Terahertz Waves - Between Electronics and Optics
Terahertz radiation is a part of the electromagnetic spectrum located between the microwaves and the infrared. The word "tera" is a prefix, indicating the large number of 10^12 hertz for the frequency. They cover the wavelengths between 30 µm and 3 mm. These terahertz (THz) waves have some unique properties which are not yet fully explored. Applying state-of-the-art optic techniques it is possible to efficiently generate and sensitively detect THz radiation. This possibility opens new fields in science and for industrial applications.
Introduction and overview
The THz spectral range covers frequencies between 100 GHz and 10 THz. It was formally known as the far-infrared. Historically, there were no man-made sources in this frequency range, only thermal emitters. Approaching the THz gap from the microwave side, there are sources around 90 GHz whose range is to be extended towards higher frequencies. On the high frequency end of the THz spectrum, there are powerful lasers in the infrared. So from a technological point of view the THz frequency band is sandwiched between electronics and optics.
Applying modern optical equipment, it is possible to generate and detect THz radiation. These methods rely on ultrafast pulsed lasers and nonlinear optics. The widespread availability of femtosecond (fs) lasers accelerated the progress in this field. Together with the method of time-domain spectroscopy (TDS), a powerful tool for the coherent detection of THz radiation is available even at room temperature. Most of today’s material investigations in the THz range are carried out by using THz-TDS. But also other techniques like the two-color-mixing of continuous wave (cw) lasers or frequency multiplying of electronic sources are available. The development of THz components is an ongoing process, leading to cheaper, more reliable and integrated solutions. New industrial applications are within reach.
On the long wavelength side, millimeter and microwaves are used for imaging applications. They are well elaborate techniques used to investigate dielectrics like plastics, clothes, ceramics or fibre-reinforced composites. Despite their high transparency in the microwaves, the probing spot size is big and hence the lateral resolution poor. Resolvable structures are typically in the centimeter range. There are no spectral absorptions of molecules reported in this frequency window. So an identification of substances such as pharmaceutics, drugs or explosives is not possible.
On the other end of the THz wavelength range, the infrared radiation is known for its highly specific absorption features. Characteristic transitions of various molecules can be probed. But due to attenuation effects like scattering losses or absorptions in packaging materials, infrared radiation cannot be applied at all experimental conditions.
THz radiation unifies the advantages of both adjacent frequency ranges, i.e. the high transparency of the dielectrics along with the spectral selectivity. This combination makes the THz waves particularly interesting and unique in the electromagnetic spectrum. For example in processes like quality inspection and in security applications, these possibilities attract a lot of interest. One major advantage of THz radiation is that the photon energy is very low, so they are non-ionizing. Unlike x-rays, which are also employed in various imaging applications, THz waves pose no health risk. No protective measures have to be taken, which is an important criterion for the implementation of a new technology.
Metals as well as conducting surfaces are perfect reflectors in the THz range. Polar liquids, for example water, strongly absorb THz waves. The penetration depth in aqueous substances is very limited. Also a high air moisture level impedes a broadband THz measurement over a long distance (>10 m). In reverse, THz waves can sensitively detect the residual moisture content of hydrophilic samples.
Terahertz Time-Domain Spectroscopy
The method of THz time-domain spectroscopy (THz-TDS) has led to a prospering future for THz technology. It is based on the pump-probe principle. The pumping laser pulse is split into two fractions, one to excite the transmitter and one to gate the receiver. The relative time delay between these two laser pulses is used to sample the THz transient. It is a coherent detection process, sampling the electric field. So the amplitude and phase is directly accessible. After a Fourier transformation of the time-domain signal, the frequency resolved amplitude is available. The ultra-short THz pulses have a broadband spectrum which is very useful for the investigation of all kind of samples. Due to the coherent detection scheme, a high signal-to-noise ratio of typically better than 1000:1 is obtained, despite the low output power of the THz system.
In a standard measurement geometry, the generated THz pulse is reflected at a surface or transmitted through a sample. This modified pulse is compared to a reference pulse without the sample for transmission geometry, or to the reflected pulse of a bare metal in reflection geometry, respectively. Evaluated quantities are amplitude, delay and spectral signatures. Changes in amplitude are related to sample properties such as porosity, absorption, thickness and homogeneity. The optical thickness (product of refractive index n times the geometric thickness d) is responsible for the observed delay. Multiple reflections within the sample may cause echo modulations for longer timescales, depending on the sample thickness. The field oscillations after the main peak contain the spectral information of the sample. A Fourier transformation connects the time traces to frequency dependent amplitude and phase. Here the presented data show the absorption lines of moisture in ambient air.
Generation and Detection of THz Radiation
The development of THz transmitters and receivers is a key topic in THz research. These components are needed to explore new applications. The progress to make the components more efficient, more compact and cheaper is important to find new applications. This development started in the 1990s with the invention of new types of THz sources. These can be roughly divided into two different classes, the ones emitting continuous wave THz radiation within a narrow frequency spectrum and the pulsed ones having a large bandwidth.
Continuous wave (cw) THz radiation can be generated by either an electronic or optic approach. The electronic sources have a large potential to become cheap and compact devices, but their output frequency range is typically located in the low frequency end of the THz spectrum. The higher their frequency gets, the lower is the output power. Optic sources on the other hand are, for example, THz gas lasers, frequency mixing in semiconductors or laser pumped nonlinear crystals. The latter ones explore nonlinear processes based on the generation of the difference frequency between two lasers in the visible or near infrared. One problem of cw THz radiation is the detection process.
The pulsed THz sources have the advantage that the detector has to be sensitive only for a short timescale. During the off-time of the receiver no further noise is integrated. Typical duty cycles are 10000:1. Pulsed THz transmitters are mostly gated by femtosecond laser pulses. The THz frequencies are generated and detected either by electro-optic conversion in nonlinear materials (dielectric crystals, semiconductor surfaces) or by photoconductive switches. These are the components which started the ongoing interest in THz in the 1990s. Still today, they are widespread and often used as they are powerful emitters and detectors for THz radiation. They will be introduced here in more detail.
A typical broadband THz pulse is a single-cycle pulse with a duration of 1 ps. This transient can be sampled as function of delay time. The detected wave form is transferred into the frequency domain by a Fourier transformation. Using pump pulses of 100 fs duration a broadband THz spectrum between 100 GHz and 4 THz is detectable. Femtosecond-based time-domain spectroscopy has the substantial advantage that the phase of the THz pulse can be precisely determined, even absolutely and not only modulo 360°. Thus, the measured delay can be directly used to measure the refractive index or the thickness of a sample, which is a very useful type of data evaluation in THz imaging.
Photoconductive antennae are metalized structures on top of a semiconductor with high ohmic dark resistivity. In the center of the metallization, a gap is kept in the antenna which isolates the two electrodes with respect the each other. If the photoconductive antenna is used as transmitter, the electrodes are biased resulting in an electric field of 106 V/m within the gap. But still no current flows If now a laser pulse illuminates the gap, free charge carriers are injected and a current transient occurs between the electrodes. It flows as long as there are free charge carriers. This period of time is determined by the laser pulse duration and the carrier lifetime of the semiconductor. This current peak in the antenna leads to an emission of an electromagnetic pulse, whose maximum frequency is given by the inverse of the time constant. The spectral shape and emission characteristics can be further designed by the shape of the antenna.
To keep the carrier lifetime as short as possible special semiconductors are used such as at low temperature grown gallium arsenide (lt-GaAs) or radiation damaged silicon on sapphire (rd-SOS). To a certain extent photoconductive antennae are transistors which are gated by laser pulses and not via electronics. The emitted THz beam is collimated by using silicon lenses mounted in direct contact with the antenna.
The detection schemes can be divided into incoherent and coherent detection. The incoherent detection just measures the THz power of the absorbed radiation. Using the coherent detection the electric field including the phase is sampled which leads to an increased sensitivity.
Photoconductive antenna can be used for the detection in a similar manner as for the generation of THz radiation. Just the electric source of the bias is replaced by a sensitive ammeter. A current is only present is the photo-excited charge carriers are present and accelerated by an electric field at the same time. The accelerating field is the incident THz pulse. Due to sample losses the detected pulse has a longer pulse duration than the emitted one. So for sure the laser pulse is short enough to sample this pulse, too. Only the electric field which overlaps with the active period of the detector contributes to the current. By shifting the relative delay between the THz pulse and the laser pulse, different parts of the THz pulse can be scanned. So the waveform is sampled stepwise. The delay is caused by a mechanical movement of a retro mirror in the laser beam. This may either be in the transmitter or receiver arm. A temporal accuracy of approximately 10 fs is obtained by using a linear stage with a positioning accuracy better than 1 µm. The travel range can reach up to 10 cm depending on the required frequency resolution. For molecular gases absorption lines as narrow as 1 GHz were resolved.
Another method for the coherent detection is electro-optic sampling. This scheme explores the birefringence in an appropriate crystal induced by an external electric field (Pockels effect). The refractive index ellipsoid of the crystal is changed by the THz pulse. If now the laser pulse overlaps with the induced birefringence in space and time, the probing laser pulse polarization will be altered. Among a variety of electro-optics crystals, zinc telluride (ZnTe) und gallium phosphide (GaP) are most often used. The sensitivity and detection bandwidth depend on the crystal thickness.
The components used in a THz-TDS system are shown in Fig. 6. It shows the laser, the delay line as well as the photoconductive transmitter and receiver. The pumping laser pulses in the transmitter arm are often modulated to reduce the noise level by using the lock-in technique.
The THz radiation is focused into the detector using parabolic mirrors. Additional lenses are typically avoided not to disturb the pulse shape by causing further modulations due to the etalon effect at the surfaces. If a small THz spot is needed a further set of parabolic mirrors produces a focus. A simple apparatus for THz imaging is obtained where the sample is moved through the focus.
Applications of THz radiation
Various feasibility studies investigating the potential applications of THz radiation have already been carried out so far. The major topics cover nondestructive testing, medicine/biology and security applications. Chemical compounds can be analyzed by exciting rotational and vibrational modes in the THz range. Investigated samples include semiconductors, polymers, bio-molecules, pharmaceuticals, drugs and explosives. For industrial applications imaging capabilities are needed as samples have to be investigated at their entire area. Besides raster scanning imaging systems also multi-pixel sensors such as antenna arrays or CCD cameras with corresponding electro-optic crystals are used.
THz radiation can uncover hidden objects. This capability makes the THz frequency range interesting for security applications. Illicit items like explosives or drugs can be detected even underneath of clothes or within non-metallic packages. For security check points and surveillance applications two different approaches are taken: The passive and the active systems. The passive ones use the emitted natural (inartificial) radiation of a body. The THz fraction of the emitted thermal radiation is detected in microbolometers. No spectral information is accessible. These systems do not require an illumination source and hence do not have any radiation safety issues, but also have a weak signal strength. On the other hand, the active systems use an additional artificial THz illumination. The recorded signals are much higher, especially if the direct reflection is detected, and not only the scattered signal. For spectroscopic investigations a high dynamic range of the THz system is necessary. Spectroscopic investigations can be carried out using broadband pulses or by tuning the illumination wavelength. This technique is used to inspect individuals as well as packages and mail.
The availability of a simple detection scheme with simultaneous identification of bio-molecules (DNA, polynucleotide, genes, proteins) is the key requirement for biotechnology. Current investigations are dedicated to the marker-free identification which would remove one major step in sample preparation. The knowledge and understanding of complex molecules can be extended by using THz data not available in standard FTIR measurements.
The field of nondestructive testing covers various industrial fields. The spectroscopic identification is of particular interest for chemical and pharmaceutical applications. In literature results on polymorphism and the concentration determination of active ingredients is reported. But THz radiation can even investigate the body of a transparent sample. Ceramics and plastics can be easily investigated. Gaps, intrusions, delimitations and substance distribution (additives, filler materials, flame retardants) can be detected. Also fibre-reinforced plastics can be tested. A prominent application is the investigation of a radar dome looking for delimitations or water inclusions. All types of products, from tablets to aeronautical spare parts, are covered with functional coatings which can be investigated by using THz radiation. Not only the coating thickness can be determined even the substance distribution can be measured. Plastic parts which are common in modern systems were successfully checked. Also food production industries have testing requirements. For them metal detectors are the standard techniques to find metallic contaminants. But non-metallic pieces can only be found by using x-rays up to now. This is a potential application for THz radiation, especially as it is non ionizing and easy to implement.