Researchers in the US have generated coherent terahertz pulses by frequency-mixing two carbon-dioxide (CO2) laser beams in a nonlinear crystal. The pulses were generated at 328.2 µm with an average output power of 260 µW and a repetition rate of 60 kHz. The corresponding peak power is 510 mW. (Applied Physics Letters 91 091108)
¡°This is the first report of high-power terahertz generation based on frequency-mixing two CO2 lasers in a bulk nonlinear crystal,¡± Yujie Ding, a researcher at Lehigh University, told optics.org. ¡°Our aim was to improve the average output powers of terahertz sources at relatively low frequencies where other approaches are either unreachable or inefficient.¡±
The team plans to use the terahertz output to measure the absorption of gases at very low pressures. ¡°Since the terahertz output is highly coherent, the pulses can propagate back and forth through the gases multiple times, which significantly improves detection,¡± explained Ding. ¡°We will also work on terahertz imaging of remote objects.¡±
CO2 lasers may not be the obvious first choice for practical terahertz generation, however Ding believes that they have many advantages. ¡°These lasers are robust, highly compact and rugged,¡± commented Ding. ¡°The conversion efficiency for converting each CO2 laser photon into a terahertz photon is one order of magnitude higher than using distributed feedback lasers. In addition, quantum cascade lasers cannot produce such a low frequency.¡±
The terahertz pulses were generated by frequency-mixing two CO2 lasers, one emitting at 10.59 µm and the other at 10.26 µm. The maximum output peak powers delivered by these lasers were 11.9 kW and 11.1 kW respectively. The team focused the two beams into GaSe crystals with lengths varying between 2 and 47 mm.
The electric fields from the two CO2 lasers at two different frequencies produce a nonlinear polarization at the difference frequency (terahertz),¡± explained Ding. ¡°As the two CO2 lasers propagate in the GaSe crystal, the terahertz radiation driven by the propagating nonlinear polarization is spatially accumulated to produce the highest output power.¡±
The team eventually hopes to achieve average terahertz output powers of a few milliwatts.