Optically pumped FIR molecular laser systems
(Photos)
60 Watt cw CO2 laser with wavelength in the range between 9.2-10.8 µm and far infrared laser with about 100 mW power in the wavelength range between 60 µm and 2000 µm.
2 kW CO2 Q-switch laser with a tunning range between 9.2-10.8 µm and far infrared laser pumped by Q-switch laser.
As a source of terahertz electric fields a high power tunable pulsed far-infrared molecular laser pumped by a TEA CO2-laser has been used [1]. Strong single line emission has been achieved in the wavelength range from 30 to 1500 µm applying NH3, CH3F, and D2O as laser active media. In Table1 the characteristics of these lines are listed together with lines of the TEA CO2 laser which are used for pumping. The most sophisticated element in the laser system is the pumping laser. While in the past we used for this purporse self-made high-power pulsed TEA-CO2 laser with a high level of suppression of electromagnetic interference, at present the a high-stability, high-power commercial TEA CO2 lasers (URANIT 104, 204) are available.
Table 1: Characteristics of the far-infrared laser lines.
Wavelength |
Frequency |
Quantum energy |
Line of CO2 |
Max. intensity |
Medium |
(µm) |
w (1012 s-1)
|
(h/2p)w (meV) |
pump laser |
(kW/cm2) |
|
|
28
|
68 |
43.4 |
500 |
NH3 |
|
35 |
54 |
34.7 |
500 |
NH3 |
|
76 |
25 |
16 |
10P(26) |
4000 |
NH3 |
90.5 |
21 |
14 |
9R(16) |
5000 |
NH3 |
148 |
13 |
8.5 |
9P(36) |
4500 |
NH3 |
250 |
7.5 |
4.9 |
9R(26) |
400 |
NH3 |
280 |
6.7 |
4.4 |
10R(8) |
1000 |
NH3 |
385 |
4.9 |
3.2 |
9R(22) |
5 |
D2O |
496 |
3.8 |
2.5 |
9R(20) |
10 |
CH3F |
The photon energies corresponding to the wavelengths in the MIR lie between 100 and 150 meV, and in the FIR between 35 and 1 meV. The radiation pulse length varies for different lines from 10 to 100 ns. The radiated power was up to about 100 kW in the frequency w range from 5·1012 s-1 to 50·1012 s-1. The radiation could be focused to a spot of about 1 mm2, with the maximum intensity reaching as high as 5 MW/cm2 which corresponds to an electric field of about 40 kV/cm inside the semiconductor samples. .
The peak intensity of each single laser pulse could be monitored with fast noncooled photodetectors based on the photon drag effect [2], on intraband µ-photoconductivity [3], or on stimulated tunneling effect in metal/semiconductor structures under plasma reflection [4] (see also the advertisement of the A.F.Ioffe Institute and the firma ARTAS [5]). The shape of the laser beam and the spatial distribution of laser radiation were controlled with a Spiricon pyroelectric camera. The strengths and the distribution of the FIR laser lines applied in this investigation as well as an image of the laser beam can be found in [1].
References