Bingwei Xu

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.

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.

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.

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.

Direct measurement of spectral phase for ultrashort laser pulses

We present an intuitive pulse characterization method that provides an accurate and direct measurement of the spectral phase of ultrashort laser pulses. The method requires the successive imposition of a set of quadratic spectral phase functions on the pulses while recording the corresponding nonlinear spectra. The second-derivative of the unknown spectral phase can be directly visualized and extracted from the experimental 2D contour plot, without any inversion algorithm or mathematical manipulation.

Group-velocity dispersion measurements of water, seawater, and ocular components using multiphoton intrapulse interference phase scan.

The use of femtosecond lasers requires accurate measurements of the dispersive properties of media. Here we measure the second- and third-order dispersion of water, seawater, and ocular components in the range of 660-930 nm using a new method known as multiphoton intrapulse interference phase scan. Our direct dispersion measurements of water have the highest precision and accuracy to date. We found that the dispersion for seawater increases proportionally to the concentration of salt. The dispersion of the vitreous humor was found to be close to that of water. The chromatic dispersion of the cornea-lens complex was measured to obtain the full dispersive properties of the eye.

Spectral phase optimization of femtosecond laser pulses for narrow-band, low-background nonlinear spectroscopy

We use experimental search space mapping to examine the problem of selective nonlinear excitation with binary phase shaped femtosecond laser pulses. The search space maps represent a graphical view of all the possible solutions to the selective nonlinear excitation problem along with their experimental degrees of success. Using the information learned from these maps, we generate narrow lines with low background in second harmonic generation and stimulated Raman scattering spectra.

Stain-free histopathology by programmable supercontinuum pulses

The preparation, staining, visualization, and interpretation of histological images of tissue is well-accepted as the gold standard process for the diagnosis of disease. These methods were developed historically, and are used ubiquitously in pathology, despite being highly time and labor intensive. Here we introduce a unique optical imaging platform and methodology for label-free multimodal multiphoton microscopy that uses a novel photonic crystal fiber source to generate tailored chemical contrast based on programmable supercontinuum pulses. We demonstrate collection of optical signatures of the tumor microenvironment, including evidence of mesoscopic biological organization, tumor cell migration, and (lymph-)angiogenesis collected directly from fresh ex vivo mammary tissue. Acquisition of these optical signatures and other cellular or extracellular features, which are largely absent from histologically processed and stained tissue, combined with an adaptable platform for optical alignment-free programmable-contrast imaging, offers the potential to translate stain-free molecular histopathology into routine clinical use.

Two-photon fluorescence excitation spectroscopy by pulse shaping ultrabroad-bandwidth femtosecond laser pulses

A fast and automated approach to measuring two-photon fluorescence excitation (TPE) spectra of fluorophores with high resolution (~2 nm) by pulse shaping ultrabroad-bandwidth femtosecond laser pulses is demonstrated. Selective excitation in the range of 675-990 nm was achieved by imposing a series of specially designed phase and amplitude masks on the excitation pulses using a pulse shaper. The method eliminates the need for laser tuning and is, thus, suitable for non-laser-expert use. The TPE spectrum of Fluorescein was compared with independent measurements and the spectra of the pH-sensitive dye 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) in acidic and basic environments were measured for the first time using this approach.

Broadband 2.12 GHz Ti:sapphire laser compressed to 5.9 femtoseconds using MIIPS

We report a self-starting prismless femtosecond Ti:sapphire ring laser whose repetition rate has been gradually increased from 1 to 2.12 GHz. A broadband spectrum extending from 650 to 1040 nm, in which 17% of the intracavity power is generated in a single-pass through the crystal, is preserved in spite of the reduction in peak power. An average power of 0.95 W was obtained for 7.5 W of pump power, with very stable operation verified over 22 hours. Pulses from this laser have been fully characterized in spectral phase, and then compressed to 5.9 femtoseconds using multiphoton intrapulse interference phase scan (MIIPS).