Spontaneous emission

About: Spontaneous emission is a research topic. Over the lifetime, 12855 publications have been published within this topic receiving 323684 citations.

Acoustically Driven Storage of Light in a Quantum Well

Abstract: The dynamics of photogenerated carriers in semiconductor structures with reduced dimensionality has been the subject of intensive investigations in recent years [1,2]. State-of-the-art band-gap engineering technologies enable us to tailor low-dimensional semiconductor systems with desirable optoelectronic properties and study the fundamental aspects of carrier dynamics. This has increased tremendously our fundamental understanding of the dynamic properties of artificial semiconductor structures and has also resulted in a wide range of novel devices such as quantum well lasers, modulators, and detectors, as well as all-optical switches. Nevertheless, the bulk band structure of semiconductors seems to dominate optoelectronic properties since the strength of interband transitions is largely governed by the atomiclike Bloch parts of the wave function [3]. Thus it appears at first glance unavoidable that strong interband optical transitions are linked to direct band-gap semiconductors with short radiative lifetimes such as GaAs, whereas long radiative lifetimes of photogenerated carriers imply utilization of semiconductors with indirect band gaps such as Si and correspondingly reduced interband absorption. Initial attempts to employ band-gap engineering in order to combine strong interband absorption with long radiative lifetimes have focused on so-called doping superlattices [4]. There, alternate n and p doping along the growth direction is utilized to combine a direct gap in momentum space with an indirect gap in real space which causes a spatial separation of photogenerated electron-hole se-hd pairs and hence considerably prolonged lifetimes. Here, we introduce a new way of band-gap engineering in which we expose a semiconductor quantum well of a direct gap material to a moving potential superlattice modulated in the plane of the well. We show that the confinement of photogenerated e-h pairs to two dimensions, together with the moving lateral superlattice, allows reversible charge separation [5]. We demonstrate that the combination of both the advantages of strong interband absorption and extremely long lifetimes of the optical excitations is achieved without affecting the superior optical quality of the quantum well material. The spatial separation of the electron-hole pairs is achieved via the piezoelectric potential of acoustic waves propagating along the surface of a semiconductor quantum well system. On a piezoelectric substrate, the elliptically polarized surface acoustic waves (SAWs) are accompanied by both lateral and vertical piezoelectric fields which propagate at the speed of sound. Those fields can be strong enough to field ionize optically generated excitons and to confine the resulting electrons and holes in the moving lateral potential wells separated by one-half wavelength of the SAW. The spatial separation dramatically reduces the recombination probability and increases the radiative lifetime by several orders of magnitude as compared to the unperturbed case. We further demonstrate that the dynamically trapped electron-hole pairs can be transported over macroscopic distances at the speed of sound and that deliberate screening of the lateral piezoelectric fields of the SAW leads to an induced radiative recombination after long storage times at a location remote from the one of e-h generation. This conversion of photons into a long lived e-h polarization which is efficiently reconverted into photons can serve as an optical delay line operating at sound velocities. The undoped quantum well samples used in our experiments are grown by molecular beam epitaxy on a (100)GaAs substrate. The quantum well consists of 10 nm pseudomorphic In0.15Ga0.85As grown on a 1 mm thick GaAs buffer and is covered by a 20 nm thick GaAs cap layer. The active area of the sample is etched into a 2.5 mm long and 0.3 mm wide mesa (see inset of Fig. 1) with two interdigital transducers (IDTs) at its ends. The IDTs are designed to operate at a center frequency fSAW › 840 MHz. They are partially impedance matched to the 50 V radio frequency (rf) circuitry using an on-chip matching network, thus reducing the insertion

Optical pumping and vibrational cooling of molecules.

Abstract: The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, cesium molecules initially formed via photoassociation of cold cesium atoms are in several vibrational levels ν of the electronic ground state. We applied a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation/spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the ν = 0 level, preventing re-excitation from that state. We observed a fast and efficient accumulation (∼70% of the initially detected molecules) in the lowest vibrational level, ν = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.

Post-selected indistinguishable photons from the resonance fluorescence of a single quantum dot in a microcavity.

Abstract: Applying continuous-wave pure resonant $s$-shell optical excitation of individual quantum dots in a high-quality micropillar cavity, we demonstrate the generation of post-selected indistinguishable photons in resonance fluorescence. Close to ideal visibility contrast of 90% is verified by polarization-dependent Hong-Ou-Mandel two-photon interference measurements. Furthermore, a strictly resonant continuous-wave excitation together with controlling the spontaneous emission lifetime of the single quantum dots via tunable emitter-mode coupling (Purcell) is proven as a versatile scheme to generate close to Fourier transform-limited (${T}_{2}/(2{T}_{1})=0.91$) single photons even at 80% of the emission saturation level.

Spontaneous Emission of Organic Molecules Embedded in a Photonic Crystal

Abstract: We report on modification of the spontaneous emission of dye molecules embedded in a threedimensional solid-state photonic crystal exhibiting a stop band in the visible range. Molecules embedded in artificial opal filled with a polymer show a dip in the fluorescence spectrum and nonexponential spontaneous decay kinetics containing both accelerated and inhibited components compared to the dye fluorescence in a reference polymer matrix. Results are interpreted in terms of redistribution of the photon density of states in the photonic crystal. [S0031-9007(98)06494-1] PACS numbers: 42.50. ‐ p

Analysis and application of theoretical gain curves to the design of multi-quantum-well lasers

Abstract: Gain/current curves for a single quantum well are calculated. The optimum well number, cavity length, threshold current, and current density of multi-quantum-well (MQW) lasers are derived in terms of this gain curve. The limiting performance of MQW lasers is found to be better than that of graded refractive index (GRIN) lasers, assuming comparable efficiencies and spontaneous emission linewidths. The optimum threshold current for an MQW laser with a 7 μm cavity and 90 percent facet reflectivity is \sim50 \mu A/μm.