There are many types of nonlinear optical techniques, and several depend on the spatial and temporal overlap of multiple optical pulses at different wavelengths on the target. Nonlinear spectroscopy results when a light field perturbs the optical properties of a molecule so that the subsequent light fields see changes in the molecular states. It is important that the different light fields interact with the molecular states over time periods that are shorter than periods associated with dephasing and population relaxation so that the effects of the first interaction are not lost. By exciting a state with one light field and probing it at a later time, nonlinear experiments allow one to investigate how quantum states evolve in time and how different states are related to each other.
Within the different nonlinear optical spectroscopy techniques, raman spectroscopy (including Coherent anti-Stokes Raman Scattering (CARS) and Stimulated Raman Scattering (SRS)) are being developed for optical biopsy applications such as distinguishing cancer cells from normal cells. In order to be able to retrieve information for such applications, the sensitivity and speed of the techniques used is very important. Fig.1 shows the different energy level of techniques where sensitivity is high.
Fig.1: Energy Level Diagram of Different Nonlinear techniques including Two-Photon Excitation (TPE) and Second Harmonic Generation (SHG)
CARS uses two synchronized picosecond sources to stimulate a strong signal response at the Anti-Stokes frequency. This happens when the wavelengths of the two lasers are separated by a wavenumber matching the Raman transition of the sample. This technique is particularly suitable to detect lipids (fats) in microscopic cellular specimens. Ti:Sapphire (Ti:S) lasers and OPOs are used nowadays for CARS experiments as they match the application requirements. However, the drawback is their size and the complexity to maintain the two lasers synchronized.
Genia Photonics’ synchronized programmable laser system uses its two laser outputs to generate CARS and SRS signals corresponding to the targeted wavenumbers. Since this compact laser system operates in the picosecond regime, the nonresonant background is eliminated allowing the acquisition of the information to be done very rapidly.