Vertical-cavity Structure Research Articles

High-performance gas sensing in metagrating-based vertical cavity with larger fabrication tolerances driven by quasi-bound states in the continuum

We proposed a novel vertical cavity structure for gas sensing, utilizing double-layer metagratings and a distributed Bragg reflector (DBR). By adjusting the position of the upper-layer grating, we were able to achieve symmetry-protected bound states in the continuum (SP-BIC) and Fabry-Pérot BIC (FP-BIC). The resonance peaks of these quasi-BICs can be tuned by modifying structural parameters like grating thickness, width, and cavity length. Notably, our structure exhibited high sensing performance while allowing for significant fabrication tolerances. Simulation results indicate that the sensitivities of the gas sensor can reach 927 nm/RIU and 1026 nm/RIU, corresponding to figure of merit (FOM) of 15450 RIU−1 and 24604 RIU−1. The proposed structure with large fabrication tolerances has potential applications in gas sensing.

High-performance gas sensor with symmetry-protected quasi-bound states in the continuum.

A high-performance optical sensor with a vertical cavity structure comprising high-contrast gratings (HCGs) and a distributed Bragg reflector was designed. The structure has two peaks with different mechanisms, among which the first peak is formed by breaking the symmetry of the structure and coupling between the incident wave and the symmetric protection mode. The joint action of the HCG resonance and Fabry-Perot resonance formed a second peak. Moreover, changing the structural parameters, such as the grating width, period, and cavity length, can tune the spectral reflection dips. The sensitivity of the designed structure was as high as 674 nm/RIU, and the corresponding figure of merit was approximately 34741. The presented gas sensor provides a method for applying a vertical cavity structure to the sensing domain.

An optical slot-antenna-coupled cavity (SAC) framework towards tunable free-space graphene photonic surfaces

The optical conductivity of single layer graphene (SLG) can be significantly and reversibly modified when the Fermi level is tuned by electrical gating. However, so far this interesting property has rarely been applied to free-space two-dimensional (2D) photonic devices because the surface-incident absolute absorption of SLG is limited to 1%–2%. No significant change in either reflectance or transmittance would be observed even if SLG is made transparent upon gating. To achieve significantly enhanced surface-incident optical absorption in SLG in a device structure that also allows gating, here we embed SLG in an optical slot-antenna-coupled cavity (SAC) framework, simultaneously enhancing SLG absorption by up to 20 times and potentially enabling electrical gating of SLG as a step towards tunable 2D photonic surfaces. This framework synergistically integrates near-field enhancement induced by ultrahigh refractive index semimetal slot-antenna with broadband resonances in visible and infrared regimes, ~ 3 times more effective than a vertical cavity structure alone. An example of this framework consists of self-assembled, close-packed Sn nanodots separated by ~ 10 nm nanogaps on a SLG/SiO2/Al stack, which dramatically increases SLG optical absorption to 10%-25% at λ = 600–1,900 nm. The enhanced SLG absorption spectrum can also be controlled by the insulator thickness. For example, SLG embedded in this framework with a 150 nm-thick SiO2 insulating layer displays a distinctive red color in contrast to its surrounding regions without SLG on the same sample under white light illumination. This opens a potential path towards gate-tunable spectral reflectors. Overall, this work initiates a new approach towards tunable 2D photonic surfaces.

Fabrication of two-color surface emitting device of a coupled vertical cavity structure with InAs quantum dots formed by wafer bonding

We fabricated a two-color surface emitting device of a coupled cavity structure, which is applicable to terahertz light source. GaAs/AlGaAs vertical multilayer cavity structures were grown on (001) and (113)B GaAs substrates and the coupled multilayer cavity structure was fabricated by wafer bonding them. The top cavity contains self-assembled InAs quantum dots (QDs) as optical gain materials for two-color emission of cavity-mode lights. The bonding position was optimized for the equivalent intensity of two-color emission. We formed a current injection structure, and two-color emission was observed by current injection, although no lasing was observed.

Dynamics of a vertical cavity quantum cascade phonon laser structure

Driven primarily by scientific curiosity, but also by the potential applications of intense sources of coherent sound, researchers have targeted the phonon laser (saser) since the invention of the optical laser over 50 years ago. Here we fabricate a vertical cavity structure designed to operate as a saser oscillator device at a frequency of 325 GHz. It is based on a semiconductor superlattice gain medium, inside a multimode cavity between two acoustic Bragg reflectors. We measure the acoustic output of the device as a function of time after applying electrical pumping. The emission builds in intensity reaching a steady state on a timescale of order 0.1 μs. We show that the results are consistent with a model of the dynamics of a saser cavity exactly analogous to the models used for describing laser dynamics. We also obtain estimates for the gain coefficient, steady-state acoustic power output and efficiency of the device.

Optically pumped ultraviolet lasing from nitride nanopillars at room temperature

A vertical cavity structure composing of an in situ grown bottom AlxGa1−xN/AlyGa1−yN distributed Bragg reflector and a top SiO2/HfO2 dielectric mirror for ultraviolet (UV) emission has been demonstrated. Close-packed nanopillars with diameters of around 500 nm have been achieved by the route of nanosphere lithography combined with inductively-coupled plasma etching. Optically-pumped UV lasing at a wavelength of 343.7 nm (3.608 eV) was observed at room temperature, with a threshold excitation density of 0.52 MW/cm2. The mechanism of the lasing action is discussed in detail. Our investigation indicates promising possibilities in nitride-based resonant cavity devices, particularly toward realizing the UV nitride-based vertical-cavity surface-emitting laser.

A grating-bicoupled plasma-wave photomixer with resonant-cavity enhanced structure

A novel terahertz plasma-wave photomixer that can improve the conversion gain and terahertz radiation power is proposed and evaluated. The photomixer is based on a high-electron mobility transistor and incorporates doubly interdigitated grating strips for the gate electrodes that periodically localize the 2D plasmons in 100-nm regions with a micron-order interval. A vertical cavity structure is formed in between the top metal grating and a terahertz mirror placed at the backside. The device features electronic tuning of plasmon characteristic frequencies, providing continuously-tunable operation below 1 THz to beyond 10 THz. Frequency-dependent finite-differential time-domain analysis demonstrates that the grating-bicoupled plasmonic structure acts as a broadband terahertz photomixer and antenna and that the vertical cavity structure effectively enhances the conversion gain and radiation power.

Lasing from GaAs Gunn devices

Lasing from a GaAs Gunn device with a Fabry–Perot (F–P) cavity and a vertical-cavity structures are demonstrated. Light output-electric field characteristic of both F–P and vertical cavity structure Gunn device shows abrupt increase in light output above a negative differential resistance (NDR) threshold, implying cavity gain overcoming over loss. Electroluminescence (EL) spectra from the F–P Gunn device measured at 95 K show a significant spectral narrowing with increasing the applied voltage pulse width, indicative of monochromatic stimulated emission due to band-to-band recombination of impact-ionized carriers. The spectra from the vertical-cavity device are in accord with the cavity resonance as defined by the distributed Bragg reflectances (DBRs). The theoretical model on carrier generation via impact ionization inside the propagating space charge domains shows clearly that the built-up excess carrier density in the cavity indeed reaches the lasing threshold.

Characteristics of stable emission GaN-based resonant-cavity light-emitting diodes

The resonant-cavity light-emitting diode (RC-LED) structure was grown by MOCVD. The structure of RC-LED consisted of a 3 λ InGaN/GaN MQW LED cavity between the top TiO 2/SiO 2 DBR (81.7%, reflectance) and the bottom AlN/GaN DBR (90.4%) stack. A stable 410 nm emission peak and a low thermally induced red-shift effect (0.12 nm/kA/cm 2) were measured by varying the injection current density. The light output power of the full RC-LED device was three times higher than the RC-LED without top TiO 2/SiO 2 DBR layers under 600 A/cm 2 inject current density. The narrow line width of 7.4 nm, emission peak localization at 410 nm, and three times higher output power were caused by the resonance effect in this vertical cavity structure.

Continuous wave observation of massive polariton redistribution by stimulated scattering in semiconductor microcavities

A massive redistribution of the polariton occupancy to two specific wave vectors, zero and approximately 3.9x10(4) cm(-1), is observed under conditions of continuous wave excitation of a semiconductor microcavity. The "condensation" of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure.

Room temperature continuous wave operation under optical pumping of a 1.48 μm vertical cavity laser based on AlGaAsSb mirror

A 1.48 μm vertical cavity structure was realised on InP substrate including a n-type AlGaAsSb/AlAsSb bottom mirror, a λ thick AlGaInAs cavity with strained quantum wells, and a fused p-type AlGaAs/GaAs top mirror. The structure exhibits laser emission under optical pumping. This was observed in continuous wave operation up to room temperature. In addition, devices were processed and electroluminescence was observed under current injection.

Dynamics of gain in vertical cavity lasers and amplifiers at 1.53 μm using femtosecond photoexcitation

Conditions for the optimization of short pulse amplification and gating times in vertical cavity laser amplifiers are elucidated using femtosecond photoexcitation. The experimental results pertaining to the temporal response are analyzed by a theoretical model taking into account the carrier heating and the Fabry–Perot nature of the cavity. A 20 dB amplification gain is obtained in a vertical cavity structure at 1.53 μm with a capacity to function at a rate >25 Gb/s.

Temperature and emission angle dependence of photoluminescence from an InGaAs/GaAs quantum well in a microcavity structure

The spontaneous emission from an InGaAs/GaAs quantum well embedded in a semiconductor vertical-cavity structure is studied. The spontaneous emission enhancement at the cavity mode is clearly identified from the photoluminescence measurement, depending on the wavelength separation between the excitonic emission and the cavity resonance mode which is tuned by changing the measurement temperature and the emission angle. The emission intensity of the cavity resonance mode is increased as the excitonic emission approaches the cavity mode, thereby obtaining evidence of coupling between the exciton and the cavity mode. The emission wavelength of the cavity mode shifts to shorter wavelengths as the emission angle increases, which is clarified with theoretical calculations performed with the transfer-matrix method.

Submilliwatt optical bistability in wafer fused vertical cavity at 1.55-μm wavelength

We report on low-power optical bistability in a vertical cavity structure at 1.55-/spl mu/m wavelength due to the bandgap resonant dispersive optical nonlinearity of InGaAsP. In this structure, a GaAs-AlAs Bragg reflector grown on GaAs and the nonlinear medium grown on InP are bonded through wafer-fusion, leading to a high-quality vertical cavity after deposition of a top dielectric mirror. This device shows interesting characteristics for optical switching applications at fiber communication wavelengths, such as a switching contrast higher than 8:1 in the reflective mode and a bistability threshold power as low as 0.6 mW. True steady-state memory effect is observed with continuous-wave input, and the device switching time is in the ns range.

Growth of vertical cavity on buried-oxide substrate by gas-source Si molecular-beam epitaxy and coupled mode luminescence of strained Si1−xGex/Si quantum wells

Successful growth of a new class of Si-based vertical cavity structure is demonstrated using a buried-oxide substrate [separation-by-implantation-oxygen (SIMOX)] by gas source Si molecular beam epitaxy. The cavity is terminated with the Si/buried-oxide and top Si/air interface mirrors. Integration of strained Si1−xGex/Si quantum wells in the vertical cavity lead to the observation of spectral coupling of luminescence to the cavity longitudinal modes. Clear oscillations were observed in reflectance and photoluminescence excitation spectra.

Wavelength sensitive tapered coupler with anti-resonant waveguides

We describe a coupler that consists of two phase-mismatched polymer waveguides coupled together by a tapered AlAs/AlGaAs vertical cavity structure. At any particular wavelength, the light is transferred from one polymer waveguide to the other via the semiconductor cavity at a unique position along the waveguide. Thus, the length of the device determines the range of wavelengths transferred. In addition to a wavelength router, the structure could be useful for wavelength selective amplifiers or multiwavelength lasers. The concept is demonstrated in a passive structure with a resolution of about 1 nm. >

Multiple wavelength light source using an asymmetric waveguide coupler

We obtained multiple wavelength light emission from a coupled waveguide structure formed from a conventional waveguide and a vertical cavity resonator. The cavity and quarter-wave mirrors are optimized to act as another waveguide, thus forming an asymmetric directional coupler. This asymmetry between the waveguide and the vertical cavity causes wavelength selectivity in the cavity light emission. By tapering the cavity, the phase-matched wavelength becomes a function of position, and pumping different regions along the length of the waveguides causes different wavelengths to be emitted from the device. We obtained narrow-band luminescence with a minimum width of 1 nm using a 75-Å quantum well in a GaAs/AlGaAs vertical cavity structure coupled to a low index polymer waveguide.

Light detection sensitivity of a vertical cavity structure used in an optical switch device

The light detection characteristics in a vertical cavity surface-emitting laser-type vertical-to-surface transmission electrophotonic device (VC-VSTEP) have been evaluated. To realize a low energy optical interconnection, not only the switching threshold energy but also the light detection sensitivity must be optimized. Maximum sensitivity is achieved when the input light wavelength and the field distribution coincide with fundamental cavity mode characteristics. Under the best matching condition, the input light energy necessary to switch the VC-VSTEP state is easily lower than 1 pJ.

VSTEP-based smart pixels

The design features and characteristics of a laser-mode vertical-to-surface transmission electro-photonic (VSTEP) device with high-intensity light output and narrow optical beam divergence are discussed. It is shown that a 20- mu m square VSTEP with an optimized vertical cavity structure demonstrates a low optical switching energy of 2.2 pJ, together with a low threshold current. In order to lower the series resistance, and to improve electrical-to-optical conversion efficiency eta /sub T/, a double mesa structure VSTEP was fabricated. The whole mesa is covered with Au to utilize the threshold reduction by photon recycling. As a result, eta /sub T/ is improved to 11% at 1 mW light output. Two approaches to achieving VSTEP-based smart pixels are presented. >

Cascadable optical switching characteristics in vertical-to-surface transmission electrophotonic devices operated as vertical cavity lasers

We examine the optical switching characteristics in vertical cavity surface-emitting laser-type vertical-to-surface transmission electrophotonic devices (VC-VSTEPs) with a pnpn structure. It is theoretically and experimentally demonstrated that both optical input/output characteristics are strongly influenced by the structure of the vertical cavity. With improvements in optical power output due to laser oscillation, optical switching speed is enhanced by two orders of magnitude compared to spontaneous emission-type VSTEPs. A 20-μm-square VSTEP with an optimized vertical cavity structure demonstrates a low optical switching energy of 2.2 pJ with a switching time of 10 ns, as low as spontaneous emission-type VSTEPs.