Ultrafast Nonlinearities In Semiconductor-Laser Amplifiers

The bound-electronic optical nonlinearities in highly excited semiconductors (i.e., semiconductor lasers) have been calculated using a two-parabolic-band model. The nonlinear absorption spectrum is first obtained using a dressed-state formalism taking into account the contributions from two-photon absorption, electronic Raman, and optical Stark effects. The nonlinear refractive index ( n 2 ) is then found by performing a Kramers-Kronig transformation on the nonlinear absorption spectrum. It is also shown that the quadratic Stark splitting of the bands leads to a shift in the quasi-Fermi levels, which introduces additional absorptive and refractive nonlinearities. The sign, magnitude, and the current-density dependence of the calculated n 2 agree well with some recently published experimental results for Al-Ga-As and In-Ga-As-P diode lasers

Third-Order Optical Nonlinearities In Semiconductors - The Two-Band Model

We calculate the coherent electronic contributions to the third-order optical response χ(3)(-ω;ω,Ω,-Ω) of bulk semiconductors in the independent-particle approximation using a simple two-band model. The formalism used to derive this response coefficient naturally accounts for all relevant contributions and, in contrast to existing results in the literature, leads to physically realistic, nondivergent expressions in the limits ω,Ω→0. Such well behaved infrared limits imply that the imaginary part of our χ(3) correctly describes the dispersion of nondegenerate absorption; indeed for Ω=0 our results are consistent with predictions from Franz-Keldysh theory. Complementing these results, we can now also unambiguously extract from the real part of χ(3) the below band gap, two-band model predictions for the nonlinear refractive index, the dc Kerr effect, and the virtual photoconductivity; all of these predict a finite, real χ(3)(0;0,0,0) as physically expected for clean, cold semiconductors. Finally, our specific results help expose more general consequences of the gauge choice when employing common approximate band-structure models

Laser-Induced Damage And The Role Of Self-Focusing

We review the influence of self-focusing on the measurement of bulk laser-induced-damage (LID) thresholds in normally transparent optical mate-rials. This role is experimentally determined by measuring the spot size and polarization dependence of LID and by observing beam distortion in the far field. Utilizing these techniques, we find that by using a tight focusing geometry in which the breakdown power is below P2, the effects of self-focusing can be practically eliminated in an LID experiment. P2 is the so-called second critical power for self-focusing, and P2 = 3.77P1, where P1 = cX2/327r2n2, where c is the speed of light in vacuum, X is the laser wavelength and n2 is the nonlinear index of refraction. This is in accordance with numerical calculations by J. H. Marburger [in Progress in Quantum Electronics, J. H. Sanders and S. Sten-holm, eds., Vol. 4, Part 1, pp. 35-110, Pergamon, Oxford (1975)]. With this knowledge we determine that damage is only partially explained by avalanche ionization and that the initiation of damage is strongly influenced by extrinsic processes. We therefore conclude that we are measuring extrinsic LID

Diffusion Of Color-Centers Generated By 2-Photon Absorption At 532-Nm In Cubic Zirconia

We have recently reported the formation of color centers in stabilized cubic zirconia (ZrO2, 18% Y2O3) by two‐photon absorption at 532 nm. Here we present the results of measurements of the transmission of the colored samples as a function of time at room temperature. The results are found to be in good agreement with theory that assumes the color centers diffuse out of the irradiated region. The initial distribution of centers is assumed to have a Gaussian profile. For this model, the diffusion equation was solved exactly and the diffusion constant obtained (∼3.4×10−8 cm2/s)

Combination of carbon nanotubes and two-photon absorbers for broadband optical limiting

New systems are required for optical limiting against broadband laser pulses. We demonstrate that the association of non-linear scattering from single-wall carbon nanotubes (SWNT) and multiphoton absorption (MPA) from organic chromophores is a promising approach to extend performances of optical limiters over broad spectral and temporal ranges. Such composites display high linear transmission and good neutral colorimetry and are particularly efficient in the nanosecond regime due to cumulative effects.Comment: 5 avril 200

Dynamic Band Unblocking And Leakage Two-Photon Absorption In Insb

An experimental demonstration of a dynamic decrease of the optical band gap of bulk n-InSb induced by picosecond, midinfrared laser pulses is reported. This occurs as a result of laser heating of the quiescent electron distribution by free-carrier absorption. The hot electrons vacate low-energy states near the conduction;band minimum,unblocking terminal states for two-photon absorption across the band gap. This \u27\u27leakage\u27\u27 two-photon absorption is detected as a consequence of changes to the refractive index and absorption coefficient of the semiconductor caused by photocarriers. The onset of dynamic band unblocking agrees with a calculation of laser-induced electron heating

Ultrafast optical nonlinearity in quasi-one-dimensional Mott-insulator Sr2CuO3{\rm Sr_2CuO_3}

We report strong instantaneous photoinduced absorption (PA) in the quasi-one-dimensional Mott insulator ${\rm Sr_2CuO_3}$ in the IR spectral region. The observed PA is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculations based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole-coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of ${\rm Sr_2CuO_3}$ for all-optical switching.Comment: 10 pages, 4 figure

Vectorial Quadratic Interactions For All-Optical Signal Processing Via Second-Harmonic Generation

Quadratic interactions involving three distinct waves can be exploited for all-optical signal processing both in the plane wave limit and in the spatial solitary-wave case. For the specific case of Type II Second-Harmonic Generation in bulk KTP, we have predicted and demonstrated all-optical transistor action with small signal gain and angular steering and switching of spatial simultons. All the phenomena rely exclusively on inputs at the same fundamental frequency

Borrmann Effect in Photonic Crystals: Nonlinear Optical Consequences

Nonlinear-optical manifestations of the Borrmann effect that are consequences of the spectral dependence of the spatial distributions of the electromagnetic field in a structure are observed in one-dimensional photonic crystals. The spectrum of the light self-focusing effect corresponding to the propagation-matrix calculations has been measured near the edge of the photonic gap.Comment: 4 pages, 3 figures, published in russian at Pis'ma v Zhurnal Eksperimental'noi i Teoreticheskoi Fiziki, 2008, Vol. 87, No. 8, pp. 461-46