Ito, Masahiko

Nishizawa, Jun-ichi

Saito, Kyosuke

Sasaki, Tetsuo

Suto, Ken

Tanabe, Tadao

Review of Scientific Instruments

English

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GaP THz wave generator and THz spectrometer using Cr:Forsterite lasers

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GaP THz wave generator and THz spectrometerusing Cr:Forsterite lasers著者 田邉  匡生journal orpublication titleReview of scientific instrumentsvolume 76number 12page range 123109-1-123109-3year 2005URL http://hdl.handle.net/10097/35613doi: 10.1063/1.2140223REVIEW OF SCIENTIFIC INSTRUMENTS 76, 123109 2005GaP THz wave generator and THz spectrometer using Cr:Forsterite lasersKen Suto and Tetsuo SasakiaSemiconductor Research Institute, Aoba 519-1176, Aramaki, Aoba-ku, Sendai 980-0845, JapanTadao Tanabe and Kyosuke SaitoDepartment of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama6–6–11–1021, Sendai 980-8579, JapanJun-ichi NishizawaSemiconductor Research Institute, Aoba 519-1176, Aramaki, Aoba-ku, Sendai 980-0845, JapanMasahiko ItoTerahertz Laboratory Co. Ltd., Maedamen 50-1, Mitsumata, Komagata-cho, Yuzawa, Akita012-0104, JapanReceived 29 March 2005; accepted 21 October 2005; published online 27 December 2005We have developed a type of THz wave generator that uses Cr:Forsterite lasers as the pump andsignal sources for difference frequency generation in GaP Cr:F source system. We confirmed thegeneration of THz waves in the frequency range from 0.3 to 7.5 THz, which is just similar to thatobtained using the THz wave generator previously developed utilizing yttrium aluminum garnet andoptical parametric oscillator OPO lasers OPO source system. A peak output power of 100 mWwas obtained from 1.2 to 5 THz when the power of the two input beams was 3 mJ each, similar tothe OPO source system. A wide measurable frequency range from below 0.6 THz to over 6 THzwas obtained by using the Cr:F source system as the light source of a spectrometer, which has themerits of simple structure, easy maintenance, and low cost compared with the OPO source system.Although the linewidth of the Cr:F source system is greater than that of the OPO source system, theTHz spectrometer still has sufficient resolution for measuring solids or liquids at roomtemperature. © 2005 American Institute of Physics. DOI: 10.1063/1.2140223In 1963, Nishizawa1,2 proposed the generation of tera-hertz THz waves via the resonance of phonons and molecu-lar vibrations to fill the frequency gap between the micro-wave and optical bands, just after the realization of asemiconductor laser, which Nishizawa also proposed andpatented.3 Loudon presented a similar proposal for THzwave generation using a uniaxial crystal4 and Yarborough etal. observed THz wave generation via dielectrics LiNbO3.5Following the development of a semiconductor GaP Ramanlaser,6 Nishizawa and Suto generated a 12-THz wave with apeak power as high as 3 W using a GaP Raman oscillatorcontaining a GaAs mixing crystal.7 Kawase et al. improvedthe efficiency of THz wave generation based on theNishizawa-Yarborough method by introducing a grating cou-pler on the LiNbO3.8 Moreover, they demonstratedfrequency-sweepable, high-power THz wave generation us-ing the parametric amplification effect of lattice vibrations inthe frequency range up to 2.5 THz using LiNbO3.9–11 Wehave developed higher-power THz wave generation fromGaP tunable over wider frequencies.12–14 The principle of ourmethod is difference frequency generation DFG via excita-tion of the phonon-polariton mode under small-angle noncol-linear phase-matching conditions. Using an yttrium alumi-num garnet YAG laser and an optical parametric oscillatorlaser as the pump and signal sources, respectively, a tunableaAuthor to whom correspondence should be addressed; electronic mail:tsasaki@hanken.jp0034-6748/2005/7612/123109/3/$22.50 76, 12310Downloaded 13 Nov 2008 to 130.34.135.83. Redistribution subject tofrequency range of 0.3–7.4 THz available by changing thecrystal length from 2.6 to 20 mm, with a linewidth of3.2 GHz, and a maximum peak power of 100 mW at2.5 THz was obtained when the pump and signal energieswere 3 mJ. When the crystal length was increased to 20 mm,the maximum power was increased to 300 mW, but the fre-quency range was reduced to below 5 THz. We constructedan automatic THz spectrometer using GaP frequency-tunableTHz wave generators and reported the spectra of variousbiomolecules, including saccharides, nucleosides, and nucle-otides, demonstrating the merits of our system for spectro-metric analysis.15–18 In this article, we report the characteris-tics of frequency-tunable THz wave generation from GaPusing Cr:Forsterite Cr:F lasers as the pump and signalsource for DFG. We present a THz spectrometer system andan example of measurements.Figure 1 shows a schematic of the experimental setupused for DFG of THz waves in a GaP crystal using Cr:Flasers as the source Cr:F source. Both the pump and signalbeams were delivered from two channel Cr:F lasers LOTISTII pumped with a double-pulse Q-switched Nd:YAG laserat 1064 nm. A Cr:F laser generates a 1.2–1.3 m wave-length beam utilizing the Cr levels in a Forsterite crystal.Cr:F source systems can be simple and small because theyuse direct excitation by the fundamental wave of a YAGlaser. The repetition rate was 10 Hz and the pulse timing ofeach channel could be controlled independently on a nano-second order. This is important because the delay timechanges with the wavelength. The wavelength of the pump© 2005 American Institute of Physics9-1 AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp123109-2 Suto et al. Rev. Sci. Instrum. 76, 123109 2005beam was fixed to 1210 nm and the linewidth was below0.01 nm with an étalon or below 0.1 nm without an étalon.The wavelength of the signal beam was variable from1210 to 1250 nm and the linewidth was below 0.1 nm. Eachbeam was 1 mm in diameter with a 22 ns pulse length, andmore than 6 mJ pulse energy per pulse. The polarization ofthe pump and signal beams was adjusted vertically and hori-zontally, respectively. The two beams were combined at avery small angle to fulfill the phase-matching condition us-ing a cubic polarizer. The undoped semi-insulating GaP crys-tal was 5 or 10 mm long in the 110 direction and 3 mmthick in the 001 direction. Both the input and output face ofthe crystal were coated with Al2O3 after mechanical polish-ing and chemical etching. The THz waves generated werecollected with a pair of off-axis parabolic reflectors and de-tected using a 4 K Si bolometer. A Ge plate was used as afilter to cut off near-IR light.Figure 2 shows the frequency dependence of the maxi-mum THz wave output peak power at various inext, where inextFIG. 1. Experimental setup for DFG of THz waves in GaP crystal using Cr:lasers.FIG. 2. Frequency dependence of the maximum THz wave output power atextvarious in for a 5-mm-long and b 10-mm-long GaP crystals.Downloaded 13 Nov 2008 to 130.34.135.83. Redistribution subject tois the external angle between the pump and signal beamsoutside the GaP crystal. The GaP crystal lengths were a 5or b 10 mm long. The two beams were attenuated to 3 mJbefore incidence. As mentioned below, the incident angleext of the pump and signal beams from the normal of theGaP input surface had to be increased to avoid total reflec-tion of the THz wave at the output surface. The ext wasselected at each frequency to obtain the maximum outputpower. THz generation was observed at frequencies from0.6 to 7.5 THz for a 5-mm-long crystal. The maximum THzwave output power was 150 mW for a 5-mm-long crystal.These values are of the same order as those obtained usinganother type of THz generator that uses YAG and OPO lasersas the source OPO source system. One of the notable fea-ture of Cr:F source system is its high output power, whichincreases to 1.5 W for a 10-mm-long crystal without causingsurface damage, when the pump and signal energies are 11.4and 11.6 mJ, respectively. Figure 3 plots the relationship be-tween inext and the generated THz wave frequency at whichthe maximum power was obtained. The incident angle extwas also changed at each point to obtain the maximumpower. The solid curve shows inext as a function of THz wavefrequency calculated using the dispersion relationship ofphonon-polaritons in GaP. Although the calculation concurswith the measurements at lower frequencies, the discrepancyincreases at higher frequencies. This may be due mainly tothe increase in ext to avoid total reflection. In Fig. 3, thesame relationship is plotted for the GaP THz generator uti-lizing an OPO laser as the pump source. The THz wavegenerator with the Cr: lasers has a larger inext than that withthe OPO laser because the lower frequencies of the incidentbeams, which cause a larger refractive index of the THzwave, need a larger angle to fulfill the phase-matching con-dition to generate a THz wave at the same frequency.Figure 4 shows a schematic diagram of an automaticmeasurement system for transmission spectroscopy using aCr:F source system as a THz light source. The equipmentused for THz transmittance spectroscopy is similar to theOPO source system reported in detail in our previouspapers.15,16 The phase-matching condition was fulfilled auto-matically with rotating and linear stages to control the beamangle and an actuator to control the wavelength of the Cr:Flaser. The GaP crystal was placed on a rotating stage and theincident angle ext was adjusted to prevent total reflection.FIG. 3. Relationship between inext and the THz wave frequency giving themaximum THz power. An output angle of  was selected for the optimumcondition.The maximum signal-to-noise S/N ratios output power/ AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp123109-3 GaP THz wave generator Rev. Sci. Instrum. 76, 123109 2005detector noise were about 5000 for a deuterium triglycinesulfate DTGS detector and over 1 000 000 for a 4 K bo-lometer. The spectrometer systems were built using thedouble-beam method to eliminate the effect of THz powerfluctuation. Actually, THz power fluctuations are 25% and10% for OPO and Cr:F source systems, respectively, so thatthe S/N ratio is better for the Cr:F source system by a factorof 2.5 even when the double-beam method is used. Pyroelec-tric DTGS detectors operated at room temperature were usedas THz wave detectors. Black polyethylene filters were usedto cut off near-IR light. The entire THz wave path waspurged with dry nitrogen or dry air to eliminate water vaporabsorption. The spectrometer measured about120100100 cm, including the source lasers and powersupplies, while the OPO source system has a large size of300100100 cm. The linewidth of the THz wave is30 GHz, while that for the OPO source system is 1.5 to3.2 GHz. Note that 30-GHz resolution is sufficient forroom temperature observations of THz spectra in solids orliquids.Figure 5 shows transmittance spectra of -D-glucosemeasured with the Cr:F source system solid line in 15-GHzsteps with a linewidth better than 30 GHz 1 cm−1, and withFIG. 4. Schematic diagram of a THz spectrometer using Cr:F lasers.FIG. 5. Transmittance spectra of -D-glucose measured using the Cr:Fsource THz spectrometer system.Downloaded 13 Nov 2008 to 130.34.135.83. Redistribution subject tothe OPO source system dashed line in 10-GHz steps with alinewidth of 1.5 GHz 0.1 cm−1, measured at 294 K bysweeping the 0.5 to 5.6 THz 23.3 to 187 cm−1 fre-quency region. The spectra were obtained by averaging 32pulses of the DTGS outputs at each frequency. Crystallineglucose purchased from Tokyo Kasei Kogyo was milled withpolyethylene PE powder and pressed into 1-nm-thick pel-lets 20 mm diam under a pressure of 2000 kg. The glucoseconcentration was 5 wt % in an approximately 300-mg PEpellet. The sample pellets were shaped into a wedge in orderto prevent THz wave resonance via an étalon effect. Themain absorption bands observed with the OPO source systemwere also observed with the Cr:F source system. Note thatthe absorption band at 1.49 THz is sharp compared to theother bands. The full width at half maximum FWHM of theband was obtained as 62 GHz using the Cr:F source systemand 50 GHz using the OPO source system.The authors are grateful to J. Shibata Terahertz Labora-tory Co. Ltd. for valuable discussions. This study was sup-ported by a Grant-in-Aid for Creative Scientific ResearchNo. 13GS0002 from the Japan Society for the Promotion ofScience for the THz-wave generation using Cr:F lasers, andthe Terahertz Optics Project for Medical Applications led byJ. Nishizawa, organized by the Ministry of Education, Cul-ture, Sports, Science, and Technology, of Japan for construct-ing the THz spectrometer.1 J. Nishizawa, Denshi Kagaku 14, 17 1963.2 J. Nishizawa, Denshi Gijutsu Sogo Kenkyusho Iho 7, 101 1965.3 J. Nishizawa, Japan Patent 273217 1957.4 R. Loudon, Proc. Phys. Soc. 82, 393 1963.5 J. Yarborough, S. Sussman, H. Purhoff, R. Pantell, and B. Johnson, Appl.Phys. Lett. 15, 102 1969.6 J. Nishizawa and K. Suto, J. Appl. Phys. 51, 2429 1980.7 K. Suto and J. Nishizawa, IEEE J. Quantum Electron. 19, 1251 1983.8 K. Kawase, M. Sato, T. Taniuchi, and H. Ito, Appl. Phys. Lett. 68, 24831996.9 K. Imai, K. Kawase, J. Shikata, H. Minamide, and H. Ito, Appl. Phys.Lett. 78, 1026 2001.10 K. Kawase, J. Shikata, K. Imai, and H. Ito, Appl. Phys. Lett. 78, 28192001.11 K. Kawase, H. Minamide, K. Imai, J. Shikata, and H. Ito, Appl. Phys.Lett. 80, 195 2002.12 T. Tanabe, K. Suto, J. Nishizawa, T. Kimura, and K. Saito, J. Appl. Phys.93, 4610 2003.13 T. Tanabe, K. Suto, J. Nishizawa, K. Saito, and T. Kimura, J. Phys. D 36,953 2003.14 T. Tanabe, K. Suto, J. Nishizawa, K. Saito, and T. Kimura, Appl. Phys.Lett. 83, 237 2003.15 J. Nishizawa, K. Suto, T. Sasaki, T. Tanabe, and T. Kimura, J. Phys. D 36,2958 2003.16 J. Nishizawa, T. Sasaki, K. Suto, T. Tanabe, K. Saito, T. Yamada, and T.Kimura, Opt. Commun. 246, 229 2005.17 J. Nishizawa, T. Sasaki, K. Suto, T. Yamada, T. Tanabe, T. Tanno, T.Sawai, and Y. Miura, Opt. Commun. 244, 469 2005.18 J. Nishizawa, K. Suto, T. Sasaki, and T. Tanno, Proc. Jpn. Acad., Ser. B:Phys. Biol. Sci. 81, 20 2005. AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp

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GaP THz wave generator and THz spectrometer using Cr:Forsterite lasers

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