Terahertz Radiation Sources for Imaging and Sensing Applications

Eric R. Mueller, Coherent Inc.

Electromagnetic radiation in the terahertz (10¹² Hz) frequency domain has unique properties that make it particularly attractive for applications ranging from biomedical imaging, national security and packaged goods inspection to remote sensing and spectroscopy. Unfortunately, however, generating terahertz radiation at any meaningful power level presents many practical hurdles.

But in recent years, a number of methods for generating both continuous-wave and pulsed terahertz radiation have been successfully developed. In turn, these are spawning the early development of terahertz applications, particularly in nondestructive imaging and atmospheric sensing.

Why terahertz radiation?

The terahertz portion of the electromagnetic spectrum lies between microwaves and the infrared. Typically, it is loosely defined to extend from about 0.3 THz (a wavelength of 1 mm) to around 10 THz (a wavelength of 30 μm). There are several reasons for the fast-growing interest in this spectral region. First, terahertz waves can penetrate a wide variety of nonconducting materials. They can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. But they cannot penetrate metal and are strongly attenuated in water.

When terahertz waves pass through any of these materials, their photon energy causes them to interact with the material via a number of mechanisms. These mechanisms often involve phonons, weak material bond vibrations and deformations, and thus are unique to the exact chemical and physical composition of the material. As a result, any internal variations in thickness, density and/or chemical composition impart information to the terahertz signal in the form of intensity (absorption) and phase (refractive index) variations.

This information can be used to reconstruct two- and three-dimensional internal images of objects that are opaque to visible or infrared light (Figure 1). Additionally, measuring absorption as a function of wavelength enables spectroscopic imaging to map chemical composition.

Figure 1. Terahertz radiation can be used to image a wide variety of materials and composite objects. For example, it can reveal information about the brain of a mouse. The optical image is on the top, terahertz amplitude transmitted through the brain is in the middle, and the terahertz phase change through the brain, bottom. Courtesy of Peter Siegel.

Why not use more readily generated microwaves for these imaging applications? First, microwaves interact only weakly with these materials, so the signal-to-noise ratio would be much lower. Second, without high-loss and cumbersome near-field imaging techniques, the lateral resolution in these images is ultimately limited by diffraction to about half the illumination wavelength.

Thus, the shorter-wavelength terahertz radiation delivers lateral (X-Y-axis) image resolution on the order of tens or hundreds of microns. This is much smaller than microwaves and more than sufficient for many real-world applications. Also, in 3-D terahertz imaging, phase/time-delay information can yield depth (Z-axis) resolution at least an order of magnitude better than the lateral resolution.

Another advantage of terahertz radiation is that it is nonionizing, with photon energies four orders of magnitude smaller than x-rays. This means that it cannot damage DNA, making it safe for human applications such as medical imaging.

Because these wavelengths are absorbed by water, terahertz imaging is very sensitive to the density of living tissue. Potential medical imaging applications include the detection of epithelial cancer and wound inspection through bandages. Moreover, some frequencies can be used for 3-D imaging of teeth and could be more accurate and safer than conventional dental x-rays.

Because terahertz radiation can penetrate fabrics and plastics, it also can be used in surveillance to remotely detect concealed weapons on a person. Some researchers have already used selective wavelength absorption as the basis for terahertz spectral imaging. This is of particular interest for security because many materials of interest, such as plastic explosives, exhibit spectral fingerprints in the terahertz range.

Terahertz amplitude (left) and phase (center) images of a Plumbago auriculata flower reveal features not seen in the visible light image (right). Courtesy of Peter Siegel, California Institute of Technology/Jet Propulsion Laboratory.

Many potential applications for terahertz imaging have been proposed in manufacturing, quality control and process monitoring. These generally exploit the fact that plastics and cardboard are transparent to terahertz radiation, so that packaged objects can be inspected for either size or chemical composition.

Generating radiation

Figure 2 shows an extended version of the photonic spectrum band, along with the typical thermal emission spectrum of a blackbody source. Higher frequencies (i.e., shorter wavelengths such as ultraviolet, visible and infrared) can be generated by direct emission sources. These include high-temperature blackbody emitters such as filament lamps, fluorescent sources and, of course, lasers. In contrast, it is difficult to create useful terahertz power levels with a blackbody source. (It is true that blackbody sources have been used as terahertz sources for many years in Fourier transform spectroscopy, but they generate about 1 nW per wave number.)

Figure 2. In the electromagnetic spectrum, terahertz radiation is sandwiched between the infrared and microwave regions (inset). A thermal (blackbody) emitter generates very low intensity at long wavelengths, including the terahertz and microwave spectral regions.

On the other hand, many of the traditional methods for generating longer wavelengths, such as radio-frequency and microwaves, rely on accelerating and decelerating electrons with an electrical field that undergoes rapid and periodic oscillation. They are thus dependent on high-speed electronic circuitry to switch the field, which becomes nearly impossible at terahertz rates. And typical semiconductor methods are not practical at terahertz frequencies because of the plethora of nonradiative mechanisms at these photon energies.

Terahertz sources

The table compares some of the most popular techniques for generating terahertz radiation. At present, only optically pumped terahertz lasers, time-domain system sources and direct multiplied sources are commercially available. (For completeness, electronic methods are included in the table, but not otherwise discussed.)

The choice of a terahertz source also will determine the type of detection scheme required. In particular, weaker sources with submilliwatt output powers complicate detection, often necessitating the use of liquid-helium-cooled bolometers or similar devices — or time-domain gating and fast Fourier transform postprocessing in the case of time-domain systems.

Direct laser methods

Because of the characteristic spectral profile of a thermal emitter (Figure 2), the only direct emission methods for producing terahertz waves use a non-Boltzmann source, involving laser technology. Four principal laser methods are at various stages of development and investigation: difference frequency mixing, terahertz parametric oscillators, molecular gas lasers and quantum cascade lasers. Of these, the molecular gas laser, often called an optically pumped terahertz laser, generates the highest power and is at the forefront of CW imaging applications development.

In its simplest embodiment, an optically pumped terahertz laser system comprises a grating-tuned CO₂ pump laser operating on a single output line, together with a terahertz gas cell mounted in a laser resonator (Figure 3). The pump beam enters the cell through an aperture in the high-reflecting resonator mirror —typically a simple metal mirror. The pump laser is tuned to the appropriate molecular vibrational absorption band, and lasing occurs between two rotational levels of the excited vibration state.

Figure 3. The optically pumped terahertz laser is one terahertz source under investigation.

First developed for military and potential communications applications, CW optically pumped terahertz lasers can be based on a number of small organic molecules, such as methanol, difluoromethane or vinyl chloride, each offering tens to hundreds of different emission lines. In addition, isotopic substitutions can further multiply the number of available lines.

Historically, individual research groups built their own optically pumped terahertz lasers, but these systems typically were very large and extremely difficult to use and maintain. The main challenge is maintaining a stable output wavelength and power from the CO₂ pump laser, along with a constant cavity length in the terahertz resonator. Today, commercial laser systems are much smaller, turnkey and reliable. A key to these improvements has been the development of permanently sealed, single-mode, frequency-stabilized, folded-cavity, radio-frequency-excited, waveguide CO₂ lasers.

Just as important has been the development of sealed terahertz gas cells that eliminate gas-transport issues, as well as passive resonator structures that are inherently stable, coupled with automated feedback stabilization technologies. As a result, the optically pumped terahertz laser can be optimized to deliver TEM₀₀ output in a single longitudinal mode, with a typical 30-second integrated linewidth of <50 kHz at 2 THz (150 μm). Over the majority of the 1- to 5-THz range, the output is greater than 50 mW and can exceed 100 mW at select lines.

One of the more interesting sources to be demonstrated in recent years is the terahertz quantum cascade laser. This is a bandgap-engineered semiconductor laser where the widths of the semiconductor quantum wells (controlled by the growth process) determine the operating frequency.
At this point, these lasers produce their most interesting powers at liquid-helium temperatures, but the upper operating temperatures and powers continue to be pushed higher.¹

It is not clear whether this technology will ever support room-temperature operation, but as its operating point is being steadily increased by researchers, its potential usefulness for many applications will inevitably increase. Indeed, the quantum cascade laser may well become an important terahertz source.

Photomixing (difference-frequency generation) is another direct-laser method that has been known for many years but has never gone beyond the research lab. This is, in part, because of its complexity and also because it generates very low power compared with an optically pumped terahertz laser. Here, two closely spaced near-IR laser beams are mixed in a low-temperature-grown GaAs crystal. Under favorable conditions, photomixing can produce CW terahertz radiation at the 10-nW level. The monochromatic output can be tuned by adjusting the input laser wavelengths.

Another nonlinear laser-based method is the terahertz parametric oscillator. In this method, a high-energy pulsed laser, such as a nanosecond Q-switched Nd:YAG, pumps a parametric oscillator that generates pulsed terahertz output. The technique relies on an optical parametric down-conversion involving lattice vibrations in materials such as LiNbO₃ or MgO-doped LiNbO₃. The process is an extension of the optical parametric oscillator (OPO) often used to generate tunable mid-IR. Unlike the infrared OPO, the equivalent terahertz process is inefficient, with pulse energies less than 1 nJ generated from hundreds of millijoules of 1.06-μm pump radiation.

Laser-enabled method

The most prominent laser-enabled approach uses a dipole antenna constructed from a near-IR absorbing semiconductor material, together with an ultrafast laser. Called a time-domain system, this hardware eliminates the need for high-speed electronic circuitry. Specifically, an incident ultrafast laser pulse is tightly focused into this switch, which is pre-biased with an external voltage supply. Absorption of the pulse creates a massive but transient population of charge carriers, allowing current to flow across the antenna. Because of fast natural recombination of electrons and holes, this current is short-lived.

This device emits a burst of electromagnetic radiation with a pulse shape that is dependent on the laser pulse. If the pulse is in the picosecond or 100-fs range, the resulting electromagnetic waveform is in the terahertz regime. The pulse is intense but with a fairly broad spectral bandwidth given by the Fourier transform of the carrier rise and decay profile. Figure 4 illustrates the basics of this time-domain system source, including the high-numerical-aperture (hemispherical) lens that collimates the terahertz output into a useful beam.

Figure 4. A time-domain system switch relies on an ultrafast laser pulse to create a short-lived intense population of charge carriers in an antenna.

The output of this device is a train of pulses whose repetition frequency is the same as that of the femtosecond pump laser; i.e., 75 to 80 MHz for typical commercial ultrafast lasers. Pulse widths are less than 1 ps, with average powers on the order of microwatts and a bandwidth of >500 GHz (FWHM). The bandwidth is often centered below 1 THz (Figure 5), but this can vary significantly, depending upon the design of the switch as well as the pumping characteristics.

Figure 5. The typical spectral content of a time-domain terahertz source is shown here.

Recently, researchers have produced measurable output at frequencies up to 30 THz with a time-domain system setup,² although the power spectral density is commensurately lower because the total average power is spread over a wider bandwidth.

Sensing and spectroscopy

Two key applications for terahertz radiation are remote sensing using optically pumped laser sources and time-domain imaging and spectroscopy using time-domain system sources. In fact, commercial optically pumped terahertz lasers are used around the world, primarily for astronomy, environmental monitoring and plasma diagnostics. For example, the Coherent SIFIR-50 system that was installed at the Antarctic Submillimeter Telescope and Remote Observatory facility at the South Pole is the local oscillator for a heterodyne terahertz receiver, where it measured interstellar singly ionized nitrogen and carbon monoxide during the polar winter.

On July 15, 2004, a Coherent optically pumped terahertz laser operating at 2.5 THz took a ride on a Delta rocket into space aboard NASA’s Aura satellite, where it has been measuring the concentration and distribution of the hydroxyl (OH) radical in the stratosphere, a critical component in the ozone cycle. (Until this mission, there was no global data for OH concentrations; only spot measurements that had been made by high-altitude balloon — using an optically pumped terahertz laser system.) The Aura laser system is less than 0.8 ft³ (0.2 m³) in volume, has a mass of less than 22 kg and consumes 120 W of power (Figure 6).

Figure 6. The Aura 2.5-THz local oscillator is monitoring OH concentrations in the stratosphere onboard a NASA satellite.

In 1995, Binbin Hu and Martin Nuss at Bell Labs created a terahertz imaging system using time-domain spectroscopy, and they coined the term T-ray for these short, broadband terahertz pulses.³ They showed that the absorption characteristics for terahertz radiation vary greatly from material to material, and that this characteristic can be used to create images. Moreover, because the T-ray pulse is so short, it can be used in a reflective, time-of-flight mode to create three-dimensional transparent reconstructions of various objects by measuring the time between pulses reflected from different areas within the object.

In a typical time-domain spectroscopy setup, the terahertz pulse is distorted by wavelength-selective absorption as it passes through a sample (Figure 7). The transmitted beam is then focused onto a detector, which is virtually identical to the emitter, except that it is has no external bias. At the detector, simultaneous irradiation by the terahertz field and a part of the original excitation laser pulse induces a fast transient photovoltaic signal.

Figure 7. This schematic illustration shows a typical time-domain system (TDS) experimental setup.

This signal pulse is integrated by the system electronics. By measuring the integrated electronic signal intensity as a function of the delay in the laser pulse beam path, the complete temporal profile of the terahertz pulse is derived. This data is then processed by fast Fourier transform analysis to yield the spectral content (i.e., frequency domain) of the terahertz signal present at the detector.

Many investigators create images with this type of setup by raster scanning through a sample and selecting a particular portion of the resulting spectrum to display at each point of the 2-D image. The richness of the data also supports many more sophisticated imaging techniques, including direct 3-D images and computed tomography.⁴

Meet the author

Eric R. Mueller is director of engineering at Coherent Inc. in Bloomfield, Conn.; e-mail: eric.mueller@coherent.com.

References

1. B.S. Williams et al (2006). High-power terahertz quantum-cascade lasers. ELECTRON LETT, Vol. 42, pp. 89-91.

2. P.Y. Han and X.-C. Zhang (1998). Coherent broadband midinfrared terahertz beam sensors. APPL PHYS LETT, Vol. 73, pp. 3049-3051.

3. B.B. Hu and M.C. Nuss (1995). Imaging with terahertz waves. OPT LETT, Vol. 20, pp. 1716-1719.

4. S. Wang and X.-C. Zhang (2004). Pulsed terahertz tomography. J PHYS D: APPL PHYS, Vol. 37, pp. R1-R36.