MHz-rate picosecond laser discretely tunable from the near-infrared to the deep ultraviolet

ABSTRACT Using YVO 4 as a Raman medium, stimulated Raman amplification of white-light continuum was successfully demonstrated. Only microjoule-level of pul se energies was needed to achieve effici ent energy conversion to the first and the second Stokes radiation. Nonlinear opti cal mixing in a series of BBO crystals was used to attain discretely tunable picoseconds laser pulses in the visible and the ultraviolet spectral regions. A potential application of this radiation for resonance Raman spectroscopy is discussed. Keywords: Raman, laser, ultrasfast, spectroscopy, picosecond, stimulated Raman scattering 1. INTRODUCTION Tunable, short-pulsed laser radiation is of great demand for a number of spectroscopic applications. One of the most challenging fundamental problems is to study the dynamics of biological molecules [1], which spans through more than 15 orders of magnitudes in time domain: from femto- and picoseconds to seconds and minutes. Vibrational Raman spectroscopy is one of the most powerful optical spectroscopic techniques, which plays a critical role in providing with a detailed understanding of how different parts of a molecule behave during its complex interaction process [2]. The ability to collect Raman spectra using excitation of tunable visible and deep-ultraviolet radiation greatly improves our ability to interrogate molecular structure and undergoing tran sformations [3]. Picosecond laser source with an extended tenability, high average power and relatively low energy per pulse, would be ideal to perform such studies. In this report, we demonstrate the principal possibility of reaching this goal by amplifying pi cosecond pulses using Raman amplifier and frequency up-converting them using a series of frequency mixing processes in traditional nonlinear optical crystals. As a result, we were able to generate at least several mWs of the average power in th e deep UV (<270-nm) spectral region. When broadband tenability is required, mo st of the time, it is considered to be necessary to continuously tune the incident radiation to a selected wavelength. However, in Raman spectroscopy, such tenability might be prohibitively expensive, since for each excitation wavelength a notch filter has to be purchased to achieve the fundamental light rejection. A cheaper alternative would be to have a set of laser wavelengths generated, which can be used for Raman spectra excitation. This approach is somewhat similar to the one used by Coherent, Inc. in their intracavity doubled Ar-ion laser. This laser has been widely used for Raman spectroscopy, providing a range of wavelengths in the visible and deep-UV; however, the overall cost and ma intenance issues coupled with the fact that this laser is not suitable for fast dynamic studies make us look for an alternative solution. Stimulated Raman scattering is a very efficient way of generating new colors of light [4]. Unlike optical parametric amplifiers [5-6], the amplified bandwidth is simply contro lled by the linewidth of the Raman transition, thus making it easy to generate transform-limited laser pulses, whose bandwidth is less than 5-10 cm