Pumped Quantum Wells Research Articles

Nuclear spin diffusion effects in optically pumped quantum wells

We studied the influence of the nuclear spin diffusion on the dynamical nuclear polarization of low dimensional nanostructures subject to optical pumping. Our analysis shows that the induced nuclear spin polarization in semiconductor nanostructures will develop both a time and position dependence due to a nonuniform hyperfine interaction as a result of the geometrical confinement provided by the system. In particular, for the case of semiconductor quantum wells, nuclear spin diffusion is responsible for a nonzero nuclear spin polarization in the quantum well barriers. As an example we considered a 57 A GaAs square quantum well and a 1000 A Al x Ga1−x As parabolic quantum well both within 500 A Al0.4Ga0.6As barriers. We found that the average nuclear spin polarization in the quantum well barriers depends on the strength of the geometrical confinement provided by the structure and is characterized by a saturation time of the order of few hundred seconds. Depending on the value of the nuclear spin diffusion constant, the average nuclear spin polarization in the quantum well barriers can get as high as 70% for the square quantum well and 40% for the parabolic quantum well. These results should be relevant for both time resolved Faraday rotation and optical nuclear magnetic resonance experimental techniques.

Intersubband Absorption in Terahertz Lasers Based on Optically Pumped Quantum Well Structures

We study the effects of microscopic dynamics of electrons on the intersubband optical absorption in terahertz lasers based on optically pumped quantum wells (THz-OPQW). The ensemble Monte Carlo method is used to include different electron scattering mechanisms in the calculation, so that the dependence of intersubband absorption lineshape on microscopic dynamics of electrons can be easily investigated. We find that electron-electron scattering is the dominant factor to determine the absorption linewidth. The real distributions of hot electrons are included to study the temporal changes of the spectra lineshape. Our findings show that the dependence of spectral lineshape on electron population essentially results from the electron scattering and the non-Fermi electron energy distribution.

Gain-assisted propagation of surface plasmon polaritons via electrically pumped quantum wells

We studied the loss compensation of surface plasmon polaritons (SPPs) with InGaAsP quantum wells at telecom wavelength. The quantum wells are buried in the vicinity of a thin Au film. The propagation length of short-range SPPs increases drastically with the gain coefficient of quantum wells, generated by a forward bias. The elongation of SPP propagation is experimentally observed via long-range SPPs, which strongly couple with the short-range SPPs. This study paves a way for electrically manipulated amplification of SPPs in plasmonic circuits.

Charging Dynamics in Electrically Pumped Quantum Wells

Injection pumping of quantum-well (QW) lasers can lead to charging of the active region due to the different electron and hole masses. To investigate the significance and physical consequences of this effect, the consequences of electrical pumping and the associated charging phenomena are investigated microscopically. The analysis reveals the necessity to include the extended barrier states energetically above the band edge in addition to the quantum-confined well states. It is shown that the barrier-state populations compensate the QW charging. The replenishing of stimulated carrier recombination under CW laser operation causes persistent charge oscillations at the plasma frequency.

Measurement of far-infrared intersubband spontaneous emission from optically pumped quantum wells

We present the results of an experiment to measure far-infrared intersubband spontaneous emission from optically pumped quantum wells. A four-level laser design scheme was implemented using a coupled triple quantum well structure for optical pumping from E1 to E4 with a CO2 laser. The phonon scattering rates were designed to optimize the far-infrared emission efficiency and population inversion between E3−E2∼30 meV. The spontaneous emission spectrum was successfully resolved to reveal an emission peak at 30.8 meV with a linewidth of 7 meV.

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 infrared intersubband emission in 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.