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Sunday, May 1, 2011

CLEO/Laser Focus World Innovation Award endorses the recent triumph of Terahertz (THz) spectroscopy and applications.

This year, the award goes to Applied Research and Photonics Inc. for its endeavor in THz device and applications. This is indeed another sign saying that THz will be a hot topic for the following few years thanks to many people’s efforts over the past two decades. Besides, we are very happy to see this field has grown into a vibrant society with its own conference – Optical THz Spectroscopy and Technology (OTST). I was there, learned many news things from researchers all over the world, and enjoyed the nice breeze from the Pacific sea in Santa Barbara.

Thinking about THz, most of us immediately connect it with a couple of concepts, including the wavelength of it is long compared with the familiar optical and even mid-IR wavelength (a wavelength of 1 micron is 300 THz while 1 THz is 300 um), the property of great penetration to soft materials like tissues, plastics and, card boards, and its application in security screening due to the sensitivity of many explosives. In addition, its low energy and non-invasive feature is perfect for authenticity test on artwork and biomedical imaging. With this field burgeoning like never before, it is worth to take a quick look on the methodologies of generating THz.

The most orthodox way to create THz laser is to find a suitable gain media and pumping source. Just like a dye laser in which an electronic transition of dye is directly related to the lasing frequency, the transitions between the rotational states of methanol gas falls right into the THz region. A very good white paper using methanol and pumped by a CO2 laser can be found in here. Of course, the gain media is not restricted to methanol; even water vapor is actually a good THz source.

Mixing two lasers with the frequency difference in the THz regime is another neat way. Considering mixing two lasers in the optical fiber, the beating that is produced by the frequency difference of these two lasers is just the source of the THz. If we can filter out the pumping lasers, then we have a useful THz radiation. A more efficient way of doing so will be mixing two lasers in the nonlinear crystals (GaAs, LiNbO3, … etc). Depending on the nonlinearity of the crystal, frequency conversion is achieved with different efficiencies. In this case, the efficiency of THz generation is proportional to how strong the nonlinearity of the crystal is. This principle is adopted by Applied Research and Photonics Inc. to the extreme. The core of its THz device is based on a polymeric nanomaterial that has very strong nonlinearity.

How about generating ultrashort THz pulses to cover a wide THz spectrum at once. A very exciting way of doing so is through air plasma. The principle is quite straightforward although there are several variations of it (figure 1).  Basically, you create air plasma in the air (by focusing intense laser pulses in air), drift the electrons away from the nuclei through different bias methods, and then the nature law takes over. Since the electrons will recombine with the nuclei through coulombic force, this process creates a transient current (or a oscillatory dipole). An oscillatory dipole/transient current in this process creates radiation that covers THz regime. Before the advance of intense lasers, researchers focus laser beams onto photoconductive antenna chips to create transient current. Actually this is still the most popular way to generate ultrashort THz pulses.

Figure 1. THz generation based on air plasma. In all scenario, air plasma is generated by intense laser pulse (red). Electrons are accelerated in the direction of laser pulse . This process creates THz in a cone fashion.  Electrons are drifted away by external DC source (b), by another laser pulse (c), and by the laser pulse itself (d). Courtesy of Thomson M., Kreb M., Loffler T., and Roskos H. in Laser & Photon Rev. 1. No. 4. 349 (2007).

Just a reminder, on Monday, CLEO has an entire session focusing on TH Sources. Besides, the award presentation will be during the Plenary Session in the same day evening. This is definitely another power boost for you to learn how it goes in this field. Hopefully, next time when we encounter THz research, we all know a bit more in their business. You can also explore THz quantum-cascade lasers, THz generation by pulse front tilting in the crystal, and more through the power of the Internet…^_^.

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

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