Vadim V. Lozovoy

Multiphoton intrapulse interference. IV. Ultrashort laser pulse spectral phase characterization and compensation

We introduce a noninterferometric single beam method to characterize and compensate the spectral phase of ultrashort femtosecond pulses accurately. The method uses a pulse shaper that scans calibrated phase functions to determine the unknown spectral phase of a pulse. The pulse shaper can then be used to synthesize arbitrary phase femtosecond pulses or it can introduce a compensating spectral phase to obtain transform-limited pulses. This method is ideally suited for the generation of tailored spectral phase functions required for coherent control experiments.

Quantitative investigation of the multiphoton intrapulse interference phase scan method for simultaneous phase measurement and compensation of femtosecond laser pulses

Femtosecond pulse characterization and compensation using multiphoton intrapulse interference phase scan (MIIPS) [Opt. Lett.29, 775 (2004)] was rigorously tested. MIIPS was found to have 3 mrad precision within the 90 nm bandwidth of the pulses. Group-velocity dispersion measurements of glass and quartz provided independent accuracy tests. Phase distortions from high-numerical-aperture objectives were measured and corrected using MIIPS, an important requirement for reproducible two-photon microscopy. Phase compensation greatly improved the pulse-shaping results through a more accurate delivery of continuous and binary phase functions to the sample. MIIPS measurements were possible through the scattering of biological tissue, a consideration for biomedical imaging.

Multiphoton intrapulse interference. II. Control of two- and three-photon laser induced fluorescence with shaped pulses

Nonlinear optical processes are controlled by modulating the phase of ultrafast laser pulses taking advantage of multiphoton intrapulse interference. Experimental results show orders of magnitude control over two- and three-photon excitation of large organic molecules in solution using specific phase functions. We show simulations on the effect of phase modulation on the second- and third-order amplitude of the electric field spectrum, and demonstrate that the observed control is not caused by simple changes in peak intensity.

Selective two-photon microscopy with shaped femtosecond pulses

Selective two-photon excitation of fluorescent probe molecules using phase-only modulated ultrashort 15-fs laser pulses is demonstrated. The spectral phase required to achieve the maximum contrast in the excitation of different probe molecules or identical probe molecules in different micro-chemical environments is designed according to the principles of multiphoton intrapulse interference (MII). The MII method modulates the probabilities with which specific spectral components in the excitation pulse contribute to the two-photon absorption process due to the dependence of the absorption on the power spectrum of E2(t) [1-3]. Images obtained from a number of samples using the multiphoton microscope are presented.

Coherent mode-selective Raman excitation towards standoff detection

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.

Multiphoton intrapulse interference 6; binary phase shaping

We demonstrate a new approach to laser control using binary phase shaping. We apply this method to the problem of spectrally narrowing multiphoton excitation using shaped laser pulses as required for selectivity in two-photon microscopy. The symmetry of the problem is analyzed from first principles and a rational solution is proposed. Successful experimental implementation and simulations are presented using 10 fs ultrashort pulses. The proposed solution is a factor of 6 better than the sinusoidal phase used previously by our group. An evolutionary learning algorithm was used to efficiently improve the solution by a further factor of 2.5 because of the greatly reduced search space afforded by binary phase shaping.

Control of Molecular Fragmentation Using Shaped Femtosecond Pulses

The possibility that chemical reactions may be controlled by tailored femtosecond laser pulses has inspired recent studies that take advantage of their short pulse duration, comparable to intramolecular dynamics, and high peak intensity to fragment and ionize molecules. In this article, we present an experimental quest to control the chemical reactions that take place when isolated molecules interact with shaped near-infrared laser pulses with peak intensities ranging from 1013 to 1016 W/cm2. Through the exhaustive evaluation of hundreds of thousands of experiments, we methodically evaluated the molecular response of 16 compounds, including isomers, to the tailored light fields, as monitored by time-of-flight mass spectrometry. Analysis of the experimental data, taking into account its statistical significance, leads us to uncover important trends regarding the interaction of isolated molecules with an intense laser field. Despite the energetics involved in fragmentation and ionization, the integrated second-harmonic generation of a given laser pulse (ISHG), which was recorded as an independent diagnostic parameter, was found to be linearly proportional to the total ion yield (IMS) generated by that pulse in all of our pulse shaping measurements. Order of magnitude laser control over the relative yields of different fragment ions was observed for most of the molecules studied; the fragmentation yields were found to vary monotonically with IMS and/or ISHG. When the extensive changes in fragmentation yields as a function of IMS were compared for different phase functions, we found essentially identical results. This observation implies that fragmentation depends on a parameter that is responsible for IMS and independent from the particular time-frequency structure of the shaped laser pulse. With additional experiments, we found that individual ion yields depend only on the average pulse duration, implying that coherence does not play a role in the observed changes in yield as a function of pulse shaping. These findings were consistently observed for all molecules studied (p-, m-, o-nitrotoluene, 2,4-dinitrotoluene, benzene, toluene, naphthalene, azulene, acetone, acetyl chloride, acetophenone, p-chrolobenzonitrile, N,N-dimethylformamide, dimethyl phosphate, 2-chloroethyl ethyl sulfide, and tricarbonyl-[eta5-1-methyl-2,4-cyclopentadien-1-yl]-manganese). The exception to our conclusion is that the yield of small singly-charged fragments resulting from a multiple ionization process in a subset of molecules, were found to be highly sensitive to the phase structure of the intense pulses. This coherent process plays a minimal role in photofragmentation; therefore, we consider it an exception rather than a rule. Changes in the fragmentation process are dependent on molecular structure, as evidenced in a number of isomers, therefore femtosecond laser fragmentation could provide a practical dimension to analytical chemistry techniques.

Pulse shaping of octave spanning femtosecond laser pulses

Phase characterization, correction and shaping of an ultra-broad-bandwidth femtosecond laser were achieved using a grating-based pulse shaper. By using MIIPS, the compensated pulses generated a second harmonic spectrum spanning over 12,260 cm-1.

Highly selective standoff detection and imaging of trace chemicals in a complex background using single-beam coherent anti-Stokes Raman scattering

A non-destructive and highly selective method of standoff detection is presented and quantitatively evaluated. The method is found to be orders of magnitude more sensitive than previous coherent spectroscopy methods, identifying concentrations as low as 2 μg/cm2 of an explosive simulant mixed in a polymer matrix. The approach uses a single amplified femtosecond laser to generate high-resolution multiplex coherent anti-Stokes Raman scattering (CARS) spectra encompassing the fingerprint region (400−2500 cm−1) at standoff distance. Additionally, a standoff imaging modality is introduced, visually demonstrating similar sensitivity and high selectivity, providing promising results toward highly selective trace detection of explosives or warfare agents.

Standoff and arms-length detection of chemicals with single-beam coherent anti-Stokes Raman scattering

The detection of chemicals from safe distances is vital in environments with potentially hazardous or explosive threats, where high sensitivity and fast detection speed are needed. Here we demonstrate standoff detection of several solids, liquids, and gases with single-beam coherent anti-Stokes Raman scattering. This approach utilizes a phase coherent ultrabroad-bandwidth femtosecond laser to probe the fundamental vibrations that constitute a molecules fingerprint. Characteristic Raman lines for several chemicals are successfully obtained from arms-length and 12 m standoff distances. The sensitivity and speed of this approach are also demonstrated.