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Two photodetectors for measuring transmission and two bulky, separated narrowband filters for picking a target gas absorption line and a non-absorbing reference from broadband emission are typically required for dual-band non-dispersive infrared (NDIR) gas sensing. Metal-dielectric-metal (MDM) metasurface plasmon cavities, precisely controllable narrowband absorbers, suggest a next-generation, nanophotonic approach. Here, we demonstrate a dual-band MDM cavity detector that consolidates the function of two detectors and two filters into a single device by employing resonant photon sorting-a function unique to metasurfaces. Two MDM cavities sandwiching a quantum well infrared photodetector (QWIP) with distinct resonance wavelengths are alternately arranged in a subwavelength period. The large absorption cross section of the cavities ensures ~95% efficient lateral sorting of photons by wavelength into the corresponding detector within a near-field region. The flow of incident photons is thus converted into two independent photocurrents for dual-band detection. Our dual-band photodetectors show competitive external quantum efficiencies up to 38% (responsivity 2.1 A/W, peak wavelength 6.9 5m) at 78 K. By tailoring one resonance to an absorption peak of NO2 (6.25 5m) and the other to a non-absorbing reference wavelength (7.15 5m), NDIR NO2 gas sensing with 10 ppm accuracy and 1 ms response times is demonstrated. Through experiment and numerical simulation, we confirm near-perfect absorption at the resonant cavity and suppressed absorption at its non-resonant counterpart, characteristic of resonant photon sorting. Dual-band sensing across the mid-infrared should be possible by tailoring the cavities and quantum well to desired wavelengths.

The integration of quantum well infrared photodetectors with plasmonic cavities has allowed for demonstration of sensitive photodetectors in the mid-infrared up to room-temperature operating conditions. However, clear guidelines for optimizing device structure for these detectors have not been developed. Using simple stripe cavity detectors as a model system, we clarify the fundamental factors that improve photodetector performance. By etching semiconductor material between the stripes, the cavity resonance wavelength was expected to blue-shift, and the electric field was predicted to strongly increase, resulting in higher responsivity than unetched stripe detectors. Contrary to our predictions, etched stripe detectors showed lower responsivities, indicating surface effects at the sidewalls and reduced absorption. Nevertheless, etching led to higher detectivity due to significantly reduced detector dark current. These results suggest that etched structures are the superior photodetector design, and that appropriate sidewall surface treatments could further improve device performance. Finally, through polarization and incidence angle dependence measurements of the stripe detectors, we clarify how the design of previously demonstrated wired patch antennas led to improved device performance. These results are widely applicable for cavity designs over a broad range of wavelengths within the infrared, and can serve as a roadmap for improving next-generation infrared photodetectors.

Purcell effect enhancement of spontaneous emission rates is demonstrated for isoelectronic trap single-photon emitters. Two-dimensional photonic crystal slabs with L3 defects were fabricated in nit...

We study the angle-dependent reflection spectra of two-dimensional photonic crystal (PC) slabs, which we use for developing mid-infrared surface-emitting lasers. The PC design produces a perfect resonance between material gain frequencies and the [Formula: see text]-point band edge in the first Brillouin zone. Hence, we expect laser emission to occur in a direction normal to the slab surface. We fabricate PC slabs, which consist of a square lattice of nanopillars with different lateral cross-sections, i.e., circles, triangles, and pentagons, and discuss their impact on the mode characteristics. We consider that our angle-resolved spectroscopic technique will be useful for characterizing mid-infrared PC slabs to be used in vertical surface-emitting quantum cascade lasers.

We propose and numerically investigate integrated photonic crystal waveguides (PhC-WGs) formed in a semiconductor slab to realize an ultrasmall and highly efficient terahertz (THz) wave source. The structure consists of a straight PhC-WG with low-group-velocity and low-dispersion (LVLD) for efficient difference frequency generation (DFG) connected to two PhC-WGs to introduce two fundamental lights into the LVLD PhC-WG. The fundamental light propagating through each PhC-WG designed to enhance their electric fields by the slow-light effect is efficiently coupled to the LVLD PhC-WG owing to the reduced refractive index differences at the boundaries of the heterostructures. The DFG from the two fundamental lights was numerically simulated, and a temporal intensity oscillation corresponding to the difference in frequency was clearly observed. By comparing the DFG intensities of the integrated structures with an LVLD PhC-WG and a strip WG, the estimated DFG intensity from the LVLD PhC-WG was more than 100 times higher than that from the strip WG. These results indicate the effectiveness of the proposed heterostructure in the application of a highly efficient THz source with an ultrasmall footprint compared with conventional materials.

We realized a wide-band wavelength conversion method through four-wave mixing in W3-type AlGaAs photonic crystal waveguides. AlGaAs exhibits a large third-order nonlinearity. Furthermore, because of its large bandgap, two-photon absorption can be avoided in the 1550 nm range. A four-wave mixing efficiency of −7 dB was obtained for a pump peak power of 7 W. Furthermore, by utilizing the two even guided bands of the W3-type photonic crystal waveguide, a conversion bandwidth greater than 38 nm was achieved with a conversion efficiency of −22 dB.

We measured angle-resolved reflection spectra of triangular-lattice photonic crystal slabs fabricated in a silicon-on-insulator wafer in the mid-infrared region. We achieved a high angle-resolution measurement by means of our homemade optical setup integrated in the sample chamber of an FT-IR spectrometer. By examining the reflection peak frequency as a function of the lateral component of the wave vector of the incident light and applying the selection rules expected from the spatial symmetry of electromagnetic eigenmodes in C6v-symmetric structures, we successfully obtained the dispersion relation and the mode symmetry of the photonic crystal slabs, which agreed well with numerical calculations by the finite element method. We also found the redistribution of diffraction loss between A1- and E1-symmetric modes, which was caused by the Dirac-cone formation due to their effective degeneracy.

The catalyst-free formation of silicon (Si) and germanium (Ge) core-shell and core-double shell nanowires (NWs) was studied for use as building blocks of high electron (hole) mobility transistors (HEMTs). Vertically aligned p-type Si (p-Si)/intrinsic Ge (i-Ge) core-shell NWs and p-Si/i-Ge/p-Si core-double shell NWs with uniform diameters were formed by combining nanoimprint lithography, Bosch etching, and chemical vapor deposition. The boron (B) doping process was used to prepare p-Si NWs. The hole gas accumulation could be reliably detected from the i-Ge shell region in the p-Si/i-Ge core-shell NW and p-Si/i-Ge/p-Si core-double shell NW arrays through the Fano resonance effect, showing that core-shell NW heterostructures can suppress impurity scattering and act as high-mobility transistor channels. This provides the possibility for the future creation of vertical high-speed transistors.

Photonic crystals (PCs) offer unique ways to control light-matter interactions. The measurement of dispersion relations is a fundamental prerequisite if we are to create various functionalities in PC devices. Angle-resolved spectroscopic techniques are commonly used for characterizing PCs that work in the visible and near-infrared regions. However, the techniques cannot be applied to the mid- and long-wavelength infrared regions due to the limited sensitivity of infrared detectors. Here, we propose an alternative approach to measuring infrared dispersion relations. We construct a high-precision angle-resolved setup compatible with a Fourier-transform spectrometer with an angle resolution as high as ${0.3}^{\ensuremath{\circ}}$. Hence, the reflection spectra are mapped to the two-dimensional (2D) photonic band structures of In($\mathrm{Ga}$,$\mathrm{Al}$)As/$\mathrm{In}\mathrm{P}$-based PC slabs, which are designed as midinfrared-PC surface-emitting lasers. We identify complex PC modes with the aid of polarization-selection rules derived by group theory. Spectral analysis makes it possible to evaluate the mode quality ($Q$) factors. Therefore, angle-resolved reflection is a useful way of optimizing 2D PC parameters for midinfrared devices.

In this study, an optical gain chip using emission-wavelength-controlled self-assembled InAs quantum dots (QDs) was developed for swept-source optical coherence tomography (SS-OCT) applications. The optical characterizations indicated that the QDs emission wavelength and optical gain spectra were controlled in the 1.1μm waveband by optimizing the QDs growth conditions. This waveband is useful for obtaining a large imaging depth of OCT because of an optimal balance between absorption and scattering in biological samples. In addition, continuous tunable lasing in the waveband was achieved by introducing the QD-based gain chip into a grating-coupled external cavity. This tunable laser was introduced into an SS-OCT setup, and the point spread function (PSF) was evaluated. The PSF position was observed to vary according to the optical path length differences. These results demonstrate the feasibility of the application of emission-wavelength-controlled QDs for SS-OCT.

We fabricated surface-emitting quantum cascade lasers with photonic crystal resonators whose active layers were strain-compensated InGaAs/AlInAs multiple quantum wells to operate at 4.3 μm. We tested two kinds of square-lattice photonic crystals consisting of circular and pentagonal InGaAs cylinders as a unit structure. We examined their output power and far-field pattern to find that both of them were improved by lowering the photonic-crystal symmetry to achieve a high extraction efficiency. The maximum output power was 50 mW at 77 K and the far-field pattern was a single peak with a divergence angle smaller than 1° for the pentagonal cylinder structure. In addition, we attributed the lasing mode to a B2-symmetric (quadrupole) eigenstate on the Γ point for the circular cylinder structure by observing the polarization dependence of the far-field pattern and comparing it with calculations by the k · p perturbation method.

Complex lightwave manipulation such as broadband absorption has been realized with metasurfaces based on laterally arranged metal-dielectric-metal cavities with different geometries. However, application of these metasurfaces for optoelectronic devices by incorporating functional dielectrics remains challenging. Here, we integrate a quantum well infrared photodetector (QWIP) with a metasurface made of a patchwork of square cavities with different dimensions arranged in a subwavelength unit cell. Our detector realizes wideband photoresponse approaching the entire responsivity spectrum of the QWIP—single-sized square cavities can utilize only 60% of the possible bandwidth—and external quantum efficiencies of up to 78% at 6.8 µm. Our highly flexible design scheme enables integration of photodetectors and metasurfaces with arbitrary arrangements of cavities selectively responding to incidence with a specific wavefront.

We recently reported an isotropic linear dispersion relation of a square-lattice photonic crystal slab fabricated in an SOI wafer. The linear dispersion of the square lattice generally suffers from the distortion due to a diffraction loss, which may reduce its performance in expected applications. We present a method to eliminate the diffraction loss and avoid the distortion by using triangular-lattice photonic crystal slabs with the six-fold rotational symmetry. The theoretical results on the dispersion and the angle-resolved reflection spectra obtained by the k-p perturbation theory and the finite element method will be compared with available experimental data.

The integration of quantum well infrared photodetectors with plasmon cavities has allowed for demonstration of sensitive photodetectors in the mid-infrared. These detectors typically use only a single type of plasmon cavity, but previous studies of metasurface absorbers has shown that combining multiple cavities in a single subwavelength period can generate complex photoresponses such as multi-band and broadband absorption. In this report, we use a simple square cavity design as a building block for demonstrating detectors with a broadened photoresponse. Device performance of simple square detectors is comparable to previous highdetectivity designs, and by using a patchwork of square cavities within a single subwavelength period, we can subsequently broaden photoresponse by 1.5 times. Absorption in these patchwork cavity detectors shows good agreement with previous reports for metasurface absorbers, and suggests that this design and fabrication process could serve as a roadmap for integrating detectors and metasurface cavities with more complex photoresponses.

Optical patch antennas sandwiching dielectrics between metal layers have been used as deep subwavelength building blocks of metasurfaces for perfect absorbers and thermal emitters. However, for applications of these metasurfaces for optoelectronic devices, wiring to each electrically isolated antenna is indispensable for biasing and current flow. Here we show that geometrically engineered metallic wires interconnecting the antennas can function to synchronize the optical phases for promoting coherent resonance, not only as electrical conductors. Antennas connected with optimally folded wires are applied to intersubband infrared photodetectors with a single 4-nm-thick quantum well, and a polarization-independent external quantum efficiency as high as 61% (responsivity 3.3 A W−1, peak wavelength 6.7 μm) at 78 K, even extending to room temperature, is demonstrated. Applications of synchronously wired antennas are not limited to photodetectors, but are expected to serve as a fundamental architecture of arrayed subwavelength resonators for optoelectronic devices such as emitters and modulators., Applications of metasurfaces for optoelectronic devices require wiring to each isolated antenna for biasing and current flow. Here, the authors report optimal wire interconnects design for controlling the optical properties and present antenna-enhanced mid-infrared photodetection incorporating a single quantum well.

Fourier transform (FT) spectroscopy is a versatile technique for studying the infrared (IR) optical response of solid-, liquid-, and gas-phase samples. In standard Fourier transform infrared (FT-IR) spectrometers, a light beam passing through a Michelson interferometer is focused onto a sample with condenser optics. This design enables us to examine relatively small samples, but the large solid angle of the focused infrared beam makes it difficult to analyze angle-dependent characteristics. Here, we design and construct a high-precision angle-resolved reflection setup compatible with a commercial FT-IR spectrometer. Our setup converts the focused beam into an achromatically collimated beam with an angle dispersion as high as 0.25°. The setup also permits us to scan the incident angle over ∼8° across zero (normal incidence). The beam diameter can be reduced to ∼1 mm, which is limited by the sensitivity of an HgCdTe detector. The small-footprint apparatus is easily installed in an FT-IR sample compartment. As a demonstration of the capability of our reflection setup, we measure the angle-dependent mid-infrared reflectance of two-dimensional photonic crystal slabs and determine the in-plane dispersion relation in the vicinity of the Γ point in momentum space. We observe the formation of photonic Dirac cones, i.e., linear dispersions with an accidental degeneracy at Γ, in an ideally designed sample. Our apparatus is useful for characterizing various systems that have a strong in-plane anisotropy, including photonic crystal waveguides, plasmonic metasurfaces, and molecular crystalline films.

We developed a compact dual-wavelength surface-emitting light source using InAs quantum dots (QDs) embedded in a vertical cavity (VC). The VC was designed to possess two optical cavity modes that resonate with the discrete emission lines of the QDs. The fabricated light source exhibited significant enhancements in the vertical light emission corresponding to the VC modes. In addition, the light source demonstrated selectivity to the enhanced emission wavelengths with changes in temperature. Compared to conventional dual-wavelength vertical external cavity surface-emitting lasers, these QD-based dual-wavelength emission devices allow for the realization of simple structures because the InAs QDs act as dual-light-emitting materials. These results can be applied to simple dual-wavelength surface-emitting light sources.

Since their initial demonstration, metasurface quantum-well infrared photodetectors (QWIPs)—intersubband detectors integrated with plasmon cavities—using only a single quantum well (Nw = 1) have been considered the highest performance design. However, experimental confirmation via comparison of detectors with different Nw has not been performed. Here, we systematically compare Nw = 1 and Nw = 3 metasurface QWIPs with identical cavities and reveal the clear superiority of Nw = 3 detectors. While signal (responsivity) decreases with additional wells as predicted, the signal-to-noise-ratio (detectivity) increases due to drastically reduced noise. Thanks to the narrow spectral linewidth unique to metasurface QWIPs, our detectors can reach detectivity beyond the theoretical limits of interband detectors; our best detectors show maximum detectivities of 6.4×1010 cm Hz1/2/W at 7.0 μm, exceeding the interband limit of 5.3×1010 cm Hz1/2/W at the same wavelength.

Photonic crystal resonators with the C4v symmetry were designed and fabricated on quantum cascade lasers with a strain-compensated multiple quantum well to achieve single-mode and vertical surface emission at 4.32 μm. Their fabrication accuracy was confirmed by high-resolution reflection spectroscopy. The maximum output power was 10 mW at 77 K. A far-field pattern with a small divergence angle below 1 degree was observed. Its main peak had a donut shape, which was attributed to the spatial symmetry of the resonance mode of the photonic crystal.

We developed a near-infrared (NIR) superluminescent diode (SLD) based on self-assembled InAs quantum dots (QDs) and demonstrated high-axial-resolution optical coherence tomography (OCT) imaging using this QD-based SLD (QD-SLD). The QD-SLD utilized InAs QDs with controlled emission wavelengths as a NIR broadband light emitter, and a tilted waveguide with segmented electrodes was prepared for edge-emitting broadband electroluminescence (EL) spanning approximately 1–1.3 μm. The bandwidth of the EL spectrum was increased up to 144 nm at a temperature of 25 °C controlled using a thermoelectric cooler. The inverse Fourier transform of the EL spectrum predicted a minimum resolution of 3.6 μm in air. The QD-SLD was subsequently introduced into a spectral-domain (SD)-OCT setup, and SD-OCT imaging was performed for industrial and biological test samples. The OCT images obtained using the QD-SLD showed an axial resolution of ~4 μm, which was almost the same as that predicted from the spectrum. This axial resolution is less than the typical size of a single biological cell (~5 μm), and the practical demonstration of high-axial-resolution OCT imaging shows the application of QD-SLDs as a compact OCT light source, which enables the development of a portable OCT system.

We fabricated and characterized a grating-coupled external cavity laser with gain chips including self-assembled InAs quantum dots (QDs) for swept-source optical coherence tomography applications. By controlling the emission wavelength of the self-assembled InAs QDs, tunable lasing at a wavelength band of 1–1.1 μm was obtained, which represents an optimal balance between absorption and scattering in biological tissues. Straight and J-shaped edgeemitting ridge waveguides (RWGs) were fabricated on a GaAs-based waveguide layer containing four InAs QDs layers. A diffraction grating with the quasi-Littrow configuration was employed as an external cavity for the fiber-coupled diodes. Electroluminescence spectra from the QD-based diodes revealed that broadband amplified spontaneous emissions appeared in a J-shaped RWG, whereas Fabry–Perot lasing occurred in the straight RWG. The external cavity was then introduced for the diode with a J-shaped RWG, and a tuning range of 65 nm centered at approximately 1100 nm was obtained from the QD gain chip with the J-shaped RWG.

Herein, we report an emission wavelength control technique for self-assembled InAs quantum dots (QDs) grown via molecular beam epitaxy using an As2 source (As2-QDs). The As2-QDs exhibited photoluminescence with a shorter center wavelength and larger bandwidth than those of the QDs grown using an As4 source. In addition, the emission center wavelength could be controlled by adjusting the time between the growth and capping of the As2-QDs. We utilized the multilayer stack of emission-wavelength-controlled As2-QDs to fabricate an electrically-driven light source and demonstrated its broadband (approximately 130 nm) emission in the 1–1.3 μm wavelength range.