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THz Photonics Technology and Its Applications

Haibo Liu, Hua Zhong, Nick Karpowicz, Xia Li, Albert Redo, Yunqing Chen, Jingzhou Xu, and X.-C. Zhang

Center for Terahertz Research, Rensselaer Polytechnic Institute, Troy, NY 12180 USAPhone/Fax:(518) 276-3079/3292; zhangxc@rpi.edu ; http://www.rpi.edu/terahertz

Haewook Han

Center for Terahertz Photonics, POSTECH, San-31 Hyoja-Dong Nam-Gu, Pohang, Kyugbuk,Korea

Yun-Sik Jin

Applied Electrophysics Group, Korea Electrotechnology Research Institute, 28-1 Sungju-DongChangwon Gyungnam, Korea 641-120

Abstract Terahertz (THz) radiation, which occupies alarge portion of the electromagnetic spectrum between themid-infrared and microwave bands, offers innovativeimaging and sensing technologies that can provideinformation, which may not be available throughconventional methods (i.e. microwave and X-raytechniques.) As THz wave (T-ray) technology improves, webelieve new THz wave sensing and imaging capabilities willimpact a range of interdisciplinary fields, including:communications, imaging, medical diagnosis, healthmonitoring, environmental control, and chemical andbiological identification. This is particularly crucial foridentifying terrorist threats in homeland security (three tofive years), and medical diagnosis or even clinical treatmentin biomedical applications (five to ten years).

Index Terms microwave photonics, THz wave, image,sensing.

I. I NTRODUCTION

Terahertz waves, like mid-infrared and microwaves inthe adjacent bands of the electromagnetic spectrum, offerinnovative imaging and sensing technologies forapplications in material characterization,microelectronics, medical diagnosis, environmentalcontrol and chemical and biological identification.Recent advances in THz science and technology make itone of the most promising research areas in the 21stcentury for sensing and imaging, as well as in other

interdisciplinary fields. Recently, government-supportedTHz wave related fundamental research in science andapplied technology has increased substantially. We

believe new T-ray capabilities will impact a broad rangeof interdisciplinary fields and industries, includingcommunication, imaging, medical diagnosis, healthmonitoring, environmental control, and chemical and

biological identification.

While microwave and X-ray imaging modalities produce density pictures, broadband T-ray imaging provides spectroscopic information in the THz frequencyrange. The unique rotational and vibrational responses ofmaterials in the THz range provide information that isgenerally absent in optical, X-ray and NMR images. ATHz wave can easily penetrate and inspect the insides ofmost dielectric materials, which are opaque to visiblelight and exhibit low contrast to X-rays, making THzimaging a useful complementary technology in thiscontext.

Recent years have seen a plethora of significantadvances as THz sources and more sensitive detectorsopen up a range of potential applications. Applications

including semiconductor and high-temperaturesuperconductor characterization, tomographic imaging,label-free genetic analysis, cellular level imaging, andchemical and biological sensing have thrust THzresearch into the limelight.

MIT Technology Review listed THz Technology in 10Emerging Tech That Will Change World in its Feb.2004 issue. The History Channel and Discovery TVChannel had 20-minute interviews and stories about THzresearch and development in their Modern Marvelsand the Greatest Gadget programs.

In 2003, the journal Nature featured the use of THzwaves for defect identification for the NASA SpaceShuttle program. NASA has installed six pulsed THzimaging systems for the non-destructive inspection ofshuttle insulation foam. APS News reported on our THzimaging for the space shuttle foam application.

Recently, the transfer of THz technology from thelaboratory to industry has accelerated. Numerousinternational societies have started special THzsymposiums or workshops for the first time. Other recentactivities include the merging of the THz Electronics

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Conference (THz Electronics) with the Infrared andMillimeter Wave Conference (IRMMW) in 2004.

II. G ENERATION AND DETCTION OF THZ WAVES

THz waves, either pulsed or continuous wave (CW),can be generated and detected by various systems. Each

of them has different output powers, bandwidths,detection efficiencies, etc.

THz time-domain spectroscopy (THz-TDS) systemsgenerate and detect picosecond THz pulses by asynchronous, coherent method using near-infraredfemtosecond lasers. Free electron lasers can also provide

pulsed THz waves with the highest power.For CW THz systems, there are five classes of CW

sources and five classes of detectors. The broadbandthermal emitter, such as glowbar, and cooled detectorlike bolometer are used in Fourier transformed far-infrared spectroscopy systems. Electronic sourcesinclude Gunn diodes, Bloch oscillators, and backward-wave oscillators. THz-wave parametric oscillators arenonlinear optical sources based on optical lasertechnology. In addition, THz lasers, including free-electron lasers, gas lasers and quantum cascade lasershave been developing over the past thirty years. Schottkydiodes are compact electronic detectors. Golay cell and

pyroelectric detector are also used for THz wavedetection. Additionally, quantum well THz detectorshave been introduced recently.

III. S ENSING WITH THZ WAVES

Spectroscopic technologies are essential for chemicaland biological sensing. The THz-TDS technology

developed in the past decade enables us to explorespectroscopic sensing in the THz band more easily. Itcovers a broad frequency range from approximately 100GHz to 40 THz. Coherent detection and high signal-to-noise ratio (SNR, up to 10,000:1) make it an effectivetechnique for sensing and identification.

Many solid materials have THz absorption fingerprints,which are from either torsion vibrations, intermolecular(crystalline lattice) vibrations, or hydrogen-bondingstretches [1]. Based on this, THz spectroscopy hasrecently been applied in the investigation of explosivesand related compounds, small biological molecules (suchas drugs, amino acids and nucleobases), and otherchemical substances.

Various explosives and related compounds, includingRDX, TNT, HMX, PETN, DNT, etc., have absorptionfingerprints in THz range [2,3]. THz waves can penetratethrough many dielectric materials, such as clothing,

paper, plastics, leather, wood, and ceramics. In addition,THz radiations have low photon energies and will notcause harmful photoionization in biological tissues.Owing to these advantages, THz technology is acompetitive method for inspecting hidden explosives.

Many groups have applied THz-TDS for explosivesensing in transmission mode. For real-field applications,reflection measurements are preferred, because bulkytargets are impossible to measure in transmission mode.In addition, reflection spectroscopy, especially diffusereflection spectroscopy, is more applicable than thetransmission method for standoff detection.

Figure 1 presents the absorption spectra of RDXfrom diffuse (Fresnel) reflection measurements (withoutany cover and with a paper cover), which agree with thetransmission measurement well. These results indicatethat THz waves have a great potential for standoffsensing of explosives concealed in packages or underclothing.

0.4 0.6 0.8 1.0 1.2 1.4 1.60.0

0.2

0.4

0.6

0.8

1.0

Transmission

Paper cover

Reflection

1.551.351.05

0.82

A b

s o r b a n c e

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Frequency (THz)

Fig. 1. Absorption spectra of RDX from a transmissionmeasurement and diffuse reflection measurements. Upper curve:transmission measurement; middle curve: diffuse reflectionmeasurement (without any cover); bottom curve: diffusereflection measurement (with a paper cover).

Another significant application of THz spectroscopy isfor pharmaceutical materials since different

polymorphisms and crystallinities of a pharmaceuticalsubstance could have different THz fingerprints. THzspectroscopy has some advantages like high SNR, highspeed, and adaptability for online use. Thesecharacteristics will make THz spectroscopy acomplementary technique for X-ray diffraction in

pharmaceutical science and industry. Here we give anexample of using THz spectroscopy to identify theinteraction of caffeine and sulfamethoxazole (SMZ), which is a drug used worldwide in the treatment of

bacterial infections. Figure 2 shows the THz absorptionspectra of bonded and mixed SMZ-caffeine.

0.5 1.0 1.5 2.0 2.50

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3 Bonded SMZ-caffeine Mixed SMZ-caffeine

Both are ~120 mgat the same w/w ratio

A b s o r b a n c e

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Frequency (THz)

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Fig. 2. Absorption spectra of bonded and mixed SMZ-caffeine. There is an evident difference between the bonded andunbonded SMZ-caffeine, because of the change of the crystalstructure after bonding.

IV. IMAGING WITH PULSED TH Z WAVES

Interest in THz imaging has increased alongside thedevelopment of THz technology. Because thewavelengths of THz waves are substantially smaller thanthose of microwave radiation, they are able to providemuch higher spatial resolution. THz radiation is alsosignificantly less susceptible to scattering than mid-infrared light, allowing improved reconstruction fidelitycompared to infrared tomography techniques. THz pulseimaging provides spatial, temporal and spectral domaininformation by applying THz -TDS techniques. It isconsidered a viable option for functional imaging, sincemany biological and chemical agents have signatures inthis spectral domain.

The earliest pulsed THz imaging experiment was

conducted by raster scanning the sample with a focusedTHz beam [4]. The change of refractive index modulatesthe timing and phase of the pulse, and the densityvariation is shown in the pulse energy change. Byanalyzing the THz pulse in frequency domain it is

possible to identify materials that have characteristicspectra. This geometry has been widely adopted inapplications such as biomedical diagnosis, imaging andsensing of mines, drugs, and explosives, and non-destructive defect identification.

Another remarkable pulsed THz imaging method isTHz tomography. THz tomography refers to the imagingtechnology in which either reflected or transmitted THzwave illumination reveals the cross section of the object.Raster scanning THz imaging in reflection geometry can

be placed in this category, as it reveals three-dimensionalinformation about a layered structure by recording pulsesreflected from each interface. For example, thistechnique has been applied in the inspection of spaceshuttle insulating foam, since defect interfaces modulatethe pulses in both the time and frequency domains. THzcomputed tomography (CT) is the adaptation of X-raytomography to the THz region [5]. Unlike traditional X-ray CT, in which only the density image of the objectcan be reconstructed, THz CT is able to reconstruct boththe reflective index and density image by performing aRadon transform on both the phase and amplitude of the

transmitted THz pulses. Pulsed THz binary lenstomography utilizes the frequency dependent focallength of a Fresnel binary lens, so that the images atdifferent depths are projected onto a single imaging

plane at corresponding frequencies that satisfy the lensequation [6].

THz 2-D electro-optic (EO) imaging [7] is a big stepforward compared to the traditional raster scan imaging.In this technique, the entire THz field modulates an

expanded probe beam on a large piece of EO sensorcrystal. The modulated beam profile is captured by aCCD camera. It greatly improves data acquisition speedand makes real time THz imaging possible. However,there are still limitations to the application of thismodality due to an insufficient SNR.

There have been other pulsed THz imaging modalities

that benefit from available technologies developed forother regions of the electromagnetic spectrum, or forsound waves. THz ranging measurement, THz syntheticaperture radar imaging, and THz interferometry, have all

been explored based upon more established radarastronomy and holography techniques in microwave andradio frequency. There is also THz reflection multistaticimaging using Kirchhoff migration, which was originallycarried out using ultrasound.

Regarding THz imaging on the microscopic scale,THz near field imaging applies near field theory to breakthrough the resolution limit set by the THz wavelength.Earlier THz near field microscopes used sub-wavelengthapertures to detect the evanescent THz wave close to thesample surface. An alternative way is to have a THzsource that is much smaller than the THz wavelength. Arecord of 150 nm resolution has been reported by using atip-type near field THz microscope, in which a STM tipwas used in the investigation [8].

V. IMAGING WITH CW THZ WAVES

Imaging with CW radiation in the THz region of theelectromagnetic spectrum was proposed in the 1970s [9],

but has gained new attention recently due totechnological advances that make it more practical. CWTHz waves for imaging may be generated through a

variety of methods, such as frequency-multiplied Gunndiode sources [10], backward-wave oscillators [11],quantum cascade lasers [12], photomixing [13] and CO 2-

pumped low-pressure gas lasers, and have a narrowspectral linewidth compared to those generated by pulsedsources. As a result, the dynamic range at a givenfrequency can be relatively large and the acquisition ofdata may be simplified and accelerated. However,unlike the case of pulsed THz, a CW system lacks time-of-flight information and will only allow spectroscopicimaging if a tunable source or an ensemble of sources isused. Despite this constraint, the information supplied

by a fixed-frequency CW system is adequate for manyapplications.

Examples of CW THz images are shown in Figures 6and 7. Figure 6 shows a 0.6 THz transmission image ofa racquetball racket inside of its case. Since the case is anon-conducting cloth material, it is partially transparentto THz radiation. The source was a frequency-multipliedGunn diode and the detector was a Golay cell.

Figure 7 shows a panel of insulating foam of the kindthat covers the external tank of the space shuttle. Thisfoam has low attenuation in the THz region and thus a

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THz imaging system is capable of locating defectshidden inside. In the sample shown, there are 29 man-made defects, of which 28 were detected. The sourcewas a frequency multiplied Gunn diode operating at 0.2THz and the detector was a Schottky diode.

(a) (b)Fig. 6. (a) Optical image and (b) 0.6 THz image of aracquetball racket in a case.

CW THz sources and detectors allow for theconstruction of compact and fast THz imaging systems.Although less information is provided relative to a

pulsed system, such an imaging system has the potentialto be useful in a wide range of industrial applications,such as in the aerospace and manufacturing industries.

(a) (b)

Fig. 7. (a) Optical image and (b) 0.2 THz image of a 0.6 m x0.6 m sample of space shuttle insulating foam with built-indefects.

VII. CONCLUSION

THz photonic technology developed rapidly over the past decade. Improved source and detector performancecontinue to enable new application areas and facilitatethe transition of THz systems from the laboratory tocommercial industry. THz spectroscopy and non-destructive evaluation might be two direct applicationsfor industrial companies.

ACKNOWLEDGEMENT

The authors wish to acknowledge the support of theU.S. Army Research Office under the MURI project, theCenter for Subsurface Sensing and Imaging Systems,under the Engineering Research Centers Program of the

National Science Foundation, and the PhotonicsTechnology Access Program under the OptoelectronicsIndustry Development Association.

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