Haowen Li

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.

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.

Single-Beam Coherent Anti-Stokes Raman Scattering for Standoff Detection

Remote Sensing: We continue to develop more sensitive systems for the remote sensing of biological and chemical trace components.

Detection of chemicals at a standoff >10 m distance based on single-beam coherent anti-Stokes Raman scattering

The fundamental difficulty of achieving a coherently enhanced sensing method at standoff distances greater than 10 meters has been solved by single-beam coherent anti-Stokes Raman scattering and by actively measuring and eliminating chromatic dispersion experienced by the broad-bandwidth (100 nm) laser pulses. Characteristic Raman spectra for several chemicals in gas, liquid, and solid states, are successfully obtained from a 12 meter standoff distance. The results obtained indicate this is a promising approach to standoff detection of chemicals, hazardous contaminations, and explosives.

Standoff chemical detection using single-beam CARS

We introduce an improved method for standoff chemical detection of films and residues on solid targets which scatter or reflect the incident light using single-beam Coherent anti-Stokes Raman Scattering.

Delivery and characterization of sub-8fs laser pulses at the imaging plane of a two-photon microscope

We demonstrate a modular and versatile experimental setup that enables straightforward compression (and then shaping) of ultrashort laser pulses at the imaging plane of a two-photon microscope. A commercially available pulse shaper is used in conjunction with a commercially available broadband Ti:Sapphire oscillator to produce sub-8fs pulses at the focus of a high-numerical-aperture objective. Automated adaptive pulse compression, based on multiphoton intrapulse interference phase scan (MIIPS), is verified in situ by shaper assisted interferometric autocorrelation. Two-photon excited fluorescence image of a mouse kidney slide is obtained to confirm microscopic sectioning capabilities.

Two-Photon Microscopy with Sub-8fs Laser Pulses

Broadband Ti:Sapphire oscillator output, guided through a calibrated shaper, is compressed down to sub-8fs at the focus of a high-NA microscope objective. The compression is verified in situ by interferometric autocorrelation at the objective focus.

Nonlinear Optical Imaging with Sub-8fs Laser Pulses

Broadband Ti:Sapphire oscillator output, guided through a pulse shaper, is compressed down to sub-8fs at the focus of a high-NA microscope objective. The compression is verified in situ by interferometric autocorrelation, and images were obtained.

Generation of high energy 4.8 fs pulses from helium-argon mixture filled hollow waveguide

Self phase modulation in hollow waveguide filled with noble gases have been successfully used to produce ultra-broad bandwidths [1]. The intense spectral phase distortions produced by self phase modulation have previously been compensated by prism pairs and adaptive pulse shaping [2]. Here we demonstrate the generation of 4.8 fs, 200 µJ intense laser pulses by multiphoton intrapulse interference phase scan (MIIPS) [3, 4].

Mode selective single-beam coherent anti-Stokes Raman scattering

Binary phase shaping is applied to single-beam CARS spectroscopy of gas mixtures, such as ambient air, and is shown to provide mode-selectivity and enhanced non-resonant background suppression capability when compared with the original technique.