TeraHertz laser generation by optically pumped Polar Molecules

Abstract

In this work we report 12 new optically pumped far-infrared (FIR) laser lines from CH 3 OH and 19 from 13 CH 3 OH. A 13 CO 2 laser of wide tunability (110 MHz) was used as pump source, and a Fabry-Perot open cavity was used as a FIR laser resonator.The interest on powerful sources of electromagnetic waves in the TeraHertz region (sometimes identified as Far Infrared (FIR)) for applications such as imaging and medical sciences [1][2][3][4][5], has been a strong motivation to reinvestigate CO 2 laser-pumped methanol as active medium for THz laser generation. Optical pumping of polar molecules is one the most efficient techniques to generate cw THz laser lines, and today thousands of discrete emissions, ranging 30µm (10THz) to 3mm (0.1THz), have been observed. Methyl alcohol and its isotopomers are the best sources of THz laser lines and the CO 2 laser the most widely used source for optical pumping. An accidental coincidence between a CO 2 laser emission and an infrared molecular absorption is a rather stringent condition in those experiments. The use of isotopic forms of carbon dioxide, as optical laser sources, greatly increases the possibility of those accidental coincidences.In this work we conducted a careful search for new laser lines in CH 3 OH and 13 CH 3 OH optically pumped by a sealed-off 13 CO 2 laser (Lasertch Model LT250 Laser Tube) [1,2]. A Fabry-Perot cavity was used both as an optoacoustic (OA) cell, to record the absorption transitions, and as a TeraHertz laser resonator. The FIR laser cavity is formed by two gold-coated copper concave mirrors (R=1.4m) separated by 100 cm. One of these mirrors is fixed while the other is coupled to a micrometer which allows tuning of the longitudinal modes of the FIR laser. For each wavelength measurement, the cavity is scanned over several longitudinal modes and the laser intensity was recorded as function of cavity length. This allows determination of the FIR laser wavelengths with an uncertainty of ±0.5µm. The FIR output power is detected by a Golay cell, after being coupled out by a 45 mirror that can be moved transverse to the axis of the FIR laser cavity. The laser also has an internal eletret microphone used for optoacoustic (OA) detection. The OA signal is used for tuning the CO 2 laser frequency into the active medium absorption. Using this apparatus, we observed 12 new laser lines, ranging from 58.1µm (5.2 THz) to 624.6µm (0.5 THz) from CH 3 OH, and 19 laser lines in the range from 42.3µm (7.1THz) to 717.7µm (0.42 THz), from 13 CH 3 OH. These lines were characterized according to wavelength, relative polarization, relative intensity (very strong (VS), strong (S), medium (M) and weak (W) according to the criteria used in [6]), optimum pressure, and infrared absorption offset frequency. The transferred Lamb-Dip technique was used to measure the absorption offset frequencies [7]. This work was supported by FAPESP, CePOF and CAPES -Brazilian government agencies.