GENERATION OF WIDELY TUNABLE FOURIER-TRANSFORM-LIMITED PULSED TERAHERTZ RADIATION USING NARROWBAND NEAR-INFRARED LASER RADIATION Jinjun Liu, Christa Haase, and Frédéric Merkt Laboratorium für Physikalische Chemie, ETH Zürich, CH-8093 Zürich, Switzerland

Narrowband THz generation in DAST via DFG [1] K. Suizu, et. al., Opt. Lett. 32, 2885 (2007) [2] K. Suizu, et. al., Proc. of SPIE 6103, 61030A (2006) Difference Frequency Generation of ns laser pulses DAST (4’-dimethylamino-N-methyl-4-stilbazolium tosylate)

Experimental setup ν 2 (tunable) ν 1 (fixed) ν THz

DAST cut for phase matching c a b θ 40 o 50 o k polarization of THz (ac) polarization of 800nm (b)

THz output power Generated by the cut DAST crystal (thickness=1 mm) NIR peak power ~16 kW, 1/10 of the damage threshold Normalized by the transmission of the vacuum window (PE), the THz cut-on filter (Sapphire) of the detector and the black PE filter used to block NIR. _____ NIR polarization perpendicular to b axis, AOI=0 o ____ NIR polarization parallel to b axis, AOI=0 o NIR polarization parallel to b axis, AOI=10 o X10 (absorption efficiency & acceptance of the detector [3]) [3] M. Rochat, Ph.D. thesis, Université de Neuchâtel, Switzerland, 2002.

Optimization of output power at 7.5 THz Optimum crystal length

Comparison with other emitters LTG-GaAs photoconductive antenna: Low damage threshold Narrow frequency range: 0-2 THz Maximum output power: 0.3 THz ZnTe crystal Narrow frequency range: 0-3 THz Damage threshold: ~1 MW/cm 2 GaP crystal Narrow frequency range: peak ~8 THz Damage threshold: ~5 MW/cm 2 S. N. Orlov et al. Quantum Electron. 37, 36 (2007).

Comparison with other crystals crystal length peak power density damage threshold 340 μm0.23 MW/cm 2 ~3 MW/cm 2 1 mm0.23 MW/cm 2 ~3 MW/cm 2 1 mm0.23 MW/cm 2 ~1 MW/cm mm0.85 MW/cm 2 ~5 MW/cm 2

Simulation of THz generation Experimental Simulated / 10  DAST [4] F. Pan, G. Knöpfle, Ch. Bosshard, S. Follonier, R. Spreiter, M.S. Wong, P. Günter, Appl. Phys. Lett. 69 (1996) 13. [5] A. Schneider, M. Neis, M. Stillhart, B. Ruiz, R.U.A. Khan, P. Günter, J. Opt. Soc. Am. B 23 (2006) [6] G. Knöpfle, R. Schlesser, R. Ducret, P. Günter, Nonlinear Opt. 9 (1995) 143. [7] M. Walther, K. Jensby, S.R. Keiding, H. Takahashi, H. Ito, Opt. Lett. 25 (2000) 911. [8] S. Follonier, M. Fierz, I. Biaggio, U. Meier, Ch. Bosshard, P. Günter, J. Opt. Soc. Am. B 19 (2002) [9] Ch. Bosshard, R. Spreiter, L. Degiorgi, P. Günter, Phys. Rev. B 66 (2002)  ZnTe [10] D.T.F. Marple, J. Appl. Phys. 35 (1964) 539. [11] T. Hattori, Y. Homma, A. Mitsuishi, M. Tacke, Opt. Commun. 7 (1973) 229.  GaP [12] M. Schall, M. Walther, P. Uhd Jepsen, Phys. Rev. B 64 (2001) [13] T. Hattori, K. Takeuchi, Opt. Expr. 15 (2007) [14] D.F. Parsons, P.D. Coleman, Appl. Opt. 10 (1971) [15] G.D. Boyd, T.J. Bridges, M.A. Pollack, E.H. Turner, Phys. Rev. Lett. 26 (1971) 387. [16] F.L. Madarasz, J.O. Dimmock, N. Dietz, K.J. Bachmann, J. Appl. Phys. 87 (2000) [17] T. Tanabe, K. Suto, J. Nishizawa, K. Saito, T. Kimura, Appl. Phys. Lett. 83 (2003) 237.

Measurement of bandwidth microwave mixer (downconversion) spectrum analyzer (BW=1 MHz) f =304GHz f LO ~304GHz f IF

Beat note and FFT microwave mixer (downconversion) spectrum analyzer (BW=1 MHz) f THz f LO f IF oscilloscope

Water absorption spectra

Pure rotational spectrum of OCS (2) MHz

Linewidth J ’=35  J ”=34 Center frequency = (8) THz [18] Absorption path length=1 m Pressure=0.5 mbar Δν pressure =4.33(15) MHz Δν Doppler, negligible FWHM~10 MHz Upper limit of Δν source =9 MHz [18] G.Yu. Golubiatnikov, A.V. Lapinov, A. Guarnieri, R. Knöchel, J. Mol. Spectrosc. 234 (2005) 190.

Pure rotational spectrum of HF J ’=4  J ”=3 Center frequency = THz [19] Δν Doppler =13 MHz FWHM=25 MHz Upper limit of Δν source =21 MHz [19] Nolt et al. J. Mol. Spectrosc. 125, 274 (1987)

Summary and outlook A new THz source with: wide tunability narrow bandwidth (~10MHz) high frequency accuracy enough output power for spectroscopy J. Liu and F. Merkt, Appl. Phys. Lett. 93, (2008) J. Liu, H. Schmutz and F. Merkt, J. Mol. Spectrosc. (2009), in press doi: /j.jms Extension to higher frequency region (>11 THz). Frequency locking of one of the ring lasers to Doppler-free I 2 transitions. Future spectroscopy Direct absorption spectroscopy using multipass cells. THz-UV double-resonance spectroscopy under jet- cooled conditions.

Acknowledgements Prof. Dr. Martin Quack (Laboratorium für Physikalische Chemie, ETH Zürich) Prof. Dr. Peter Günter, Dr. Arno Schneider (Institute for Quantum Electronics, ETH Zürich)

DAST for THz generation DAST (4’-dimethylamino-N-methyl-4-stilbazolium tosylate)  large birefringence (Δn = 800 nm)  large nonlinear optical susceptibilities: χ 111 = 1230±130; χ 122 = 166±16; χ 311 = 239±32; χ 212 = 135± nm)  large electro-optic coefficients: r 111 = 77±8; r 221 = 42±4; r 113 = 15±2; r 122 = 17.0± nm) DAST (4’-dimethylamino-N-methyl-4-stilbazolium tosylate)  large birefringence (Δn = 800 nm)  large nonlinear optical susceptibilities: χ 111 = 1230±130; χ 122 = 166±16; χ 311 = 239±32; χ 212 = 135± nm)  large electro-optic coefficients: r 111 = 77±8; r 221 = 42±4; r 113 = 15±2; r 122 = 17.0± nm) F. Pan, et. al., Appl. Phys. Lett., 69, 13 (1996) A. Schneider, et. al., J. Opt. Soc. Am. B, 23, 1822 (2006) Optical Rectification of fs laser pulses

Frequency stabilization Coherent Ti:Sa Ring Laser PD1 PD2 Lock-in / Piezo Driver/ Oscillator / RF Rriver Stabilized He-Ne AOM Lock-in / Oscillator PZTConfocal Fabry-Perot Cavity

Phase mismatching of DAST Taniuchi et al. Electro. Lett. 36, 1414 (2000) Han et al. Opt. Lett. 25, 675 (2000)

Consideration in DFG

Looking for phase matching 1.Construct index of refraction ellipsoid; 2.Define wave vector by polar angle and azimuthal angle ; 3.Determine the intersection of the ellipsoid and the plane normal to, an ellipse with major axis n max and minor axis n min. 4.For nonzero components of the nonlinear susceptibility tensor, calculate the coherent lengths Note that: 5.Maximize as a function of and. n2n2 n3n3 n1n1 n min n max Ferguson, Math. Geol. 11, 329, (1979)

NIR and THz properties of DAST Pan et al. Appl. Phys. Lett. 69, 13 (1996) M. Walther et al. Opt. Lett. 25, 911 (2000) Bosshard et al. Phys. Rev. B 66, (2002). ?... n 1 =2.43 n 2 =1.72 n 3 =1.72 α THz =50 cm -1 n 1 =2.43 n 2 =1.72 n 3 =1.72 α THz =50 cm -1

Calculated THz peak power using the cut DAST crystal (see above) pumped by 5kW NIR peak power with different angle between the wave vector (k) and the c- axis of the crystal. The calculation is based on measured indices of refraction and χ 122 component of the nonlinear susceptibility tensor. Predicted THz Radiation

Calculated THz peak power as a function of θ. Note the log scale. Predicted tuning curves

Conversion efficiency (top) Measured THz peak power as a function of NIR peak power used to pump the DAST crystal. The error bar for the THz peak power is estimated to be 10% of the absolute value. The dotted line represents the least-squares fit assuming a quadratic relationship. (bottom) THz conversion efficiency as a function of NIR peak power. The dotted line represents a linear fit. (top) Measured THz peak power as a function of NIR peak power used to pump the DAST crystal. The error bar for the THz peak power is estimated to be 10% of the absolute value. The dotted line represents the least-squares fit assuming a quadratic relationship. (bottom) THz conversion efficiency as a function of NIR peak power. The dotted line represents a linear fit.

Outline Motivation (generation of THz radiation in the DAST crystal via DFG) Experimental setup Phase-matching condition Comparison of different THz emitters Measurement of bandwidth Proof-of-principle spectroscopic measurement Conclusions and outlook [2] A. de Lange, E. Reinhold, and W. Ubachs, Phys. Rev. A 65, (2002). [3] Y. P. Zhang, C. H. Cheng, J. T. Kim, J. Stanojevic, and E. E. Eyler, Phys. Rev. Lett. 92, (2004). [4] L. Wolniewicz, J. Chem. Phys. 103, 1792 (1995).