Femtosecond coherent anti-Stokes Raman spectroscopy for chemical identification and detection

This thesis presents the application of femtosecond coherent anti-Stokes Raman scattering (fs-CARS) technique, and one of its recent improvement, femtosecond adaptive spectroscopic techniques for CARS (FAST CARS), for the detection and identification of chemicals and substances. The fs-CARS technique replies on temporally delaying the probe pulse to eliminate the unwanted four-wave mixing (FWM) background signal. The intense femtosecond pulses excite the molecule into vibrational modes and create coherence between the Raman level and the ground level. This coherence enhances the interaction between the probe pulse and the molecule, and significantly increases the anti-Stokes signal.; In this work, fs-CARS is first investigated for chemical solution identification. Special attention is addressed to the specificity of the technique. It is shown that the Fourier transformation of the time-resolved CARS trace provides spectral characteristic information of the molecule. The femtosecond adaptive pulse shaping technique, when applied to the Stokes pulse, confirms that the transform-limited pulses are optimal for coherence preparation. A variation of the fs-CARS system, which uses a photonic crystal fiber (PCF) to generate the Stokes and probe pulses, eliminates the need for a regenerative Ti:sapphire amplifier and significantly reduces the system cost and complexity.; Although it is a powerful high-precision spectroscopic tool, the fs-CARS is difficult to apply to targets with rough surfaces, such as samples in powder form or substances such as anthrax spores. The scattering from the surface reduces the CARS signal, but increases the FWM background. The novel FAST CARS technique is demonstrated to significantly increase the CARS/FWM ratio. The use of optimized probe pulse together with infrared pump and Stokes pulses suppresses the FWM signal and increases the CARS signal. The spectral signature of anthrax spores can be extracted in as fast as 50 ms, and real-time identification of anthrax spores in 1 second is achieved.; Finally, I present my research work on the application of quantum well intermixing (QWI) technique to asymmetric twin waveguide (ATG) lasers. The QWI technique effectively shifts the absorption band of the active material within the taper region, and reduces the threshold current of the laser by ∼18%.