Publications

Optical parametric chirped-pulse amplifiers utilizing a 300-ps Nd:YAG pump system, a tunable 1.6-μm fiber signal, and KNbO3, KTA, RTP, or BBO nonlinear crystals were designed and built. Gain >109, and peak powers >30GW were obtained. © 2005 Optical Society of America.

Terahertz radiation from optically-induced plasmas on metal, semiconductor, and dielectric surfaces is compared to electron-hole plasma radiation from GaAs and Ge. Electro-optic sampling and electric-field probes measure radiated field waveforms and distributions to 0.350 THz.

The spatial, spectral and temporal properties of self-focusing 798-nm 100-fs pulses in air were experimentally measured. It was measured using high-resolution, single-shot techniques at a set propagation distance of 10.91 m. The data were taken over an extended energy range and can thus be used to test the validity of physical models. The experimental results show that significant spatial, spectral and temporal changes occur at intensities lower than than those required for strong ionization of air.

Abstract not provided.

Measurement and signal intelligence demands has created new requirements for information management and interoperability as they affect surveillance and situational awareness. Integration of on-board autonomous learning and adaptive control structures within a remote sensing platform architecture would substantially improve the utility of intelligence collection by facilitating real-time optimization of measurement parameters for variable field conditions. A problem faced by conventional digital implementations of intelligent systems is the conflict between a distributed parallel structure on a sequential serial interface functionally degrading bandwidth and response time. In contrast, optically designed networks exhibit the massive parallelism and interconnect density needed to perform complex cognitive functions within a dynamic asynchronous environment. Recently, all-optical self-organizing neural networks exhibiting emergent collective behavior which mimic perception, recognition, association, and contemplative learning have been realized using photorefractive holography in combination with sensory systems for feature maps, threshold decomposition, image enhancement, and nonlinear matched filters. Such hybrid information processors depart from the classical computational paradigm based on analytic rules-based algorithms and instead utilize unsupervised generalization and perceptron-like exploratory or improvisational behaviors to evolve toward optimized solutions. These systems are robust to instrumental systematics or corrupting noise and can enrich knowledge structures by allowing competition between multiple hypotheses. This property enables them to rapidly adapt or self-compensate for dynamic or imprecise conditions which would be unstable using conventional linear control models. By incorporating an intelligent optical neuroprocessor in the back plane of an imaging sensor, a broad class of high-level cognitive image analysis problems including geometric change detection, pattern recognition, and correlated feature extraction can be realized in an inherently parallel fashion without information bottlenecking or external supervision, Using this approach, we believe that autonomous control systems embodied with basic adaptive decision-theoretic capabilities can be developed for imaging and surveillance sensors to improve discrimination in stressing operational environments.

Stable self-channelling of ultra-powerful (P0 ∼ 1 TW-1 PW) laser pulses in dense plasmas is a key process for many applications requiring the controlled compression of power at high levels. Theoretical computations predict that the transition zone between the stable and highly unstable regimes of relativistic/charge-displacement self-channelling is well characterized by a form of weak instability that involves bifurcation of the propagating energy into two channels. Recent observations of unstable behaviour with femtosecond 248 nm pulses reveal a mode of bifurcation that corresponds well to these theoretical predictions. It is further experimentally shown that the use of a suitable longitudinal gradient in the plasma density can eliminate this unstable response and restore the efficient formation of single stable channels.

Research is presented on infrared (IR) and near infrared (NIR) sensitive sensor technologies for use in a high speed shuttered/intensified digital video camera system for range-gated imaging at "eye-safe" wavelengths in the region of 1.5 microns. The study is based upon nonlinear crystals used for second harmonic generation (SHG) in optical parametric oscillators (OPOs) for conversion of NIR and IR laser light to visible range light for detection with generic S-20 photocathodes. The intensifiers are "stripline" geometry 18-mm diameter microchannel plate intensifiers (MCPIIs), designed by Los Alamos National Laboratory and manufactured by Philips Photonics. The MCPIIs are designed for fast optical shuttering with exposures in the 100-200 ps range, and are coupled to a fast readout CCD camera. Conversion efficiency and resolution for the wavelength conversion process are reported. Experimental set-ups for the wavelength shifting and the optical configurations for producing and transporting laser reflectance images are discussed.

Summary form only given. It has been shown in 3 + 1 dimensional Kerr-nonlinearity self-focusing models, that group velocity dispersion is responsible for the temporal pulse-splitting of ultrashort pulses during propagation. Previous experiments have demonstrated pulse splitting due to the Kerr nonlinearity for short pulse propagation in bulks or gaseous media. However, studies in gaseous media are often in a focused geometry, or use pressurized gaseous media. This experiment elucidates the relationship between pulse splitting and spot-size change and does not use any optic to initiate self-focusing. We find pulse splitting occurs at a distance merely 0.7x the diffraction length and occurs before spatial collapse to a filament. In addition, multiple pulse splitting is also observed. Peak fluence information from the beam-profile is monitored, indicating nonlinear loss mechanisms. We believe this is the first data on multiple pulse-splitting events in air.

Summary form only given. Numerous groups have demonstrated that tabletop high peak power femtosecond lasers are capable of inducing nonlinear self-focused propagation in atmosphere at 800 nm. The phenomenon unfailingly exhibits (1) light concentration in long single or multiple filaments of the order of 150 μm diameter and tens of meters in length; (2) conical emission associated with these filaments has a considerably wider spectral content than the original laser pulse. Conical emission became apparent after the filaments were formed. While the divergence angle of these conical emissions has been studied, unfortunately there is no reasonable model proposed that can qualitatively describe (Brodeur et al, 1996; Nibbering et al, 1996) even the most basic features such as divergence angles of the different colors. Furthermore, the color ordering of these conical emissions can be changed upon changing the chirp of the launched pulse. In this paper, we present conical emission data to show its behavior as the pulse is chirped. In addition, we also present the spectral distribution of the conical emissions and how it depends on chirp. Finally, we compare our result with numerical result of Gaeta (Phys. Rev. Lett. vol. 84, pp. 3582-3585, 2000).

Laboratory experiments utilizing different near-infrared (NIR) sensitive imaging techniques for LADAR range gated imaging at eye-safe wavelengths are presented. An OPO/OPA configuration incorporating a nonlinear crystal for wavelength conversion of 1.56 micron probe or broadcast laser light to 807 nm light by utilizing a second pump laser at 532 nm for gating and gain, was evaluated for sensitivity, resolution, and general image quality. These data are presented with similar test results obtained from an image intensifier based upon a transferred electron (TE) photocathode with high quantum efficiency (QE) in the 1-2 micron range, with a P-20 phosphor output screen. Data presented include range-gated imaging performance in a cloud chamber with varying optical attenuation of laser reflectance images.

Abstract not provided.

Abstract not provided.

Efficient (1.2% yield) multikilovolt x-ray emission from Ba(L) (2.4--2.8{angstrom}) and Gd(L) (1.7--2.1{angstrom}) is produced by ultraviolet (248nm) laser-excited BaF{sub 2} and Gd solids. The high efficiency is attributed to an inner shell-selective collisional electron ejection. Much effort has been expended recently in attempts to develop an efficient coherent x-ray source suitable for high-resolution biological imaging. To this end, many experiments have been performed studying the x-ray emissions from high-Z materials under intense (>10{sup 18}W/cm{sup 2}) irradiation, with the most promising results coming from the irradiation of Xe clusters with a UV (248nm) laser at intensities of 10{sup 18}--10{sup 19}W/cm{sup 2}. In this paper the authors report the production of prompt x-rays with energies in excess of 5keV with efficiencies on the order of 1% as a result of intense irradiation of BaF{sub 2} and Gd targets with a terawatt 248nm laser. The efficiency is attributed to an inner shell-selective collisional electron ejection mechanism in which the previously photoionized electrons are ponderomotively driven into an ion while retaining a portion of their atomic phase and symmetry. This partial coherence of the laser-driven electrons has a pronounced effect on the collisional cross-section for the electron ion interaction.

UV filaments in air have been examined on the basis of the diameter and length of the filament, the generation of new spectral components, and the ionization by multiphoton processes. There have been numerous observations of filaments at 800 nm. The general perception is that, above a critical power, the beam focuses because nonlinear self-lensing overcomes diffraction. The self-focusing proceeds until an opposing higher order nonlinearity forms a stable balance.