Vadim V Lozovoy

ABSTRACT A laser system using ultrashort laser pulses is provided. In another aspect of the present invention, the system includes a laser, pulse shaper and detection device. A further aspect of the present invention employs a femtosecond laser and binary pulse shaping (BPS). Still another aspect of the present invention uses a laser beam pulse, a pulse shaper and a SHG crystal.

ABSTRACT We compensate high-order dispersion of laser pulses with ~100-nm bandwidth at the focal plane of a laser-scanning two-photon microscope. Phase distortion correction accounts for significant increase of two-photon excitation fluorescence and second harmonic generation signal.

ABSTRACT Ultrashort pulses are expected to be beneficial for multiphoton microscopy. To utilize their advantages, however, chromatic dispersion must be compensated. We use multiphoton intrapulse interference phase scan (MIIPS) to measure and then eliminate phase distortions of pulses with a FWHM spectral bandwidth greater than 100 nm. Once compensated, the transform limited pulses (

ABSTRACT Spectral phase comb shaping is shown to be a powerful concept for phase-only generation and in-situ characterization of arbitrary optical pulse sequences, where the temporal shape of every pulse in the train is controlled independently.

Nonlinear optical applications depend on pulse duration and coherence of the laser pulses. Characterization of high-repetition rate pulsed laser sources can be complicated by their pulse-to-pulse instabilities. Here, we introduce and demonstrate experimentally a quantitative measurement that can be used to determine the pulse-to-pulse fidelity of ultrafast laser sources. Numerical simulations and experiments illustrate the effect of spectral phase and amplitude noise on second and third harmonic generation.

ABSTRACT A LIBS system is demonstrated using a 100m cavity Yb fiber oscillator producing ~ 70ps, 320nj clusters of 50-100fs sub-pulses at 2MHz. A new empirical model for femtosecond ablation is presented to explain the LIBS signal intensity’s non-linear dependence on pulse fluence by accounting for the Gaussian beam’s spatial distribution. This model is compared to experimental data and found to be superior to linear threshold fits. This model is then used to measure the ablation threshold of Cu using a typical amplified Ti:Sapphire system, and found to reproduce previously reported values to within ~ 20%. The ablation threshold of Cu using the Yb fiber oscillator system was measured to be five times lower than on the amplified Ti:Sapphire system. This effect is attributed to the formation of nanostructures on the surface, which have previously been shown to decrease the ablation threshold. The plasma lifetime is found to be ~ 1ns, much shorter than that of nanosecond ablation, further indicating that the decreased threshold results from surface effects rather than laser-plasma interaction. The low threshold and high pulse energy of the Yb fiber oscillator allows the acquisition of LIBS spectra at megahertz repetition rates. This system could potentially be developed into a compact, fiber-based portable LIBS device taking advantage of the benefits of ultrafast pulses and high repetition rates.

ABSTRACT We report design parameters to eliminate or minimize the spatial distortion for shaped femtosecond pulses after a Fourier-transform pulse shaper, theoretically and experimentally. We conclude that all distortions are avoided with correct pulse shaper setup.

ABSTRACT The excellent chemical identification ability of Raman based spectroscopy provides a versatile and widely applicable method to identify hazards within a complex chemical environment. However, the small spontaneous Raman scattering cross section can limit standoff detection of trace quantities. Coherent anti-Stokes Raman scattering provides the same chemical specificity, but with the potential for much greater signal due to coherent addition in the non-linear spectroscopy. Utilizing CARS, we have demonstrated μg/cm2 level detection of an explosive simulant using a single laser producing less than 8mW of laser power in the near IR. This detection level was achieved on a simulant present as only a small part within a polymer mixture. In addition, we present standoff chemical images of trace compounds within a complex chemical environment, which effectively demonstrate the unique capabilities of the method. Further, the temporal pulse shape can be tailored to excite specific Raman transitions, adding versatility to this method.

Page 1. Amplitude and Phase Shaping of Ultra-broad-bandwidth Femtosecond Laser Pulses Bingwei Xu, Yves Coello, D. Ahmasi Harris, Vadim V. Lozovoy, and Marcos Dantus ... 4. B. Xu, JM Gunn, JM Dela Cruz, VV Lozovoy, and M. Dantus, JOSA.B 23, 750-759 (2006). ...

We present experimental measurements of localized surface plasmon emission from individual silver nanoparticles and small clusters via accurately delayed femtosecond laser pulses. Fourier transform analysis of the nanoplasmonic coherence oscillations reveals different frequency components and dephasing rates for each nanoparticle. We find three different types of behavior: single exponential decay, beating between two frequencies, and beating among three or more frequencies. Our results provide insight into inhomogeneous and homogeneous broadening mechanisms in nanoplasmonic spectroscopy that depend on morphology and nearby neighbors. In addition, we find the optical response of certain pairs of nanoparticles to be at least an order of magnitude more intense than the response of single particles.

ABSTRACT Self phase modulation in hollow waveguide filled with noble gases have been successfully used to produce ultra-broad bandwidths. The intense spectral phase distortions produced by self phase modulation have previously been compensated by prism pairs and adaptive pulse shaping. Here we demonstrate the generation of 4.8 fs, 200 muJ intense laser pulses by multiphoton intrapulse interference phase scan (MIIPS).

ABSTRACT We describe a pulse shaping technique for synthesis of optical pulse sequences, deemed suitable for pump-probe experiments and multidimensional spectroscopy. It enables straightforward programming, manipulation, and self-characterization of multi-pulse waveforms via one-dimensional phase-only shaping.

ABSTRACT Optimum control of a chemical reaction typically involves a number of nonlinear optical processes caused by a strong shaped laser pulse. Here we explore the role that three-pulse four-wave mixing techniques can play in unraveling the different nonlinear photophysical and photochemical processes that take place in the presence of the shaped laser pulse. Two types of experiments are included here. The first illustrates how electric field interactions (up to third order) manipulate the electronic, vibrational, and rotational coherence and population transfer processes in the sample molecules. The second set of experiments explores strong field effects using off-resonance four-wave mixing. Evidence of alignment and structural deformation is presented.

ABSTRACT form only given. Preparation and probing of coherent superposition of multiple quantum states leading to the generation of quantum coherences in condensed phase systems has been the topic of intense research for over a decade and is one of the most interesting and least understood of the quantum phenomena. Evolution of ultrashort broadband excitation sources has over the years provided us with new tools for revisiting problems in every field pertaining to light matter interaction. One such problem is the formation of electronic coherence through the reversible non-radiative electronic coupling and superposition of electronic states. Understanding the role of electronic coherence in condensed phase systems is central to elucidating the mechanism of energy transfer in photosynthetic pigment protein complexes [1] and several tubular nanostructures [2]. The recent observation of long-lived electronic coherence in chemical and biological system at room temperatures [3] forces one to reconsider the existing theories on energy transfer and devise an efficient and smart way in designing of synthetic light harvesting systems.Here we explore the linear and nonlinear optical response of solvated fluorescent molecules under saturation conditions by studying the changes in fluorescence and stimulated emission upon interacting with a pulse pair generated by pulse shaper assisted phase and amplitude modulation. In particular we focus on the observation of long lived oscillations that are detected in the stimulated emission signal during an interferometric measurement. Interferometric time-delay scans are measured while simultaneously monitoring the fundamental, fluorescence and stimulated emission, as shown in Fig. 1a. The pump pulse is fixed at time zero while the probe pulse is scanned. All signals are normalized to unity at time zero when the system is being interrogated by a single transform-limited pulse. The early portion of the scan (for IJ <; 60 fs) has strong- interferometric modulation caused by the linear optical interference between the laser pulses. The different asymptotic levels of fluorescence and stimulated emission are due to the nonlinear effect of strong field excitation. The most interesting feature is the observed long lived oscillations in the stimulated emission signal which persists beyond ~150 fs. The observed π phase shift of the delayed modulation indicates that the physical source of the observed long lived oscillations is most likely caused by the induced linear polarization of the medium induced by the electronic coherence between ground and excited state. A simulation (using phenomenological formulas) of the stimulated emission showing the saturation, appearance and phase shift of the signal is shown in Fig 1b.

We report the detection of characteristic Raman lines for several chemicals using a single-beam coherent anti-Stokes Raman scattering (CARS) technique from a 12 meter standoff distance. Single laser shot spectra are obtained with sufficient signal to noise ratio to allow molecular identification. Background and spectroscopic discrimination are achieved through binary phase pulse shaping for optimal excitation of a single vibrational mode. These results provide a promising approach to standoff detection of chemicals, hazardous contaminants, and explosives.

Laser-based molecular identification has reached a new level of performance allowing real-time identification and quantification of mixtures containing isomers and enantiomers. The ROC curves of this method for absolute molecular identification will be discussed.

Interaction of intense laser pulses with atoms and molecules is at the forefront of atomic, molecular, and optical physics. It is the gateway to powerful new tools that include above threshold ionization, high harmonic generation, electron diffraction, molecular tomography, and attosecond pulse generation. Intense laser pulses are ideal for probing and manipulating chemical bonding. Though the behavior of atoms in strong fields has been well studied, molecules under intense fields are not as well understood and current models have failed in certain important aspects. Molecules, as opposed to atoms, present confounding possibilities of nuclear and electronic motion upon excitation. The dynamics and fragmentation patterns in response to the laser field are structure sensitive; therefore, a molecule cannot simply be treated as a "bag of atoms" during field induced ionization. In this article we present a set of experiments and theoretical calculations exploring the behavior o...

We formulate a simple strategy for mitigation of laser-induced damage through pulse shaping and demonstrate experimentally the effect of laser pulse duration on the degree of optically induced damage for two-photon microscopy imaging. We use a broadband Ti:Sapphire laser source, aided with a shaper, and adjust both the laser pulse duration and energy to maintain constant two-photon excitation efficiency. The damage is assessed by the dynamics of two-photon excited autofluorescence intensity and sample morphology during prolonged laser exposure. We observe that for a 5-mum layer of skin tissue the damage rate is independent of the pulse shape, which suggests that the primary damage (bleaching) mechanism stems from the two-photon excitation itself. For optically thick dried blood samples, taken as another example, the data suggests that the damage is driven by one-photon absorption. In the later case, it is favorable to use shorter laser pulses to mitigate photodamage while maintaining adequate intensity of two-photon excited autofluorescence.

ABSTRACT We report a new method for standoff chemical detection based on single ultrafast pulse excitation for remote coherent anti-Stokes Raman spectroscopy. Mode-selective and background-free excitations were achieved through optimal binary phase pulse shaping.

High-resolution two-photon excitation spectroscopy of natural and synthetic fluorescent biological molecules is demonstrated using an ultra-broad-bandwidth (over 400 nm) femtosecond laser. Selective excitation was achieved using a series of specially designed phase and amplitude masks.

ABSTRACT We demonstrate the measurement of transient dispersion in fused silica by RT-MIIPS. The results are validated via Fourier Transform Spectral Interferometry. The observed dispersion modulation is explained within a theoretical model.