Georgi I. Petrov

Raman microscopy analysis of phase transformation mechanisms in vanadium dioxide

Raman microscopy is used to study the evolution of vibrational modes of vanadium dioxide single crystals and thin films in the vicinity of the phase transition. The results support the electron correlation model of phase transformation.

Comparison of coherent and spontaneous Raman microspectroscopies for noninvasive detection of single bacterial endospores

Single bacterial spores were analyzed by using nonlinear Raman microspectroscopy based on coherent anti-Stokes Raman scattering (CARS). The Raman spectra were retrieved from CARS spectra and found to be in excellent agreement with conventionally collected Raman spectra. The phase retrieval method based on maximum entropy model revealed significant robustness to external noise. The direct comparison of signal amplitudes exhibited a factor of 100 stronger CARS signal, as compared with the Raman signal.

Efficient third-harmonic generation in a thin nanocrystalline film of ZnO

Nonlinear optical conversion is studied in thin films of wide-bandgap materials. Very high conversion efficiency to the third-harmonic radiation is achieved for an unamplified femtosecond Cr4+:forsterite laser in a submicron-thick film of a nanocrystalline ZnO pulsed-laser-deposited on a fused silica substrate.

Enhancing red-shifted white-light continuum generation in optical fibers for applications in nonlinear Raman microscopy.

We report an efficient red-shifted continuum generation of picosecond pulses in conventional optical fibers. By using a novel highrepetition rate, high-energy oscillator operating at the fundamental wavelength of 1064 nm, we achieved more than 60% of the output energy in the spectral range from 1150 to 1300 nm, perfectly suitable for broadband coherent anti-Stokes Raman spectroscopy.

Bright emission from a random Raman laser

Random lasers are a developing class of light sources that utilize a highly disordered gain medium as opposed to a conventional optical cavity. Although traditional random lasers often have a relatively broad emission spectrum, a random laser that utilizes vibration transitions via Raman scattering allows for an extremely narrow bandwidth, on the order of 10 cm−1. Here we demonstrate the first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium, with conversion efficiencies on the order of a few percent. Furthermore, Monte Carlo simulations are used to study the complex spatial and temporal dynamics of nonlinear processes in turbid media. In addition to providing a large signal, characteristic of the Raman medium, the random Raman laser offers us an entirely new tool for studying the dynamics of gain in a turbid medium.

Nonlinear optical microscopy analysis of ultrafast phase transformation in vanadium dioxide

The nonlinear optical properties of solid-solid phase transformation in vanadium dioxide are studied. It is found that the efficiency of the third-harmonic optical signal generated from the surface of the material increases by 1.5 orders of magnitude as a function of this phase transformation. Microscopy studies show the hysteresis of the phase transformation on a micrometer-size scale.

Analytical Capabilities of Coherent Anti-Stokes Raman Scattering Microspectroscopy

Nonlinear Raman scattering is an emerging spectroscopy technique for non-invasive microscopic imaging. It can produce a fluorescence background free vibrational spectrum from a microscopic volume of a sample providing chemically specific information about its molecular composition. We analyze the ability of nonlinear Raman microspectroscopy to detect low concentrated molecular species and evaluate its applicability to study complex solutions.

Detecting anthrax in the mail by coherent Raman microspectroscopy.

In this report, we show the collection of spatial information through a turbid medium by coherent Raman microspectroscopic imaging. In particular, the technique is capable of identifying anthrax endospores inside a sealed paper envelope.

Detection of Bacillus subtilis spores in water by means of broadband coherent anti-Stokes Raman spectroscopy

Broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy is used for detection of bacterial spores in aqueous solution. Polarization CARS spectroscopy is employed to suppress the non-resonant background. CARS spectrum recorded in the spectral region from 700 to 1900 cm(-1) exhibits all the characteristic features of spontaneous Raman spectrum taken for a solid powder and resembles that one of the dipicolinic acid, which is considered to be the major component of bacterial spores, including anthrax.

Human tissue color as viewed in high dynamic range optical spectral transmission measurements

High dynamic range optical-to-near-infrared transmission measurements for different parts of human body in the spectral range from 650 to 950 nm have been performed. Experimentally measured spectra are correlated with Monte Carlo simulations using chromaticity coordinates in CIE 1976 L*a*b* color space. Both a qualitative and a quantitative agreement have been found, paving a new way of characterizing human tissues in vivo. The newly developed experimental and computational platform for assessing tissue transmission spectra is anticipated to have a considerable impact on identifying favorable conditions for laser surgery and optical diagnostics, while providing supplementary information about tissue properties.