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Physical Review B

Coherent population trapping in intersubband photocurrent spectra

M. Z. Maialle, Marcos H. Degani

Published Apr 11, 2011• 5 citations
Publisher

Abstract

We present results from numerical simulations of the photocurrent generated by intersubband optical transitions in a double quantum well coupled with a continuum of extended states. The photocurrent spectra are obtained directly from the time-dependent Schr\"odinger equation for the coherent regime without any adjusting parameters in the calculations other than the ones that define the physical system, also in a nonperturbative way and without basis-set expansions or truncations. A realistic representation of a three-level system in the Lambda (``$\ensuremath{\Lambda}$'') configuration is investigated when two bound states in each quantum well are coupled by exciting fields via an excited quasibound state. Resonance between the exciting fields and the quantum states leads to coherent effects such as Rabi-dressed states, electromagnetically induced transparency, and population trapping, which are investigated in terms of the photocurrent spectral changes; that is, the coherent optical dynamics can be seen from the photocurrent signal. An excitation scheme involving two-photon absorption was proposed to produce the population-trapping effects using only one exciting field.

DOI
10.1103/physrevb.83.155308

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We present results from numerical simulations of the photocurrent generated by intersubband optical transitions in a double quantum well coupled with a continuum of extended states. The photocurrent spectra are obtained directly from the time-dependent Schr"odinger equation for the coherent regime without any adjusting parameters in the calculations other than the ones that define the physical system, also in a nonperturbative way and without basis-set expansions or truncations. A realistic representation of a three-level system in the Lambda (``\ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ'') configuration is investigated when two bound states in each quantum well are coupled by exciting fields via an excited quasibound state. Resonance between the exciting fields and the quantum states leads to coherent effects such as Rabi-dressed states, electromagnetically induced transparency, and population trapping, which are investigated in terms of the photocurrent spectral changes; that is, the coherent optical dynamics can be seen from the photocurrent signal. An excitation scheme involving two-photon absorption was proposed to produce the population-trapping effects using only one exciting field.
We investigate the conduction intersubband linear response of a n-type quantum well driven by one or two infrared optical fields with arbitrary strengths. The fields couple the third subband to the second energy subband and the ground subband, forming a Λ system. We investigate the dependences of the absorption spectrum on the pump field strengths. The results for the equal Rabi frequencies can be understood in terms of quantum coherence processes of the intersubband transitions, such as coherent population trapping, and electromagnetically induced transparency. The effects of different Rabi frequencies on the absorption spectrum due to fields are also discussed.
A dressed-state study of lasing without population inversion from a three-level atom interacting with a bichromatic laser field in the ladder configuration is being formulated. The system is allowed to be dressed by both laser-field photons (double dressing). The temporal behavior of the system under consideration is being explored numerically. Although the use of dressed-basis density matrix equations introduces major complexity into the equations, this procedure has crucial importance to the understanding of the atomic spectrum and the underlying physics of the processes involved. The system is found to possess a gain without population inversion, as well as a regular gain with inversion for various transitions, both in the transient and steady-state regimes. Enhancement of the refractive index accompanied by vanishingly small absorption was also found. The frequency response of various transitions is explored via numerical Fourier analysis. Some transitions are found to exhibit a dispersion-like line shape, clear evidence of a quantum-coherent interference effect. The spectrum consists of two distinct quintets centered about the coupling and probe-laser frequencies. The spectral features consisting of each quintet are located symmetrically (at resonance conditions only) around the line center at positions shifted from the center by the generalized Rabi and double Rabi frequencies.
Here, I discuss the interaction of two matched ultrashort pulses with a \ensuremath{\Lambda}-like system in which the upper intermediate state is replaced by a manifold of discrete states. Many of these states are simultaneously accessed by the broad bandwidth of the two excitation pulses. The equations of motion for the probability amplitudes of the levels involved in the excitation process are solved analytically in the quasi-impulsive regime. In this regime, the excitation pulses are made shorter than the characteristic time of the excited manifold; population is then transferred to the manifold without Rabi cycling. Under the appropriate conditions, this ``multi-\ensuremath{\Lambda}'' system can exhibit coherence effects such as coherent population trapping and electromagnetically induced transparency of one of the excitation pulses.
In this work we theoretically investigate a Λ-like three-level system. Our model consists of a onedimensional quantum well with a nearby continuum. The Λ level structure is formed by the ground state (a valence band state) and two excited states (both in conduction band), one being a localized and the other a quasi-bound state which is interacting with the continuum. An infrared (IR) field is used to drive the excited states into dressed states creating Autler-Townes doublets. We solve the semiconductor Bloch equation, in real space and in time domain, to follow the interband optical excitation dynamics. The optical absorption and the photocurrent spectra are calculated for different potential barriers separating the well and the continuum. We show how this affects the Autler-Townes doublets since this is a possible way of changing the relationship between the IR Rabi frequency and the dephasing rates.
Summary form only given. The phenomenon of "dark resonances" or coherent population trapping is a well known concept in optics and laser spectroscopy. It can be considered as the basis for such effects as electromagnetically induced transparency and lasing without inversion. We report the observation of the novel spectral features which arise in systems where multiple quantum superposition states are coupled and interact coherently. These novel resonances correspond to the recently predicted "double-dark states". In the experiment, an optically thin ensemble of rubidium-37 atoms in zero magnetic field is excited in a /spl Lambda/ configuration with circularly polarized light from a pair of 789 nm diode lasers.
The phenomenon of electromagnetically induced transparency, which was discovered recently in atomic systems, is demonstrated for the first in a subband quantum well system. Applying strong coupling field (Rabi frequency is on the same order as the intersubband linewidth), which is two- photon-resonant with the 1 - 3 intersubband transition, produces dramatic change in the 1 - 2 intersubband absorption profile. This effect can be accounted for in terms of 1 - 2 and 2 - 3 dipoles being driven into coherence by a strong coupling field; it is not related to self- induced transparency or spectral hole-burning (the frequency of the coupling field is quite different from that of 1 - 2 resonance) and, similar to Fano-type interference, is a pure result of destructive interference of probability amplitudes.