Photoacoustic spectroscopy using quantum-cascade lasers

Photoacoustic spectra of ammonia and water vapor were recorded by use of a continuous-wave quantum-cascade distributed-feedback (QC-DFB) laser at 8.5 mum with a 16-mW power output. The gases were flowed through a cell that was resonant at 1.6 kHz, and the QC-DFB source was temperature tuned over 35 nm for generation of spectra or was temperature stabilized on an absorption feature peak to permit real-time concentration measurements. A detection limit of 100 parts in 10(9) by volume ammonia at standard temperature and pressure was obtained for a 1-Hz bandwidth in a measurement time of 10 min.     

Frequency stabilization of quantum-cascade lasers by use of optical cavities

We report a heterodyne beat with a linewidth of 5.6+/-0.6 Hz between two cavity-stabilized quantum-cascade lasers operating at 8.5 microm . We also present a technique for measuring this beat that avoids the need for extreme isolation of the optical cavities from the environment, that of employing a third servo loop with low bandwidth to force one cavity to track the slow drifts and low-frequency fluctuations of the other. Although it is not fully independent, this technique greatly facilitates heterodyne beat measurements for evaluating the performance of cavity-locked lasers above the bandwidth of the third loop.     

Cavity ringdown spectroscopy using mid-infrared quantum-cascade lasers

Cavity ringdown spectra of ammonia at 10 parts in 10(9) by volume (ppbv) and higher concentrations were recorded by use of a 16-mW continuous-wave quantum-casacde distributed-feedback laser at 8.5 mum whose wavelength was continuously temperature tuned over 15 nm. A sensitivity (noise-equivalent absorbance) of 3.4x10(-9) cm(-1) Hz(-1/2) was achieved for ammonia in nitrogen at standard temperature and pressure, which corresponds to a detection limit of 0.25 ppbv.     

High-resolution (Doppler-limited) spectroscopy using quantum-cascade distributed-feedback lasers

Lasing characteristics were evaluated for distributed-feedback quantum-cascade (QC) lasers operating in a continuous mode at cryogenic temperatures. These tests were performed to determine the QC lasers' suitability for use in high-resolution spectroscopic applications, including Doppler-limited molecular absorption and pressure-limited lidar applications. By use of a rapid-scan technique, direct absorbance measurements of nitric oxide (NO) and ammonia (NH>(3)) were performed with several QC lasers, operating at either 5.2 or 8.5 microm. Results include time-averaged linewidths of better than 40 MHz and long-term laser frequency reproducibility, even after numerous temperature cycles, of 80 MHz or better. Tuning rates of 2.5 cm(-1) in 0.6 ms can be easily achieved. Noise-equivalent absorbance of 3 x 10(-6) was also obtained without optimizing the optical arrangement.     

Design of nonlinearity-enhanced quantum-cascade lasers

We present simulations of mid-infrared quantum-cascade lasers (QCL) with optimized second-harmonic generation (SHG). The optimized design was obtained utilizing techniques from supersymmetric quantum mechanics with both material-dependent effective mass and band nonparabolicity. Two-photon processes are analyzed for resonant cascading triple levels designed for enhancing SHG. Nonunity pumping efficiency from one period of the QCL to the next is taken into account by including all relevant carrier scattering mechanisms between the injector/collector and active regions. Carrier transport and power output of the structure are analyzed by self-consistently solving rate equations for the carriers and photons. Current-dependent linear optical output power is derived based on the steady-state photon population in the active region. The SH power is derived from the Maxwell equations with the phase mismatch and modes overlapping included. Due to stronger coupling between lasing levels, the optimized structure has both higher linear and SH output powers. The optimized structure can be fabricated through digitally grading the submonolayer alloys by molecular beam epitaxy (MBE) technique.     

Terahertz tomography using quantum-cascade lasers

The interfaces of a dielectric sample are resolved in reflection geometry using light from a frequency agile array of terahertz quantum-cascade lasers. The terahertz source is a 10-element linear array of third-order distributed-feedback QCLs emitting at discrete frequencies from 2.08 to 2.4 THz. Emission from the array is collimated and sent through a Michelson interferometer, with the sample placed in one of the arms. Interference signals collected at each frequency are used to reconstruct an interferogram and detect the interfaces in the sample. Because of the long coherence length of the source, the interferometer arms need not be adjusted to the zero-path delay. A depth resolution of 360 μm in the dielectric is achieved with further potential improvement through improved frequency coverage of the array. The entire experiment footprint is <1 m×1 m with the source operated in a compact, closed-cycle cryocooler.     

Second-harmonic generation in GaAs-based quantum-cascade lasers

We report second-harmonic (SH) and sum-frequency generation in GaAs-based quantum-cascade lasers. A doping dependence study of the second-order susceptibility in one of the investigated structure is shown. We also demonstrate that grating-coupled surface emission is a highly efficient way to couple out the SH radiation.     

Second-harmonic generation in GaAs-based quantum-cascade lasers

We report second-harmonic (SH) and sum-frequency generation in GaAs-based quantum-cascade lasers. A doping dependence study of the second-order susceptibility in one of the investigated structure is shown. We also demonstrate that grating-coupled surface emission is a highly efficient way to couple out the SH radiation.     

Nature of charge transport in quantum-cascade lasers

The first global quantum simulation of semiconductor-based quantum-cascade lasers is presented. Our three-dimensional approach allows us to study in a purely microscopic way the current-voltage characteristics of state-of-the-art unipolar nanostructures, and therefore to answer the long-standing controversial question: Is charge transport in quantum-cascade lasers mainly coherent or incoherent? Our analysis shows that (i) quantum corrections to the semiclassical scenario are minor and (ii) inclusion of carrier-phonon and carrier-carrier scattering gives excellent agreement with experimental results.     

Frequency modulation (FM) spectroscopy

Frequency modulation (FM) spectroscopy is a new method of optical heterodyne spectroscopy capable of sensitive and rapid measurement of the absorption or dispersion associated with narrow spectral features. The absorption or dispersion is measured by detecting the heterodyne beat signal that occurs when the FM optical spectrum of the probe wave is distorted by the spectral feature of interest. A short historical perspective and survey of the FM spectroscopy work performed to date is presented. Expressions describing the nature of the beat signal are derived. Theoretical lineshapes for a variety of experimental conditions are given. A signal-to-noise analysis is carried out to determine the ultimate sensitivity limits.     

Wavelength modulation spectroscopy based on quasi-continuous-wave diode lasers

A modified wavelength modulation spectroscopy（WMS）based on the self-heating effect of the tunable diode laser when driven in quasi-continuous-wave（QCW）mode is investigated.A near-infrared distributed feedback（DFB）diode laser working at the QCW mode is employed as the QCW light source,and CO2 is selected as the target gas.The characteristic of the QCW second harmonic（2f）line profile is analyzed through a comparison with that of the traditional CW WMS with the same system.A noise-equivalent absorbance of 3.2×10-5 Hz-1/2 for CO2 at 1.58μm is obtained with 18-m optical path.The QCW WMS lowers the dependence on lasers and expands selectivity,thus verifying the feasibility of the method.     

High frequency wavelength modulation spectroscopy with diode lasers

High frequency wavelength modulation spectroscopy with diode lasers is accomplished by dithering the drive current at RF frequencies as high as 250 MHz. This technique is useful for fast and sensitive detection of absorption lines in the near-and mid-infrared spectral regions. Detection of 300 MHz wide spectral features corresponding to 1% changes in transmission is accomplished in time intervals as short as 500 ns. A potential application is for fast reading of information contained in frequency domain optical memories based upon photochemical hole burning.     

半绝缘等离子体波导太赫兹量子级联激光器工艺研究

采用气态源分子束外延设备生长了GaAs/AlGaAs束缚态到连续态跃迁结构的太赫兹（THz）量子级联激光器（QCL）有源区结构,研究了半绝缘等离子体波导THz QCL的器件工艺,采用远红外傅里叶变换光谱仪以及探测器测量了器件的电光特性.器件激射频率为32 THz,10 K下的阈值电流密度为275 A/cm². 关键词： 太赫兹 量子级联激光器 波导 器件工艺     

太赫兹量子级联激光器材料生长及表征

采用气态源分子束外延方法生长了束缚态到连续态跃迁太赫兹量子级联激光器（terahertz quantum-cascade laser,简称THz QCL）有源区结构,并且采用电化学CV仪、霍尔测试仪以及高分辨X射线衍射对材料的质量进行表征,得出THz QCL有源区具有很高的晶体质量.另外,采用蒙特卡罗方法模拟了共振声子THz QCL器件的I-V曲线,分析了在不同偏压下子能级的对齐状况和电子的输运特征. 关键词： 太赫兹 量子级联激光器 分子束外延 X射线衍射     

Perfectly phase-matched third-order distributed feedback terahertz quantum-cascade lasers

We report a novel laser cavity design in third-order distributed feedback (DFB) terahertz quantum-cascade lasers based on a perfectly phase-matching technique. This approach substantially increases the usable length of the third-order DFB laser and leads to narrow beam patterns. Single frequency emissions from 151 apertures (5.6 mm long device) are coherently added up to form a narrow beam with (FWHM≈6×11°) divergence. A similar device with 40 apertures shows more than 5 mW of optical power with slope efficiency ～140 mW/A at 10 K pulsed operation.     

Frequency stabilization of multiple lasers and Rydberg atom spectroscopy

In this paper we report details of the apparatus and experimental techniques used to excite Rydberg states of ⁷Li using multiple diode lasers. Special attention is paid to frequency stabilization of the lasers and we show how three lasers can be stabilized using the fluorescence from a single atomic state. Laser spectroscopy of the 8p,9p, and 11p–15p states is then performed to determine the quantum defects of these states. Our measurement precision exceeds that of previous measurements of these defects by as much as a factor of 25. This work substantially extends our previous measurement of the 10p quantum defect, and we compare our measured defects with recent theoretical calculations for the np states across the range 8≤n≤15. The agreement with theory is excellent.     

Application of terahertz quantum-cascade lasers to semiconductor cyclotron resonance

Quantum-cascade lasers operating at 4.7, 3.5, and 2.3 THz have been used to achieve cyclotron resonance in InAs and InSb quantum wells from liquid-helium temperatures to room temperature. This represents one of the first spectroscopic applications of terahertz quantum-cascade lasers. Results show that these compact lasers are convenient and reliable sources with adequate power and stability for this type of far-infrared magneto-optical study of solids. Their compactness promises interesting future applications in solid-state spectroscopy.     

Second-harmonic generation in three-well and bound-to-continuum GaAs-based quantum-cascade lasers

We present efficient second-harmonic generation in state of the art GaAs-based quantum-cascade lasers with nonlinear output powers up to 100 μW. The nonlinear output was significantly improved by applying an AlGaAs waveguide structure, which guides both, the fundamental and nonlinear light. We further show the influence of the doping in the active region on the nonlinear light generation by comparing two similar structures with different doping levels. PACS 42.55.Px; 42.65.Ky; 81.05.Ea     

Distributed-feedback terahertz quantum-cascade lasers with laterally corrugated metal waveguides

We report the demonstration of distributed-feedback terahertz quantum-cascade lasers based on a first-order grating fabricated via a lateral corrugation in a double-sided metal ridge waveguide. The phase of the facet reflection was precisely set by lithographically defined facets by dry etching. Single-mode emission was observed at low to moderate injection currents, although multimode emission was observed far beyond threshold owing to spatial hole burning. Finite-element simulations were used to calculate the modal and threshold characteristics for these devices, with results in good agreement with experiments.