We experimentally demonstrated the scalability of the terahertz-wave parametric oscillator by using a pump beam with a wide aperture and a high pulse energy. Terahertz-wave absorption by the LiNbO3 crystal in the oscillator is substantially suppressed by employing a surface-emitting cavity configuration. We also improved the conversion efficiency by increasing the parametric interaction in the noncollinear phase-matching geometry. A pump depletion of 54% and a conversion efficiency of 0.9 × 10-6 are achieved. A maximum terahertz output of 382 nJ/pulse was achieved at 1.46 THz using a 8.0-mm-diameter pump beam with a pulse energy of 465 mJ/pulse

Guo, R.

Ikari, T.

Ito, H.

Minamide, H.

JEOS:RP - Journal of the European Optical Society Rapid publications

J O U R N A L  O FTORH E  E U R O P E A N  P T I C A L  S O C I E T YA PID  PU B LIC AT IO N SJournal of the European Optical Society - Rapid Publications 5, 10054 (2010) www.jeos.orgEnergy scalable terahertz-wave parametric oscillatorusing surface-emitted configurationTomofumi Ikariikari@riken.jpRIKEN Advanced Science Institute, 519-1399, Aramaki Aoba, Aoba-ku, Sendai 980-0845, JapanRuixiang Guo RIKEN Advanced Science Institute, 519-1399, Aramaki Aoba, Aoba-ku, Sendai 980-0845, JapanHiroaki Minamide RIKEN Advanced Science Institute, 519-1399, Aramaki Aoba, Aoba-ku, Sendai 980-0845, JapanHiromasa Ito RIKEN Advanced Science Institute, 519-1399, Aramaki Aoba, Aoba-ku, Sendai 980-0845, JapanGraduate School of Engineering, Tohoku University, Sendai 980-8577, JapanWe experimentally demonstrated the scalability of the terahertz (THz)-wave parametric oscillator by using a pump beam with a wide apertureand a high pulse energy. THz-wave absorption by the LiNbO3 crystal in the oscillator is substantially suppressed by employing a surface-emitting cavity configuration. We also improved the conversion efficiency by increasing the parametric interaction in the noncollinear phase-matching geometry. A pump depletion of 54% and a conversion efficiency of 0.9× 10−6 are achieved. A maximum THz output of 382 nJ/pulsewas achieved at 1.46 THz using a 8.0 mm-diameter pump beam with a pulse energy of 465 mJ/pulse. [DOI: 10.2971/jeos.2010.10054]Keywords: terahertz-wave, parametric oscillation, surface emission, energy enhancement1 INTRODUCTIONOver the last two decades, many THz applications have beenproposed in an increasingly wide variety of fields, includ-ing information and communication technology, biology andmedical science, nondestructive evaluation and national secu-rity [1]–[5]. High-power widely tunable THz sources are re-quired in practical applications of THz technologies. For in-stance, security imaging at airports requires high power toachieve enough penetration through clothing and luggage tobe able to identify weapons and possibly explosives in real-time.We have focused on developing widely tunable THz paramet-ric sources that use phonon-polariton scattering in a LiNbO3crystal [6]–[8]. These sources can generate monochromatic ra-diation in the frequency range 1 THz–3 THz at room temper-ature. Several schemes have been implemented to enhancethe THz-wave output energy. Output energy enhancementusing a Si prism array and an injection technique has beendemonstrated [9]. A Si prism coupler not only increases theoutput energy but it also generates unidirectional radiationwith frequencies in the range 1 THz–3 THz, enabling THz-wave parametric oscillators (TPOs) to be used for spectro-scopic and multifrequency imaging [3, 10]. The output energyand parametric gain have been increased by cryogenicallycooling LiNbO3 [11]. An enhanced efficiency was achieved byusing LiNbO3 waveguide confinement [12]. A top-hat pumpbeam was shown to enhance the THz-wave from a THz–waveparametric generator [13].Using high-energy pumping in conjunction with a large pumpspot size is effective for enhancing the output energy in non-collinear phase matching geometry [14]. However, in conven-tional THz-wave parametric generators, the high absorptionloss in the LiNbO3 crystal prevents energy up-scaling. Fur-thermore, up-scaling TPOs has not been investigated.In this report, we investigate increasing the output energy andthe efficiency of a surface-emitting (SE) TPO by simultane-ously increasing the aperture size for the pump beam andusing a higher pump beam energy. In the SE configuration,a THz wave is emitted from the surface at which the pumpand idler beams reflect [15]. Thus, the area from which THzradiation is emitted from can be scaled by varying the pumpspot size. Both the conversion efficiency and the output en-ergy were increased by this technique. By performing experi-ments using several spot sizes of the pump beam, we demon-strate that these enhancements in the output energy and effi-ciency are due to increasing the interaction volume in a non-collinear geometry.2 EXPERIMENTAL SETUPFigure 1(a) shows the experimental set-up for our SE-TPOwhich is configured from two rectangular and one trape-zoidal MgO:LiNbO3 crystals. The two rectangular crystalswere placed on either side of the trapezoidal crystal. Thec-axes of these crystals are parallel to each other. The endsurfaces that the pump and idler beams pass through wereantireflection coated for the wavelengths of the pump andidler beams, respectively. The pump laser was a Q-switchedNd:YAG laser with an output wavelength of 1.064 µm. TheReceived April 16, 2010; published November 08, 2010 ISSN 1990-2573Journal of the European Optical Society - Rapid Publications 5, 10054 (2010) T. Ikari et al.THz wave65 deg.mM1 M2Idler50mTSMgO:LiNbO3 cPump beam Beam stopNd:YAG(a)THz waven0=1AirkpkTIdlernTHz ≈ 5.2nP,i≈2.15°64.3°MgO:LiNbO3kiθCrit.65Pump(b)(c)FIG. 1 (a) Schematic diagram of SE-TPO. M1 and M2: cavity mirrors, TS: telescope.(b) The noncollinear phase matching condition (left) and schematic diagram of threebeams for the perpendicular generation at 1.5 THz (right). kp, ki , and kT show thewave vectors of the pump, idler and THz, respectively. (c) Schematic diagram of SE-TPOenabling a larger pump beam and high energy pumping. The cavity is based on theoff-axis pump double-pass singly resonant oscillator.polarization of the pump beam was parallel to the c-axes ofthe crystals. Two flat cavity mirrors, M1 and M2, were used forthe idler wave resonance. These cavity mirrors were speciallyfabricated to have a high transmittance at 1.064 µm and a highreflectivity above 1.068 µm. To enable higher energy pumping,an amplified longitudinal multimode Nd:YAG laser was usedthat can generate pulses with energies of up to 80 mJ/pulseand with a pulse length of 25 ns. The pump beam was com-pressed or expanded using a Galilean-telescope configurationto produce collimated beams with full width half maximum(FWHM) diameters of 1.0 mm, 1.7 mm and 2.2 mm.THz waves were generated normal to the output surface bysetting the incident angles of the pump and idler beams tothe crystal surface as shown in Figure 1(b). In the polariton-scattering process, idler (Stokes) and signal (THz) waves aregenerated from the pump (near-infrared) wave in a directionconsistent with noncollinear phase-matching, kp = ki + kT ,where kp, ki, and kT are the wave vectors of the pump, idler,and THz waves, respectively. Figure 1(b) shows incident an-gles of pump and idler beams to the crystal surface. For per-pendicular emission at 1.5 THz, the idler beam incident angleis set to 65◦, (i.e. the phase-matching angle, δ1.5THz) for 1.5 THzgeneration and the pump beam incident angle is set to 64.3◦(i.e. δ1.5THz − θ1.5THz). These angles exceed the critical angle(θcrit = 27.7◦) of the interface between the MgO:LiNbO3 crys-tal (np,I ≈ 2.15) and air (n = 1), ensuring that both the idlerand pump beams are totally reflected at the surface.We designed another SE-TPO cavity that had a much higherpumping energy and a wider pump beam size, as shown inFigure 1(c). An isosceles trapezoidal MgO:LiNbO3 crystal wasused. The base angle of the trapezoidal crystal was 65◦ andits bottom length was 50 mm. The cavity and crystal lengthalong the idler beam axis is half that of the cavity shown inFigure 1(a). To counteract the reduction in the gain length, thepump beam traverses the crystal twice. So, propagating lengthof the pump beam in the crystal is same as that of the cav-ity shown in Figure 1(a). M is a high-performance mirror thathighly reflects (> 95%) only the idler wavelength and HR is ahigh-reflectivity dielectric mirror that highly reflects (> 98%)both the pump and idler beams. The pump laser was a Q-switched Nd:YAG laser with the pulse duration of 12 ns, max-imum pulse energy of 500 mJ/pulse, and beam size of 8.0 mm.THz pulse energy was measured by 4K Si-Bolometer. Thegenerated THz-wave was focused on the 4-K Si bolometerthrough THz aspherical lens. The bolometer produces an out-put of 1 V for a (pulsed) input energy of 101 pJ with 200 timesvoltage amplify. The sensitivity for single shot was experi-mentally verified. When measuring the output, transmittance-calibrated thin metallized Mylar films were used to ensurethat the output signal was below the saturation level of thebolometer.3 RESULT AND DISCUSSIONSFigure 2 shows the THz-wave output energy at 1.46 THz. Themaximum output achieved was 382 nJ/pulse when the pumpenergy was 465 mJ/pulse and the pump beam was 8 mm indiameter. The inset of Figure 2 is magnified view for pumpbeams with sizes of 1.0 mm, 1.7 mm and 2.2 mm. The outputfrom a SE-TPO has previously been reported to be 8 nJ/pulsefor a pump energy of 23 mJ/pulse with a pump beam diam-eter of 1.0 mm [15]. The THz output energy was re-calibratedusing latest sensitivity.The approximately 50 times higher output energy achieved inthe present study was obtained by increasing the pump en-ergy by a factor of 20. The enhancement in the output energyexceeds linear scaling of the pump energy, indicating that theconversion efficiency was increased.Figure 3(a) shows the dependence of the conversion efficiencyon the pump fluence. A maximum conversion efficiency of0.9× 10−6 was achieved at a pump fluence of 0.92 J/cm2 for a8.0 mm pump beam, which corresponds to a pump energy of465 mJ/pulse; these are the same conditions at which the max-imum THz-wave output energy was observed. A conversionefficiency of 0.9× 10−6 corresponds to a photon conversion ef-10054- 2Journal of the European Optical Society - Rapid Publications 5, 10054 (2010) T. Ikari et al.Fig. 2400/ pu ls e]401.7 8.0 mm200300e rg y  [n J/0 20 40 600201.0 2.2100u tp ut  en e0 100 200 300 400 5000Pump energy [mJ/pulse]Ou  FIG. 2 THz output at various pump beam size and magnified view for pump beam sizeof 1.0 mm, 1.7 mm and 2.2 mm. The maximum output was obtained at 382 nJ/pulse,a pump beam size of 8.0 mm (FWHM) and 465 mJ/pulse.Fig. 31.0y  1 78.0 mm(a)0.5n  e ff i ci en cy0-6 ]2.2 .  1.0 0.0Co nv er si on[ x14060i on  [ %]C8.0 mm2.2 1.7 (b)20u mp de pl et i1.0 0 1 2 30Pump fluence [J/cm2]PuFIG. 3 (a) The THz-wave output conversion efficiency. (b) Pump depletion for variouspump spot sizes. The figure shows a reduction in the threshold and an increase in theslope, indicating an improvement in the efficiency.ficiency of 0.017%, since the frequency of the THz wave is 200times lower than that of the pump beam. For the two smallerbeam sizes, 1.0 mm and 1.7 mm, the maximum pump fluencewas limited by the damage threshold of the crystal surfaces,which is typically about 2.6 J/cm2. For the pump beam sizes of2.0 mm and 8.0 mm, the maximum input energy was limitedby the available Nd:YAG laser output pulse energy of 60 mJand 500 mJ respectively.Figure 3(b) shows the dependence of the pump depletion onthe pump beam size when 1.46 THz was generated. We defineFig. 42[ J/ cm2 ]1l d f lu en ce  [0 0 0 2 0 4 0 6 0 8 1 00T hr es ho. . . . . .Beam diameter [cm]FIG. 4 Measured threshold fluence at difference beam spot sizes. The solid line is afitted curve based on an inverse quadratic function.the pump depletion as:H ≈ (Em − Ea)/Em (1)where Em is the pump pulse energy after it has passed throughthe misaligned cavity and Ea is the pump energy after it haspassed through the aligned cavity. When the cavity is aligned,the reduction of pump energy was observed. The some ofpump energy was converted into idler and THz-wave energy.Eais reduced energy due to the parametric conversion. In con-trast, misaligning the cavity can suppress the nonlinear con-version. Em is the input pump energy without depletion. Themaximum observed pump depletion was 54% at a pump flu-ence of 0.92 J/cm2 for a pump beam size of 8.0 mm. This is thelargest depletion observed in our TPO research. Figures 3(a)and 3(b) show that the oscillation threshold decreases and theslope efficiency increases as the width of the pump beam isincreased.The spectral width was increased by increasing the pumpbeam size. The measured THz-wave spectral width estimatedfrom the idler waves was greater than 500 GHz above0.5 J/cm2 when a pump beam size of 8.0 mm was used.Injection seeding or an intracavity grazing-incidence gratingcould be used to reduce the spectral width [12].Figure 4 shows a reduction in the threshold. In a singly res-onant OPO, the threshold is inversely proportional to thesquare of the effective interaction length [16]. The solid line isa fitted curve that is an inverse quadratic function of the beamsize. The resonating idler and nonresonating THz beams arecrossed with the pump beam using a noncollinear geometry.If we assume that the effective interaction length is propor-tional to the pump beam diameter and transverse interactionvolume is not significant affect the threshold fluence, we canroughly estimate that the threshold is inversely proportionalto the beam diameter. The threshold fluence reduces drasti-cally in small beam diameter. This result shows that the im-provement in the efficiency is due to increasing the interac-tions.10054- 3Journal of the European Optical Society - Rapid Publications 5, 10054 (2010) T. Ikari et al.The merit of SE-TPO is energy scaling by expanding the ra-diation area with large aperture pumping. The investigationof the dependence of the output-coupling efficiency on thepump beam size is important to discuss the possibility of thefurther upscaling. The external efficiency (η) for a nonresonantwave as a function of pump depletion (H)isη =ωTHzωp· Loss−1 · H (2)where Loss includes the absorption in the crystal and Fresnelreflection at the surface of the THz wave. From Eq. (2), we canestimate Loss by measuring the conversion efficiency η andthe pump depletion H by considering ωT/ωp = 5.2× 10−3.η and H can be obtained by simultaneous measurements andthey are shown in Figures 3(a) and 3(b). The output-couplingefficiency (Tr) is given by the inverse of Loss. The output-coupling efficiency was measured for different beam diam-eters and it were ranges from Tr = Loss−1 = 2.5 × 10−4to 5.0× 10−4 for all beam diameters. This demonstrates thatthe transmission efficiency is not strongly dependent on thepump beam diameter, indicating that the energy can be scaledup further. We note that this results including the error relatedto the term of ωT/ωp in Eq. (2), because the relatively widespectral bandwidth for 8.0 mm pump beam size, which was500 GHz centered at 1.46 THz.4 CONCLUSIONSUnlike difference frequency generation, which requires a dualfrequency tunable source, the TPO uses only a single laser. Weproposed the method to utilize the full energy of a Nd:YAGlaser to increase the output energy. We have shown that in thenoncollinear phase matching geometry, the threshold fluenceand pump depletion depend on the pump beam size. Whenthe beam size is increased, the threshold fluence decreasesand the pump depletion increases. With a large pump beamsize (diameter: 8.0 mm) and a high pump energy, we obtaineda pump depletion of 54% with 0.96 J/cm2. We also demon-strated that the output coupling loss does not depended onthe pump beam size in the SE configuration.ACKNOWLEDGEMENTSThe authors thank Dr. C. Otani of RIKEN Sendai and Prof.K. Kawase of Nagoya University for discussions on bolome-ter calibration, and Prof. S. Okajima and Dr. K. Nakayama inChubu University for providing the CW THz source for exper-imental verification of the bolometer sensitivity. The authorsthank Mr. Shoji of Teraphotonics Laboratory at RIKEN Sendaifor polishing the crystals and C. Takyu for coating the crystalsurface.References[1] M. Tonouchi, “Cutting-edge terahertz technology” Nat. Photonics1, 97–105 (2007).[2] H. Yoneyama, M. Yamashita, S. Kasai, K. Kawase, H. Ito, andT. Ouchi, “Membrane device for holding biomolecule samples forterahertz spectroscopy” Opt. Commun. 281, 1909–1913 (2008).[3] K. Kawase, Y. Ogawa, Y. Watanabe, and H. 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Energy scalable terahertz-wave parametric oscillator using surface-emitted configuration

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Energy scalable terahertz-wave parametric oscillator using surface-emitted configuration

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JEOS:RP - Journal of the European Optical Society Rapid publications