Over 4000 nm bandwidth of mid-IR supercontinuum generation in sub-centimeter segments of highly nonlinear tellurite PCFs.

We report broad bandwidth, mid-IR supercontinuum generation using a sub-cm (8 mm) length of highly nonlinear tellurite microstructured photonic crystal fiber (PCF). We pump the fiber at telecommunication wavelengths by using 1550 nm, 100 fs pulses of energy E=1.9 nJ. When coupled in the PCF, these pulses result in a supercontinuum (SC) bandwidth of 4080 nm extending from 789 to 4870 nm measured at 20 dBm below the peak spectral power. This bandwidth is comparable or in excess of previously reported spectra for other nonlinear glass fiber formulations despite the significantly shorter fiber length. In addition, besides offering a convenient pump wavelength, short fiber lengths enable smoother SC spectra, lower dispersion, and reduced material absorption at longer wavelengths making the use of this PCF particularly interesting.

Application of optical trapping to beam manipulation in optofluidics.

We introduce a novel method of attaining all-optical beam control in an optofluidic device by displacing an optically trapped microsphere through a light beam. The micro-sphere causes the beam to be refracted by various degrees as a function of the sphere position, providing tunable attenuation and beam-steering in the device. The device itself consists of the manipulated light beam extending between two buried waveguides which are on either side of a microfluidic channel. This channel contains the micro-spheres which are suspended in water. We simulate this geometry using the Finite Difference Time Domain method and find good agreement between simulation and experiment.

Surface organization and nanopatterning of collagen by dip-pen nanolithography.

Collagen is a key fibrous protein in biological systems, characterized by a complex structural hierarchy as well as the ability to self-assemble into liquid crystalline mesophases. The structural features of collagen influence cellular responses and material properties, with importance for a wide range of biomaterials and tissue architectures. The mechanism by which fibrillar collagen structures form from liquid crystalline mesophases is not well characterized. We report positive printing of collagen and a collagen-like peptide down to 30-50-nm line widths, using the atomic force microscopy technique of dip-pen nanolithography. The method preserved the triple-helical structure and biological activity of collagen and even fostered the formation of characteristic higher levels of structural organization. The "direct-write" capability of biologically relevant molecules, while preserving their structure and functionality, provides tremendous flexibility in future biological device applications and in proteomics arrays, as well as a new strategy to study the important hierarchical assembly processes of biological systems.

Facial frequency manipulation normalizes face discrimination in AD.

People with AD have deficient contrast sensitivity and impaired face discrimination. The authors presented photographs of unfamiliar faces of three different sizes to enhance the low, middle, or high facial frequency information (cycles per face). Patients with AD demonstrated normal discrimination of small faces only, indicating that impaired contrast sensitivity at low facial frequencies contributes to their poor face discrimination.

Turbulence-aberration correction with high-speed high-gain optical phase conjugation in sodium vapor.

Optical aberrations that are due to high-speed turbulence in the aero-optical regime are corrected with optical phase conjugation based on coherent population trapping in sodium vapor. Experimental measurements of an unheated, forced helium jet in air have demonstrated aberration correction by a factor of 7.8 at a forcing frequency of 18kHz with an optical power gain of 32.

Distortion-free gain and noise correlation in sodium vapor with four-wave mixing and coherent population trapping.

We observed optical gain as great as 30 with nearly distortion-free beam propagation in optically dense sodium vapor, using four-wave mixing. Moreover, 15-dB classical noise correlations were seen in the amplified probe and conjugate beams. To achieve this performance in such a strongly absorbing medium, one must suppress unwanted absorption and self-focusing effects. This is accomplished with coherent population trapping.

Model of amplified scattering in photorefractive media: comparison of numerical results and experiment.

We implement a random-walk scattering model in a nonlinear beam-propagation code to simulate fanning in photorefractive crystals. The characteristics of the scatterers were verified by comparisons of the measured firstorder statistics of the scattered intensity with a theoretical expression of the probability-density function based on a random-walk scattering model. Both the size and the separation of the scatterers can be varied, enabling us to tailor our numerical simulations to experimental measurements of nonamplified and amplified scattering. We show that a one-dimensional model does not accurately simulate amplified scattering for crystals with a nondiagonal electro-optic tensor and that surface scattering alone may not account for the angular dependence of the amplified scattering distributions.

Laser beam cleanup with photorefractive two-beam coupling.

The effectiveness of laser beam cleanup with photorefractive two-beam coupling is evaluated. We compare the measured effectiveness of the method with that of spatial filtering and find that the method works well for a highly aberrated beam approximately 700 times diffraction limited, whereas for moderately aberrated beams the effectiveness of the method is the same as that of spatial filtering.

Photorefractive surface waves.

A laser beam propagating in a photorefractive crystal generates a refractive-index profile that deflects the beam to one side. This nonlinear deflection can be balanced by total internal reflection at the crystal surface to produce self-induced photorefractive surface waves. Theoretical and experimental evidence for this effect is given.

Photorefractive two-beam coupling optimal thresholding filter for additive signal-dependent noise reduction.

Computer simulations of photorefractive thresholding filters for the reduction of artifact or dust noise demonstrate an increase in signal-to-noise ratio (SNR) of 70% to 95%, respectively, of that provided by the Wiener filter for inputs with a SNR of approximately 3. These simple, nearly optimal filters use a spectral thresholding profile that is proportional to the envelope of the noise spectrum. Alternative nonlinear filters with either 1/ν or constant thresholding profiles increase the SNR almost as much as the noise-envelope thresholding filter.

Analysis of the dual discrimination ability of the two-port photorefractive joint transform correlator.

An all-optical joint transform correlator featuring two operative correlation planes(ports) with complementary performance is presented. We present the theory of operation, derive the input-output characteristics, and demonstrate computer simulations and experimental results. The two-port joint transform correlator is based on simultaneous use of two photorefractive wave-mixing architectures. The first port uses two-beam coupling, and the second port uses four-wave mixing. The performance of the two ports depends on an experimentally controlled beam intensity ratio and the photorefractive coupling coefficient. With appropriate selection of these parameters, the first port is capable of high discrimination, while simultaneously the second offers a low discrimination output. Our results show that the two-beam coupling port can achieve peak-to-noise and signal-to-noise ratio values better than the phase-only correlator, whereas the four-wave-mixing port performs similarly to the classical joint transform correlator. This leads to a potential application in which the correlator could be set up so that in one port a general class is detected (interclass) and, in the other, the specific item in a class is detected (intraclass).

Solitonlike optical switching in a circular fiber array.

We show numerically that a solitonlike propagation behavior in a self-focusing nonlinear fiber array can be used for efficient optical switching. Initial phase modulation of only five fibers is used for switching control.

Photorefractive mixing of amplitude-modulated signals.

We analyze the circumstances under which a real-time hologram written with amplitude-modulated beams behaves as a linear mixer. We calculate nonlinear correction terms that can be used to analyze the linearity of a variety of photorefractive time-integrative devices.

Real-time holographic frequency-division demultiplexing: theoretical aspects.

The multiplicative and low-band-pass filtering characteristics of real-time holograms in photorefractive media are proposed for use for frequency-division demultiplexing. Several aspects of the frequencydivision demultiplexer are discussed and derived. These aspects are baseband demodulation of optical images, demultiplexing limits, heterodyne cross talk, and nonlinear cross-talk effects that are due to second- and high-order nonlinearities. The nonlinear cross talk is calculated in two different approaches. Both approaches indicate that nonlinear effects enhance the performance of this demultiplexer.

Photorefractive phase-conjugate technique for measuring surface granularity.

A self-aligned photorefractive autocorrelator is implemented for the real-time use of second-order statistics for texture analysis. We demonstrate the measurement of the length scale of the granularity of a rough surface.

All-optical nonlinear joint Fourier transform correlator.

We present the first all-optical nonlinear joint transform correlator based on a square-law receiver in the Fourier plane. Our device uses a photorefractive limiting quadratic processor. The compressional nonlinearity associated with the transfer function of the limiting quadratic processor enables the correlator to detect signals embedded in Gaussian and non-Gaussian noise. In the limiting region this device correlates the phase-only information of the input. This is the first time to our knowledge that photorefractives or real-time holography has been used in the correlation of the phase-only information. We demonstrate the operation of this device experimentally, and we evaluate its performance throughcomputer simulation for various forms of noise.