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Photonics 2014: 12th International Conference on Fiber Optics and Photonics © OSA 2014
Optical Parametric Amplifier Based Efficient Terahertz Generation in DAST Crystal using Optical Rectification K. S. Raoa, M. Venkatesha, , K. Thirupugalmanib ,S. Brahadeeswaranb and A.K. Chaudharya,* a
Advanced Center of Research in High Energy Materials(ACRHEM),University of Hyderabad, Hyderabad-500046(India) . b Department of Physics, Bharathidasan Institute of Technology, Anna University, Tiruchirapalli-620024(India). *Email:
[email protected],
[email protected]
Abstract: We report generation of an efficient THz signal of the order of 4.82µW from an indigenously grown organic crystal DAST using tunable wavelength between 1150-1600nm ranges obtained from 40fs, 1 kHz OPA. OCIS codes :( 190.4710) Optical nonlinearities in organic materials; (190.7110) Ultrafast nonlinear optics; (190.4223) Nonlinear wave mixing:
1. Introduction In recent years the generation and detection of powerful tunable terahertz (THz) waves using nonlinear optical processes attracted several research groups as important molecules such as proteins, pharmaceuticals, explosives and narcotic drugs etc., which are of interest to scientific community, exhibit their characteristic finger prints in this region of electromagnetic spectrum, especially in the frequency range of 0.1 to 10 THz. The generation of tunable THz radiation is achieved by means of different optical processes such as PC antennas, E-O sampling, DFG and Optical rectification (OR) etc [1-4]. Amongst all these techniques, the OR is one of the preferred techniques for the generation of powerful THz signal using ultrashort pulses. The THz enhancement by OR can be achieved by equating the phase velocity matching conditions between the generated THz pulse and corresponding group index of optical pump pulses[5]. As compared to other nonlinear crystals such as ZnTe, GaSe, CdTe etc. the second order nonlinear coupling coefficient of DAST is much more higher [6,7]. We report the generation of efficient THz signal from indigenously grown organic nonlinear crystal 4-N, Ndimethylamino-4’-N’-methyl-stilbazoliumtoyslate (DAST). It is a non centrosymmetric molecular crystal possesses a large optical nonlinearity. It belongs to monoclinic space group Cc, point group m, with four molecules units for unit cell [8,9]. The DAST crystal has high nonlinearity coefficient broad transmission between 0.8µm to THz range. Therefore it has attracted many researches. By selecting different wavelengths we can generate the wide range of efficient THz radiation. Table.1. NLO coefficients d=1/2(2)(-2ω, ω, ω) of DAST determined by Marker-Fringe experiments [10] λ(nm) d111(pm/V) d122(pm/V) d212(pm/V) 1318 1010110 969 5312 1542 29015 413 253 The nonlinear coupling coefficient deff value of DAST crystal is higher at 1318nm as compared to that at 1542nm. The efficient THz pulses can be achieved in nonlinear organic crystals when the velocity matching condition is fulfilled with generated THz pulses and optical pump beams because the low dielectric constants allow the phase velocity of THz beam and the group velocity of the optical pump beam[11]. 2. Experimental The DAST was successfully synthesized in the laboratory by adding the reactants 4-picoline, methyl 4toluenesulphonate in equimolar ratio and using piperidine as a catalyst. The synthesized starting material was then purified by re-crystallization process several times before proceeding for the growth with methanol as solvent. The temperature of the bath (equipped with a controlled accuracy ±0.01 ºC) was kept at 30 ºC at which the concentration of the solution was about 19.8 g/l. Later the temperature of the solution was increased to 40 ºC for complete dissolution and it was at this temperature for about 5 h. Subsequently, the temperature of the solution was slowly lowered to 30 ºC with a cooling rate of 0.10 ºC/h. The growth was performed by isothermal slow evaporation technique at 30 ºC. Transparent single crystals of DAST were harvested after a period of about 10 days. A typical DAST crystal (with a dimension of 3 x 3 x 1 mm3) thus grown in the laboratory is shown in Fig. 1. The DAST crystals were then studied for their THz generation efficiency. The experimental layout employed for this study is shown in the Fig. 2. The required tunability of pump wavelength was obtained from an optical parametric amplifier (TOPAS-C, Coherent). The input wavelength for OPA was supplied from regenerative amplifier which provides an output wavelength of 800nm, pulse duration 40fs and repetition rate 1 kHz. The tunable optical pump pulses obtained between 1150-1600 nm was vertically polarized and separated out from their
S5A.28.pdf
Photonics 2014: 12th International Conference on Fiber Optics and Photonics © OSA 2014
corresponding Idler and residual second harmonics using suitable wavelength separators. The tunable pump beam was then allowed to fall on a 0.2 mm thick DAST crystal to generate the THz signal and its second harmonics. A Teflon sheet and silicon lens of thickness of 2 and 10 mm respectively were used as filters for the residual incident unconverted and SHG signals. The generated THz was detected by the pyro-electric detector (Gentec) which was placed 5cm away from the crystal whereas second harmonic signal was detected in transmission mode using USB 4000 Spectrometer (Endor), which can detect the signals in the range between 200 and 1100 nm. The pyro-electric detector consists of an optical chopper which is used to modulate the optical pump pulses at the rate 25 Hz. The modulated pump beam was allowed to fall on the DAST crystal. The generated THz signal was detected with the help of a pyro-electric head (sensor) connected to a T-Rad system which consists of pre amplifier and lock –inamplifier and used for the signal processing. Finally the output of the T-Rad was fed to the PC for monitoring the strength of the generated THz signal.
Fig.1. The picture of DAST crystal
Fig. 2. Experimental set up
3. Results and discussion Figures 3(a) and (b) show the generation process of THz in the DAST crystal using tunable pump wavelength between 1150-1475nm and 1500-1600nm range, respectively. The duration and repetition rate of these pulses are 40fs and 1 kHz respectively. The diameter of incident laser beam is 2mm which is vertically polarized. It provides the fluency of 0.96 mJ/cm2 correspond to incident power of the order of 30 mW. The generated power of THz signal exhibited wavelength dependence and lied between 0.83 – 4.82µW. Fig. 3(a) showed the strength of generated THz power as a function of wavelength tunability between 1150- 1300 nm range which is linear in nature. After crossing the 1300 nm wavelength the THz power started falling and this trend continued up to 1475 nm. The maximum strength of the generated THz signal was of the order of 4.83 µW which was recorded at 1300 nm while minimum strength of the THz signals of the order of 0.83 µW at 1475nm. Once again the power of THz signal showed growth between 1500 and 1600nm range.
Figs. 3 a and b. THz output power vs wavelength (c) THz output power (d) SH signal intensity with rotation of crystal at 1300nm
In case of THz generation using OR the second harmonic signal (SHS) is also generated. It is due to shuttling of generated electromagnetic energy with respect to wave propagation direction of THz and SHS with respect to the incident pump laser wavelength. The second harmonic signal generated by DAST crystal get transmitted and reflected from the crystal surface. The group vvelocity matching condition between THz pulses and optical pump pulses at1300 nm wavelength in DAST crystal is responsible for the efficient generation of THz signal[5,12]. The effect of rotation of crystal plays crucial role to distinguish the THz signal from the SHS at wavelength 1300 nm since the nonlinear coupling coefficient deff and transmission of THz signal in DAST crystal are higher than other wavelength range. As a result, the measured strength of generated THz was of the order of 4.82 µW, with the
S5A.28.pdf
Photonics 2014: 12th International Conference on Fiber Optics and Photonics © OSA 2014
experimental conversion efficiency Ƞ= 0.0161%. Figures 3 (c) and (d) showed the polar graph of generated THz power and corresponding SHS intensity which were collected by USB 4000 spectrometer with respect to the orientation of the DAST crystal in its transmission mode. The input fluency of the laser pulse is 0.668 mJ/cm2. The generated THz power was less when the DAST crystal generated higher second harmonic signal. This means that the generated energy due to sum frequency and OR processes occurred simultaneously with some small group velocity mismatching. The generation could be controlled by the deff value of the crystal orientation. However, the pointing vector which shows the direction of energy flow of generated signal is affected by walk-off effect. When the walk-off angle between generated THz signal and wave propagation direction is very less then the corresponding strength of THz signal is very high since it possesses higher walk-off angle with respect to SHG. It is also true in other direction when strength of SHG is more than the THz signal. Figures 3 (c) and (d) clearly revealed the effect of generation of second harmonic signal with respect to the vertical rotation of the crystal. The generation mechanism of THZ and SHS are shown in Figures 3 (c) and (d). The polar graph is divided in to four quadrants where the position of lobs shows the strength of the generated signals. It is clearly shown that at 0 o the strength of THz signal is less whereas the strength of SHS is comparatively higher. In second quadrant the maximum power of THz signal is located between 90-120o, whereas there is another maximum of SHS which lies once again in the first quadrant between 30-90o. Similarly, in third quadrant the strength of the generated THz signal is relatively low and located at 180o. However, the strength of SHS is lowest and located at 180o. The entire energy transfer mechanism is repeated in the third and fourth quadrants too and shown in Figures 3 (c) and (d). The rotation angle from 90-270o in Fig.3c and 270-90o in Fig.3d showed that the curve is far away from the origin of the polar plot and vice versa in other side of the polar plot. Hence it indicates that higher the strength of the second harmonic signal lesser the generated THz power. 4. Conclusions We have reported the potential use of indigenously grown DAST crystal to generate efficient and wide range of THz signal using pump wavelength tunable between 1150-1600 nm ranges obtained by OPA. We have also demonstrated the effect of vertical crystal rotation on the generation of SHS and THz signals at 1300nm wavelength. These results exhibited the shuttling of electromagnetic radiation between THz to SHS. The higher efficiency of the generated THz signals could be attributed to the nonlinear coupling coefficients deff values of the DAST crystal. Acknowledgements The authors (KSRao, MV and AKC) gratefully acknowledge D.R.D.O., Ministry of Defense, Govt. of India, India, for the financial support and KT and SB would like to acknowledge DST (SR/S2/CMP-0028/2011 Dated: 0112-2012) for the financial support. Our special thanks to Dr. S.V. Rao, ACRHEM, University of Hyderabad for providing TOPAS facility. References: [1]
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