Far-infrared CH3F Stark laser.

This Letter reports oscillation of a far-infrared (FIR) waveguide laser on individual Stark components of the 496-,Am transition in CH3 F by the application of an electric field across the laser medium. Stark shifting of vibrational-rotational absorption lines into coincidence with C02-laser pump lines giving rise to FIR oscillation in the rotational transitions was first observed in ammonia.' Recently much attention has been given to the effect of electric fields on the strong 496-gm FIR transition in CH3 F; a frequency shift was observed, 2'3 and frequency modulation was achieved by use of the Stark effect.3 The use of rectangular metal-dielectric waveguides for optically pumped FIR lasers provides a simple and effective way of applying an electric field to the laser medium and has been previously used to enhance the power output of the 119-gm CH30H laser line.4 A metal-dielectric waveguide laser was constructed of a pair of copper plates 8 mm thick, 64 mm wide, and 2.03 m long, which was optically polished on the widewaveguide faces. These plates were separated by 6mm-square pieces of common window glass epoxied to the copper pieces with the smooth window surfaces inside the guide. The glass pieces were separated by 35 mm. A fixed flat copper mirror was placed about 1 mm from the end of the waveguide. A 1-mm input coupling hole was centered in this mirror and was covered with a 2-mm-thick slab of NaCl. A 7-mm-diameter output coupling hole was spaced 14 mm from the center of the mirror and was covered with a 0.5-mm-thick piece of crystal quartz. A movable flat copper mirror was positioned with a micrometer on the far end of the laser. A 25-W, grating controlled, piezoelectrically tuned, cw CO2 laser was used to drive the FIR laser. The FIR laser oscillated very well at wavelengths shorter than 600 Am (the output power on the 496-Am line was 3 mW) but refused to oscillate on the strong 700-gm line of methyl alcohol. The calculated losses at 1-mm wavelength are less than 2%/o/m, 5 and the failure of the 700-gm line to oscillate indicates that the waveguide might be somewhat more lossy than was expected. The polarization of the FIR laser was parallel to the copper surface of the waveguide, as expected. The electric Stark field is, of course, perpendicular to the copper plates; hence the dc Stark field is always perpendicular to the FIR-laser field. The electric field of the pump radiation from the CO2 laser can be oriented either parallel or perpendicular to the Stark field. Thus there are two cases to consider: (A) pumping transitions with AM = 1 and FIR transitions with AM = +1 and (B) pumping transitions with AM = 0 and FIR transitions with AM = ±1. The energy levels of the 496-gm transition in CH 3F are given in Fig. 1. The tabulated matrix elements for the dipole moment of the symmetric top molecule6 give the relative intensity of the single Stark components of a transition (J,M) (J 1, M F 1) as shown by the equations: