Showing papers on "Spontaneous emission published in 1991"

Observation of squeezed states generated by four-wave mixing in an optical cavity

Abstract: Squeezed states of the electromagnetic field have been generated by nondegenerate four-wave mixing due to Na atoms in an optical cavity. The optical noise in the cavity, comprised of primarily vacuum fluctuations and a small component of spontaneous emission from the pumped Na atoms, is amplified in one quadrature of the optical field and deamplified in the other quadrature. These quadrature components are measured with a balanced homodyne detector. The total noise level in the deamplified quadrature drops below the vacuum noise level.

Analysis of semiconductor microcavity lasers using rate equations

Abstract: The rate equations for a microcavity semiconductor laser are solved and the steady-state behavior of the laser and some of its dynamic characteristics are investigated. It is shown that by manipulating the mode density and the spontaneous decay rates of the cavity modes, the threshold gain can be decreased and the modulation speed can be improved. However, in order to fully exploit the possibilities which the modification of the spontaneous decay opens up, the active material volume in the cavity must be smaller than a certain value. Threshold current using different definitions, population inversion factor, L-I curves, linewidth, and modulation response are discussed. >

Modification of spontaneous emission rate in planar dielectric microcavity structures.

Abstract: The spontaneous emission rate and radiation pattern for a thin quantum well sheet enclosed by a one-dimensional dielectric microcavity have been calculated. By placing the sheet in the node (antinode) position of the cavity standing wave, the spontaneous emission in the direction normal to the sheet will be decreased (enhanced). In the former case the theory predicts that the spontaneous lifetime can be increased more than a factor of 10. In the latter case both theory and experiments confirm that such a simple structure can couple a substantial amount (30\char21{}90 %) of the spontaneous emission into the cavity resonant mode. In both cases, however, theory predicts that the spontaneous emission lifetime will increase for structures without guided modes.

Microcavity semiconductor laser with enhanced spontaneous emission

Abstract: A metal-clad optical waveguide with a semiconductor microcavity structure is proposed to increase the coupling efficiency of spontaneous emission into a lasing mode (spontaneous emission coefficient \ensuremath{\beta}) and to increase a total spontaneous emission rate simultaneously. Such a microcavity semiconductor laser with enhanced spontaneous emission has novel characteristics, including high quantum efficiency, low threshold pump rate, broad modulation bandwidth, and intensity noise reduced to below the shot-noise limit (amplitude squeezing).

Spontaneous emission factor of a microcavity DBR surface-emitting laser

Abstract: The spontaneous emission factor (SEF) of a microcavity distributed Bragg reflector (DBR) surface-emitting laser has been obtained theoretically to investigate the possibility of the thresholdless lasing operation. Formulas expressing the spontaneous emission in a three-dimensional microcavity were obtained. By introducing the distribution of mode density in wavevector space, it is shown that the radiation pattern of spontaneous emission is deeply modified by the microcavity and is different from that in free space. Based on this result, the SEF and the emission lifetime are calculated as a function of emission spectral width and the size of the active region. It is found that the SEF exceeds 0.1, even though the spectral width is as large as 30 nm when the transverse size is smaller than 0.5 mu m and the DBR reflectivity is larger than 90%. >

Well-barrier hole burning in quantum well lasers

Abstract: The reported wide variations in the damping behavior of quantum well lasers are explained by a novel theory of nonlinear gain, well-barrier hole burning. In the model a spatial hole develops perpendicular to the active region involving carriers moving between the wells and the barrier/confinement layers. The modified rate equations describing well-barrier hole burning are presented. An analytical approximation for the nonlinear gain coefficient epsilon , valid only under certain conditions, is given. A numerical solution is given for the case of high photon densities and large capture-times. It is shown how well-barrier hole burning explains the measurements of the increased spontaneous emission from the barrier/confinement region above threshold. Various higher-than-expected damping rates reported in some quantum well lasers are shown to be consistent with the model. >

Quantum efficiency and excited-state relaxation dynamics in neodymium-doped phosphate laser glasses

Abstract: Radiometrically calibrated spectroscopic techniques employing an integrating-sphere detection system have been used to determine the fluorescence quantum efficiencies for two commercially available Nd3+-doped phosphate laser glasses, LG-750 and LG-760. Quantum efficiencies and fluorescence lifetimes were measured for samples with various neodymium concentrations. It is shown that the effects of concentration quenching are accurately described when both resonant nonradiative excitation hopping (the Burshtein model) and annihilation by cross relaxation are accounted for by Forster–Dexter dipole–dipole energy-transfer theory. The Forster–Dexter critical range for nonradiative excitation hopping was found to be RDD = 11 A, while the critical range for cross relaxation was close to RDA = 4 A in these glasses. The quantum efficiency at low Nd3+ concentrations was (92 ± 5)%, implying a nonradiative relaxation rate of 210 ± 150 s−1 for isolated ions. Improved values for the radiative lifetimes and the stimulated emission cross sections for these glasses were also deduced from the measurements.

Coherent atomic mirrors and beam splitters by adiabatic passage in multilevel systems.

Abstract: We study atomic-beam deflection by adiabatic passage between Zeeman ground levels via Raman transitions induced by counterpropagating ${\mathrm{\ensuremath{\sigma}}}^{\ifmmode\pm\else\textpm\fi{}}$-polarized lasers. We show that complete population transfer between the ground states can be achieved, which corresponds to the scattering of the atomic wave packet into a single final momentum state by absorption and induced emission of laser photons. Although the lasers can be resonant, the excited state(s) are never populated during the adiabatic transfer, which suppresses the effects of spontaneous emission and preserves the coherence of the atomic wave function. This scheme has attractive features as a beam splitter and mirror for atomic interferometry.

Coherence, amplification, and quantum effects in semiconductor lasers

Abstract: Line Broadening of Semiconductor Lasers (C. Henry). Modulation and Noise Spectra of Complicated Laser Structures (O. Nilsson, et al.). Frequency Tunability, Frequency Modulation, and Spectral Linewidth of Complicated Structure Lasers (Y. Yoshikuni). Spectroscopy by Semiconductor Lasers (M. Ohtsu & K. Nakagawa). Coherent Detection Using Semiconductor Lasers: System Design Concepts and Experiments (T. Hodgkinson). Traveling--Wave Semiconductor Laser Amplifiers (T. Saitoh & T. Mukai). Semiconductor Laser Amplifiers in High--Bit--Rate and Wavelength--Division--Multiplexed Optical Communication Systems (R. Jopson & T. Darcie). Injection--Locked Semiconductor Laser Amplifiers (S. Kobayashi). Photon Statistics and Mode Partition Noise of Semiconductor Lasers (P. Liu). Squeezed--State Generation by Semiconductor Lasers (Y. Yamamoto, et al.). Generation of Photon--Number--Squeezed Light by Semiconductor Incoherent Light Sources (M. Teich, et al.). Controlled Spontaneous Emission in Microcavity Semiconductor Lasers (Y. Yamamoto, et al.). Index.

Evidence for quantum confinement in the photoluminescence of porous Si and SiGe

Abstract: We have used anodization techniques to process porous surface regions in p-type Czochralski Si and in p-type Si0.85Ge0.15 epitaxial layers grown by molecular beam epitaxy. The SiGe layers were unrelaxed before processing. We have observed strong near-infrared and visible light emission from both systems. Analysis of the radiative and nonradiative recombination processes indicate that the emission is consistent with the decay of excitons localized in structures of one or zero dimensions.

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

Abstract: In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination. Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives d...

Optical fiber amplifier

Abstract: An optical fiber amplifier in which light generated within an Er doped optical fiber by spontaneous emission is eliminated or suppressed. By inserting a wavelength demultiplexer in the Er doped optical fiber, the light generated within the optical fiber by spontaneous emission can be effectively eliminated. As an alternative, the Er doped optical fiber is arranged in a coil form with a predetermined radius of curvature to thereby effectively eliminate light due to spontaneous emission on the longer wavelength side than the signal light. Further, by doping the center portion of the core of an optical fiber with aluminum and the whole body of the core uniformly with erbium, it is made possible to suppress the generation of light by spontaneous emission without causing an adverse effect on the amplification performance of the signal light.

Measurement and calculation of spontaneous recombination current and optical gain in GaAs-AlGaAs quantum-well structures

Abstract: Experimental determinations have been made of the peak optical gain as a function of spontaneous recombination current density for GaAs quantum wells of width 25 and 58 A bounded by AlGaAs barriers. These data were obtained from measurements of spontaneous emission spectra, observed through narrow windows in the 50‐μm‐wide contact stripes of oxide isolated lasers, using only a single reference value of the optical absorption coefficient above the band edge to calibrate the measurements in absolute units. These results are in good agreement with gain‐current curves calculated using a model which includes unintentional monolayer well width fluctuations, band‐gap narrowing and intraband carrier‐carrier scattering. The characteristic intraband scattering time is calculated from first principles as a function of electron energy and carrier density on the basis of a 2‐dimensional Auger‐type process. This lifetime gives a much better representation of our observed spontaneous spectra than a lifetime which is sim...

Quantum dynamics of a two-state system in a dissipative environment

Abstract: An analytical study of the dissipative Landau-Zener model is presented. The model where two energy levels at constant speed are brought to cross is a standard model used to describe a large variety of phenomena. In many cases of interest the presence of coupling of the two-state system to an environment is of importance, the accounting for which shall be done here from first principles. Analytical results for the excitation transition from the ground state of the two-state system at large negative times to the excited state at large positive times (as well as the opposite, decay, transition) are obtained in terms of the speed by which the two levels approach each other, the energy gap between the adiabatic energies, the coupling strength to the environment, and its temperature. For the excitation transition we find the following results: In the slow-passage limit of small sweeping speed it is shown that adiabaticity is limited to low temperatures and the quantitative adiabatic criterion is established. Particularly, at zero temperature there is no influence of the environment on the transition probability as a consequence of a compensation property shown to be peculiar to the linear-sweep model. The transition is at low temperatures due to quantum tunneling and, with an increase in temperature, an intermediate region appears where the transition is dominated by thermally assisted transitions across the energy gap before finally at high temperatures a saturated regime is reached with equal population of the levels. In contrast to the dependence on the temperature the dependence of the transition probability as a function of coupling strength is nonmonotonic with maximum influence at intermediate strength. In the fast-passage limit with rapid sweep speed there is no influence of the environment on the transition probability. For the decay transition the adiabatic limit does not exist for the linear-sweep model for physically relevant spectra of the environment and the decay transition is dominated by spontaneous emission, except in the fast-passage limit and the high-temperature limit where the decay transition probability equals the excitation probability.

Inhibition of spontaneous emission noise in lasers without inversion.

Abstract: I demonstrate a very large degree of quenching of spontaneous emission noise in different laser systems that work on mechanisms other than the population inversion between the bare states of the atom. Such laser systems have much narrower linewidths. I give explicit results for phase diffusion for four different model systems.

InAs quantum dots in a single-crystal GaAs matrix.

Abstract: We directly synthesize InAs microclusters embedded in a single-crystal GaAs matrix by molecular-beam epitaxy. Fractional monolayers of InAs are deposited on terraced (001) GaAs surfaces and subsequently overgrown with GaAs. Growth conditions are adjusted in situ by reflection high-energy electron diffraction to those favoring step-flow nucleation of both Ga and In adatoms. The resulting microscopic structural configuration is studied by double-crystal x-ray diffractometry and high-resolution electron microscopy. These experiments reveal that InAs growth takes place in fact by nucleation of In adatoms on step edges. An array of isolated InAs clusters of subnanometer size (quantum dots) is thereby formed within the GaAs matrix. The interface of the InAs clusters is in registry with the surrounding GaAs matrix and is thus defect-free. Several spectroscopic techniques, such as transmission, cw photoluminescence, photoluminescence excitation, and picosecond photoluminescence, are applied to get insight into the optical properties of this system. We show that the optical response of excitons attached to the InAs dots is determined by the zero-dimensional symmetry of the system. This effect is most evident when comparing the spontaneous emission of InAs dots and InAs planes, which in either case results from the relaxation of excitons to the emitting state followed by their radiative recombination. The reduced translational symmetry causes a progressive release of wave-vector conservation, thus modifying the selection rules that uniquely determine the interaction of excitons with phonons (relaxation) and photons (recombination).

Spontaneous emission and the concept of effective area in a very short optical cavity with plane-parallel dielectric mirrors

Abstract: Spontaneous emission in a model cavity with very short length and with plane-parallel dielectric mirrors is calculated quantum-mechanically under perturbation approximation Outgoing modes as well as guided modes are considered Results on the directivity and the rate of emission into outer space are compared with the estimate by Purcell's method This leads to the concept of "effective area" of the "cavity mode", which has a simpie physical interpretation

Observation of spontaneous emission lifetime change of dye‐containing Langmuir–Blodgett films in optical microcavities

Abstract: We have studied the spontaneous emission properties of Langmuir–Blodgett films containing dye molecules confined in Fabry–Perot microcavity structures having wavelength thickness. The presence of the cavity causes great modifications in the emission spectrum and emission intensity in the direction of the cavity axis. A clear change in spontaneous emission lifetime occurs, which is also attributed to the microcavity effect.

Spontaneous emission from a dipole in a semiconductor microcavity

Abstract: The effect of a semiconductor microcavity on the radiative spontaneous recombination of an electron‐hole pair strategically placed (by virtue of a quantum well) in the microcavity is considered. First‐order perturbation theory is used in the quantum mechanical calculation of the spatially anisotropic radiation rate and shows a strong influence of the cavity, and dipole position in the cavity, on the spontaneous photon emission process. Calculations are compared with previous experiments [T. J. Rogers, D. G. Deppe, and B. G. Streetman, Appl. Phys. Lett. 57, 1858 (1990)].

Intensity noise and polarization stability of GaAlAs-GaAs surface emitting lasers

Abstract: The intensity noise characteristics of GaAlAs-GaAs vertical cavity surface emitting (SE) lasers have been investigated. A low-intensity noise of -145 dB/Hz was obtained with an output power of 2.2 mW. The spontaneous emission factor is estimated to be 2*10/sup -5/, both from the bias dependence of the intensity noise and from the relative mode intensity below threshold. This is in good agreement with theoretical expectation. The possibility of low-noise operation of a microcavity SE laser with an extremely low output power of less than approximately 100 mu W is shown. The polarization stability of the surface emitting laser is demonstrated by measuring the intensity fluctuations of an individual polarization state. >

Vacuum Field Fluctuations and Spontaneous Emission in the Vicinity of a Dielectric Surface

Abstract: The electromagnetic field is quantized on the basis of the classical spatial modes of a system geometry in which half of space is filled by a dielectric of constant refractive index and the other half of space is empty. The vacuum field fluctuations and spontaneous emission rates are evaluated as functions of position and polarization both inside and outside the dielectric. Particular attention is given to the variations of these quantities in the vicinity of the interface. The source-field operator is derived for emission by atoms inside and outside the dielectric, in the direction perpendicular to the interface.

Improved performance due to suppression of spontaneous emission in tensile-strain semiconductor lasers

Abstract: It is shown that application of biaxial tension to the active region of a bulk or quantum well semiconductor laser can significantly enhance TM gain compared to TE gain and reduce the threshold current density, due to suppression of spontaneous emission polarised in the growth plane of the laser structure. The differential gain is enhanced compared to unstrained structures and a larger peak gain can be achieved than in comparable structures under biaxial compression.

Evolution of the band gap and the dominant radiative recombination center versus the composition for ZnSe1−xTex alloys grown by molecular beam epitaxy

Abstract: We report a systematic study of the optoelectronic properties of ZnSe1−xTex alloys grown by molecular beam epitaxy over the entire range of compositions. The band‐gap energy as a function of the composition presents a minimum at x≂0.65. The main luminescence emission observed at 5 K becomes narrower and closer to the band‐gap energy as we increase the Te content. The linewidth and the difference between the emission peak and band‐gap energy decrease significantly with increasing x and present a break in the slope at x≂0.65.

Radiative transition probalities for all vibrational levels in the X 1Σ+ state of HF

Abstract: Recent analyses have led to an experimentally-based potential energy curve for the ground state of HF which includes nonadiabatic corrections and which joins smoothly to the long-range potential at an accurately determined dissociation limit. Using this potential curve and a new ab initio dipole moment function, accurate radiative transition probabilities among all vibrational levels of the ground state of HF have been calculated for selected rotational quantum numbers. Comparisons of Einstein A spontaneous emission coefficients, dipole moment absorption matrix elements, and Herman-Wallis factors for absorption bands are presented.

Spontaneous emission and laser oscillation properties of microcavities containing a dye solution

Abstract: An experimental study on spontaneous and stimulated emission properties of planar optical microcavities confining an organic dye solution is reported. The lasing threshold in an input‐output curve became indistinct by decreasing the distance between a pair of dielectric reflectors. The coupling ratio of spontaneous emission into a laser mode was as large as 0.2 for a cavity half a wavelength distance. Differences between the spontaneous emission dominant regime and the stimulated emission dominant regime were also examined with emission spectra and emission lifetime measurements.

Theoretical investigation of the second-order harmonic distortion in the AM response of 1.55 mu m F-P and DFB lasers

Abstract: Numerical calculations of the second-order harmonic distortion in the amplitude modulation-response of Fabry-Perot, and distributed feedback (DFB) lasers are presented, and the influence of several nonlinearities, such as longitudinal spatial hole burning, gain suppression, and relaxation oscillations are considered. This analysis is valid for modulation frequencies ranging from a few megahertz to well beyond the resonance frequency of the relaxation oscillation. The distortion of Fabry-Perot lasers for which the effects of spontaneous emission and gain suppression can be clearly illustrated is investigated. The distortion of DFB lasers where the emphasis is on the influence of spatial hole burning and its combination with other nonlinearities is discussed. Various effects are discussed. >

Transfer-Matrix Formulation of Spontaneous Emission Noise of DFB

Abstract: Absfract-A unified formulation of the spontaneous emission noise in semiconductor DFB lasers is presented by using a transfer-matrix approach. Analytical expressions for the noise power per unit frequency bandwidth below threshold and the spontaneous emission rate into the lasing mode are obtained based on the Green’s function method. Three DFB laser structures are analyzed: 1) a standard DFB structure with facet reflectivities, 2) a multisection DFB structure composed of n sections, which models a phase-shifted DFB laser and a multielectrode (tunable) DFB laser, and 3) a periodic layered DFB structure which models a surface-emitting DFB laser. It is shown that the spontaneous emission noise of a complicated DFB laser structure can be calculated easily by the transfer matrix of each section of the structure and its derivative to frequency.