Near-field radiative heat transfer between macroscopic planar surfaces

Near-field radiative heat transfer allows heat to propagate across a small vacuum gap in quantities that are several orders of magnitude greater then the heat transfer by far-field, blackbody radiation. Although heat transfer via near-field effects has been discussed for many years, experimental verification of this theory has been very limited. We have measured the heat transfer between two macroscopic sapphire plates, finding an increase in agreement with expectations from theory. These experiments, conducted near 300 K, have measured the heat transfer as a function of separation over mm to $\mu$m and as a function of temperature differences between 2.5 and 30 K. The experiments demonstrate that evanescence can be put to work to transfer heat from an object without actually touching it

High-power femtosecond optical pulse compression by using spatial solitons

We demonstrate a novel pulse-compression technique that uses the self-confinement of two-dimensional spatial solitons propagating in bulk nonlinear media to increase the spectral bandwidth followed by a grating pair for recompression. Output pulses of 19-fs duration with 0.6-,J energies are routinely obtained at a repetition rate of 8.6 kHz. Unlike other high-energy compression methods, soliton compression offers both high repetition rates and a potentially unlimited wavelength range. Femtosecond pulse compression techniques that employ self-phase modulation in an optical fiber to generate spectral bandwidth have developed to the point where it is now possible to generate optical pulses as short as 6 fs.1 However, fiber damage thresholds and parasitic higher-order nonlinear processes typically limit the amount of energy that can effectively be compressed to less than 10 nJ. Applications such as mode-selective excitation of coherent phonons by means of impulsive stimulated Raman scattering 2 and strong-field physics'-' require new methods of compression that produce shortduration optical pulses while maintaining high energies. In recent years progress has been made in extending the energy range of compressed pulses. Efforts by Rolland and Corkum, who used self-phase modulation in bulk materials, have succeeded in generating 100-,J, 24-fs pulses. In this Letter we report on a new method of pulse compression, which produces 19-fs, 0.6-,uJ optical pulses at a repetition rate of 8.6 kHz. Our method relies on the self-trapping and stable propagation of two-dimensional bright spatial optical solitons in bulk nonlinear media. In close analogy with temporal solitons, in which the balancing of group-velocity dispersion and self-phase modulation lead to dispersion-free propagation, 9 the balancing of diffraction by the spatial nonlinear index profile results in diffraction-free propagation.' 0 Although selftrapping of beams in three dimensions is unstable and leads to catastrophic self-focusing, recent experiments have demonstrated the stable propagation of two-dimensional spatial solitons in CS 2 liquid"' and in guided-wave geometries.12l' 4 The self-trapped propagation of the spatial soliton itself maintains the high intensity necessary for large phase modulation, which generates the necessary bandwidth for pulse compression. Unlike other high-energy compression methods, soliton compression offers both high repetition rates and a potentially unlimited wavelength range. The basic experimental apparatus for generating and compressing spatial solitons is as follows. Pulses of 75-fs duration and 0.1-nJ energies from a balanced colliding-pulse mode-locked ring dye laser operating at 620 nm were amplified to 30 /.tJ at a repetition rate of 8.6 kHz in a two-stage optical amplifier pumped by a 20-W copper-vapor laser. To achieve these pulse energies, we used a dye cell in the second stage.' 5 Following recompression to 75 fs with a two-prism sequence in a double-pass geometry, the pulses were spatially filtered to improve beam quality and ensure the formation of clean spatial solitons. The energy throughput of the prism sequence-spatial filter was 11 /%J. We chose an 8-mm-thick piece of bulk fused silica as the nonlinear medium, which has a positive nonlinear index (n 2 = 2.7 x 10-16 cm 2 /W), as required for bright spatial solitons as well as minimal linear and twophoton absorption. Pulses were focused on the front face of the glass in an elliptical profile by a cylindrical-spherical lens combination. We used beam diameters of w = 900 gum (l/e peak intensity) in the long dimension (which we denote x; see the graph o

Renormalized Energies of Superfluorescent Bursts from an Electron-Hole Magneto-plasma with High Gain in InGaAs Quantum Wells

We study light emission properties of a population-inverted 2D electron-hole plasma in a quantizing magnetic field. We observe a series of superfluorescent bursts, discrete both in time and energy, corresponding to the cooperative recombination of electron-hole pairs from different Landau levels. The emission energies are strongly renormalized due to many-body interactions among the photogenerated carriers, exhibiting red-shifts as large as 20 meV at 15 T. However, the magnetic field dependence of the lowest Landau level emission line remains excitonic at all magnetic fields. Interestingly, our time-resolved measurements show that this lowest-energy burst occurs only after all upper states become empty, suggesting that this excitonic stability is related to the `hidden symmetry' of 2D magneto-excitons expected in the magnetic quantum limit.Comment: 5 pages, 4 figure

Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma

We investigate photoluminescence from a high-density electron-hole plasma in semiconductor quantum wells created via intense femtosecond excitation in a strong perpendicular magnetic field, a fully-quantized and tunable system. At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. In addition, changes in the linewidth, carrier density, and magnetic field scaling of the PL spectral features correlate precisely with the onset of random directionality, indicative of cooperative recombination from a high density population of free carriers in a semiconductor environment

Characterization of thermal effects in the Enhanced LIGO Input Optics

We present the design and performance of the LIGO Input Optics subsystem as implemented for the sixth science run of the LIGO interferometers. The Initial LIGO Input Optics experienced thermal side effects when operating with 7 W input power. We designed, built, and implemented improved versions of the Input Optics for Enhanced LIGO, an incremental upgrade to the Initial LIGO interferometers, designed to run with 30 W input power. At four times the power of Initial LIGO, the Enhanced LIGO Input Optics demonstrated improved performance including better optical isolation, less thermal drift, minimal thermal lensing and higher optical efficiency. The success of the Input Optics design fosters confidence for its ability to perform well in Advanced LIGO

Small optic suspensions for Advanced LIGO input optics and other precision optical experiments

We report on the design and performance of small optic suspensions developed to suppress seismic motion of out-of-cavity optics in the Input Optics subsystem of the Advanced LIGO interferometric gravitational wave detector. These compact single stage suspensions provide isolation in all six degrees of freedom of the optic, local sensing and actuation in three of them, and passive damping for the other three

Phase Effects in the Diffraction of Light: Beyond the Grating Equation

Diffraction gratings affect the absolute phase of light in a way that is not obvious from the usual derivation of optical paths using the grating equation. For example, consider light which encounters first one and then the second of two parallel gratings. If one grating is moved parallel to its surface, the phase of the light diffracted from the grating pair is shifted by 2π each time the grating is moved by one grating constant, even though the geometric path length is not altered by the motion. This additional phase shift must be included when incorporating diffraction gratings in interferometers

Sub-nanosecond, time-resolved, broadband infrared spectroscopy using synchrotron radiation

A facility for sub-nanosecond time-resolved (pump-probe) infrared spectroscopy has been developed at the National Synchrotron Light Source of Brookhaven National Laboratory. A mode-locked Ti:sapphire laser produces 2 ps duration, tunable near-IR pump pulses synchronized to probe pulses from a synchrotron storage ring. The facility is unique on account of the broadband infrared from the synchrotron, which allows the entire spectral range from 2 cm-1 (0.25 meV) to 20,000 cm-1 (2.5 eV) to be probed. A temporal resolution of 200 ps, limited by the infrared synchrotron-pulse duration, is achieved. A maximum time delay of 170 ns is available without gating the infrared detector. To illustrate the performance of the facility, a measurement of electron-hole recombination dynamics for an HgCdTe semiconductor film in the far- and mid infrared range is presented.Comment: 11 pages with 9 figures include

Theory for the ultrafast ablation of graphite films

The physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses are analyzed using a microscopic electronic theory. We describe the nonequilibrium dynamics of electrons and lattice by performing molecular dynamics simulations on time-dependent potential energy surfaces. We show that graphite has the unique property of exhibiting two distinct laser induced structural instabilities. For high absorbed energies (> 3.3 eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold (> 2.0 eV/atom) ablation occurs via removal of intact graphite sheets.Comment: 5 pages RevTeX, 3 PostScript figures, submitted to Phys. Re

Spatial Symmetry of Superconducting Gap in YBa2Cu3O7-\delta Obtained from Femtosecond Spectroscopy

The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa2Cu3O7-\delta thin films are reported. This bulk-sensitive spectroscopy, combining with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By the orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally.Comment: 8 pages, 4 figures. To be published in Physical Review B, Rapid Communication