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Annual Meeting Optical Society of America

Reduced absorption mechanisms in semiconductor quantum wells

Dong‐Seon Lee, Kevin J. Malloy

Published Jan 1, 1992• 0 citations

Abstract

Ultrahigh refractive indices without absorption1 and reduced absorption2 have been predicted and observed for atomic systems. We apply these considerations to semiconductor structures with complicated energy bands and strong phonon interactions. Bulk semiconductors offer limited flexibility, while quantum well structures offer the ability to tailor the dielectric response through control of the conduction subband states and the separation between heavy and light hole states. To observe high indices without absorption, Raman pumping from the ground conduction state should result in a coherent state at the bottom of heavy hole and light hole bands. The high index then exists between the first excited conduction subband and the range of separation between the holes. Since this scheme gives a large refractive index change, we can observe results by photo- or electro-reflectance. However, interactions between subbands and the strong phonon scattering may inhibit the coherent state. To observe reduced absorption without changes of refractive index, two conduction subbands should be coupled by a CO2 laser. Reduced absorption should be observable from the bottom of the hole state to the range between the two dressed states of the conduction subbands. This approach is simpler than coherent pumping, and modulated transmission offers the best chance for observation.

DOI: 10.1364/oam.1992.wb6

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Ultrahigh refractive indices without absorption1 and reduced absorption2 have been predicted and observed for atomic systems. We apply these considerations to semiconductor structures with complicated energy bands and strong phonon interactions. Bulk semiconductors offer limited flexibility, while quantum well structures offer the ability to tailor the dielectric response through control of the conduction subband states and the separation between heavy and light hole states. To observe high indices without absorption, Raman pumping from the ground conduction state should result in a coherent state at the bottom of heavy hole and light hole bands. The high index then exists between the first excited conduction subband and the range of separation between the holes. Since this scheme gives a large refractive index change, we can observe results by photo- or electro-reflectance. However, interactions between subbands and the strong phonon scattering may inhibit the coherent state. To observe reduced absorption without changes of refractive index, two conduction subbands should be coupled by a CO2 laser. Reduced absorption should be observable from the bottom of the hole state to the range between the two dressed states of the conduction subbands. This approach is simpler than coherent pumping, and modulated transmission offers the best chance for observation.

When two electron conduction subbands in a doped quantum well are coupled by a coherent laser pumping, electromagnetically induced transparency and Rabi splitting in the interband absorption spectrum of a probe light are found for steady electron dressed states. The effect of light-electron interaction on electron and hole eigenenergies and states are calculated using a generalized density-functional approach. The change of absorption coefficient with the pump field amplitude is studied and explained.

Optical interband excitonic absorption of semiconductor quantum wells (QW’s) driven by a coherent pump field is investigated on the basis of semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW’s. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron–hole pairs in the QW structure is low, the pump field induces an Autler–Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron–hole pairs is increased, the split contrast (the ratio between the maximum and the minimum values) is decreased, because the exciton effect is suppressed at higher densities owing to the many-body screening.

Summary form only given. We investigate theoretically the relation between generalized Rabi oscillations involving optical transitions in three band systems and Raman coherences. The system under consideration is a conventional GaAs semiconductor quantum well in which only the lowest subband of the conduction, the heavy hole bands need to be taken into account. We consider two simultaneous, strong sub-picosecond pulses (770 fs) of opposite circular polarization, spectrally centered at the hh and the lh-excitons respectively.

A strong infrared field tuned in resonance with dipole-allowed intersubband transitions in quantum-well conduction bands can act to coherently mix or dress the resonant states. The effects of dressing two excited conduction subbands on the linear response of a weak probe field nearly resonant with one of the excited subbands and the partially occupied ground subband is theoretically treated to arbitrary order in the coupling-field strength. Three critical coupling-field strengths are identified that demark: (i) the minimum coupling required to observe changes in the linear response, (ii) the onset of a resolvable doublet in the response, and (iii) the strong-coupling limit in which the line doubling is fully developed. Analytic expressions for these three critical fields are obtained in terms of the Rabi frequency and the phenomenological damping parameters associated with states in the three subbands.

Two-photon absorption spectroscopy was performed in GaAs quantum wells with photon energy \ensuremath{\Elzxh}\ensuremath{\omega} near half the band gap

{\mathrm{E}}_{\mathrm{g}}

. For the case of incident light polarized perpendicular to the confinement direction \ensuremath{\epsilon}^\ensuremath{\perp}z^, the onset of two-photon absorption occurs at the 2P state of the lowest-energy heavy-hole exciton and the absorption increases linearly with 2\ensuremath{\Elzxh}\ensuremath{\omega}-

{\mathrm{E}}_{\mathrm{g}}

. For \ensuremath{\epsilon}^\ensuremath{\parallel}z^, the two-photon spectrum is stepped with superpositions of sharp 1S exciton features at odd \ensuremath{\Delta}N light-hole--conduction-band transitions. These results are used to accurately determine exciton binding energy, successive subband energy splittings, and band offsets.

In a self-consistent field formalism we investigated the weak-probe optical intersubband response of a three-subband semiconductor quantum well pumped by a strong coherent laser beam. We showed that, when the pumping photon energy is somewhat above the energy separation between the two lowest subbands, a combination of hole-burning effects and coherent pump–probe wave interactions can lead to both optical gain without population inversion and an Autler–Townes-like doublet in the probe optical spectrum. These two absorption peaks, whose positions are blueshifted as the pump intensity is increased, are due to the two-photon absorption process and light-induced intersubband transitions between the two upper states. It is also demonstrated that, in a stepwise two-photon pumping scheme, the optical gain with population inversion can also occur in the quantum-well system when a modestly strong pump field is present.