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21 Jun 2007
Although many applications could potentially benefit from terahertz radiation, finding a convenient way to generate it has been problematic. Anselm Deninger and Thomas Renner of TOPTICA Photonics weigh up the merits of using distributed feedback diodes and femtosecond fibre lasers.
The terahertz (THz) region is the last "final frontier" of the electromagnetic spectrum. Over the last few years, in-depth THz research has opened our eyes to new opportunities in medicine, security screening and material science. The high carrier frequency also has great potential for high-speed wireless communication.
Unfortunately, generating intensive, directional THz radiation has been difficult, as THz waves are too long for direct optical techniques but too short for electronic devices. Recent work has shown us that indirect approaches based on tunable diode lasers or femtosecond lasers could be the key to finally closing the THz gap (see table for a comparison of approaches).
Spectral properties
THz radiation covers the frequency range from 300 GHz to 10 THz (equivalent to wavelengths between 1 mm and 30 µm) and sits between the far infrared and microwaves. In this spectral range, electromagnetic radiation is strongly absorbed by molecular rotation and libration transitions. Depending on the size of the molecules, these chemically sensitive absorption lines are found in the high-frequency (small molecules) or low-frequency (macromolecules) range of the THz spectrum.
Water is the most dominant absorber and, depending on the application, can turn out to be a blessing or a curse. Air moisture, for example, limits the propagation of THz rays to a few metres, but on the other hand, the characteristic "fingerprint" allows high-precision humidity-sensitive measurements to be made.
The second characteristic feature of THz radiation is scattering, and in turn penetration depth. Similar to X-rays, THz waves pass through numerous amorphous substances – but without any ionizing effects. The ability to penetrate clothing, paper or plastics, combined with spectral sensitivity, makes THz radiation interesting for a multitude of spectroscopic and imaging applications.
Terahertz generation
There is no straightforward optical or electronic way to generate THz rays. Lasers that emit THz radiation directly must counter the thermal population of the upper lasing level and usually require cryogenic temperatures. Quantum cascade semiconductors and CO2 laser-pumped gas lasers can provide discrete lines in the upper THz frequency range but still struggle with low THz frequencies and wavelength tunability.
Electronic devices offer attractive output power levels at frequencies up to around 100 GHz, but higher frequencies are only reached with frequency multipliers and considerable power losses. Frequency tuning, if possible, only spans a limited range.
Today, the spectroscopically important frequencies between 0.5 and 5 THz are reached by indirect, laser-based techniques. The two most common approaches rely on fast modulation of the photocurrent in semiconductor antennas, or on photoconductive switches that are irradiated with ultrashort pulsed lasers. Both of these methods use lasers in the visible or near-infrared spectral range: tunable continuous-wave (CW) diode lasers in the former case, and femtosecond fibre lasers in the latter.
Tunable CW THz radiation
Tunable CW THz radiation can be generated by difference frequency mixing two lasers with adjacent wavelengths. The beams are superimposed on a semiconductor antenna structure, such as gallium arsenide (GaAs), which acts as a THz emitter. The two-colour laser beam generates a photocurrent in the semiconductor, modulated at the difference frequency of the lasers. This beat signal, in turn, induces an electromagnetic wave at the THz difference frequency.
Here, the lasers of choice are distributed feedback (DFB) diode lasers. DFB diodes comprise a grating structure integrated within the active section of the semiconductor. The grating restricts the laser's emission to a single longitudinal mode and shifting the grating pitch (either thermally or electrically) tunes the wavelength. Thermal tuning can produce wide continuous frequency scans in excess of 1000 GHz.
DFBs emitting between 850 and 860 nm are particularly useful for generating tunable THz radiation. The main benefits are the source's high output power, wide frequency tuning and an emission wavelength below the bandgap of GaAs. Combining central wavelengths of 853 and 855 nm results in a THz tuning range from 0 to 2 THz. Diodes at 855 and 860 nm can deliver a difference frequency range between 0.6 and 2.9 THz. These THz frequencies cover the absorption signatures of a wide range of industrially relevant gases, chemical agents and explosives. Figure 1 shows an illustrative realization of a two-colour laser system.
The optical power of such a state-of-the-art laser system is around 75 mW per two-colour fibre output. Modern photo-mixers convert this into between 50 and 1000 nW of THz power, which is usually enough for spectroscopic applications. THz imaging, albeit not in real time, has also been demonstrated successfully with these laser systems.
If an experiment requires higher power levels (and the damage threshold of the photomixer tolerates it), the set-up in figure 1 can be changed to a master-oscillator power-amplifier configuration. Here, both DFB lasers are simultaneously coupled into a semiconductor amplifier, which emits a near-diffraction-limited output beam with 500–1000 mW power. The tuning range and spectral properties of the DFB lasers remain unaltered.
Certain measurement techniques for precise detection and quantification of hazardous gases require exact control of the THz frequency. On the laser side, this calls for accurate stabilization of the emission frequency. TOPTICA has developed a locking scheme based on a quadrature interferometer, which permits a highly precise, computerized adjustment of the THz frequency to a resolution of 1 MHz.
Ultrafast broadband THz radiation
Pulsed THz waves are obtained using a combination of femtosecond lasers and a semiconductor antenna that acts as a THz emitter. The incident femtosecond pulse generates free-charge carriers, which may be accelerated by internal or external electric fields. The current onset and switch-off induces a transient, spectrally broad electromagnetic field. Putting this in numbers, a 100 fs pulse in the near-infrared range corresponds to a spectral width of 4–5 THz.
The most established emitter technology uses GaAs antennas, which are excited around 800 nm. There are however many other approaches, some of which also function in the telecoms wavelength range around 1550 nm.
One option is an ultrafast fibre laser as it is compact, inexpensive and reliable. Femtosecond lasers based on Erbium-doped glass fibres emit spectrally broad pulses at 1550 nm with durations of <100 fs and more than 250 mW average power. Second harmonic generation converts the fundamental wavelength into the sensitivity range of GaAs antennas. At 775 nm, today's femtosecond fibre lasers achieve a pulse width of 150 fs and an average power in excess of 100 mW, which is compatible with available antennas.
In an experiment carried out in collaboration with EKSPLA UAB of Lithuania, the signal-to-noise ratio (SNR) of THz spectra generated by a TOPTICA fibre laser was compared with a Ti:sapphire laser. Using an otherwise identical experimental set-up, we found that the fibre laser increased the SNR by a factor of 2.
Spoilt for choice
The demands of an individual experiment will dictate whether pulsed or CW THz radiation is necessary. Ultrafast THz radiation lends itself to time domain spectroscopy. Pulsed THz waves provide broadband spectral data within the shortest times and optical time-of-flight measurements reveal depth information of the sample.
CW THz, on the other hand, offers a higher spectral power and consequently, better frequency resolution. This method is the preferred choice for measuring narrow spectral signatures via frequency domain spectroscopy. The required lasers are also cheaper than currently available femtosecond sources.
Applications
Advances in THz generation techniques have paved the way for new applications, with the most promising fields being sensing and communication.
The ability of THz radiation to penetrate opaque substances and the associated spectral sensitivity are advantageous for security-related applications. The ultimate goal is to visualize concealed toxic or explosive substances that are hidden beneath clothing or within paper envelopes.
A related field is medical imaging, where THz radiation is being used to detect cancer. Unfortunately, the high water content of tissue limits the penetration depth to a few millimetres so current research is focusing on near-surface investigations such as skin examination and wound healing.
Industry is also using THz spectroscopy to study materials. For example, quality control in pharmaceuticals, semiconductor inspection and automotive production all harness THz radiation to detect material flaws in compound substances concealed, for instance, behind synthetics or thin layers of varnish.
Finally, the communication sector represents a major potential market for THz radiation. Today's research is targeting data transfer rates of 40 Gbit/s and it appears obvious that faster transfer rates will require higher carrier frequencies. A frequency of 100 GHz seems only a decade away, however there is still a lack of suitable transmittance lines. When it comes to free-space communication channels (satellite communication, hot spots for wireless download of high data videos), the technology requirements are already clear.
Conclusion
THz spectroscopy and communication are highly attractive cutting-edge technologies but today, the expectations still outweigh the technological feasibility. In particular, more development is required to produce powerful yet affordable THz sources. Frequency mixing using DFB diode lasers and femtosecond fibre lasers are promising advances and are currently being used in both research laboratories and initial industrial installations.
• This article originally appeared in the June 2007 issue of Optics & Laser Europe magazine.
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