Menaka De Zoysa

Single-peak narrow-bandwidth mid-infrared thermal emitters based on quantum wells and photonic crystals

We experimentally demonstrate single-peak narrow-bandwidth thermal emission with a quality factor (Q factor) of more than 100 at a wavelength of 9.1 μm. The emission is significantly suppressed at all other wavelengths. Our emitter is based on an intersubband transition in a multiple quantum well structure combined with a single high-Q resonant mode in a two-dimensional photonic crystal slab, which allows strong light-matter interaction only at a specific wavelength. Strong thermal emission is exhibited only in a limited angular range (∼20°) from the normal direction. Our results have potential applications in bio- and environmental sensors.

Realization of narrowband thermal emission with optical nanostructures

The control of thermal emission spectra using optical resonances has been attracting increased attention both with respect to fundamental science and for various applications, including infrared sensing, thermal imaging, and thermophotovoltaics. In this mini-review, we describe the recent experimental demonstrations of narrowband thermal emission with optical nanostructures, including metallic cavities, metamaterials, and all-dielectric photonic crystals. The spectral features of the controlled thermal emission (e.g., wavelength, linewidth, peak emissivity, and angular characteristics) are strongly dependent on the choice of both materials and structures of the emitters. Through the appropriate design of optical nanostructures, arbitrary shaping of thermal emission spectra, from single-peak ultra-narrowband (Q>100) emission for midinfrared sensing to a stepwise emissivity spectrum for thermophotovoltaics, has been successfully realized.

Design of single-mode narrow-bandwidth thermal emitters for enhanced infrared light sources

We design efficient thermal emitters based on intersubband transitions (ISB-Ts) in quantum wells and two-dimensional photonic crystal (PC) slabs that have single-mode, very narrowband emission with high emissivity. Our design strategy involves positioning a single isolated mode of the PC within the absorption range of the ISB-T, where the mode’s radiation rate is precisely matched with the absorption rate of the ISB-T. The optimized design for this class of thermal emitters has a single-peak emission with a quality factor of ∼600, an emissivity of ∼0.9, and a radiation divergence cone of ∼20°, surpassing, by a large margin, the performance of previous designs. We also demonstrate, for practical application purposes, that the required input power for our best-performing emitter to reach a given temperature threshold is less than a factor of 200 compared to that of a blackbody.

Filter-free nondispersive infrared sensing using narrow-bandwidth mid-infrared thermal emitters

We experimentally demonstrate filter-free nondispersive infrared (NDIR) sensing of organic solvents using single-peak narrow-bandwidth mid-infrared thermal emitters. Our emitters are based on multiple quantum wells (MQWs) and two-dimensional (2D) photonic crystal (PC) slabs, and show a single thermal emission peak with a quality factor of over 100 at the fingerprint wavelength (around 9 µm) of the target organic solvents. Using these narrow-bandwidth thermal emitters and commercial pyroelectric sensors without any optical bandpass filters, we successfully distinguish and determine the concentration of the target solvents among other solvents.

Improved efficiency of ultra-thin µc-Si solar cells with photonic-crystal structures

We investigate the improvement of the conversion efficiency of ultra-thin (~500nm-thick) microcrystalline silicon (μc-Si) solar cells incorporating photonic-crystal structures, where light absorption is strongly enhanced by the multiple resonant modes in the photonic crystal. We focus on the quality of the intrinsic μc-Si layer deposited on the substrate, which is structured to form a photonic crystal at its upper surface with a period of several hundred nanometers. We first study the crystalline quality from the viewpoint of the crystalline fraction and show that the efficiency can be improved when the deposition conditions for the μc-Si layer are tuned to give an almost constant crystalline fraction of ~50% across the entire film. We then study the influence of the photonic-crystal structure on the crystalline quality. From transmission-electron microscope images, we show that the collision of μc-Si grains growing at different angles occurs when a photonic-crystal structure with an angular surface is used; this can be suppressed by introducing a rounded surface structure. As a result, we demonstrate an efficiency of 8.7% in a ~500-nm thick, homo-junction μc-Si solar cell, which has only ~1/4 the thickness of typical μc-Si solar cells. We also discuss the possibility of further improving the efficiency by performing calculations that focus on the absorption characteristics of the fabricated cell structure.

Experimental Demonstration of Quasi-resonant Absorption in Silicon Thin Films for Enhanced Solar Light Trapping

We experimentally demonstrate that the addition of partial lattice disorder to a thin-film microcrystalline silicon photonic crystal results in the controlled spectral broadening of its absorption peaks to form quasi resonances: increasing light trapping over a wide bandwidth while also reducing sensitivity to the angle of incident radiation. Accurate finite-difference time-domain simulations are used to design the active-layer photonic crystal so as to maximize the number of its absorption resonances over the broadband interval where microcrystalline silicon is weakly absorbing before lattice disorder augmented with fabrication-induced imperfections is applied to further boost performance. Such a design strategy may find practical use for increasing the efficiency of thin-film silicon photovoltaics.

Near-infrared–to–visible highly selective thermal emitters based on an intrinsic semiconductor

A Si nanorod array enables the concentration of thermal emission in the near-infrared range while suppressing other components. Control of the thermal emission spectra of emitters will result in improved energy utilization efficiency in a broad range of fields, including lighting, energy harvesting, and sensing. In particular, it is challenging to realize a highly selective thermal emitter in the near-infrared–to–visible range, in which unwanted thermal emission spectral components at longer wavelengths are significantly suppressed, whereas strong emission in the near-infrared–to–visible range is retained. To achieve this, we propose an emitter based on interband transitions in a nanostructured intrinsic semiconductor. The electron thermal fluctuations are first limited to the higher-frequency side of the spectrum, above the semiconductor bandgap, and are then enhanced by the photonic resonance of the structure. Theoretical calculations indicate that optimized intrinsic Si rod-array emitters with a rod radius of 105 nm can convert 59% of the input power into emission of wavelengths shorter than 1100 nm at 1400 K. It is also theoretically indicated that emitters with a rod radius of 190 nm can convert 84% of the input power into emission of <1800-nm wavelength at 1400 K. Experimentally, we fabricated a Si rod-array emitter that exhibited a high peak emissivity of 0.77 at a wavelength of 790 nm and a very low background emissivity of <0.02 to 0.05 at 1100 to 7000 nm, under operation at 1273 K. Use of a nanostructured intrinsic semiconductor that can withstand high temperatures is promising for the development of highly efficient thermal emitters operating in the near-infrared–to–visible range.

On-chip integration and high-speed switching of multi-wavelength narrowband thermal emitters

We experimentally demonstrate the high-speed, on-chip wavelength switching of thermal emission in the mid-infrared region. Our device consists of multiple integrated thermal emitters of different colors, each of which is composed of quantum wells and a photonic crystal. On current injection, the device exhibits narrowband (Q > 70) thermal emission with low electric power consumption. By applying a reverse bias to each section of the device, we achieve high-speed (>kHz) switching of multiple thermal emission wavelengths, opening a route towards compact, highly efficient mid-infrared light sources for various sensing applications.

Structural Optimization of Photonic Crystals for Enhancing Optical Absorption of Thin Film Silicon Solar Cell Structures

We carry out the structural design of photonic crystals to enhance the optical absorption of thin-film microcrystalline silicon (μc-Si) solar cells using two methods. First, by exhaustive search, we choose a structure with the largest absorption within the investigated patterns. Then we employ a sensitivity analysis to finely modulate the structure for further increase of the optical absorption. The obtained μc-Si solar cell structure with a photonic crystal in this work has more than twice as much optical absorption as that without a photonic crystal.

Enhanced efficiency of ultrathin (∼500 nm)-film microcrystalline silicon photonic crystal solar cells

Enhancing the absorption of thin-film microcrystalline silicon solar cells at 600–1000 nm wavelengths is very important to the improvement of the energy conversion efficiency. This can be achieved by creating a large number of resonant modes utilizing two-dimensional photonic crystal band edges, which exceeds the Lambertian limit of absorption in random textures. We focus on suppressing the parasitic absorption of back-reflector metal and doped layers in photonic crystal microcrystalline silicon solar cells. We achieve a high active-area current density of 22.6 mA cm−2 for an ultrathin (~500 nm)-film silicon layer and obtain an active-area efficiency of ~9.1%, as independently confirmed by the CSMT of AIST.