Optically pumped intersubband lasers based on quantum wells

A far infrared (FIR) laser based on intersubband transitions in quantum wells is proposed where a pumping laser is used to create population inversion in the structure. The goal is to develop a structure which operates essentially as a 4-level laser, to minimize bottlenecking of the lower laser state. Multiple quantum wells can be used in the active laser of these structures to enhance the laser gain and the minimum required reflectivity in the cavity structure. The possibility of using both conduction and valence band quantum-well structures are investigated. Our study shows that, due to high intersubband scattering rates in the valence band structure, the creation of population inversion is more difficult and requires a high pumping power density while in the conduction band structure, population inversion can be achieved by a moderate pumping power density. The maximum population inversion in the conduction band structure is estimated to be 2.1×10¹¹ cm², which requires a pumping power density 2 kW cm^-2 for a single quantum well. The threshold power as well as the minimum required reflectivity of the cavity structure for the conduction band scheme are estimated for different well numbers     

光抽运甲基氟远红外激光器

一、引言 用CO_2激光抽运的远红外激光器诞生在1970年,至今已在30多种分子中获得了1000多条远红外激光谱线,波长范围从40微米到2毫米,复盖了整个远红外区。与电激励远红外激光器相比,光抽运的远红外激光器不存在放电起伏、热漂移和分子分解等现象。因此,具有单色性好,噪声低等优点,特别适用于通讯、雷达、等离子体诊断、光频标与光频测量、光谱学、天文学等领域。     

Optically pumped intersublevel MidInfrared lasers based on InAs-GaAs quantum dots

We propose an optically pumped laser based on intersublevel transitions in InAs-GaAs pyramidal self-assembled quantum dots. A theoretical rate equations model of the laser is given in order to predict the dependence of the gain on pumping flux and temperature. The energy levels and wave functions were calculated using the 8-band k/spl middot/p method where the symmetry of the pyramid was exploited to reduce the computational complexity. Carrier dynamics in the laser were modeled by taking both electron-longitudinal optical phonon and electron-longitudinal acoustic phonon interactions into account. The proposed laser emits at 14.6 /spl mu/m with a gain of g/spl ap/ 570 cm/sup -1/ at the pumping flux /spl Phi/=10/sup 24/ cm/sup -2/ s/sup -1/ and a temperature of T=77 K. By varying the size of the investigated dots, laser emission in the spectral range 13-21 /spl mu/m is predicted. In comparison to optically pumped lasers based on quantum wells, an advantage of the proposed type of laser is a lower pumping flux, due to the longer carrier lifetime in quantum dots, and also that both surface and edge emission are possible. The appropriate waveguide and cavity designs are presented, and by comparing the calculated values of the gain with the estimated losses, lasing is predicted even at room temperature for all the quantum dots investigated.     

Mid-infrared phase modulation via Stark effect on intersubbandtransitions in GaAs/GaAlAs quantum wells

Mid-infrared (~10 μm) electrorefractive effects of intersubband transitions in step GaAs/GaAlAs quantum wells are experimentally analyzed. A method for studying infrared electrorefraction of a two-dimensional structure via the Stark effect is used. The anomalous dispersion is measured over the entire spectral range of the transition and is found to be consistent with the Kramers-Kronig relations in the case of a Lorentzian absorption shape. A solution of Maxwell's equations in the multiquantum well structure is detailed and leads to a good agreement with experimental data. A standard interferometer and a lock-in technique allow a quasi-direct determination of phase and amplitude modulation with very good sensitivity. A theoretical approach based on a Lorentzian model and the solution of Maxwell's equations for a multiple QW (quantum well) structure agreed well with the data     

Nd:YAG laser pumped infrared dye laser

Efficient operation of a high peak power and high-average power infrared dye laser, continuously tunable over the range of 10810-12160 Å, has been achieved by pumping with an Nd:YAG laser. A peak power as high as 4 MW with an average power of 390 MW was obtained.     

Optically pumped “immersion-lens” infrared light emitting diodes based on narrow-gap III–V semiconductors

Spectral and power characteristics of light emitting diodes (LEDs) for the 3.3-to 7-µm range with GaAs LED pumping are presented. The LEDs consist of narrow-gap In(Ga)As, InAsSb(P), or InAs layers on a n ⁺-InAs substrate (band width ～λ_max/10) or on InSb (band width ～ 1 µm). The LEDs equipped with an immersion lens exhibit a conversion efficiency as high as 0.08–3 mW/A, which is comparable to or exceeds the highest reported data for the injection LEDs.     

Waveguide effect of GaAsSb quantum wells in a laser structure based on GaAs

The waveguide effect of GaAsSb quantum wells in a semiconductor-laser structure based on GaAs is studied theoretically and experimentally. It is shown that quantum wells themselves can be used as waveguide layers in the laser structure. As the excitation-power density attains a value of 2 kW/cm² at liquid-nitrogen temperature, superluminescence at the wavelength corresponding to the optical transition in bulk GaAs (at 835 nm) is observed.     

Broadly tunable external cavity laser diodes with staggered thickness multiple quantum wells

Widely tunable low-threshold current laser diodes fabricated from an engineered multiple-quantum-well (MQW) gain structure consisting of three compressively strained In/sub 0.2/Ga/sub 0.8/As wells of different thicknesses are reported. Using a grating in an external cavity, a continuous-wave tuning range of 70 nm (911-981 nm) is measured for a 155-/spl mu/m semiconductor cavity length device at a current of 32 mA. This is the lowest reported bias current for a semiconductor laser with this broad a tuning range. A maximum continuous wave tuning of 80 nm (901-981 nm) has been measured at a bias current of 95 mA. At long wavelengths, a suppression of amplified spontaneous emission and preferential population of the lowest energy well were observed.     

Optically detected impurity D and D transitions in GaAs quantum wells

We have carried out a far-infrared magnet-optical study on shallow donor states confined in GaAs quantum wells (QWs), applying a recently developed optical detection technique. We have observed, in addition to cyclotron resonance, the 1s → 2p₊ transition of neutral donors (D⁰), and singlet and triplet transitions of negative donor ions (D⁻); the latter observation verifies the existence of D⁻ ions in well-only doped QWs under optical pumping. This is the first observation of optically detected impurity resonances in confined systems and demonstrates the power and utility of this technique for such studies.     

Comparison of GaInNAs laser diodes based on two to five quantum wells

We have investigated GaInNAs-GaAsN multiquantum-well (MQW) lasers with two-QW (DQW), three-QW (TQW), and five-QW (5QW) active regions and emission in the 1.3-/spl mu/m range. A solid-source molecular beam epitaxy system has been used to grow the structures. Operation of a GaInNAs 5QW laser is reported. Low threshold currents of 22 (DQW) to 52 mA (5QWs) and external efficiencies of 0.25 W/A (DQWs) to 0.16 W/A (5QWs) per facet are realized under CW operation. T/sub 0/-values of 121 K are obtained.     

A dual-color injection laser based on intra- and inter-band carriertransitions in semiconductor quantum wells or quantum dots

A new type of semiconductor injection laser capable of simultaneously generating radiation in the mid-infrared (MIR) (λ~10 μm) and near-infrared (NIR) (λ~0.9 μm) spectral regions is proposed. The MIR emission is a result of intersubband (intraband) electron transitions within a three-level conduction band in a quantum well or a quantum dot. The NIR emission, on the other hand, is due to conventional interband recombination of injected electrons and holes into the conduction and valence bands, respectively. The conditions for population inversion in the intersubband emission process are determined by an appropriately engineered energy structure for a three-level system in the conduction band of a quantum well or dot structure: for the quantum-well-based system, the structure has an asymmetric funnel shape to provide long electron-phonon lifetime at the third (top) energy level. Under high carrier injection, NIR interband emission depopulates the conduction ground level of the quantum well, thereby stabilizing the electron concentration at this level-a necessary condition fur the operation of the MIR laser. This paper discusses the calculation of the population inversion conditions, the requisite gain, and threshold current for MIR laser operation. We also present a preliminary design of the laser structure with a composite waveguide that accommodates both mid- and NIR stimulated emission     

Optical effects based on intersub-band-transitions in quantum wells

Unipolar devices based on intersub-band-transition have been developed in the last decade [Appl. Phys. Lett. 65 (1994) 2901]. However, this technique will not be directed applicable for the longer wavelengths corresponding to Tetrahertz frequencies. In this work we analyze a THz device based on the intersub-band-transitions of an asymmetric double quantum well. We first study the intersub-band optical absorption in superlattice made of asymmetric double quantum wells tailored as a three level system. By applying an external electric field we obtain the Wannier-Stark ladder and can tune the transition energies between the sub-bands to reach the THz absorption between the two excited sub-bands. Although the dipole moments are big, the relative THz absorption to the other frequencies is small. However, reported lifetimes for this system encourage the possibility of getting population inversion, which is the main condition to design a laser. With this goal, it is presented a detailed study of the geometric design of the asymmetric double quantum well by performing an accurate calculation of the energies and wave functions, the dipole moments and the electron-LO-phonon interaction form factors, which are important ingredients of the scattering rates results. Furthermore, we analyze the role of electron-LO-phonon scattering in THz devices.     

Optically pumped cw CH2DOH fir laser: New lines and frequency measurements

We have measured the output powers and relative polarizations of 66 cw FIR laser lines from CH₂DOH (including 50 not previously reported), which were optically pumped by a CO₂ laser. The frequencies of 43 of these lines were measured relative to stabilized CO₂ lasers.     

New HF laser pumped molecular lasers in the middle infrared

Stimulated emission has been observed in five molecules optically pumped by a pulsed HF laser. Emission was observed in¹⁵N2O,¹⁴N¹⁵NO, and¹⁵N¹⁴NO at 4.6 μm, HCOOH at 5.7 μm, and in¹³CS2at 6.9 μm. Lasing pulse shapes and delays after the pump pulse were measured. Lasing due to rotational relaxation induced by collisions with He was observed in¹⁵N¹⁴NO.     

Optically pumped FIR lasers: Frequency and power measurements and laser magnetic resonance spectroscopy

Optically pumped FIR lasers are currently in use in both frequency metrology and laser magnetic resonance spectroscopy programs in the NBS Boulder labs. The laser for use in frequency metrology is a CW 71 μm methyl alcohol waveguide laser with over 100 mW output for frequency synthesis. Another laser with an intracavity absorption cell for laser spectroscopy has been constructed and is nearly transversely pumped. The metrology technique used to measure the frequency of these lasers is briefly reviewed and a unique power meter is described.     

Optically pumped far-infrared emission in the cis 1, 2- difluoroethene laser

37 new far-infrared laser frequencies have been obtained in CO2laser excited cis 1, 2-C2H2F2, with several strong emissions in the terahertz region. Competing transitions have enabled assignments to be made to the v4and2v_{7}vibration bands.     

钠蒸气中基于Na-Na碰撞能量转移的光泵级联辐射

本文报道了钠蒸气中基于Na-Na近共振碰撞能量转移而产生的光泵受激辐射。当双光子共振激发4d能级时,可探测到起始于4f能级的受激辐射;而当进行4f能级的偶极禁戒双光子共振激发时,则可产生起始于4f能级的受激辐射及其跟随的级联受激辐射。文中对有关过程进行了讨论。     

Nonlinear level crossing in the infrared transitions of optically pumped far infrared laser media

In conjunction with the Stark field induced power enhancement in optically pumped far infrared lasers, absorption transitions in a degenerate two level system under the influence of an weak static Stark field have been analyed. A third order theory to the density matrix formalism leads to zero field level crossing signals for absorption in rate equation approximation and in Doppler limit. For fast relaxation among Zeeman sublevels, the zero field level crossing signal arises from population effect. Optoacoustic measurements have been carried out on the ν₅,^qQ(16,8) absorption line of CH₃OH which produces the familiar 119-μm laser with the CO₂ 9P(36) pump line. The optoacoustic absorption signals observed are well reproduced by the theoretical expression.