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

The coherent artifact in modern pulse measurements

We simulate multi-shot intensity-and-phase measurements of unstable ultrashort-pulse trains using frequency-resolved-optical-gating (FROG) and spectral phase interferometry for direct electric-field reconstruction (SPIDER). Both techniques fail to reveal the pulse structure. FROG yields the average pulse duration and suggests the instability by exhibiting disagreement between measured and retrieved traces. SPIDER under-estimates the average pulse duration but retrieves the correct average pulse spectral phase. An analytical calculation confirms this behavior.Comment: submission to Opt. Let

Numerical modeling of thermal refraction in liquids in the transient regime

We present the results of modeling of nanosecond pulse propagation in optically absorbing liquid media. Acoustic and electromagnetic wave equations must be solved simultaneously to model refractive index changes due to thermal expansion and/or electrostriction, which are highly transient phenomena on a nanosecond time scale. Although we consider situations with cylindrical symmetry and where the paraxial approximation is valid, this is still a computation-intensive problem, as beam propagation through optically thick media must be modeled. We compare the full solution of the acoustic wave equation with the approximation of instantaneous expansion (steady-state solution) and hence determine the regimes of validity of this approximation. We also find that the refractive index change obtained from the photo-acoustic equation overshoots its steady-state value once the ratio between the pulsewidth and the acoustic transit time exceeds a factor of unity

All-Optical Switching Devices Based On Large Nonlinear Phase-Shifts From 2Nd Harmonic-Generation

We show that the large nonlinear phase shifts obtained from phase-mismatched second harmonic generation can be used to implement all-optical switching devices such as a nonlinear Mach-Zehnder interferometer and a nonlinear directional coupler

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

3-D IR imaging with uncooled GaN photodiodes using nondegenerate two-photon absorption

We utilize the recently demonstrated orders of magnitude enhancement of extremely nondegenerate two-photon absorption in direct-gap semiconductor photodiodes to perform scanned imaging of 3D structures using IR femtosecond illumination pulses (1.6 um and 4.93 um) gated on the GaN detector by sub-gap, femtosecond pulses. While transverse resolution is limited by the usual imaging criteria, the longitudinal or depth resolution can be less than a wavelength, dependent on the pulsewidths in this nonlinear interaction within the detector element. The imaging system can accommodate a wide range of wavelengths in the mid-IR and near-IR without the need to modify the detection and imaging systems.Comment: 9 pages, 6 figure

Te Deum

Te Deum is a concert work for orchestra (woodwinds in pairs plus contrabassoon, four horns, two trumpets, two trombones, tuba, percussion, and strings) and a large mixed chorus. The text is derived from The Book of Common Prayer of the Church of England. In the interest of intelligibility, the text setting is mostly syllabic. The single through-composed movement, lasting about eleven minutes, is divided into three principal sections. Section two contains the primary climax of the work. The basic musical material of the piece is a set of motives which are altered and combined in various ways