Mid-infrared Intersubband Research Articles

Measurement of semiconductor laser gain and dispersion curves utilizing Fourier transforms of the emission spectra

A new technique for the measurement of semiconductor laser gain and dispersion spectra is presented. The technique is based on an analysis of the subthreshold emission spectrum by Fourier transforms. Applications of this method to AlGaInP-based interband laser diodes and mid-infrared intersubband quantum cascade lasers are discussed. A good agreement between the measured dispersion of the refractive index and tabulated values in the literature was found.

Mid-infrared intersubband electroluininescencein InAs/AlSb cascade structures

Mid-infrared intersubband electroluminescence is reported in InAs single quantum wells embedded in InAs/AlSb quantum cascade structures. The observed emission energy is in good agreement with calculations based on the multiband k/spl middot/p theory. The dominant polarisation of the emitted light is perpendicular to the quantum well layers.

Intersubband χ/sup 3/ in coupled InGaAs-AlGaAs multiple quantum wells

The third-order nonlinearity, /spl chi//sup 3/(/spl omega/,/spl omega/,-/spl omega/), is measured for a mid-infrared intersubband transition in strained InGaAs-AlGaAs multiple quantum wells (MQW's). The high conduction band offset of this system allows an intersubband transition at 3.1 /spl mu/m. The level structure of the quantum well is designed to include a meta-stable trapping level, resulting in a peak saturation intensity of 6 MW/cm/sup 2/ at Brewster's angle, approximately 20 times lower than would be found in a square quantum well with similar linewidth. A near-resonant n/sub 2/ of 8.4/spl times/10/sup -7/ cm/sup 2//W at 3.1 /spl mu/m is calculated. The off-resonant n/sub 2/ is also calculated and shown to be attractive at wavelengths as short as 1.55 /spl mu/m.

Ultrafast heating and cooling of electron plasmas in GaInAs/AlInAs quantum wells after intersubband excitation

Abstract Energy exchange between nonequilibrium carriers and a two-dimensional electron plasma and subsequent carrier cooling are studied in femtosecond pump-probe experiments. (n = 1) electrons excited to the (n = 2) conduction subband via the midinfrared intersubband transition undergo intersubband scattering and subsequently transfer a large amount of energy to unexcited (n = 1) electrons resulting in a hot Fermi distribution within 2 ps. The observed heating rate increases with the plasma density whereas the subsequent cooling proceeds within 40 ps by emission of optical phonons and is found to be density independent.

Mid-infrared intersubband electroluminescence from a single-period GaAs/AlGaAs triple barrier structure

This letter reports the observation of intersubband electroluminescence from a single-period resonant tunneling structure. Intersubband emission (λ≈8.4 μm), with a full width at half maximum of 7 meV, was observed from a GaAs/AlGaAs triple barrier structure with quantum well widths of 66 and 33 Å. The emission was coupled out of the sample by a metallic grating with a period of 5 μm deposited on the surface. The intensity of emission follows the resonance behavior in the I–V characteristics. As the temperature increases from 10 to 200 K, the emission intensity decreases monotonically by a factor of ∼2 and the emission energy shifts down slightly (ΔE≈2.7 meV). The temperature dependence of the emission energy is explained by a combination of thermal broadening of the electron distribution and the nonparabolicity of the conduction bands.

Investigation of mid-infrared intersubband stimulated gain under optical pumping in GaAs/AlGaAs quantum wells

We have investigated the mid-infrared intersubband stimulated emission under optical pumping in GaAs/AlGaAs coupled quantum wells. The quantum wells exhibit four levels bound in the conduction band. The energy between the ground and first excited subband is close to the optical phonon energy enabling population inversion. Intersubband stimulated gain between subbands E3 and E2 is observed around 14 μm wavelength by optically pumping the E1−E3 intersubband transition at 10 μm. The gain measurement is performed by time-resolved pump-probe experiments using a two-color picosecond free-electron laser. The dependence of the intersubband stimulated emission is analyzed as a function of the pump intensity, and the pump and probe wavelengths. We show that very large intersubband stimulated gain can be achieved at liquid nitrogen temperature in a 2 mm thick waveguide. The stimulated gain is resonant with the pump wavelength with a broadening ≈25 meV. The experimental results are explained with a nonperturbative matrix density treatment of a three-level system.

Mid-infrared intersubband emission from optically pumped asymmetric coupled quantum wells

Mid-infrared optical emission due to intersubband transitions between excited conduction subbands of a coupled quantum well structure is studied. The emission process is based on optical pumping of free carriers from the ground subband into the third subband followed by a radiative transition from the third subband into the second subband and a fast phonon assisted relaxation into the ground subband. We have observed spontaneous emission at 14 μm that persists up to room temperature. Our results indicate that population inversion between conduction subbands and large stimulated gain can be achieved.

Hot electron effects in optically-pumped mid-infrared intersubband semiconductor laser

A theoretical investigation of an optically-pumped mid-infrared intersubband semiconductor laser is presented. The influence of electrons and dopant ions on the conduction band structure is simulated with a self-consistent Poisson–Schrödinger solver. Electron-polar optical phonon interactions are calculated by using a macroscopic phonon model with electromagnetic boundary conditions. In order to assess the influence of the electronic temperature on the device optical performances, electron dynamics under optical pumping are investigated within a rate equation model where particle and energy flow equations are derived from Boltzmann's equation with Fermi statistics. Our calculations show that population inversion between the first and second excited states can occur at 77 K under intersubband optical excitation.

Simulation of optically pumped mid-infrared intersubband semiconductor laser structures

A theoretical self-consistent investigation of optically pumped mid-infrared intersubband semiconductor laser with hot electron effects is presented. Electron dynamics under optical pumping are investigated within a rate equation formulation where particle and energy flow equations are derived from Boltzmann’s equation using Fermi statistics. Electron-polar optical phonon interactions with suitable screening are calculated by using a macroscopic model with slab and interface phonon modes. Our calculations show that despite hot electron effects, population inversion between the first and second excited states can occur at low temperatures under intersubband optical excitation. It is anticipated that lasing in the mid-infrared can be achieved with asymmetric quantum well structures optimized for electron concentrations exceeding 1011/cm2.

Design and performance optimization of optically-pumped mid-infrared intersubband semiconductor lasers

We show that self-consistent simulations of asymmetric coupled double-quantum wells (ACQW) can be used efficiently to increase significantly the optical performances of optically-pumped mid-infrared intersubband semiconductor lasers by increasing the electron concentration in the quantum well. Our calculations, which account for the screening effect and degenerate statistics, show optical gain in excess of 100/cm for pumping intensity of 39 kW/cm/sup 2/ can be achieved. Owing to hot electron effects, the stimulated gain at 77 K is insensitive to the phonon resonance for pumping intensities larger than 400 kW/cm/sup 2/.

Intersubband mid-infrared emission in optically pumped quantum wells

Optical emission between excited subbands of asymmetric coupled GaAs/AlGaAs quantum wells is investigated. The structures are designed to present an energy separation between the ground subband and the first excited subband close to the LO-phonon energy in order to enhance the non-radiative relaxation between those states. Optical pumping at 10 μm is used to excite electrons in the third subband. A mid-infrared emission at 14- μm is shown to take place between the third and second subbands. The spontaneous emission is observed up to room temperature. The radiative efficiency is ≈60- nW/W in lightly doped samples. We also predict that this situation is very effective for achieving almost complete population inversion as well as large stimulated gains. The possibility of mid-infrared intersubband emission under band-to-band optical pumping is also discussed.

Observation of optically pumped midinfrared intersubband luminescence in a coupled quantum wells structure

We have observed midinfrared emission due to intersubband transitions in a coupled quantum wells structure. The excitation process involves optical pumping of the carriers from the ground subband into the third subband and a generation of a population in the third subband. The luminescence is produced by a radiative transition of the photoexcited carriers from the third to the second subband, followed by a fast relaxation into the ground subband due to a resonance LO-phonon assisted relaxation process. The emission signal persists from low temperatures up to room temperature with a maximum efficiency at 190 K.

Observation of infrared intersubband emissionin optically pumped quantum wells

Mid-infrared intersubband emission is investigated in GaAs/AlGaAs asymmetric coupled quantum wells exhibiting an energy separation between the ground and first excited subbands close to the LO-phonon energy. The authors show that an infrared emission occurs between excited subbands under intersubband optical pumping.

Vertical transition quantum cascade laser with Bragg confined excited state

A new midinfrared (λ∼4.5 μm) intersubband quantum cascade laser based on a vertical transition is reported. A superlattice graded gap region was incorporated in the design to provide strong electron confinement in the upper state using a Bragg reflector. Pulsed operation at 100 K is reported with a threshold current density of Jth=3 kA/cm2 and a measured slope efficiency of 300 mW/A.

Mid-infrared field-tunable intersubband electroluminescence at room temperature by photon-assisted tunneling in coupled-quantum wells

Electroluminescence associated with intersubband transition energies greater than the optical phonon energy and at temperatures as high as 300 K is reported. The mid-infrared (λ∼5 μm) luminescence is generated by electron injection into AlInAs/GaInAs coupled-quantum wells via an AlInAs/GaInAs pseudoquaternary graded gap alloy. Optical powers up to a few nanowatts and linear in the drive current have been measured. The large Stark shift of the luminescence peak demonstrates the photon-assisted tunneling nature of the transition. The temperature independence of the luminescence slope efficiency in the 10–100 K range and the linearity of the optical power versus drive current provide strong evidence of population inversion.

Switching between two mid-infrared intersubband absorption energies in double quantum wells

We present a new efficient modulators structure in which electron tunneling between asymmetric coupled quantum wells is used to populate or deplete the ground level for the infrared absorption, leading to externally controlled infrared absorption at two different wavelengths (here 10.2 μm and 11.4 μm). The temperature dependence of the modulation depth is measured. The experimental results are corroborated by calculations based on a self-consistent model. Monochromatic electron transfer infrared modulators are also presented.