D. V. G. L. N. Rao

Passive all-optical diode using asymmetric nonlinear absorption

Saturable and reverse saturable absorptions are well-known phenomena, originating from the imaginary component of the third order nonlinear optical susceptibility. We note that structures with an axially asymmetric nonlinear absorption can be easily realized from saturable and reverse saturable absorption materials arranged in tandem. In this paper, the basic transmission behavior of such a structure is worked out. Detailed numerical simulations demonstrate passive all-optical diode behavior, and the results are verified experimentally. The principle will work for all light polarizations, has no phase-matching restrictions, and can be extended to a large number of available nonlinear media for possible applications.

Optical Fourier processing using photoinduced dichroism in a bacteriorhodopsin film.

The photoinduced dichroism in bacteriorhodopsin films has been investigated in view of its application for optical Fourier processing. A simple optical system for Fourier processing is demonstrated that does not require precise alignment and vibration isolation. The basic principle of operation relies on the intensity dependence of photoinduced dichroism in a bacteriorhodopsin film. Experimental results with Fourier processing are shown for edge enhancement.

Phase contrast imaging using photothermally induced phase transitions in liquid crystals

Phase contrast imaging is performed for live biological species using photothermal induced birefringence in dye doped liquid crystals. Using typical 4-f configuration, when liquid crystal cell is at back focal plane of Fourier lens, low spatial frequencies at center of Fourier spectrum are intense enough to induce local liquid crystal molecules into isotropic phase, whereas high spatial frequencies on the edges are not intense enough and remain in anisotropic phase. This results in π∕2 phase difference between high and low spatial frequencies. This simple, inexpensive, all-optical, user-friendly, self-adaptive phase contrast imaging technique using low-power laser offers several distinct advantages.

Medical image processing using transient Fourier holography in bacteriorhodopsin films

Real time image processing is demonstrated by recording and reconstructing the transient photoisomerizative grating formed in the bR film using Fourier holography. Desired spatial frequencies including both high and low band in the object beam are reconstructed by controlling the reference beam intensity. The results are in agreement with a theoretical model based on photoisomerization grating. We exploit this technique to process mammograms in real-time for identification of microcalcifications buried in the soft tissue for early detection of breast cancer. A feature of the technique is the ability to transient display of selected spatial frequencies in the reconstructing process which enables the radiologists to study the features of interest.

Third‐order nonlinear optical interactions of C60 and C70

We measured the third‐order nonlinear optical susceptibility χ(3) for C60 and C70 solutions as a function of concentration using degenerate four‐wave mixing with picosecond pulses at 532 nm. From the observed minimum we were able to obtain the real and imaginary parts of χ(3). The real part is negative at this wavelength. Values of the real and imaginary components of the molecular hyperpolarizability (γ) calculated are in the range 10−31 esu.

Coherent population oscillations and superluminal light in a protein complex

We observed superluminal light in aqueous solution of the protein complex bacteriorhodopsin (bR) at 647.1 nm wavelength where it exhibits reverse saturable behavior, exploiting the technique of coherent population oscillations (CPO). With a modulation frequency of 10 Hz, the signal pulse through a 1 cm path cell is ahead by 3 msec relative to the reference pulse, corresponding to a group velocity of -3.3 m/sec. Following our early work on slow light in the same sample at the saturable wavelength 568.2 nm, we now explicitly observed the narrow spectral hole in the absorption band of the stable B state and further, demonstrated a close correlation between the profile of the hole and the corresponding pulse delay for various modulation frequencies. A similar behavior is observed for superluminal light versus antihole blown in the absorption band.

The complex optical response of arrays of aligned multiwalled carbon nanotubes

The optical properties of periodic and nonperiodic arrays of aligned multiwalled carbon nanotubes are presented. Experimental analysis indicates a complex optical response that is attributed to both the individual carbon nanotube scatterers and also to the array ensembles. These studies indicate that by controlling the geometry and spacing of the arrays, it is possible to create structures that respond very strongly to specific wavelengths or bands of wavelengths. Structures such as these may form the basis for numerous applications in energy conversion. This would include highly efficient solar energy conversion as well as sensitive, finely tuned detectors that can respond to predetermined wavelength bands ranging from the ultraviolet to the infrared region. Experimental, theoretical and modeled results are discussed as they apply to these applications. Challenges and issues are discussed.

Fourier phase contrast multimodal optical microscopy for real time display of phase and fluorescence at the same time

Microorganisms, cells and thin tissue sections are transparent and not visible to view in ordinary microscope. Techniques such as phase contrast and Normarski/Differential interference contrast microscopy transform the phase variation information into intensity distribution to reveal the details of internal structures. Similarly fluorescence microscope uses intrinsic or extrinsic chromophores to reveal specific and hidden details. Advances achieved in recent years have greatly improved the versatility of microscopes to obtain more insightful information about different physiological functions that occur at cellular level. Understanding the cell response, involving both structural and functional changes within the cell, dictates ability to image cell structure and function at the same time. We report a novel optical Fourier phase contrast multimodal optical microscopy technique for real time display of phase and fluorescence features of biological specimens at the same time. It combines the principles of (a) Fourier phase contrast microscopy which exploits monochromaticity, intensity and phase coherence of the laser beam via optical Fourier transform and photoinduced birefringence of dye doped liquid crystal for phase contrast imaging, and (b) common-path multimodal optical microscopy for co-registered imaging of phase and fluorescence features of biological specimens in real time using a single optical path, single light source, and single camera with no requirement of image registration. Further the instrument also enables co-registered imaging of fluorescence and spatial filtering facilitating simultaneous display of structural and functional information. This comprehensive microscope has the capability of simultaneously providing both structural and functional information in a streamlined simplified design and may find applications in high-throughput screening and automated microscopy.

All-Optical Fourier Phase Contrast Microscope

Investigators have found new ways to provide the biological community with more powerful tools.

Nonlinear absorption of tetrabenzporphyrins

We studied the nonlinear absorption properties of tetrabenzporphyrins (TBP). Two dimensional delocalized pi- electron systems such as the macrocyclic dyes tetrabenzporphyrins exhibit nonlinear absorption. Their nonlinear absorption and refraction are due mainly to a mechanism that involves the formation of large populations of excited states. This takes place in the spectral regions between B and Q bands. The relevant time frame in which the nonlinear absorption takes place is determined by the dipole dephasing times in the case of the excited singlet states (which is of order picosecond). TBPs have been proposed as candidates for applications in optical power limiting. We have monitored the nature of the nonlinear absorption of TBP samples with picosecond laser pulses at 532 nm wavelength. Open aperture picosecond Z-scan measurements obtained with solutions of TBP in tetrahydrofuran (THF) exhibit reverse saturable absorption (RSA). Our experiments also reveal RSA for a thin film of TBP in a polymethyl methacrylate (PMMA) matrix. On the other hand we observed that for films deposited using the Langmuir-Blodgett (LB) technique the RSA is decreased. A simple extrapolation of solution measurements to thin film samples might result in erroneous conclusions. We investigated the nonlinear absorption of TBP in THF-water mixture to elucidate the role of molecular aggregates in the nonlinear properties of TBP samples. The RSA observed in the case of THF solutions is lost in the mixture.