Final Report Title: Fundamental studies of electric-field-induced coherent Raman scattering

A rather novel method has been studied, referred to here as electric-field-induced coherent Raman scattering (E-CRS). Two beams of laser pulses of frequencies, whose energy difference matches to the Raman transition, produce an infrared (IR) coherent beam corresponding to the Raman transition energy in the presence of a direct-current (dc) electric field. The IR light may be considered as an anti-Stokes wave generated by the electric fields of two laser beams and a third field at zero frequency, i.e., the dc electric field. The same laser beams also produce conventional coherent anti-Stokes Raman scattering (CARS) regardless of the presence of the dc electric field. The electric field can be extracted by the signal ratio (IR vs. CARS), which depends only on the electric field strength. This method, using nanosecond laser beams, has the potential to be a powerful tool to reveal rich high-speed dynamics in discharge plasmas. Such potential has been demonstrated by revealing rapid breakdown mechanisms of nanosecond-pulsed dielectric barrier discharges generated in open air. Our experimental observations have revealed that, in the pre-breakdown phase of the nanosecond DBD discharge, the externally applied fast-rising electric field is strongly enhanced near the cathode due to large accumulation of space charge, which then strongly enhances ionization near the cathode. Once a sufficiently large number of ionizations take place, the location of peak ionization forms a front and propagates toward the cathode with strong optical emission, which establishes the discharge. This process is essentially different from the well-known Townsend mechanism for slower discharges, in which ion transport to the cathode for continuous generation of secondary electrons is considered to be a prerequisite for discharge breakdown. Objectives and contents The ultimate targets of this study are to measure both macroscopic electric field and microscopic electric field (for estimating electron density) by E-CRS with nanosecond temporal resolutions, to reveal rich high-speed dynamics in discharge plasmas. In the period of the funding, we have successfully revealed very high-speed discharge dynamics in high-pressure discharges with E-CRS method, while successful measurements of microscopic electric field, as well as reducing the environmental pressure, still requires further study probably with a more sensitive measurement system. This report, based on the successful results, has components related to: (a) description of the electric field measurement method and (b) successful demonstration revealing highspeed dynamics in nanosecond-pulsed discharges. a) Electric-field-induced coherent Raman scattering (E-CRS) In this section, I describe the measurement method, electric-field-induced coherent Raman scattering. This method was first proposed and demonstrated by a Russian group in mid 90’s [1-3] for hydrogen molecules. After that, no one else has worked on this method. Very recently, I have revisited the subject and successfully measured the electric field in hydrogen. With E-CRS method, our group has revealed very fast discharge dynamics in repetitively pulsed nanosecond discharges [4] and first demonstrated the feasibility of this method in an open air environment by using nitrogen molecules [5]. Fig. 1 Schematic energy diagram for the electric field measurement. The schematic energy diagram of the Raman transition is shown in Fig. 1. Two beams of laser pulses of frequencies ω1 and ω2, whose energy difference matches to the Raman transition, produce an infrared (IR) coherent beam corresponding to the Raman transition energy in the presence of a direct-current (dc) electric field E. The IR light may be considered as an anti-Stokes wave generated by the electric fields of two laser beams and a third field at zero frequency, i.e., the dc electric field. The same laser beams also produce conventional coherent anti-Stokes Raman scattering (CARS) regardless of the presence of the dc electric field. The coherent Raman scattering beam intensities, which we denote by IIR and ICARS, may be written [1,5] as   2 2 1 2 1 E I I N N C I ex g IR   (1) and   2 2 1 2 2 I I N N C I ex g CARS   , (2) where C1 and C2 are constants, I1 and I2 the intensities of incident two laser beams of frequency ω1 and ω2, and Ng and Nex the number densities of nitrogen molecules at the ground and excited levels involved in the transition. By eliminating the number densities from Eqs. 1 and 2, one obtains CARS IR I I I C C E 1