Chandra S. Yelleswarapu

Shining Light on the Dark Side of Imaging: Excited State Absorption Enhancement of a Bis-styryl BODIPY Photoacoustic Contrast Agent

A first approach toward understanding the targeted design of molecular photoacoustic contrast agents (MPACs) is presented. Optical and photoacoustic Z-scan spectroscopy was used to identify how nonlinear (excited-state) absorption contributes to enhancing the photoacoustic emission of the curcuminBF2 and bis-styryl (MeOPh)2BODIPY dyes relative to Cy3.

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.

Dual plane in-line digital holographic microscopy

We report a dual plane in-line digital holographic microscopy technique that exploits the method of subtraction of average intensity of the entire hologram to suppress the zero-order diffracted wave. Two interferograms are recorded at different planes to eliminate the conjugate image. The experimental results demonstrate successful reconstruction of phase objects as well as of amplitude objects. The two interferograms can be recorded simultaneously, using two CCD or CMOS sensors, in order to increase the acquisition rate. This enhanced acquisition rate, together with the improved reconstruction capability of the proposed method, may find applications in biomedical research for visualization of rapid dynamic processes at the cellular level.

Nonlinear photoacoustics for measuring the nonlinear optical absorption coefficient

We report a novel photoacoustic Z-scan (PAZ-scan) technique that combines the advantages offered by the conventional Z-scan method and the sensitivity of the photoacoustic detection. The sample is scanned through the focused laser beam and the generated photoacoustic signal is recorded using a 10 MHz focused ultrasound transducer. Since the signal strength is directly proportional to the optical absorption, PAZ-scan displays nonlinear behavior depicting the nonlinear optical absorption of the material. Among many advantages, our experiments on mouse blood show that PAZ-scan can potentially be used as a standard technique to calibrate contrast agents used in theranostics in general and photoacoustics in particular.

Self-diffraction in bacteriorhodopsin films for low power optical limiting

We demonstrated a novel technique for low power optical limiting using self-diffraction in bacteriorhodopsin (bR) films. A cw Ar-Kr laser is used as the pump (input beam, 568 nm) and the output is the first order self-diffracted beam with an observed efficiency of about 0.01%. Input beam intensity is varied over three orders of magnitude in the range of milliwatt to watts per cm2 with output clamped at eye safe level of about 0.13 mW/cm2. Threshold intensity for limiting is governed by the saturation intensity of M-state of bR and hence can be varied by choosing films with different lifetimes.

Nonlinear scattering studies of carbon black suspensions using photoacoustic Z-scan technique

Nonlinear scattering properties of carbon black suspensions (CBS) are studied using nanosecond photoacoustic (PA) and optical z-scan techniques. When the laser is operated in multi-pulse mode, no nonlinear behavior is observed in PAZ-scans. However, in the single-pulse mode, we observed the nonlinear scattering in both PAZ and optical z- scans. Our results are in agreement with the well-known bleaching effect in CBS and demonstrate the importance of pulse repetition frequency for studying nonlinear scattering using photoacoustics. The effective nonlinear extinction coefficients of CBS were determined, and we found that PAZ-scan data are more sensitive and offer information on higher nonlinearities.

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.

Slow light in bacteriorhodopsin solution using coherent population oscillations.

Slow light is demonstrated in liquid phase in an aqueous bacteriorhodopsin (bR) solution at room temperature. Group velocity as low as 3 m/s (all the way to c) is achieved by exploiting the photoisomerization property of bR for coherent population oscillations. Slow light in the liquid phase offers several advantages over solids or vapors for a variety of applications: (i) shorter lifetimes of the M state facilitate slow light at higher modulation frequencies, (ii) convection makes it possible to obtain large signal delays even at high input powers, and (iii) solution concentration is another convenient parameter to vary the signal delay over a wide range.

All-optical spatial filtering with power limiting materials

We demonstrated that a power limiting mechanism could potentially be used for self-adaptive, all-optical Fourier image processing. Reverse saturable absorbers like porphyrins are chosen due to their fluence dependent power limiting property, which triggers at relatively low intensities. At low input intensities, below the power-limiting threshold, the 4-f configuration will image the object onto the CCD camera without any spatial frequency filtering. As the input intensity is increased above the threshold level, dc and low spatial frequencies are blocked resulting in edge-enhanced images containing high spatial frequencies. The incident intensity sets the higher limit on the band of frequencies blocked. In addition, the use of the same experimental setup for both power limiting experiments and optical image processing demonstrates that in the case of any bright image bearing laser beam, the sensitive detectors are protected, by blocking the intense low spatial frequencies.