Spectral Tuning Range Research Articles

Multi-wavelength injection locked semiconductor comb laser

Quantum dot lasers on silicon have gained significant interest over the past decade due to their great potential as an on-chip silicon photonic light source. Here, we demonstrate multi-wavelength injection locking of InAs/GaAs quantum dot Fabry–Perot (FP) lasers both on GaAs and silicon substrates by optical self-injection via an external cavity. The number of locked laser modes can be adjusted from a single peak to multiple peaks by tuning wavelength dependent phase and mode spacing of back-injected light through a Lyot filter. The multi-wavelength injection locked laser modes exhibit average optical linewidth of ∼ 20 kHz , which are narrowed by approximately three orders of magnitude from their free-running condition. Furthermore, multi-wavelength self-injection locking via an external cavity exhibits flat-top optical spectral properties with approximately 30 stably locked channels under stable operation over time, where the frequency detuning is less than 700 MHz within 40 min. Particularly, FP lasers by direct epitaxial growth on silicon substrates are self-injection locked as a flat-top comb source with tunable free spectral range from approximately 25 to 700 GHz. The reported results emphasize the great potential of multi-wavelength injection locked lasers as tunable on-chip multi-wavelength light sources.

Extended tuning of distributed-feedback lasers in a bias-tee circuit via waveform optimization for MHz-rate absorption spectroscopy

Variations in injection-current waveform are examined using diplexed radio-frequency modulation with continuous-wave distributed-feedback (CW-DFB) lasers, with the aim to maximize the spectral tuning range and signal-to-noise ratio for MHz-rate laser absorption spectroscopy (LAS). Utilizing a bias-tee circuit, laser chirp rates are shown to increase by modulating the AC input voltage using square waves instead of sine waves and by scanning the laser below the lasing threshold during the modulation period. The effect of waveform duty cycle and leading-edge ramp rate are further examined. A spectral scan depth on the order of 1 cm−1 at a scan frequency of 1 MHz is achieved with a representative CW-DFB quantum cascade laser near 5 μm. Distortion of high-frequency optical signals due to detector bandwidth is also examined, and limitations are noted for applications with narrow spectral features and low-bandwidth detectors. Based on common detection system limitations, an optimization approach is established for a given detection bandwidth and target spectra. A representative optimization is presented for measurements of sub-atmospheric carbon monoxide spectra with a 200-MHz detection system. The methods are then demonstrated to resolve transient gas properties (pressure and temperature) via LAS at MHz rates in a detonation tube and shock tube facility. An appendix detailing a first-order model of high-speed distributed feedback laser tuning dynamics is also included to support the experimental observations of this work.

Optical frequency comb with tunable free spectral range based on two Mach-Zehnder modulators cascaded with linearly chirped fiber Bragg grating and phase modulator

Optical frequency comb with tunable free spectral range based on two Mach-Zehnder modulators cascaded with linearly chirped fiber Bragg grating and phase modulator

Engineering Near-Infrared Light Emission in Mechanically Exfoliated InSe Platelets through Hydrostatic Pressure for Multicolor Microlasing.

γ-indium selenide (InSe) is a van der Waals semiconductor and holds great potentials for low-energy-consumption electronic and optoelectronic devices. Herein, we investigated the hydrostatic pressure engineered near-infrared (NIR) light emission of mechanically exfoliated γ-InSe crystals using the diamond anvil cell (DAC) technique. A record-wide spectral tuning range of 185 nm and a large linear pressure coefficient of 40 nm GPa-1 were achieved for spontaneous emissions, leading to ultrabroadband microlasing spectrally ranging from 1022 to 911 nm. This high emission tunability can be attributed to the compression of the soft intralayer In-Se bonds under high pressure, which suppressed the band gap shrinkage by increasing the interlayer interaction. Furthermore, two band gap crossovers of valence (direct-to-indirect) and conduction bands were resolved at approximately 4.0 and 7.0 GPa, respectively, resulting in pressure-sensitive emission lifetime and intensity. These findings pave the pathways for pressure-sensitive InSe-based NIR light sources, sensors and so on.

DFT studies on the structural, electronic, and optical properties of Na2ZnP2O7 compound

First principles density functional theory calculations were applied to determine the structural, electronic, elastic, and optical properties of disodium zinc diphosphate (Na2ZnP2O7) compound. The calculated lattice parameters of the Na2ZnP2O7 compound are found to be a = 7.75 Å and c = 10.20 Å, which are in reasonable agreement with available experimental data. From the calculated energetics and elastic properties, the formation of the Na2ZnP2O7 compound is energetically favorable and elastically stable. The calculated electronic band structure and density of states indicate that the Na2ZnP2O7 compound is a wide gap semiconductor with a direct band gap of 4.95 eV. Such a wide bandgap energy has a broad tunable spectral range, making this compound suitable for absorbing or emitting ultraviolet (UV) light. The different contributions of the electronic orbitals are discussed using the partial density of states, which shows significant contribution from the Zn atom with minor contribution from the Na, P and O atoms. The optical properties such as dielectric function, optical conductivity, refractive index, absorption coefficient, reflectivity, and electron energy loss are calculated and discussed.

Controlling the Shape Anisotropy of Monoclinic Nb12O29 Nanocrystals Enables Tunable Electrochromic Spectral Range.

Electrochromic smart windows that modulate the solar transmittance in a wide and selective spectral range can optimize building energy efficiency. However, for conventional materials such as bulk transition metal oxides, the electrochromic spectral range is constrained by their crystal structure with limited tunability. Herein, we report a method to control the shape anisotropy of monoclinic Nb12O29 nanocrystals and obtain a tunable electrochromic spectral range. We demonstrate the synthesis of monoclinic Nb12O29 nanorods (NRs), extending one-dimensionally along the b direction, and monoclinic Nb12O29 nanoplatelets (NPLs), extending two-dimensionally along the b and c directions. Upon electrochemical reduction accompanied by Li insertion, the NR films show increasing absorbance mostly in the near infrared region. In contrast, the NPL films show increasing absorbance in the near infrared region first followed by increasing absorbance in both visible and near infrared regions. To elucidate the influence of shape anisotropy, we used density functional theory to construct the lithiated structures of monoclinic Nb12O29 and in these structures we identified the presence of square planar sites and crystallographic shear sites for Li insertion. By calculating the theoretical spectra of the lithiated structures, we demonstrate that the Li insertion into the square planar sites results in absorption in the near infrared region in both NRs and NPLs due to their extension in the b direction, while the subsequent insertion of Li into the crystallographic shear sites leads to absorption in both visible and near infrared regions, which only occurs in NPLs due to their extension in the c direction.

High-Responsivity, Fast, and Self-Powered Narrowband Perovskite Heterojunction Photodetectors with a Tunable Response Range in the Visible and Near-Infrared Region.

In recent years, narrowband photodetectors (PDs) have been widely used in color imaging, spectral detection or discrimination, defense, and scientific research due to their special spectral selective responses. In this work, by combining organic-inorganic hybrid perovskite layers of different band gaps and thicknesses, a series of narrowband perovskite heterojunction PDs with a continuously adjustable spectral range in the visible and near-infrared range are designed and prepared. The PDs can achieve a narrowband photoresponse with a full width at half-maximum (FWHM) of less than 50 nm and a light rejection ratio (between 780 and 532 nm) of over 1100 and exhibit excellent photoresponse performances with an external quantum efficiency (EQE), responsivity (R), and detectivity (D*) as high as 50.3%, 0.331 A/W, and 4.27 × 1010 Jones, respectively. More importantly, the photoresponses of the PDs at zero bias are as good as those at the reverse bias voltages, indicating the outstanding self-powered property. In addition, a fast response time of ∼180/∼200 μs is obtained in the narrowband perovskite heterojunction, and the response speed nearly remains constant for different PDs in the whole tunable wavelength range, demonstrating the suitable and stable structure of the heterojunction, as well as the high crystalline quality of the perovskite layers. This work definitely provides a simple strategy for designing low-cost, high-photoresponsivity, fast speed, and self-powered narrowband PDs with a tunable spectral range.

Integrating lattice and gap plasmonic modes to construct dual-mode metasurfaces for enhancing light–matter interaction

Photonic structures with optical resonances beyond a single controllable mode are strongly desired for enhancing light-matter interactions and bringing about advanced photonic devices. However, the realization of effective multimodal photonic structures has been restricted by the limited tunable range of mode manipulation, the spatial dispersions of electric fields or the polarization-dependent excitations. To overcome these limitations, we create a dualmode metasurface by integrating the plasmonic surface lattice resonance and the gap plasmonic modes; this metasurface offers a widely tunable spectral range, good overlap in the spatial distribution of electric fields, and polarization independence of excitation light. To show that such dual-mode metasurfaces are versatile platforms for enhancing light-matter interactions, we experimentally demonstrate a significant enhancement of second-harmonic generation using our design, with a conversion efficiency of 1–3 orders of magnitude larger than those previously obtained in plasmonic systems. These results may inspire new designs for functional multimodal photonic structures.

Carrier Blocking Layer Materials and Application in Organic Photodetectors.

As a promising candidate for next-generation photodetectors, organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using solution processes and a tunable spectral response range, making them particularly attractive for large area image sensors on lightweight flexible substrates. Carrier blocking layers engineering is very important to the high performance of OPDs that can select a certain charge carriers (holes or electrons) to be collected and suppress another carrier. Carrier blocking layers of OPDs play a critical role in reducing dark current, boosting their efficiency and long-time stability. This Review summarizes various materials for carrier blocking layers and some of the latest progress in OPDs. This provides the reader with guidelines to improve the OPD performance via carrier blocking layers engineering.

Electrically reconfigurable non-volatile metasurface using low-loss optical phase-change material.

Active metasurfaces promise reconfigurable optics with drastically improved compactness, ruggedness, manufacturability and functionality compared to their traditional bulk counterparts. Optical phase-change materials (PCMs) offer an appealing material solution for active metasurface devices with their large index contrast and non-volatile switching characteristics. Here we report a large-scale, electrically reconfigurable non-volatile metasurface platform based on optical PCMs. The optical PCM alloy used in the devices, Ge2Sb2Se4Te (GSST), uniquely combines giant non-volatile index modulation capability, broadband low optical loss and a large reversible switching volume, enabling notably enhanced light-matter interactions within the active optical PCM medium. Capitalizing on these favourable attributes, we demonstrated quasi-continuously tuneable active metasurfaces with record half-octave spectral tuning range and large optical contrast of over 400%. We further prototyped a polarization-insensitive phase-gradient metasurface to realize dynamic optical beam steering.

Electrically driven frequency blue-chirped emission in Fabry–Perot cavity quantum cascade laser at room temperature

We present a method to produce a fast frequency swept laser emission from a monolithic mid-infrared laser. A commercially available Fabry–Pérot cavity quantum cascade laser (QCL) operating at a wavelength of 8.15 μm was electrically driven by a current pulse with a 10 μs duration and a slow front rising time of ∼2 μs. Due to the switching of the lasing emission from the vertical to the diagonal transition in the QCL and a strong quantum-confined Stark effect energy shift of the diagonal transition, the frequency of the emitted light was blue-shifting as the injection current continues to raise above the threshold. The temporal evolution of the laser spectrum was measured by a high-resolution step-scan Fourier transform infrared spectrometer. The blue-chirped emission was strongly influenced by the heatsink temperature due to the high thermal sensitivity of the threshold current and slope efficiency. By optimizing carefully the QCL operating temperature and the amplitude of the current pulse, we demonstrate a high-speed self-sweeping laser emission under room temperature operation conditions, reaching the spectral tuning range of ∼25 cm−1 within 1.8 μs.

Nonvolatile, Reconfigurable and Narrowband Mid-Infrared Filter Based on Surface Lattice Resonance in Phase-Change Ge2Sb2Te5.

We propose a nonvolatile, reconfigurable, and narrowband mid-infrared bandpass filter based on surface lattice resonance in phase-change material GeSbTe. The proposed filter is composed of a two-dimensional gold nanorod array embedded in a thick GeSbTe film. Results show that when GeSbTe transits from the amorphous state to the crystalline state, the narrowband reflection spectrum of the proposed filter is tuned from 3.197 m to 4.795 m, covering the majority of the mid-infrared regime, the peak reflectance decreases from 72.6% to 25.8%, and the corresponding quality factor decreases from 19.6 to 10.3. We show that the spectral tuning range can be adjusted by varying the incidence angle or the lattice period. By properly designing the gold nanorod sizes, we also show that the quality factor can be greatly increased to 70 at the cost of relatively smaller peak reflection efficiencies, and that the peak reflection efficiency can be further increased to 80% at the cost of relatively smaller quality factors. We expect that this work will advance the engineering of GeSbTe-based nonvalatile tunable surface lattice resonances and will promote their applications especially in reconfigurable narrowband filters.

Metamaterial Enhancement of Metal-Halide Perovskite Luminescence.

Metal-halide perovskites are rapidly emerging as solution-processable optical materials for light-emitting applications. Here, we adopt a plasmonic metamaterial approach to enhance photoluminescence emission and extraction of methylammonium lead iodide (MAPbI3) thin films based on the Purcell effect. We show that hybridization of the active metal-halide film with resonant nanoscale sized slits carved into a gold film can yield more than 1 order of magnitude enhancement of luminescence intensity and nearly 3-fold reduction of luminescence lifetime corresponding to a Purcell enhancement factor of more than 300. These results show the effectiveness of resonant nanostructures in controlling metal-halide perovskite light emission properties over a tunable spectral range, a viable approach toward highly efficient perovskite light-emitting devices and single-photon emitters.

Real-time en-face Gabor optical coherence tomographic angiography on human skin using CUDA GPU.

We recently proposed an optical coherence tomographic angiography (OCTA) algorithm, Gabor optical coherence tomographic angiography (GOCTA), which can extract microvascular signals from a spectral domain directly with lower computational complexity compared to other algorithms. In this manuscript, we combine a programmable swept source, an OCT complex signal detecting unit, and graphics process units (GPU) to achieve a real-time en-face GOCTA system for human skin microvascular imaging. The programmable swept source can balance the A-scan rate and the spectral tuning range; the polarization-modulation based complex signal detecting unit can double the imaging depth range, and the GPU can accelerate data processing. C++ and CUDA are used as the programming platform where five parallel threads are created for galvo-driving signal generation, data acquisition, data transfer, data processing, and image display, respectively. Two queues (for the raw data and en-face images, respectively) are used to improve the data exchange efficiency among different devices. In this study, the data acquisition time and data processing time for each 3D complex volume (256×304×608 pixels,) are 405.3 and 173.7 milliseconds respectively. To the best of our knowledge, this is the first time to show en-face microvascular images covering 3×3 mm2 at a refresh rate of 2.5 Hz.

One-way and near-absolute polarization insensitive near-perfect absorption by using an all-dielectric metasurface.

In this Letter, we numerically propose the one-way perfect absorption of near-infrared radiation in a tunable spectral range with high transmission in the neighboring spectral ranges. This functionality is obtained by using a two-dimensional, guided-mode resonance-based grating-waveguide metasurface that acts as a frequency-selective reflector, a spacer dielectric, and an absorbing oxide layer. Within the bandwidth of the excited guided-mode resonance excited at 1.82 µm (with a full-width at half-maximum of 19 nm), we confirmed perfect absorption when light was incident from one of the two opposite directions, whereas in the other direction, perfect reflection was observed. The forward-to-backward absorption ratio reached as high as 60, while the thickness of the entire structure was on the order of the operating wavelength. In addition to the spectral tunability of the excited resonances and their bandwidths, our proposed device supports transparency windows with 65% transmission in the adjacent frequency bands. Our 2D grating is also verified to enable near-absolute insensitivity to the polarization state of incident light. Geometrical parameter modification also gives our design great tunability, as we also designed a device with a 300 nm absorption/reflection linewidth.

Tunable Stokes laser based on the cascaded stimulated polariton scattering and stimulated Raman scattering in RbTiOPO4 crystal.

Stimulated polariton scattering (SPS) and stimulated Raman scattering (SRS) in ${{\rm RbTiOPO}_4}$RbTiOPO4 (RTP) crystal are combined in an intracavity-pumped Stokes parametric oscillator (SPO) to extend the tunable Stokes laser spectral range. The pumping laser wavelength is 1064 nm from a diode-end-pumped acousto-optically Q-switched Nd:YAG laser. By the SPS process in the SPO, the SPS-Stokes wave can be discontinuously tuned in the range of 1075.7-1076.0 nm, 1077.7-1080.4 nm, 1081.8-1082.2 nm, and 1084.8-1087.8 nm, respectively. By the following SRS process in the same RTP crystal, the laser wavelength is further shifted in the range of 1107.7-1108.1 nm, 1109.0-1112.7 nm, 1114.3-1115.1 nm, and 1117.8-1121.1 nm, respectively. A maximal average output power of 970 mW is achieved for the SRS-Stokes wave at the peak wavelength of 1118.8 nm. It is obtained when the diode power is 7.9 W, and the pulse repetition frequency (PRF) is 10 kHz.

Near‐Infrared Tunable Surface Lattice Induced Transparency in a Plasmonic Metasurface

AbstractCollective coherent scattering at the surface of a plasmonic nanoparticle array is shown to induce tunable transparency windows at the localized plasmon band. Broadband phase measurements show that the enhanced transmission is accompanied by a large anomalous dispersion, which leads to a group delay as large as within only 40 nm thick sample. This effect occurs over a wide tunable spectral range of , and appears for two distinct counter‐propagating surface waves. The experimental observations are in good agreement with calculations based on coupled dipole approximation (CDA) and with finite‐difference time‐domain (FDTD) simulations. This study opens the door for implementation in the fields of sensing, displays, optical buffering, tunable filtering, and nonlinear optics.

Ultrabroad tuning range (100 nm) of external-cavity continuous-wave high-power semiconductor lasers based on a single InGaAs quantum well.

An external cavity in Littrow configuration based on a reflective diffraction grating and a high-power semiconductor laser based on an asymmetric heterostructure with low optical loss was studied. A continuous-wave optical output power of 13 W with a linewidth of 0.15 nm was achieved for an external-cavity laser. It was shown that a decrease in the length of the laser chip to 1500 µm makes it possible to expand the tuning range of the lasing spectrum to 100 nm, while the maximum side-mode suppression reaches 45 dB, and at the edges of the spectral tuning range it is no worse than 30 dB. It was shown that the main factors in reducing the optical power when tuning the laser spectrum are an increase in the threshold current and internal optical loss of the laser chip.

Intra-pulse H2O absorption diagnostic for temperature sensing in a rapid compression machine

An in-situ temperature diagnostic based on intra-pulse absorption spectroscopy has been developed using two pulsed quantum cascade lasers (QCLs) centered at 5.46 and 5.60 μm for rapid compression machine (RCM) experiments. Pulsed mode operation of the QCLs yielded a broad spectral tuning range (1.8–2.3 cm−1), through which spectral line-shapes of two H2O ro-vibrational transitions were resolved at high pressure conditions in the RCM (15–20 bar). Based on the resolved line-shapes, a calibration-free two-line thermometry method was used to determine the gas temperature. A high temporal resolution of 10 μs was achieved through a pulse repetition frequency of 100 kHz. The diagnostic was validated through measurements of temperature rise during the first-stage ignition of n-pentane/air mixtures. Thereafter, temperature rise during the first-stage ignition of iso-octane/air mixtures was quantified for the first time and compared with the calculated temperature rise using a chemical kinetic model.

Hybrid Light Emitters and UV Solar-Blind Avalanche Photodiodes based on III-Nitride Semiconductors.

In the last two decades, remarkable progress has been achieved in the field of optoelectronic devices based on III-nitride semiconductors. In terms of photonics applications in the visible-UV spectral range, III-nitrides are one of the most promising materials. For instance, emerging gallium nitride (GaN)-based micro-light-emitting diode (LED) technology for high-resolution display, and UV photo-detection for environmental monitoring, health, and medical applications. In this work, hybrid micro/nano-LEDs with integration of II-VI quantum dots by means of lithography and nano-imprinting patterning techniques are demonstrated, and high-performance red/green/blue and white emissions are achieved. Consequently, plasmonic nanolasers are designed and fabricated using a metal-oxide-semiconductor structure, where strong surface plasmon polariton coupling leads to the efficient lasing with a low excitation threshold from the visible to UV tunable spectral range. Furthermore, performance-improved AlGaN UV solar-blind avalanche photodiodes (APDs) with a separate absorption and multiplication structure by polarization engineering are reported. These APDs deliver a record-high avalanche gain of up to 1.6 × 105 . These newest advances in nano/micro-LEDs, nanolasers, and APDs can shed light on the emerging capabilities of III-nitride in cutting-edge applications.