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Laser-Based Terahertz Generation & Applications

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Laser-Based Terahertz Generation & Applications Optical Spectroscopy Laser-based terahertz generation & applications Marion Lang, Anselm Deninger, Toptica Photonics AG, Gräfelfi ng, Germany Superior to alternative methods, optoelectronic techniques propel tera- hertz applications out of the lab and into the real world. One physical effect involves two laser beams at adjacent frequencies focusing on a semiconductor; another effect involves an ultrafast laser pulse separating charge carriers in a photoconductive switch. In combination with suitably structured antennas, these two effects generate electromagnetic radiation in the diffi cult-to-access terahertz region. Some of the principles of continuous-wave (CW) and pulsed terahertz sources are described here while exploring emerging applications. The label “terahertz-gap” for this part of tify different crystalline forms (polymorphs) the electromagnetic spectrum is used to of the active component. Further applica- pinpoint the lack of suitable sources and tions include the detection of counterfeit detectors in this frequency range. Thanks drugs and food quality monitoring. Because to modern laser technologies, however, terahertz radiation penetrates packaging robust, compact and cost-effi cient sources materials made of paper or plastics, pills can for terahertz imaging and spectroscopy are be analyzed in the blister, and food can be now readily available. The sources produce tested through air-tight packaging. terahertz radiation with suffi cient intensity Many applications benefi t from the imag- for scientifi c and industrial measurement ing capabilities of terahertz radiation. Tera- tasks, enabling many new applications, hertz waves can be focused with mirrors including basic research, material analysis, and lenses. Scanning a sample with a homeland security, and process control in terahertz beam delivers images with mil- biology, pharmacy and medicine. limeter-range resolution. Depending on the method used, the depth resolution is 1 Can you see what I see? Figure 1: Terahertz spectroscopy enables even higher – by as much as two orders of the detection of hazardous substances magnitude. Because their photon energies The terahertz spectrum lies between the far in envelopes and parcels range from 0.4 to 40 meV, terahertz rays infrared and radio- or microwaves; this cor- do not have any ionizing effect (in contrast responds to frequencies between 100 GHz terahertz region – so-called “fi ngerprints” to gamma rays) and are considered biologi- and 10 THz or wavelengths between 3 mm – that allow their chemical composition cally innocuous. Metals are not transpar- and 30 μm. Terahertz radiation penetrates identifi cation. Many explosives and toxic ent for terahertz waves due to their high many opaque materials, especially non- agents feature characteristic absorption refl ectivity. Organic matter is also opaque metal and non-polar materials such as tex- lines between 0.5 and 5 THz. This ena- to terahertz light, as the ever-present water tiles, paper and plastics. Compared to visible bles the detection of not only illicit drugs strongly absorbs terahertz rays. and near-infrared light, terahertz radiation like cocaine, ecstasy and opiates, but also scatters less due to its longer wavelength, explosives like TNT, HMX and RDX in paper 2 Terahertz sources resulting in a higher penetration depth. envelopes and parcels (fi gure 1). In the Most interestingly, many substances exhib- development and production of pharma- The generation of terahertz radiation at it characteristic spectral signatures in the ceuticals, terahertz spectroscopy can iden- spectroscopically relevant frequencies from 0.5 to 5 THz is challenging. This range cannot be directly accessed with semicon- ductor lasers, as no semiconductors with suitable band gaps exist. One direct opti- cal source is the quantum-cascade-laser (QCL). A QCL generates frequencies above 1 THz but requires operating temperatures of 40 K, and therefore a He cryostat for cooling. QCLs with less complex nitro- gen cooling are limited to frequencies above 2 THz. Voltage-controlled oscillators (VCOs), in conjunction with frequency mul- tipliers, achieve frequencies up to several Figure 2: A photomixer with spiral antenna converts laser light into terahertz radiation 100 GHz. In the terahertz regime, however, 36 Photonik international · 2012 Originally published in German in Photonik 4/2012 Optical Spectroscopy Figure 3: Compact two-color CW laser Figure 4: Absorption signatures of α-lactose monohydrate (left) and the plastic for terahertz generation explosive RDX (right), measured with CW terahertz spectroscopy this technique proves ineffi cient, technically tion creates free charge carriers in the semi- 2.2 Pulsed terahertz techniques complex, and very expensive. conductor, and an applied voltage acceler- (time-domain terahertz) Optoelectronic techniques offer an effi - ates them towards the metal electrodes. cient and cost-effective method for tera- This gives rise to a photocurrent, which is Pulsed terahertz waves are generated with hertz generation: near-infrared laser light is modulated at the difference frequency of femtosecond lasers. When focused on a focused on a special semiconductor struc- the two lasers. The antenna then translates non-linear crystal or a photoconductive ture (fi gure 2). The result is a photocurrent, this beat frequency into a new electromag- switch, the fs-pulse produces free charge which in turn is the source for the terahertz netic wave – the terahertz wave. carriers, which are accelerated by an external wave. A continuous-wave (CW) laser pro- On the laser side, DFB (distributed feed- electrical fi eld. This change in current induces duces narrow-band terahertz radiation; fem- back) diodes are perfect sources. They a transient electromagnetic fi eld. tosecond pulses convert into a broadband can be tuned over more than 1000 GHz The properties of the laser pulse determine terahertz spectrum. These two techniques and thus yield difference frequencies from the bandwidth of the terahertz spectrum: are explored further in the next sections. 0 to 2 THz or from 1 to 3 THz. Figure 3 in the near infrared a pulse duration of shows a compact two-color CW laser that 100 fs corresponds to a spectral width of 2.1 CW terahertz techniques consists of two fi ber-coupled DFB diodes 4 to 5 THz. Different emitters are available, at 1.5 μm and a polarization-maintaining which are suitable for laser excitation at (frequency-domain terahertz) fi ber combiner. 800 nm (GaAs antennas) or at 1550 nm Optical heterodyning of two CW lasers with CW terahertz systems offer a narrow (organic crystals DAST or DSTMS1 and adjacent wavelengths (e.g. 853 nm/855 nm linewidth on the order of 1 MHz, which InGaAs/InP antennas). Erbium-doped ultra- or 1546 nm/1550 nm) on a dedicated in turn results in a high spectral resolu- fast fi ber lasers (fi gure 6) operating either at antenna generates terahertz radiation at tion. This allows measurements of nar- the fundamental wavelength of 1550 nm or the difference frequency. The two-color row signatures, such as trace gases at frequency-doubled to 780 nm are a perfect laser beam is focused onto a photomixer, low pressure, or spectra of organic sol- match for these emitters. The lasers achieve a metal-semiconductor-metal structure, at ids. Figure 4 shows very precise absorp- pulse lengths of less than 100 fs and aver- the center of the antenna. The laser radia- tion measurements of α-lactose and the age output powers of more than 100 mW. plastic explosive Similar to the CW laser depicted in fi gure 3, RDX. Photomixers these compact and robust sources even fi t can also be used transportable terahertz systems. to detect CW tera- In a typical time-domain setup, the laser hertz radiation. This very effi cient 1 DAST (4-N, N-dimethylamino- 4‘-N‘-methyl-stilbazoli- um tosylate) or DSTMS (4-N, N-dimethylamino-4‘-N‘- technique achieves methyl-stilbazolium 2, 4, 6- trimethylbenzenesulfonate) signal-to-noise ratios of more than 70 dB and conse- quently enables fast measurements with a dwell time of only a few millisec- onds per frequency point. A complete spectrum can then be acquired in less than 2 minutes. Figure 5: Comparison of two CW terahertz measurements Figure 5 shows with different integration times. The red curve was acquired a comparison with an integration time of 300 ms per frequency step, the between a conven- Figure 6: Pulsed femtosecond fi ber blue curve with 3 ms/step. The measurement time is reduced tional and a fast laser – the engine for time-domain from about three hours to less than two minutes CW terahertz scan. terahertz systems Originally published in German in Photonik 4/2012 Photonik international · 2012 37 Optical Spectroscopy Figure 8: Experimental setup for pulsed Figure 7: Comparison of frequency- and time-domain terahertz spectroscopy. The terahertz spectroscopy with a femto- frequency-domain spectrum offers a high resolution, the time-domain measurement second fi ber laser and fi ber-pigtailed a higher bandwidth. The peaks are absorption lines of water vapor. Both spectra terahertz antennas were acquired with GaAs antennas typically in the millisecond to second range (fi gure 8). With semiconductors, terahertz pulse is split into two paths: one path leads for a complete spectrum. This compares spectroscopy allows determination of essen- to the terahertz emitter, which converts the to minutes to hours required to record the tial properties like charge carrier density laser light into terahertz pulses. After inter- same spectrum with a CW terahertz system. or DC conductivity [2]. Another emerging acting with the sample, the terahertz light is Figure 7 highlights the differences between application in the fi eld of industrial process focused onto the detector, such as a second both methods in a spectrum of water vapor. control assesses paper humidity in paper photoconductive switch made of GaAs or The pulsed system attains a bandwidth of production lines. Here, terahertz-based InGaAs, or an electro-optical crystal. The sec- 5 THz, which comes at the expense of sig- techniques provide safe alternatives to the ond part of the pulse travels to the detector nal-to-noise ratio and frequency resolution.
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