Lasing from excitons in quantum wires

Abstract

Stimulated optical emission from the lowest exciton state in atomically smooth semiconductor quantum wires is observed for the first time. The wires are formed by the T intersection of two 7 nm GaAs quantum wells. The optical emission wavelength is nearly independent of pump levels. This absence of band-gap renormalization in the laser emission indicates a marked increase in the stability of the exciton in one dimension. PACS numbers: 78.45.+h, 73.20.Dx, 78.55.Cr The superior performance of quantum well (QW) semiconductor lasers over heterostructure lasers has directed considerable attention towards lower-dimensionality quantum wire (QWR) and quantum box structures. Carrier confinement to one or even zero dimensions is expected to give rise to sharp peaks in the density of states. This should lead to a variety of interesting optical properties such as increased exciton binding [1,2], enhanced optical nonlinearities [3], narrower gain spectra, and higher differential gain [4]. These striking physical phenomena are of importance for novel optoelectronic devices. Quantum wire geometries have been proposed as a route to lower threshold lasers having reduced threshold temperature sensitivity and increased modulation bandwidth [4][5][6]. However, the fabrication of QWR structures with precisely controlled dimensions is a very challenging task. Growth on patterned substrates [7] has been used to fabricate QWRs exhibiting optical signatures of carrier confinement to one dimension. In that work, stimulated emission from QWRs was first demonstrated. However, the relatively large size (80-100 by 10 nm) of the structures results in the occupation of many one-dimensional (ID) subbands, and due to band-filling effects only the higher-order transitions were observed.The existence of QWR states was recently also demonstrated at the T intersection of two GaAs QWs [8]. The T concept originally proposed by Chang et al. [9] was realized by the cleaved edge overgrowth (CEO) method, a molecular beam epitaxy (MBE) technique that uses high-quality regrowth on the cleaved edge of a multilayer sample [10]. The method is capable of producing nearly perfect structures with atomic control in two dimensions. The quantum mechanical bound state of an electron at two intersecting QWs is illustrated in Fig. 1. Near the T intersection confinement is somewhat relaxed leading to a smaller kinetic contribution to the total energy. A carrier in such a bound state is free to move along the line defined by the intersecting planes of the two QWs.This Letter reports the first observation of stimulated optical emission using exciton recombination in QWRs formed at intersecting GaAs QWs. Exciton emission in the ID quantum limit, i.e., from the ground state exciton, is apparent in these structures at low optical pump power densities of 600 W/cm 2 , where the adjacent QWs show no stimulated emission. Equally striking is the near constancy of the photon emission energy of the QWRs under large changes of pump power, ranging over 2 orders of magnitude to 3 k...

Photoluminescence sidebands of carbon nanotubes below the bright singlet excitonic levels

“…7͒ are 1D materials that possess various unique properties. 8 The strong confinement in the radial direction ͑ϳ1 nm͒ and the weak dielectric screening inside SWNTs give rise to very large exciton binding energies of ϳ0.5 eV, [9][10][11][12][13] which is much larger than those of InGaAs quantum wires 5,14 and larger than or comparable to those of -conjugated polymers. 15,16 So far, various groups have studied the ex-ph interactions in SWNTs both in terms of theory 17,18 and experiment [19][20][21][22] and have revealed the existence of a phonon sideband approximately 200 meV above the energy level of the singlet bright exciton ͑which has an s envelope and termed a "1u" state based on symmetry 12,13 but we simply call it "E ii " hereafter for the ith single-particle subband͒.…”

Many-Body Renormalization of Semiconductor Quantum Wire Excitons: Absorption, Gain, Binding, and Unbinding

“…Recent interest has focused on low dimensional excitons in artificially structured semiconductor quantum well or wire systems where carrier confinement may substantially enhance the excitonic binding energy leading to novel optical phenomena. In this Letter we consider the formation, stability, and optical properties of one dimensional (1D) excitons in semiconductor quantum wires, a problem which has attracted a great deal of recent experimental [1][2][3] and theoretical [4-6] attention. Our motivation has been a number of recent puzzling experimental observations [1,2], which find the photoluminescence emitted from an initially photoexcited semiconductor quantum wire plasma to be peaked essentially at a constant energy independent of the magnitude of the photoexcitation intensity.…”

“…In this Letter we consider the formation, stability, and optical properties of one dimensional (1D) excitons in semiconductor quantum wires, a problem which has attracted a great deal of recent experimental [1][2][3] and theoretical [4-6] attention. Our motivation has been a number of recent puzzling experimental observations [1,2], which find the photoluminescence emitted from an initially photoexcited semiconductor quantum wire plasma to be peaked essentially at a constant energy independent of the magnitude of the photoexcitation intensity. This is surprising because one expects a strongly density-dependent "red shift" in the peak due to the exchange-correlation induced band gap renormalization (BGR) (i.e.…”

“…We find that, in agreement with experimental observations, our calculated effective excitonic energy (indicating the luminescence peak frequency) remains essentially a constant (with an energy shift of less than 0.5 meV) as a function of 1D carrier density n for n < n c ∼ 3 × 10 5 cm −1 with the system making a Mott transition from an insulating exciton gas of bound electron-hole pairs (n < n c ) to an EHP (n > n c ) at n = n c . For n > n c we find strong optical gain in the calculated absorption spectra.For our results to be presented here we have considered quantum wire parameters [1] corresponding to the T-junction structure of width 70Å in both transverse directions with only the lowest 1D subband occupied by the carriers. But our results and conclusions should be generically valid for arbitrary 1D quantum wire confinement (e.g.…”

Intrasubband and intersubband electron relaxation in semiconductor quantum wire structures

“…quantum wire lasers), and also because of their fundamental significance as examples of quasi-one-dimensional (Q1D) electron liquids. Among the important research milestones in semiconductor quantum wires are the observation [1] of one dimensional plasmons via the inelastic light scattering spectroscopy and the verification of the predicted acoustic linear plasma dispersion relation [2] in one dimension, the observation of pronounced one dimensional Fermi edge singularities in the optical spectra [3], the quantum wire excitonic laser operation [4] and its theoretical understanding [5]. With improving materials growth and nano-fabrication techniques one expects a wide range of one dimensional experimental phenomena and projected applications in semiconductor quantum wire systems.…”