Nanosecond-Pumped Resonance Raman Random lasing from Micro-/Nano-granular Materials toward Chemical Detection from Distance

Abstract

For security purpose, there is an increasing demand of techniques capable of detecting and identifying trace amount of explosive or dangerous chemicals from a safe distance. A laser-based standoff Raman detection is one of the promising spectroscopic techniques that affords quick detection of targeted molecules with high specificity. However, conventional Raman detection is primarily based on weak spontaneous emission, which undermines the vitality of such technique to perform at remote distance. Therefore, this research studied a new type of much stronger Raman generation process, called Raman random lasing (RRL), in which the optical amplification of Raman signal can be achieved. The RRL process requires stimulated Raman scattering (SRS) as a gain mechanism, and relies on multiple scattering in powder medium to supply optical feedback inside the gain medium. In principle, Raman lasing differs from conventional laser as it requires ultrashort excitation to create population inversion on a virtual state rather than a real electronic state. Thus, a nanosecond UV laser at wavelength 355 nm, and pulse duration 10 ns was used as a pump to excite RRL. Barium nitrate powder with high gain coefficient was chosen as a disordered Raman-active medium with Raman peaks similar to those of many explosive compounds. The RRL was excited near electronic resonance condition to achieve higher Stokes signal for SRS process. The behavior of resonant Raman scattering signal at pump pulse intensity, ranging from 0.12 to 21.30 MW/cm2, showed nonlinear response resulting from the competition between absorption loss and SRS gain mechanisms in the excited medium. The RRL threshold can be founded for a loosely- packed powder sample at pump intensity around 18 MW/cm2, where the SRS gain surpasses loss. The efficiency of the RRL signal at the highest pump intensity of 21.30 MW/cm2 is 4.25 x 104, which is much higher than conventional Raman efficiency of typically 10%. It was found that the RRL power grows within the gain radius, comparable to the laser pump spot size of 2 mm. The sample region outside gain radius absorbed RRL energy, causing high loss. Furthermore, sample packing structure played an important role for RRL generation. The coherent backscattering measurement results show that the pump beam energy can penetrate longer into a loosely-packed sample with lower absorption loss, when compared with a closely-packed sample. The finite different time domain run in COMSOL Multiphysics was also used to investigate energy penetration and storing inside a random medium. The simulation results also support that a low density sample allows higher pump pulse energy to propagate inward. On the contrary, a denser sample can provide greater multiple scattering to prolong light energy confinement inside the random medium. As such, it should exist an optimum particles density which gives a balance between pump energy penetration and degree of multiple scattering. Lastly, it was demonstrated that the RRL for a loosely-packed sample was efficient as the Raman spectrum can be detected at standoff distance using a CCD camera.