An Introduction to Terahertz Spectroscopy- TOPTICA PHOTONICS

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Introduction

The superiority of optoelectronic techniques over alternative methods has extended the scope of terahertz applications beyond the laboratory into real-world applications. Terahertz radiation can be produced using a combination of two effects; one is the focusing of two laser beams of consecutive frequencies at a semi-conductor. The other is the seperation of photoconductive charge carriers using an untrafast laser.

When these two effects are integrated with antennas that are structured appropriately, electromagnetic radiation can be generated across notoriously difficult terahertz region. This article explains the principles of continuous-wave (CW) and pulsed terahertz sources, and explores a few emerging applications of these techniques.

The absence of suitable sources and detectors in the terahertz frequency range of the electromagnetic spectrum is termed as “terahertz gap”. However, the availability of advanced laser technologies that provide robust, cost- effective and compact sources for use in terahertz imaging and spectroscopy has began to eliminate this gap.

The intensity of terahertz radiation generated by these sources is suitable for both industrial and scientific measurements. The availability of terahertz sources enables a number of new applications such as material analysis, research, pharmaceutical process control, medicine, biology, and homeland security. Figure 1 shows how terahertz spectroscopy is used to detect hazardous substances contained in parcels or envelopes.

Figure 1. Terahertz spectroscopy enables the detection of hazardous substances in envelopes and parcels

Terahertz Spectrum

In the electromagnetic spectrum, terahertz radiation spans from 100 GHz to 10 THz or from 3mm to 30µm wavelengths. This range positions terahertz radiation between the far infrared and radio- or microwaves. This radiation is capable of penetrating through a number of opaque materials, particularly non-polar and non-metal materials such as plastics, paper and textiles.

A number of toxic agents and explosives are known to exhibit distinct absorption lines within the 0.5 to 5 THz range. Based on these characteristic absorption lines, a number of illicit drugs like ecstasy, opiates and cocaine can be detected and explosives like RDX, HMX and TNT present in envelopes can also be detected.

Terahertz spectroscopy can also be used for developing and producing pharmaceutical drugs by the identification of various polymorphs (crystalline forms) of the active component. Due to the ability of terahertz radiation to penetrate plastic packaging material to analyze pills it can be used in the detection of fake drugs.

This radiation can also be used for testing the quality of food that is in air-tight packages. A majority of the applications of terahertz radiation are based on its imaging capabilities. Mirrors and lenses can be used to focus terahertz waves. Image resolution in the order of mm is possible in terahertz-based scanning of a sample.

Higher depths of resolution, down to two orders of magnitude, may be achieved by suitably altering the method used. Unlike gamma rays, terahertz radiation does not exhibit an ionizing effect because their photon energies vary from 0.4 to 40 meV.

Due to this reason these frequencies are considered to be biologically innocuous. Due to the high reflectivity of metals they are not transparent to terahertz waves. The water that is always present in organic matter completely absorbs terahertz rays making organic matter opaque to this radiation.

Terahertz Sources

The challenge lies in generating terahertz radiation within spectroscopically relevant frequency ranges of 0.5 to 5THz. Direct access to this range is not possible using semiconductor lasers because semiconductors with a suitable band gap are not available.

The quantum-cascade-laser (QCL) is a direct optical source for this radiation and can be used for generating frequencies beyond 1THz under an operating temperature of 40K with cooling using a He cryostat. QCLs that use less complex nitrogen cooling can also be used but they are restricted to frequencies beyond 2THz.

By combining voltage-controlled oscillators (VCOs) with frequency multipliers, it is possible to acheive frequencies of several 100 GHz. However, this method is not efficient for terahertz production making it very expensive.

An efficient and economic method for generating terahertz is optoelectronics. Terahertz radiation is generated by focusing near-infrared laser light on a semiconductor, resulting in a photocurrent that is a source for terahertz waves (Figure 2). The two terahertz techniques are continuous-wave (CW), which produces narrow-band radiation, and the conversion of femtosecond pulses into radiation.

Figure 2. A photomixer with a spiral antenna converts laser light into terahertz radiation

CW Terahertz Techniques or Frequency-Domain terahertz

When two CW lasers whose wavelengths are adjacent (853 and 855 nm or 1546 and 1550 nm) are subjected to optical heterodyning on a dedicated antenna, terahertz radiation is generated at the frequency difference. A laser beam composed of two wavelengths is focused on a photomixer at the center of the antenna.

The photomixer is essentially a metal-semiconductor-metal structure. Free charge carriers are created in the semiconductor due to the incidence of the laser radiation, if a voltage is applied these charge carriers accelerate towards the metal electrodes.

This movement produces a photocurrent. The beat frequency is then converted into a new electromagnetic wave by the antenna. The perfect sources on the laser side are the DFBs (distributed feedback diodes), which can be tuned beyond 1000 GHz to produce distinct frequencies between 0 and 2 THz, and 1 and 3THz. A compact two-color CW laser is shown in Figure 3. There are two fiber-coupled DFBs at 1.5 µm, and a polarization-maintaining fiber combiner in this laser.

Figure 3. Compact two-color CW laser for terahertz generation

A narrow linewidth of 1MHz is produced by the CW terahertz systems resulting in a high spectral resolution that facilitates the measurement of narrow signatures of organic solid spectra or low pressure trace gases. Extremely precise absorption measurements of the plastic explosive RDX and a-lactose are shown in Figure 4.

Figure 4. Absorption measurements of a-lactose monohydtrate (left) and plastic explosive RDX right), measured with CW terahertz spectroscopy.

Highly efficient detection of CW terahertz radiation may be done using photomixers. The signal-to-noise ratio achieved by this method is greater than 70 dB and is capable of performing rapid measurements with a dwell time in the order of a few ms per frequency point. An entire spectrum can be achieved within 2 minutes. A comparison between conventional scan and a quick CW terahertz scan is shown in Figure 5.

Figure 5. Comparison of two CW terahertz measurements with different integration times. The red curve was acquired with an integration time of 300ms per frequency step, the blue curve with 3ms/step. The measurement time is reduced from about three hours to less than two minutes

Pulsed Terahertz Techniques

Femtosecond lasers are used for producing pulsed terahertz waves. When the fs-pulse is incident on a photoconductive switch or a non-linear crystal, free charge carriers are produced. The charge carriers can then be accelerated by the application of external electric field. A transient electromagnetic field is induced by the change in current.

The bandwidth of the terahertz spectrum depends on the properties of the fs-pulse: an fs-pulse in the near infrared region for a 100 fs duration produces a spectrum with a width of 4-5THz.

There are many types of emitters available: GaAs antennas are used for laser excitation at 800 nm; organic crystals DAST or DSTMS and InGaAs/InP antennas used for laser excitation at 1550 nm. Figure 6 shows an erbium-doped ultrafast fiber laser that operates at the basic wavelength of 1550 nm or at a frequency-doubled wavelength of 780 nm, which can be used for either of the emitters.

Figure 6. Pulsed femtosecond fiber laser

A pulse length under 100 fs and an average output of over 100 mW can be achieved by these lasers. These laser sources are compact, strong and capable of fitting transportable terahertz systems. The laser pulse is split into two paths in a normal time-domain setup: one path is routed to the terahertz emitter which converts the laser light into terahertz pulses. Once it interacts with the sample, the terahertz light is focused onto the detector; an electro-optical crystal or a second photoconductive switch made of GaAs or InGaAs.

After the second path passes through a variable delay stage, it is routed to the detector. The field amplitude of the terahertz wave is measured by the detector by scanning it with a narrower laser pulse. By applying a fast Fourier transform a spectrum of the sample is achieved. Depending on the bandwidth of the laser excitation semiconductor antennas can produce frequencies of 4THz, and organic crystal emitters can produce frequencies greater than 10THz.

Comparison of CW and Pulsed Terahertz

The choice of the terahertz system depends on the specific requirements such as bandwidth, measurement speed and resolution. For precise frequency resolution and very high signal-to-noise ratios, frequency-domain spectrometers are the ideal choice. Owing to the fact that these systems do not require a delay stage they have a very small footprint. Also, the cost of DFB diode lasers is much less than fiber lasers.

Considering the advantages of the time-domain terahertz platforms they provide high bandwidth at shorter measurement times. Bandwidths of up to 10 THz can be achieved by using a commercial laser and a DAST-crystal. The range of the measurement times varies between ms and s for a complete spectrum and is dependent on the number of average traces.

In contrast the measurement time of the CW terahertz system varies from minutes to hours for recording the same spectrum. A comparison of the two methods for a water vapor spectrum is shown in Figure 7. It can be seen that the pulsed system achieved a bandwidth of 5 THz but at the cost of the frequency resolution and the signal-to- noise ratio.

Figure 7. Comparison of frequency- and time-domain terahertz spectroscopy. The frequency-domain spectrum offers a high resolution, the time-domain measurement a higher bandwidth. The peaks are absorption lines of water vapor. Both spectra were acquired with GaAs antennas.

Applications of Terahertz Spectroscopy

Terahertz spectroscopy has already been used for several industrial applications such as detection of toxic gases. Characteristic rotational bands are exhibited by numerous gas molecules, and these bands can be precisely identified by a terahertz spectrometer even when the gases are present as a mixture of other gases or in a spectrally “cluttered” background. Such detections are useful in subway stations or public buildings.

Quick and accurate detection without any false positives or false negatives is critical in the detection of trace amounts of chemical agents and industrial toxins. False alarms by cleaning agents, glues, perfume, exhaust fumes or paints cannot be risked. The detection of acid and toxic gases present in a disaster zone is another application of terahertz spectroscopy.

Harmful gases like hydrogen cyanide, carbon monoxide and hydrochloric acid are produced by burning plastic substances. Identification and concentration measurement of these gases can be achieved by terahertz spectroscopy from a safe distance and in the presence of black smoke, which does not let visible light pass through.

Another potential application of terahertz spectroscopy is material analysis, for example in the process and QC of the production of plastic compounds. The various additives present in the material can be measured by this method since polymers, such as polystyrene, polyethylene or polypropylene, are transparent to terahertz radiation, as shown in Figure 8.

Figure 8. Experimental setup for pulsed terahertz spectroscopy with a femtosecond fiber laser and fiber-pigtailed terahertz antennas

Terahertz spectroscopy can also be used for determining critical properties like DC conductivity or charge carrier density of semiconductors. Assessment of paper humidity during paper production is another potential industrial application of terahertz spectroscopy. This method is a safe alternative to the measurement via radioactive beta emitters that is being used currently.

Conclusion

Moving beyond the challenges in generating terahertz spectrum, more and more generation techniques for terahertz radiation are now being widely accepted. Novel techniques using optoelectronic sources that use robust laser modules are setting the trend for efficient terahertz applications in various fields. Such techniques have a promising future by employing the properties of terahertz radiation.

This information has been sourced, reviewed and adapted from materials provided by Toptica Photonics.