Robust quantum dot exciton generation via adiabatic passage with frequency-swept optical pulses

The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We report on the robust population inversion in a single quantum dot using an optical technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser pulses whose frequency is swept through the resonance. This observation in photoluminescence experiments is made possible by introducing a novel optical detection scheme for the resonant electron hole pair (exciton) generation.     

Rabi waves and Rabi wavepackets in one-dimensional quantum dot chain: Excitation,propagation, reflection

We theoretically investigate the spatial propagation of Rabi oscillations (Rabi waves) in one-dimensional inhomogeneous quantum dot (QD) chain. QD-chain is modeled by the set of two-level quantum systems with tunnel coupling between neighboring QDs. The space propagation of Rabi waves in the form of traveling waves and localized wave packets is considered. It is shown, that the propagation of Rabi waves is accompanied by the transfer of the inversion and quantum correlations along the QD-chain and by their reflection and mutual transformations at the spatial inhomogeneities of QE-chain. The effect is potentially applicable in quantum nanooptics and quantum informatics.     

Radiative coupling of intersubband transitions in GaAs/AlGaAs multiple quantum wells

Radiative coupling of resonantly excited intersubband transitions in GaAs/AlGaAs multiple quantum wells can have a strong impact on the coherent nonlinear optical response, as is shown by phase and amplitude resolved propagation studies of ultrashort electric field transients. Upon increasing the driving field amplitude, strong radiative coupling leads to a pronounced self-induced absorption, followed by a bleaching due to the onset of delayed Rabi oscillations. A many-particle theory including light propagation effects accounts fully for the experimental results.     

Femtosecond mid-infrared spectroscopy of low-energy excitations in solids

Recent progress in the generation and spectroscopic application of femtosecond pulses in the wavelength range between 3 and 25 m has led to new insight into low-energy excitations of solids, in particular semiconductors, semiconductor nanostructures, and high-T_C superconductors. The ultrafast nonequilibrium dynamics of such excitations has been observed in real-time and the underlying microscopic interactions have been analyzed. This article gives a brief overview of this exciting field of ultrafast science with the main emphasis on pulse generation and applications in semiconductor research where femtosecond intersubband Rabi flopping in quantum wells and quantum coherent electron transport in quantum cascade devices have been observed. PACS 78.67.De; 73.21.Fg; 07.57.Hm; 42.65.Re     

Nonstationary parametric amplification of polychromatic radiation propagating in an extended absorbing resonant medium

An experimental and theoretical study of the two-wave interaction (of a probe wave and a pumping wave) of wide-band multimode laser pulses with an optically dense extended resonant medium (metastable neon atoms in a positive glow-discharge column) is presented. Depending on the pumping power and characteristics of the medium, various regimes of interaction between the field and the matter, leading to qualitatively different types of amplification spectra, are realized: a cooperative interaction regime at low pumping powers corresponds to amplification spectra consisting of two peaks, which lie on both sides of the absorption line; in the regime of nonstationary Rabi oscillations (Rabi flopping), the amplification appears in the form of an isolated peak at a frequency close to the resonance in the substance; under certain interaction conditions, a dispersive shape of amplification spectra is observed. A significant gain in the probe field (by several times at a length of 10 cm) is obtained as a result of a nonstationary population inversion upon rotation of the Bloch vector of the resonant medium under the action of a strong pumping field in the course of propagation of the two waves.     

Intensity‐dependent ultrafast dynamics of injection currents in unbiased GaAs quantum wells

The intensity dependence of optically‐induced injection currents in unbiased GaAs semiconductor quantum wells grown in [110] direction is investigated theoretically for a number of well widths. Our microscopic analysis is based on a 14 × 14 band k · p method in combination with the multisubband semiconductor Bloch equations. An oscillatory dependence of the injection current transients as function of intensity and time is predicted and explained. It is demonstrated that optical excitations involving different subbands and Rabi flopping are responsible for this complex dynamics. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rapid adiabatic passage without level crossing

We present a method for achieving complete population transfer in a two-state quantum system via adiabatic time evolution in which, contrary to conventional rapid adiabatic passage produced by chirped pulses, there occurs no crossing of diabatic energy curves: there is no sign change of the detuning. Instead, we use structured pulses, in which, in addition to satisfying conditions for adiabatic evolution, there occurs a sign change of the Rabi frequency when the detuning is zero. We present simulations that offer simple geometrical interpretation of the two-dimensional motion of the Bloch vector for this system, illustrating how both complete population inversion and complete population return occur for different choices of structured pulses.     

Single-qubit lasing and cooling at the Rabi frequency

For a superconducting qubit driven to perform Rabi oscillations and coupled to a slow electromagnetic or nanomechanical oscillator we describe previously unexplored quantum optics effects. When the Rabi frequency is tuned to resonance with the oscillator, the latter can be driven far from equilibrium. Blue detuned driving leads to a population inversion in the qubit and a bistability with lasing behavior of the oscillator; for red detuning the qubit cools the oscillator. This behavior persists at the symmetry point where the qubit-oscillator coupling is quadratic and decoherence effects are minimized. There the system realizes a "single-atom-two-photon laser."     

Population difference study of few-cycle laser pulse propagating in near-resonant two-level atom medium

We have theoretically studied the propagation of few-cycle laser pulse in near-resonant two-level atom medium by solving Maxwell-Bloch equations with a predictor-corrector finite-difference time-domain method. Hyperbolic secant pulses with area 2π, 4π and 6π are adopted as the initial inputs. For large pulse area, the carrier-wave Rabi flopping occurs in the faster split 2π pulse and causes spectra broadening which subsequently increases the induced population difference. Combined with the contribution of periodically varying carrier-envelop phase, the maximum of population difference gives rise to an interesting phenomenon.     

Quantum state manipulation of single-Cesium-atom qubit in a micro-optical trap

Based on single Cesium atoims trapped in a 1064 nm microscopic optical trap we have exhibited a single qubit encoded in the Cesium ＂clock states＂. The single qubit initialization, detection and the fast state rotation with high efficiencies are demonstrated and this state manipulation is crucial for quantmn information processing. The ground ~ates Rabi flopping rate of 229.0 ± 0.6 kHz is realized hy a two-photon Raman process. A clock states dephasing time of 3.0 ± 0.7 ms is measured, while all irreversible homogeneous dephasing time of 124 ± 17 ms is achieved by using the spin-echo technique. This well-controlled single atom provides an ideal quantmn qubit and quantmn node for quantum information processing.     

Geometric Population Inversion in Rabi Oscillation

The relations between geometric phases and population inversion in Rabi oscillation are investigated for all possible cases. The results show that the population inverse is an elliptically symmetric distribution as a function of the difference of geometric phases so as resiliently to rebut certain types of computational and experiment errors in geometric quantum computation.     

Microcavity effect on the pump-probe intersubband response of multiple-quantum-well structures

We study theoretically the coherent pump-probe intersubband response of a multiple quantum well (MQW) embedded in a semiconductor microcavity. An n-type doped MQW structure with two subbands in the conduction band is considered. Self-consistent numerical calculations are performed for realistic systems employing a semiclassical approach based on the transfer matrix formalism and the so-called sheet model. They show that in the strong coupling limit the pumping of the system leads to evolution of the intersubband cavity polariton doublet into a Mollow-type spectrum. By using appropriate angles, both the pump and the probe light can be tuned into resonance with the cavity mode. In this double-resonance case, simultaneously with a dramatic enhancement of the Rabi flopping frequency, a strong selective enhancement of distinct parts of the Rabi sidebands is possible.     

Phase-resolved nonlinear response of a two-dimensional electron gas under femtosecond intersubband excitation

Strong electric-field transients resonant to intersubband transitions in n-type modulation-doped GaAs/AlGaAs quantum wells induce coherent Rabi oscillations, which are demonstrated by a phase-resolved measurement of the light emitted by the sample. The time evolution of the intersubband polarization is influenced by Coulomb-mediated many-body effects. The subpicosecond period and the phase of the Rabi oscillations are controlled by the properties of the midinfrared driving pulse.     

Quantum interference effects on the probe amplification without and with inversion in a four-level system

A four-level system driven by two coherent fields is considered. It is shown that in the presence of an incoherent pump, the probe gain at a short wavelength can be achieved due to the quantum interference. Our density matrix calculation provides the conditions for probe amplification from different origins, including gain without population inversion on any state basis, gain with population inversion on the dressed-state basis, and gain with population inversion on the bare-state basis. Also, by controlling the Rabi frequency of the coupling field a total change from non-inversion to inversion can be achieved which does not depend on the intensity of the incoherent pump.     

Coherent control and polarization readout of individual excitonic states

We present measurements and simulations of coherent control and readout of the polarization in individual exciton states. The readout is accomplished by transient four-wave mixing detected by heterodyne spectral interferometry. We observe Rabi oscillations in the polarization, which show half the period of the Rabi oscillations in the population. A decrease of the oscillation amplitude with pulse area is observed, which is not accompanied by a change in the dephasing time. This suggests the transfer of the excitation to other states as the origin of the Rabi-oscillation damping. Detuning of the excitation enables the control of the polarization in phase and amplitude and is in qualitative agreement with simulations for a two-level system. Additionally, simultaneous Rabi flopping of several spatially and energetically close exciton states is demonstrated.     

半导体量子器件物理讲座 第六讲 半导体量子阱激光器

量子阱结构是半导体光电子器件的核心组成部分,它是半导体光电子集成的重要基础,文章在描述了量子结构的态密度,量子尺寸效应,粒子数反转的基础上,介绍了量子阱导质结构激光器的工作原理,器件结构,器件性能,并对其在可见光激光器和大功率激光器件中显现出来的优越性作了进一步的说明。     

Effects of atomic motion in a standing-wave laser field on the Rabi oscillations

We study the effects of the two-level-atom motion in a standing-wave laser field on the Rabi oscillations. In the presence of the resonance optical Stern–Gerlach effect, the atomic wave packet, centered initially at a node of the standing wave, is shown to evolve in such a way that the atomic population inversion remains zero when the initially de-excited atom moves between the nodes, then collapses to the ground level upon crossing the nodes, and practically returns to zero after that. This coherent population trapping is explained in the dressed-state picture. The Doppler–Rabi resonance, i.e., maximum Rabi oscillations at large values of the atom–field detuning, becomes possible if the detuning is equal to the Doppler shift. A simple formula for the population inversion is derived in the Raman–Nath approximation.     

Enhancement of coherent population transfer in a double Λ-type four-level system via a train of pulse pairs

We propose a flexible way to significantly enhance population transfer efficiency with a train of time-separated pump-Stokes pulse pairs when the non-adiabatic coupling between two degenerate adiabatic states exists in a double Λ-type four-level system, where the pump and Stokes pulses in each pair can be applied in either counterintuitive or intuitive order. It is shown that the needed Rabi frequency for achieving complete population transfer can be reduced dramatically with the increase of number of pump-Stokes pulse pairs, which results from temporal constructive quantum interference between the sequential transitions and subsequent coherent accumulation; moreover, an arbitrary coherent superposition between the two lower states can be realized by suitably tuning the Rabi frequency and the time delay between each pump-Stokes pair. The method may find applications in control of chemical reactions, quantum optics, and quantum information processing.     

Theoretical Investigation of Lasing without Inversion in a Multiple Quantum Well System

In this study, the influence of exciton spins relaxation (ESR) on spatial evolution of amplification without population inversion in a four-level GsAs multiple quantum wells is discussed theoretically. The role of propagation distance on absorption, dispersion and group index of weak probe light in the presence and absence of ESR is then analyzed. It is found that exciton spin relaxation (spin decoherence) which resulted from many-particle interaction in semiconductors have essential roles to adjusting the absorption and dispersion properties of weak probe light. Moreover, it is realized that the propagation distance in the presence of ESR can lead to switching between subluminal and superluminal light propagation. Our study provides a suitable treatment for the next generation of all-optical systems in semiconductor nanostructures.