The transient detection microscope

Up-converted photorefractive optical transient detection with femtosecond laser pulses

We report on experimental demonstration of optical transient detection (OTD) based on photorefractive two-wave mixing of femtosecond pulses. The demonstrated technique also combines nonlinear-crystal-based OTD with up-conversion from infrared into the visible range. The approach enables measurement of phase changes of a dynamic signal in the infrared using GaP- or Si-based detectors while suppressing stationary background. Experimental results reveal existence of the relation between input phases in the infrared and output phases in the visible wavelength range. We further present experimental evidence of additional merits of up-converted transient phase analysis under noisy conditions, such as residual continuous-wave emission affecting the ultrashort pulses from the laser.

Interferometric measurement of complex-field changes in transient detection imaging

We report an experimental method that combines nonlinear-crystal-based transient detection imaging (TDI) with interferometric complex-field retrieval. The system allows measuring both phase and amplitude of a dynamic scene while suppressing stationary background. Theoretical and experimental results prove the linear relation existing between input and output phases, as well as the benefits of phase analysis for both detection and measurement with high resolutions of λ/30, even under noisy conditions. Additionally, we present experimental evidence of the remarkable ability of the technique to detect phase sign changes in the scene -what we call differential-phase TDI- showing great detection sensitivity and no calibration requirements.

Single-exposure optical focusing inside scattering media using binarized time-reversed adapted perturbation

Light scattering inhibits high-resolution optical imaging, manipulation and therapy deep inside biological tissue by preventing focusing. To form deep foci, wavefront shaping techniques that break the optical diffusion limit have been developed. For in vivo applications, such focusing must provide high gain, high speed, and a high focal peak-to-background ratio. However, none of the previous techniques meet these requirements simultaneously. Here, we overcome this challenge by rapidly measuring the perturbed optical field within a single camera exposure followed by adaptively time-reversing the phase-binarized perturbation. Consequently, a phase-conjugated wavefront is synthesized within a millisecond, two orders of magnitude shorter than the digitally achieved record. We demonstrated real-time focusing in dynamic scattering media, and extended laser speckle contrast imaging to new depths. The unprecedented combination of fast response, high gain, and high focusing contrast makes this work a major stride toward in vivo deep tissue optical imaging, manipulation, and therapy.

Time-reversed adapted-perturbation (TRAP) optical focusing onto dynamic objects inside scattering media

The ability to steer and focus light inside scattering media has long been sought for a multitude of applications. To form optical foci inside scattering media, the only feasible strategy at present is to guide photons by using either implanted¹ or virtual^2-4 guide stars, which can be inconvenient and limits potential applications. Here, we report a scheme for focusing light inside scattering media by employing intrinsic dynamics as guide stars. By time-reversing the perturbed component of the scattered light adaptively, we show that it is possible to focus light to the origin of the perturbation. Using the approach, we demonstrate non-invasive dynamic light focusing onto moving targets and imaging of a time-variant object obscured by highly scattering media. Anticipated applications include imaging and photoablation of angiogenic vessels in tumours as well as other biomedical uses.

Relationship between parameters of bacteriorhodopsin film and behavior of optical novelty filters

To continue our earlier research on novelty filters in a system of incoherent light [Opt. Lett. 30,81 (2005)], we discuss the relationship between parameters of a bacteriorhodopsin film and the quality of a novelty filter image. For both fixed and moving velocities of the input image, differences in the novelty filter's image as a function of thickness, lifetime of the M state, and molecular concentration are displayed, and the optimal ranges of parameters of the bR film that correspond to the entire novelty filter image and obvious gray-level differences in the image are given. The method can be used to design high-quality novelty filter images in incoherent light systems.

Optical novelty filter using bacteriorhodopsin film

A new kind of optical novelty filtering by an incoherent light system is presented that is based on a special property of dynamic complementary suppression modulated transmission in bacteriorhodopsin film. By simplifying the energy system, we establish a theoretical model, and our experimental results are compared with those reached by numerical simulation of the novelty filter.

Tracking novelty filter at 780 nm based on a photorefractive polymer in a two-beam coupling geometry

We have constructed an all-optical tracking novelty filter based on the dynamic holographic properties of an efficient and fast infrared-sensitive photorefractive polymer. The photorefractive polymer was used in a two-beam coupling geometry. The polymer had a gain coefficient of 175 cm(-1) at a wavelength of 780 nm and an applied field of 72 V/microm. In contrast to what has been observed in photorefractive crystals, the gain coefficient and the filter contrast are largely independent of the writing beam's intensity ratio. We show images of a swinging pendulum observed through the novelty filter.

Novelty filter that uses a bacteriorhodopsin film

We propose a new novelty optical filter that uses a bacteriorhodopsin film. This filter is based on the time-dependent nonlinear diffraction efficiency of real-time holograms recorded in the film. As soon as the signal beam carrying a pattern is diffracted by the polarization hologram recorded in the bacteriorhodopsin film, it begins to erase the hologram and suppresses the diffraction of the beam at the position of the stationary part of the pattern. This filter enhances only leading edges of moving patterns. In this system undesired scattered light, which is orthogonally polarized to the diffracted beam, is discriminated by a polarizer.

Fiber-optic acoustic Fourier transducer for audio sound processing

We demonstrate a fiber-optic acoustic transducer operating in the audio-frequency regime. The device is made of an array of 120 multimode optical fibers and a photorefractive novelty filter. Each fiber in the array acts as a cantilevered mechanical resonator. The resonant frequencies of the fibers logarithmically sample the acoustic spectrum from approximately 100 Hz to 5 kHz. Laser light is injected into all fibers simultaneously and is reflected from the end of each fiber. An optical novelty filter extracts the acoustic information from the reflected light. The output of the novelty filter is essentially a Fourier transform of the acoustic signal. The background intensity in the transducer output corresponds to a driving amplitude of approximately 50 A. We describe holographic storage of complex sound patterns that use a LiNbO(3) crystal and an acoustic transducer.

Novelty imaging system with a desired long-time scale using BaTiO(3) and a controlled shutter sequence

A novelty imaging system using photorefractive crystal BaTiO(3) and a controlled shutter sequence, which can be applied to the slow-motion detection of the object, is proposed. In this system the observation time scale is set to the desired long value uninfluenced by the time constant of the crystal that is used. This system utilizes a photorefractive beam-fanning effect in the BaTiO(3) crystal, so it is constructed simply by using only the object beam without suffering from the external disturbance that is intrinsic when the reference beam is used. Moreover, the visibility for an opaque object is also maintained by the use of an additional random-phase plate. We examine the basic characteristics and determine the appropriate conditions of the system.

Nonlinear Optical Materials

The current state of materials development in nonlinear optics is summarized, and the promise of these materials is critically evaluated. Properties and important materials constants of current commercial materials and of new, promising, inorganic and organic molecular and polymeric materials with potential in second- and third-order nonlinear optical applications are presented.