper. The absorber is a periodic array of cells which includes three patterns of resistive film on two dielectric substrates. To design the array’s cell, a pattern optimization procedure based on the application of a binary optimization algorithm was utilized. The outcome of this procedure is a pixelated pattern of resistive film yielding the maximum possible absorption bandwidth (BW) and a high insensitivity to the polarization of the incident wave. Our pattern-optimized design achieved a 90%-absorption rate over a wide frequency BW of 1.6 to 23.7 GHz (175% fractional BW) under normal incidence. Moreover, the absorber operates over a wide range of incidence angles for obliquely incident waves.

Defect depth-profiling in kesterite absorber by means of chemical etching and surface analysis

A method to probe the depth morphology, defect profile and possible secondary phases in a thin film semiconductor is presented, taking a standard Kesterite film as an example. Using a top-down approach based on a previously reported controlled Methanol-Br2 chemical etching, well-defined slabs of a state of the art Kesterite absorber are fabricated. The analysis of their morphology both by Scanning Electron Microscopy and 3D optical Profilometry reveals the extent of a previously reported poor film morphology toward the back interface, and we are able to determine that more than 50% of a standard absorber is disconnected from the substrate. More importantly, these etched films are subsequently analyzed by surface sensitive techniques such as X-ray Photoelectron Spectroscopy and UV-Raman analysis. An accurate composition profile is established, and for the first time, a direct observation of the defects’ nature and their depth profiling in Kesterite is made possible. While VCu are found with a constant amount throughout the absorber, indicating a homogenous carrier concentration, a prevalence of the ZnSn defect is observed with a steep gradient toward the back interface, associated with an increase in the SnSe2 secondary phase. With bulk defects being often pointed out as the intrinsic limitation of this material, this result highlights what possibly is the main impediment of Kesterite solar cells, and a critical point to address in the design of future devices. Beyond the case of Kesterite absorbers, the method presented here offers a combination of simplicity, tunability and versatility making a straightforward transfer to other emerging thin film absorbers feasible, and it could possibly be an important tool in their future performance assessment and comparison.

Spectral reflectance optimization for planar perovskite solar cells

Perovskite solar cells (PSC) have been under development for a few years only, and quickly they have achieved high record efficiencies, above 25 %, attracting great attention as a possible alternative to commercial Si solar cells. However, very few articles have dealt with the optical losses in this type of solar cells, but an important factor to be taken in account for developing highly efficient extra-thin solar cells, such as Perovskite Solar Cells, is the optical reflectance in the solar spectrum for these devices. Then, in this work, we do calculations for planar multilayer perovskite solar cells using the Optical Matrix method for determining the expected reflectance as a function of wavelength, using refraction index and extinction coefficients for each of the materials that typically constitute these solar cells. Our results show that for very thin perovskite absorber layers (less than 350 nm), the optical losses can become very high. Even for a typical perovskite thickness above 450 nm, the total average reflectance in the solar spectrum can be as high as 10 %, approximately.

Wheat flour-derived nanoporous carbon@ZnFe2O4 hierarchical composite as an outstanding microwave absorber

Thin and lightweight absorbers with strong absorption capacity are future development directions in the field of electromagnetic wave (EMW) absorption. Based on two microwave-absorbing mechanisms, the three-dimensional (3D) porous carbon materials derived from wheat flour were prepared via pyrolysis carbonization. Then, ZnFe2O4 nanospheres were synthesized and distributed on the surface of carbon skeleton by simple solvothermal method. Results showed that microwave absorption performance was significantly improved due to synergistic effects between ZnFe2O4 nanospheres and carbon matrix. With absorber thickness of only 1.8 mm and low filler loading of 20 wt%, the minimum reflection loss could reach as low as −54.1 dB at 14.1 GHz. The widest effective absorption bandwidth ranged from 9.2 GHz to 15.0 GHz (5.8 GHz) with a thickness of 2.5 mm when the filler loading was 10 wt%, covering 36% of entire measured bandwidth. As an absorber, 3D porous carbon@ZnFe2O4 composite exhibited ideal microwave absorption properties because of synergistic effects between dielectric and magnetic loss, well-matched impedance, interfacial polarization, Debye relaxation process as well as multiple reflection and scattering. Considering excellent EMW absorption performance, the porous carbon@ZnFe2O4 composite fabricated by a simple method can provide a new inspiration for designing structure and composition of lightweight microwave absorbers.

Extending the view of ArH+chemistry in diffuse clouds

Context.One of the surprises of theHerschelmission was the detection of ArH+towards the Crab Nebula in emission and in absorption towards strong Galactic background sources. Although these detections were limited to the first quadrant of the Galaxy, the existing data suggest that ArH+ubiquitously and exclusively probes the diffuse atomic regions of the interstellar medium.Aims.In this study, we extend the coverage of ArH+to other parts of the Galaxy with new observations of itsJ= 1−0 transition along seven Galactic sight lines towards bright sub-millimetre continuum sources. We aim to benchmark its efficiency as a tracer of purely atomic gas by evaluating its correlation (or lack of correlation as suggested by chemical models) with other well-known atomic gas tracers such as OH+and H2O+and the molecular gas tracer CH.Methods.The observations of theJ= 1−0 line of ArH+near 617.5 GHz were made feasible with the new, sensitive SEPIA660 receiver on the APEX 12 m telescope. Furthermore, the two sidebands of this receiver allowed us to observe theNKaKc= 11,0−10,1transitions of para-H2O+at 607.227 GHz simultaneously with the ArH+line.Results.We modelled the optically thin absorption spectra of the different species and subsequently derived their column densities. By analysing the steady state chemistry of OH+and o-H2O+, we derive on average a cosmic-ray ionisation rate,ζp(H), of (2.3 ± 0.3) × 10−16s−1towards the sight lines studied in this work. Using the derived values ofζp(H) and the observed ArH+abundances we constrain the molecular fraction of the gas traced by ArH+to lie below 2 × 10−2with a median value of 8.8 × 10−4. Combined, our observations of ArH+, OH+, H2O+, and CH probe different regimes of the interstellar medium, from diffuse atomic to diffuse and translucent molecular clouds. Over Galactic scales, we see that the distribution ofN(ArH+) is associated with that ofN(H), particularly in the inner Galaxy (within 7 kpc of the Galactic centre) with potentially even contributions from the warm neutral medium phase of atomic gas at larger galactocentric distances. We derive an average ortho-to-para ratio for H2O+of 2.1 ± 1.0, which corresponds to a nuclear spin temperature of 41 K, consistent with the typical gas temperatures of diffuse clouds.

Highly stable ethanol-based Cu2ZnSnS4 (CZTS) low-cost thin film absorber: Effect of solution aging

Aging is an important factor that can affect the grain size, surface morphology and optical band gap of the thin films. Especially, it is thought that these important parameters will have a strong impact on the solar cell energy conversion ratio in Cu2ZnSnS4 (CZTS) absorber based photovoltaic devices. With this aim the presented study reports the effect of the aging time on the structural, morphological and optical characteristics of p-type quaternary CZTS absorber materials, produced by using a solution assisted dip-coating process on the glass substrate and annealed at 515 °C without using any dangerous environment (such as sulphurization or selenization). The kesterite crystal structure was determined by X-ray diffraction (XRD) analyses for all the CZTS samples. To support the observed kesterite crystal structure of the CZTS, the main vibration modes and chemical composition of the samples were investigated by Raman and XPS tools. Besides, the comprehensive surface analyses were conducted to better understanding of the morphology variation within the CZTS thin films depending on the aging time of the CZTS precursor solution by using SEM and AFM tools. Generally, large grain size and solution stability of absorber material have important impacts on the solar cell energy conversion solar cells. Furthermore, the band gap of the CZTS thin film shifts toward the optimal energy value (from 1.82 eV to 1.56 eV), observed from the optical studies. This last result is highly attractive for the CZTS based solar cell applications.

Cornstalk-derived macroporous carbon materials with enhanced microwave absorption

Biomass transformation is being considered as a green and sustainable strategy for carbon-based functional materials in many fields. To produce porous structure favorable for microwave absorption, we demonstrate herein the successful synthesis of macroporous carbon materials with cornstalk as a biomass precursor. It is found that two kinds of typical biological structures in cornstalk, linear vascular bundles and parenchyma cells, can be well preserved during high-temperature pyrolysis. Mercury intrusion porosimetry and N2 adsorption indicate that these cornstalk-derived carbon materials have very high porosity, which is mainly from desirable macroporous structure rather than conventional micro/mesopores. Electromagnetic (EM) analysis reveals that dielectric loss is the only pathway for the consumption of EM energy, and high pyrolysis temperature favors strong dielectric loss through conductive loss and interfacial polarization loss. Meanwhile, pyrolysis temperature also affects the matching degree of characteristic impedance. When the pyrolysis temperature reaches 750 °C, good dielectric loss and impedance matching endow the sample (CSC-750) with excellent microwave absorption performance, including strong reflection loss, broad response bandwidth, and relatively thin absorber thickness. The advantages of macroporous structure are further highlighted in impedance matching and multiple reflection by comparing with a macropore-free counterpart.

Acoustic characterizations of Helmholtz resonators with extended necks and their checkerboard combination for sound absorption

This study presents a thin absorber for low-frequency noise mitigation based on a Helmholtz resonator with an extended neck (HREN). An analytical model is established based on the transfer matrix method and the equivalent medium model to predict the acoustic characteristics of the HREN-based absorber, and is validated by experiments and simulations. The acoustic properties of the HREN are characterized comprehensively, including the effects of the radius and length of the extended neck. It is demonstrated that a feature of the HREN is its thin thickness (around 1/30th wavelength of the operating frequency), and its resonance frequency can be flexibly tuned by adjusting the geometry of the extended neck. The HREN can achieve effective low-frequency absorption in a constricted space, but is only effective within a narrow bandwidth. To overcome the limitation, a checkerboard absorber consisting of alternately distributed inhomogeneous HRENs is investigated. Results show that the HRENs in a checkerboard absorber operate almost independently, resulting in a dual-band sound absorber, when the adjacent HREN units are largely dissimilar. In contrast, a bandwidth extension of effective absorption can be obtained if the adjacent HRENs in the checkerboard absorber are strongly coupled. The absorber with a broadened absorption bandwidth exhibits a quasi-perfect absorption (the absorption coefficient above 0.85) within the bandwidth between the two absorption peaks induced by two corresponding HRENs.

Ba-acceptor doping in ZnSnN2 by reactive RF magnetron sputtering: (002) faceted Ba–ZnSnN2 films

Zn-IV-N2 semiconductors display outstanding optoelectronic behavior and hence developed as an effective thin film absorber materials in photovoltaic devices. However, alkaline-earth metallic dopants can improve the performance of Zn-IV-N2 semiconductors. In view of this, Barium (Ba) acceptor was successfully doped into ZnSnN2 crystal lattice with various dopant concentrations (∼0–7%) by reactive RF magnetron co-sputtering at 450 °C. The orthorhombic structured Ba doped ZnSnN2 (Ba:ZTN) films with preferential orientation along (002) plane were obtained. Hall measurement studies revealed that the type of conduction is changed from the conventional n-type to p-type via Ba doping. The deposited Ba:ZTN films exhibited high hole concentration in the range of 1018-1019 cm−3 with low resistivity of ∼10−1 Ω cm and a reasonable mobility of ∼0.5–5 cm2 V−1 s−1. The doping of Ba at either Zn or Sn site would be responsible for the p-type conductivity in Ba:ZTN films. The variations in the surface properties such as morphology and topography of ZnSnN2 on Ba doping were described by FE-SEM and AFM analysis. Further analysis by UV–Vis–NIR and X-ray photoelectron spectroscopies were also performed in order to reveal the optical performance and chemical bonding, composition of Ba:ZTN thin films. The existence of Ba2+ ions in Ba:ZTN was confirmed by the obtained binding energies from XPS analysis. From the collective results, it is suggested that Ba:ZTN could be employed as effective p-type layer in thin film solar cells.

Perfect mid-infrared dual-band optical absorption realized by a simple lithography-free polar dielectric/metal double-layer nanostructure.

A perfect mid-infrared dual-band absorber based on a very simple lithography-free polar dielectric/metal double-layer nanostructure is demonstrated experimentally. Silicon dioxide (SiO2) is chosen as the top polar dielectric, which is deposited through room-temperature plasma enhanced chemical vapor deposition to protect the bottom metal layer. A nearly 100% absorption is obtained at the wavelength of ∼ 10 µm due to the constructive interference resonance, which is related to the SiO2 thickness but insensitive to the light polarization or incident angle. Another enhanced absorption is observed experimentally at ∼ 8 µm under oblique incidence. Both numerical simulation and analytical calculation show that such absorption enhancement is induced by the excitation of the Berreman mode, where the refracted light propagates almost horizontally within the SiO2 layer. Different from the interference-induced absorption, the Berreman mode induced absorption exists even for a very thin absorber and is sensitive to the light polarization and incident angle.

Design of Ti3C2Tx/TiO2/PANI multi-layer composites for excellent electromagnetic wave absorption performance

Ti3C2Tx MXene is an excellent electromagnetic wave (EMW) absorber with excellent electrical conductivity and abundant surface functional groups. In this research, Ti3C2Tx/TiO2/PANI multi-layer composites were successfully synthesized by HCl and LiF etching, one-step hydrothermal method and in-situ polymerization. Ti3C2Tx can provide more electron transfer paths due to its unique multilayer structure and high specific surface area. The growth of TiO2 particles on the surface of Ti3C2Tx through hydrothermal reaction enhances the interface polarization, and then polyaniline (PANI) is doped on the surface of Ti3C2Tx where TiO2 particles are grown by in-situ polymerization. Due to the excellent dielectric properties and synergistic effects of the material itself, Ti3C2Tx/TiO2/PANI composites have excellent EMW absorption property. In this study, the Ti3C2Tx/TiO2/PANI composites showed the strong reflection loss (RL) of at 13.92 GHz, which was −65.61 dB, and the thickness was only 2.18 mm. Moreover, the composites also exhibit a wide absorption band, with an effective absorption bandwidth (RL < -10 dB) of 5.92 GHz (11.84 GHz to 17.76 GHz) at 2.10 mm. The results show that the Ti3C2Tx/TiO2/PANI composites are expected to become EMW absorber with thin thickness and high absorption strength.

RCS reduction of microstrip antenna using split square loop thin absorber

This study proposes an electrically thin frequency-selective surface-based microwave absorber without using resistive surface and lumped resistors to make it simple and inexpensive. The ground plane of the microstrip antenna (MSA) serves as the metallic back of the absorber. The top two layers of the proposed absorber are printed on both sides of the top FR4 sheet in a square annular ring form to encircle the square patch. Both simulations and measurements are carried out to obtain the scattering parameters, which are used to retrieve the absorption coefficient, relative permittivity (ɛ r), and relative permeability (μ r). An equivalent circuit model for the proposed absorber is constructed and validated. The absorption performance is insensitive to the angle of incidence (θ) up to 50°. The absorption mechanisms are studied based on the surface current distribution, electric field distribution, and constitutive parameters. The absorber is then integrated with an MSA operating at 4.2 GHz, which gives a maximum monostatic radar cross-section (RCS) reduction of 15 dB. Numerical and experimental results of the radiation performance and the RCS of the absorber integrated antenna structure have been presented. A very good agreement between simulation and measurement results is observed.

Cu(In,Ga)Se2 thin film solar cells produced by atmospheric selenization of spray casted nanocrystalline layers

Cu(In,Ga)Se2 (CIGS) is suitable absorber material for thin film photovoltaic owing to its excellent thermo-chemical stability and demonstration of high power conversion efficiency of 23.35% for lab-scale devices. The manufacturing of CIGS thin film solar cells need to be economical for commercially viability. Non-vacuum approach for the synthesis of the CIGS thin film absorber is desired for its cost-effective benefits over vacuum based processes. We report development of solution-processed CIGS absorber layer involving spraying of sonochemically prepared nanocrystal ink to obtain pristine CIGS nanocrystal layer, followed by single-step atmospheric pressure selenization utilizing Se pallets as a Se vapor source. Influence of selenization conditions and NaF doping on grain growth of the CIGS thin film absorber is studied in detail by structural, morphological, compositional and electrical characterization. Solar cells prepared under optimized conditions yielded a maximum efficiency of 6.7% without employing any thermal pre-treatment and toxic Potassium Cyanide etching.

Dielectric, magnetic, and microwave absorbing properties of Ag-plated terapod-like ZnO whiskers

Thin thickness, lightweight, wideband and high absorption intensity are highly desirable properties for the next-generation electromagnetic wave absorbing materials. This study investigates the dielectric, magnetic, and microwave absorbing properties of terapod-like ZnO whisker (ZnOW) modified by Ag nanocrystals (Ag-ZnOW), synthesized by vacuum evaporation method. The complex permittivity and permeability were measured at a microwave frequency range of 2–18 GHz. Ag nanocrystals plated on the surface of whiskers enhance the magnetic loss of ZnOW. The Ag-ZnOW possesses much broader bandwith of absorbing peak than that of unplated ZnOW. The maximum reflectivity (R) is about −30.27 dB at 11.40 GHz and the corresponding bandwidth below −10 dB is more than 5.25 GHz with a matching thickness of 3.0 mm. The calculated results show that the peak of R shifts to lower frequency region with the increase of thickness, and the Ag-ZnOW is an excellent candidate for designing lightweight and multipurpose microwave absorbers.

Spatial and spectral mode mapping of a dielectric nanodot by broadband interferometric homodyne scanning near-field spectroscopy

We investigate the optical properties of nanostructures of antimony sulfide (Sb2S3), a direct-bandgap semiconductor material that has recently sparked considerable interest as a thin film solar cell absorber. Fabrication from a nanoparticle ink solution and two- and three-dimensional nanostructuring with pattern sizes down to 50 nm have recently been demonstrated. Insight into the yet unknown nanoscopic optical properties of these nanostructures is highly desired for their future applications in nanophotonics. We implement a spectrally broadband scattering-type near-field optical spectroscopy technique to study individual Sb2S3 nanodots with a 20-nm spatial resolution, covering the range from 700 to 900 nm. We show that in this below-bandgap range, the Sb2S3 nanostructures act as high-refractive-index, low-loss waveguides with mode profiles close to those of idealized cylindrical waveguides, despite a considerable structural disorder. In combination with their high above-bandgap absorption, this makes them promising candidates for applications as dielectric metamaterials, specifically for ultrafast photoswitching.