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Far-infrared properties of DAST Markus Walther,* Kasper Jensby, and Søren Rud Keiding Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark
Hidenori Takahashi and Hiromasa Ito Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katakira, Aoba-ku, Sendai-shi, 980-8577 Japan Received April 7, 2000 Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorption coeff icient of the organic ionic salt 4-N, N-dimethylamino-40 -N 0 -methyl-stilbazolium tosylate (DAST). This promising organic electro-optic material exhibits strong absorption and dispersion for frequencies above 1 THz at both room temperature and 83 K. No reduction in the absorption strength is observed when DAST is cooled, suggesting a single-phonon origin of the resonances. A simple vibration of the anion and cation of the salt is suggested as the origin of the exceptionally strong far-infrared absorption and the high-frequency electro-optic properties of DAST. 2000 Optical Society of America OCIS codes: 160.4330, 320.7110, 300.6270.
The past decade has seen increasing applications of tailored organic materials in nonlinear optics. Most organic molecules with high hyperpolarizability b will crystallize in centrosymmetric crystals as a result of the strong dipole –dipole interaction and will consequently not exhibit second-order nonlinearity x 2 . However, by incorporating highly nonlinear molecules as cations in organic salts1 one can, depending on the anion, obtain noncentrosymmetric crystals with large macroscopic second-order optical nonlinearity. One of the best examples is the organic salt 4-N, N-dimethylamino-40 -N 0 -methyl-stilbazolium tosylate (DAST),2 whose chemical structure is shown in Fig. 1. Large DAST crystals of good optical quality can be grown, and the combination of low dielectric constant and high nonlinearity 3 – 5 makes DAST crystals promising candidates for high-speed modulation and frequency mixing applications, including generation and detection of terahertz (THz) radiation. The most recent application of DAST crystals has been for high-frequency electro-optic modulation,6,7 detection, and generation of THz radiation, either through difference-frequency generation (DFG) mixing of two simultaneously oscillating lines from an electronically tuned Ti:Al2 O3 laser8 or by use of femtosecond optical pulses.9,10 The modulation frequencies are approaching 0.1 THz, and the frequencies of the difference-frequency generated THz beams are well into the THz regime 共n 苷 1.4 THz兲; knowledge of the linear optical properties of DAST becomes important for phase-matching and efficiency considerations. In this Letter we report the index of refraction and the absorption coeff icient of DAST in the range from 0 to 3 THz (0 to 90 cm21 ). The data were obtained by THz time-domain spectroscopy.11 Only limited information is available on the properties of molecular crystals in the far-infrared (THz) spectral region, and, in addition to practical applications, the far-infrared properties of DAST provide an illustrative example of the very strong and characteristic interaction between far-infrared radiation and molecular crystals. We use a standard THz spectrometer11,12 with an additional focus added that allows us to investigate 0146-9592/00/120911-03$15.00/0
small samples (minimum sample diameter, ⬃0.5 mm). To ensure linear polarization of the THz pulse, a wire grid polarizer is inserted between the THz emitter and the sample. The ratio of the detected vertical-tohorizontal field amplitude is smaller than 1:250. The present setup can generate and detect THz radiation with a high signal-to-noise ratio (S/N) from 0.1 to 4 THz. The samples are mounted in a f low cryostat equipped with high-resistivity silicon windows. The temperature can be controlled to better than 0.1 K from 78 to 400 K. The temperature is measured close to the sample with a calibrated silicon diode. The DAST crystals examined were high-quality crystals grown by controlled temperature lowering of a saturated methanol solution containing small seed crystals.7 The crystalline quality was controlled and conf irmed in a polarization microscope. The two crystals used had thicknesses of 465 6 3 and 435 6 3 mm, with the crystallographic c axis perpendicular to the f lat surface of the crystals. The c axis and the
Fig. 1.
Chemical structure of DAST.
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dielectric z共3兲 axis coincided, and we report the optical properties of the x共1兲 and y共2兲 axes, corresponding to the complex index of refraction nˆ m 苷 nm 1 i共am c兾2v兲, where m is equal 1 or 2. The x共1兲 axis of the dielectric tensor nearly coincided with the crystallographic a axis, and the y共1兲 axis coincided with the b axis of the crystals. Figures 2 and 3 show the index of refraction and the absorption coeff icient obtained by rotation of the crystal in the linearly polarized THz beam. In each figure, the index and absorption are shown both at room temperature and at 83 K. Good agreement between the observed indices, extrapolated to zero frequency, and the square root of the static dielectric constants6 can be observed. We observed several strong resonances in the spectrum, and only a modest decrease in absorption was observed when the sample was cooled to low temperatures. The positions of the resonances shifted to slightly higher frequencies and the linewidth narrowed when the crystal was cooled, as a result of smaller anharmonicity at lower temperatures. Associated with the strong resonance absorption is a highly dispersive index of refraction, with as much as 100% change in index on resonance. With a conventional far-infrared spectrometer the power transmitted through the sample at 1 THz would be only 1024 of the incident power, precluding accurate evaluation of the absorption coeff icient. In the THz time-domain spectrometer the amplitude is detected and in the present example 1% of the incident amplitude is transmitted, allowing us to determine both the absorption coeff icient and the index of refraction. Because of the high absorption coefficient observed in DAST, and the thickness of the crystals, an accurate determination of the peak absorption is still problematic. An improved value of the peak absorption can, however, be obtained if we use the measured index of refraction to estimate the linewidth and strength, as illustrated in Fig. 2, where the index of refraction and the absorption coeff icient were fitted simultaneously to the standard expression for a Lorentzian oscillator. Good agreement was obtained both at room temperature and at LN2 temperature, except for the peak value of the absorption coefficient. The maximum absorption coeff icient measurable can be obtained from the S/N of the THz time-domain spectrometer. Assuming that the crystal absorbs all the THz radiation at a given frequency, the spectral amplitude at this frequency is given by the spectral noise level, N. This signal was compared with the spectral amplitude without the crystal, S. From this we obtained an apparent absorption coeff icient a 苷 2d21 ln共S兾N兲, where d is the thickness of the crystal. For the given case 共d 苷 435 mm兲 the spectral S/N was of the order of 5 3 102 at 1 THz, corresponding to a maximum measurable absorption coefficient of 250 cm21 . The measured peak absorption near 1 THz in Figs. 2 and 3 and the absorption above 1.5 THz in Fig. 3 are therefore too small, and the fitted Lorentzian line shape is a better estimate of the peak absorption. The validity of the absorption strength obtained from the fitting procedure is corroborated by the good agreement between the measured and the fitted indices of refraction.
In the DAST crystals, we expect that the simple vibration of the anion –cation pair corresponding to the reststrahlen band is the main cause of the strong absorption observed in the far infrared. From a simple estimate of the transverse optical mode frequency,13 vTO 2 苷 Ne2 共e` 1 2兲2 兾9me0 共edc 2 e` 兲, we obtained room-temperature resonance frequencies of vTO i 2p 苷 2.28, 1.13, 1.50 THz, where i 苷 1, 2, 3, corresponding to the main axis of the dielectric tensor. Density N was obtained from the volume of the unit cell,14 V 苷 2.098 nm3 with four DAST molecules per unit cell; m is the reduced mass of the anion –cation pair and is equal to 100 u. The estimates include the full unscreened ionic charge and are in qualitative agreement with the observed resonances in Figs. 2 and 3. When the temperature was lowered to 83 K the linewidth was reduced by approximately a factor of 2 and the peak absorption increased, indicating that the integrated absorption was constant. These observations strongly indicate the single-phonon character of the absorption process as opposed to the strongly temperature-dependent multiphonon processes often observed in the far infrared. As for other electro-optic crystals,15 we expect that the large linear electro-optic coeff icients4 arise from the strong polar low-frequency vibrations of the ionic crystal and the polarizable electronic structure corresponding to the conjugated p-electron structure on the stilbazolium cation. This simple picture can be supported by investigation of the size and symmetry of the linear electro-optic tensor: An
Fig. 2. Index and absorption for DAST close to the 关100兴兾a direction. The dotted curves are calculated from a standard Lorentzian line shape.
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electro-optic modulation and loss owing to linear absorption of the modulating field. H. Takahashi and H. Ito, appreciate the support of the Projects for Creative Telecommunication Development Program of the Telecommunications Advancement Organization, Japan, and of the Regional Consortium R&D program conducted by the New Energy and Industrial Technology Development Organization, Japan. S. R. Keiding’s e-mail address is
[email protected]. ¨ ¨ *Permanent address, Fakultat fur Physik, ¨ Albert-Ludwigs Universitat, D-79104 Freiburg, Germany. References
Fig. 3. Index and absorption for DAST along the 关010兴兾b direction.
optical beam polarized along the stilbazolium cation (a axis) is efficiently modulated by a low-frequency field along the direction of the polar vibration (c axis), corresponding to a large nonlinear optical element r113 . The opposite geometry, corresponding to the element r331 , results in weak modulation. Unfortunately, eff icient electro-optic modulation is limited at higher frequencies by the strong far-infrared absorption from the polar vibrations. Consequently, the practical phase-matched applications of DAST crystals will correspondingly be limited to the low-loss region below 0.5 THz. However, identifying the polar vibration of the organic salt as the cause of the strong far-infrared absorption also suggests new strategies for the synthesis of new electro-optic materials. Changing the weight of the anion –cation pair allows one to optimize the balance between resonant enhancement of the
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