Tunable CO2 Lasers

Non-spectroscopic high-power CO2 lasers have been constructed with a continuous output up to tens of kW. Such lasers are used industrially for 

Lasers with CO as an active medium can also be constructed. This laser type yields a large number of lines in the region 5.1-5.6 /zm. In order to achieve efficient operation, the discharge tube must be cooled to low temperatures which complicates practical use. It is simpler to use HF or DF lasers, which give lines in the 2.8-4.0 /zm region. Such lasers are examples of chemical lasers for which the active molecules are formed in the discharge tube from the supplied gases H2/D2 and SF0. 

As we have seen above, lasing has been achieved for a very large number of atomic and molecular lines. A list of these lines can be found in [8.64]. 

The carbon dioxide laser is the most efficient gas laser, with a wall-plug efficiency of up to 20%. It worlcs in the IR region aromid 10 pm. In numerous apphcations a non-tmied CO2 laser is used. However, from a spectroscopic point of view, the fact that it can be fine-tuned and continuously tuned at high gas pressures is very useful. In Fig. 3.30 a level diagram and a practical arrangement for a tuned CO2 laser are shown, together with a diagram of the available fines. The CO2 molecule has three fundamental modes of 

Non-spectroscopic high-power CO2 lasers have been constructed with a continuous output of up to tens of kW. Such lasers ai e used industrially for cutting, welding, hardening, etc. High-power pulsed CO2 lasers ai e also used for fusion research. CO2 lasers were discussed in [8,80-8.82]. 

---

H. Monobe, Y. Shimizu, Anisotropic photoconduction of triphenylene-based DLC in aligned domains by wavelength tunable CO2 laser irradiation. Mol. Cryst. Liq. CrysL 542,... 

Research has been carried out in Britain on the use of tunable CO2 lasers to excite chemical bonds in the production of chemical compounds and in the U.S.A. lasers are being used for the separation of isotopes of materials such as Uranium. 

Due to the coincidence of the CO2 laser lines and the caracteristic absorption spectrum of numerous air pollutant molecules, a tunable CO2 laser is a superior light source for a differential absorption LIDAR. An additional advantage of a tunable CO2 laser is the atmospheric window in the 9-11 pm wavelength range w hich these lasers are capable of utilizing. 

Methanol trimers were not observed in excitation spectra. Probably the excitation is so sharply peaked that it was missed by the coarse grid of frequency points obtainable from a line tunable CO2 laser. Also for the trimer excitation only one CO2 laser frequency yields excitation - a very strong excitation signal, by the way. 

A tunable CO2 laser has been combined with an atomic-force-microscopy (AFM) microscope to form an apertureless near-held-imaging system [17]. This technique can produce spahal resoluhon of up to A/lOO with high throughput however, the tunable range of the CO2 laser is hmited to a region of the IR spectrum that is not parhcularly informahve for most IR chromophores (2300 cm ). 

Repond, P., and Sigrist, M. W. (1996). Photoacoustic spectroscopy on trace gases with continuously tunable CO2 laser, Appl. Optics 35, 4065-4085. 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

Molecular lasers, which can oscillate on many rotational transitions, can be tunded to the different lines 8) by inserting a grating inside the cavity. The CO2 and N2O lasers, for instance, have an oscillating line for nearly every wave number of the 9 and 10 nm regions. Therefore vibrational bands of many molecules are entirely covered by these linesWith such a tunable NjO laser Oppenheim et studied the absorption of 65 rotational lines in NjO. 

Kanamori et al. (17) and Wood et al. (18) have reported on the use of infrared tunable diode lasers to detect by absorption the SO and CO2 photochemical fragments respectively. 

CO2 laser A continuous or pulsed source of coherent radiation normally tunable through the CO2 vibration-rotation band centered near 10.6 pm. 

Since there are only few - and expensive - tunable lasers available for the infrared range, they are not popular for routine applications. However, there are instances where CO2 lasers (with a high efficiency) are used to excite emission spectra (Belz et al., 1987). Semiconductor lasers have been developed for monitoring atmospheric trace gases (Grisar et al., 1987). 

Details of the continuous wave, axial flow, line tunable, extended cavity CO2 laser system, and of our alignment procedure, are given in ref. [14]. To access a wavelength near 12.0 /im required operating the laser on the P-branch of the 01 1 — [11 0,03 0]/ hot-band of [22], and using the laser beam co-... 

IR measurements on doped Si and shown that the subsurface mobile carriers can be probed by their response to an IR near-field with a spatial resolution of 30nm [48]. The group of Havenith presented a scanning near-field infrared microscopy (SNIM) system this is an IR s-SNOM set-up based on a continuous-wave optical parametric oscillator (OPO) as an excitation source with a much wider tunability compared to the usually applied CO2 lasers [49]. With this set-up, a subsurface pattern of implanted gallium ions in a topographically fiat silicon wafer was imaged with a lateral resolution of <30 nm. 

With the demonstration of the stimulated emission associated with these states [119], and also with the requirement of quantum computing, a keen interest has developed on the actual lifetimes of the donor excited states in silicon. Population inversion of donor electrons has been achieved by non-resonant pumping with a CO2 laser with discrete energies tunable in the 124meV range, and by resonant pumping at the energies of the donor absorption lines with a FEL [118]. These experiments have shown the importance... 

---