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Nonlinear optical susceptibilities of diglycinyl thiourea for frequency conversion and optical limiting applications
Chemical Physics Letters 491 (2010) 248–253
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Nonlinear optical susceptibilities of diglycinyl thiourea for frequency conversion and optical limiting applications T.C. Sabari Girisun a,b, S. Dhanuskodi a,* a b
School of Physics, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India Department of Physics, Bishop Heber College, Tiruchirappalli 620 017, India
a r t i c l e
i n f o
Article history: Received 11 February 2010 In final form 7 April 2010 Available online 10 April 2010
a b s t r a c t Organic nonlinear optical material diglycinyl thiourea (DGT) has been synthesized and single crystals have been grown from the aqueous solution by solvent evaporation technique. Powder second harmonic generation (SHG) study shows that DGT is 2.5 times efficient than KDP and is phase matchable. The thirdorder nonlinear optical and optical limiting properties of DGT have been investigated using a 532 nm second harmonics of diode-pumped Nd:YAG laser (1064 nm, 50 mW). The magnitude of nonlinear refractive index, nonlinear absorption coefficient and third-order susceptibility was found to be in the order of 108 cm2/W, 103 cm/W and 106 esu, respectively. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Extensive research in organic functional group materials is an expanding area of research because of their interesting nonlinear optical effects extended to optical parametric amplifiers, optical parametric oscillators, Q-switched intracavity second harmonic devices, high optical damage threshold and other electro-optical applications [1–5]. Even though intensive efforts in the field have been made for about 30 years, it is still very attractive to search for new nonlinear optical materials with various practical interests. At present, inorganic crystals such as lithium niobate and potassium dihydrogen phosphate (KDP) are generally used in nonlinear optical devices. The search is for new nonlinear optical materials applied to ultraviolet (UV), even vacuum ultraviolet (VUV) and far infrared (FIR) regions, as well as direct second harmonic generation (SHG) of diode laser [6]. Optical limiters for low power CW lasers are also very important in applications such as prevention of optical damage to very sensitive sensors. For example, the human eye will be damaged permanently if exposed to a laser beam of power 1–5 mW for a few seconds. Information about the damage level of the sensor is necessary to determine the required limiting level of the device. Amino acid family crystals have over the years been subjected to extensive investigations by several researchers for their nonlinear optical properties. Although some of the amino acids are already reported to have NLO activity, the amino acid glycine based crystals are now a days shown greater interest for NLO applications [7]. Glycine is the simplest amino acid. Unlike other amino acids, it has no asymmetric carbon atom and is optically inactive. * Corresponding author. Fax: +91 0431 2407045. E-mail address: [email protected] (S. Dhanuskodi). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.04.014
The organic ligands such as thiocyanate SCN, urea CO(NH2)2 and thiourea CS(NH2)2 are more dominant in NLO effect. Thiourea molecule is an interesting matrix modifier due to its large dipole moment and its ability to form an extensive network of hydrogen bonds. Thiourea crystallizes in the centrosymmetric space group Pbnm and SHG inactive [8]. Combinations of amino acids with organic ligands are promising materials for optical second harmonic generation. The only work reported along these lines was that giving preliminary data for the diglycinyl thiourea (DGT) as an efficient NLO material [9]. Recently, a similar type of organic complex of resorcinol and urea- was identified to be an excellent second order NLO material. Such type of materials is extremely promising for NLO applications and in particular thiourea-based materials are already available in literature [10,11]. In continuation of our work on thiourea-based compounds, in the present study thiourea has been coordinated with glycine, with an idea of improving the nonlinear response. This Letter reports the nonlinear optical properties of the DGT by SHG Kurtz powder technique and Z-scan experiment. The SHG efficiency, SHG particle variation, the nonlinear refractive index, nonlinear absorption coefficient and third-order nonlinear susceptibility were reported. The optical limiting behavior of the sample was discussed in detail. The present work is on the design of a novel optical limiter for the low power regime. 2. Experimental 2.1. Synthesis and crystal growth The DGT was synthesized by taking glycine and thiourea in the molar ratio 2:1 according to the following reaction,
T.C. Sabari Girisun, S. Dhanuskodi / Chemical Physics Letters 491 (2010) 248–253
249
Fig. 1. (a) Solubility curve and (b) Single crystal of DGT.
2C2 H5 NO2 þ CH4 N2 S ! C5 H10 N4 S þ 2H2 O The prepared thiourea solution is mixed with glycine solution using a magnetic stirrer and is stirred continuously for 2 h at 45 °C. This saturated solution is taken in a clean beaker and left for evaporation. The material is further purified by the repeated recrystallization process. Solubility is one of the most promising factors which influence the growth of a crystal. The solubility of the DGT is determined with water as the solvent. On reaching saturation, the equilibrium concentration of the material is determined gravimetrically. The above procedure is repeated for every 5 °C in water, methanol and ethanol from 25 to 45 °C and the solubility curve for DGT is plotted (Fig. 1a). From the solubility, it can be inferred that the material has a moderate solubility at 30 °C. By slow evaporation technique, with water as a solvent, optically good quality single crystals (12 4 3 mm3) were obtained in a period of 28 days (Fig. 1b).
2.2. Structural and nonlinear optical studies The grown crystal have been crushed to a uniform fine powder and subjected to powder X-ray diffraction using a Bruker AXS D8 Advance powder X-ray diffractometer, (CuKa, 1.5406 Å) and was scanned in the reflection mode in the 2h range 10–80°.
The first and the most widely used technique for confirming the SHG from prospective second order NLO materials is the Kurtz and Perry powder technique [12] to identify the materials with noncentrosymmetric crystal structures. A preliminary study of the powder SHG measurements were performed using a modified Kurtz technique (Nd:YAG 1064 nm, 8 ns, 10 Hz). The crystalline sample was powdered to different particle sizes in the range 53– 250 lm. To make relevant comparisons with known SHG materials, KDP was also ground and sieved into the same particle size range. The powdered samples were filled air-tight in separate micro-capillary tubes of uniform bore of about 1.5 mm diameter. The input laser energy incident on the capillary tube was chosen to be 3 mJ, an energy level optimized not to cause any chemical decomposition of the sample. The SHG output (532 nm) in each case was measured as the average of a few pulses to eliminate slight variations of input power. The transmittance through the aperture as a function of the distance between the sample and the beam waist is called Z-scan curve. The Z-scan technique offers a useful path to probe both the nonlinear refractive index and the nonlinear absorption coefficient of a sample. The nonlinear refractive index of DGT was determined by the closed Z-scan method developed by Sheik-Bahae et al. [13] where a linearly polarized GAUSSIAN beam from a diodepumped Nd:YAG laser of wavelength 532 nm was used as the excitation source for the Z-scan technique. The GAUSSIAN profiled laser
250
T.C. Sabari Girisun, S. Dhanuskodi / Chemical Physics Letters 491 (2010) 248–253
beam was focused on a 1 mm quevette containing the 1 mM concentration DGT solution by lens, f = 3.5 cm to produce a beam waist x0 of 15.35 lm. As the sample is scanned through the beam, the far field profile shows intensity variation across the beam profile, which is recorded through an aperture. Open aperture Z-scan was performed on the material in order to probe the nonlinear absorption mechanisms. Also, the experiment was repeated with pure solvent (water) to measure its contribution and no significant feature was observed in either the open or the closed Z-scan traces. The experimental set-up for the demonstration of optical limiting of the laser beam by DGT under CW laser illumination is very similar to the standard Z-scan geometry. Additionally a polarizer analyzer combination (PA) was used to vary the input power. The cuvette containing the nonlinear medium is placed just after the focal point. This beam is then made to fall on the optical sensor, a photo detector in this case. The input laser intensity is varied systematically and the corresponding output intensity values were measured by the photo detector.
3. Results and discussion
pffiffiffi 2 2 DT b¼ I0 Leff
where DT is the one valley value at the open aperture Z-scan curve. The value of b will be negative for saturable absorption and positive for two photon absorption. The real and imaginary parts of the third-order nonlinear optical susceptibility v(3) are defined as
Revð3Þ ðesuÞ ¼ 104 ðe0 C 2 n20 n2 Þ=p ðcm2 =WÞ
ð5Þ
Imvð3Þ ðesuÞ ¼ 102 ðe0 C 2 n20 kbÞ=4p2 ðcm=WÞ
ð6Þ
where e0 is the vacuum permittivity, n0 is the linear refractive index of the sample and c is the velocity of light in vacuum. The third-order nonlinear optical susceptibility is thus
vð3Þ ¼
ð1Þ
where S is the aperture linear transmittance and is calculated using the relation
S ¼ 1 expð2r 2a =x2a Þ
ð2Þ
where ra is the aperture and xa is the beam radius at the aperture. The nonlinear refractive index is given
ð7Þ
vð3Þ
ð8Þ
L4 N
where N is the density of molecules. The term L is the local field factor which in the Lorentz approximation is given by L ¼ ðn20 þ 2Þ=3 where n0 is linear refractive index of the medium. The normalized transmittance obtained from the closed (Fig. 2a) and open (Fig. 2b) Z-scan experiment as a function of
a Normalised Transmittance
DT p—v ¼ 0:406ð1 SÞ0:25 jD/j
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðReðvð3Þ ÞÞ2 ðImðvð3Þ ÞÞ2 :
Also the second order hyperpolarizability which describes the nonlinear induced polarization per molecule, is related to the third-order bulk susceptibility as [12]
ch ¼
From the powder X-ray diffraction data, the lattice parameters have been calculated (Table 1). There is a close agreement with the reported values from single crystal X-ray diffraction. The effective nonlinear refractive coefficient n2 is calculated by n = no + Dn = no + n2I, where n is the total refractive index, no is the linear refractive index without illumination, Dn is the total nonlinear refractive index change induced by illumination light, and I is the intensity of the GAUSSIAN beam inside the sample. For the calculation of the nonlinear refractive index the following standard relations are utilized [13]. The difference between the normalized peak and valley transmission (DTp–v) is written in terms of the on axis phase shift |D/| at the focus as,
ð4Þ
1.4 1.2 1 0.8 0.6 0.4 0.2 0
D/ n2 ¼ KI0 Leff
-15
ð3Þ
Table 1 Cell parameters of DGT. Data
DGT
System a (Å) b (Å) c (Å) a (deg) b (deg) c (deg)
Present Monoclinic 5.113 (2) 11.988 (4) 5.564 (3) 90 112 90
[9] Monoclinic 5.091 11.980 5.45 90 111.92 90
b Normalised Transmittance
where K = 2p/k (k is the laser wavelength), I0 is the intensity of the laser beam at focus (Z = 0), Leff is the effective thickness of the sample, [Leff = [1 exp(aL)]/a], L is the thickness of the sample and a is the linear absorption coefficient. From the open aperture Z-scan data, the nonlinear absorption coefficient is estimated
-10
-5
0 z (mm)
5
10
0 z (mm)
5
10
15
1.14 1.12 1.1 1.08 1.06 1.04 1.02 1 0.98 -15
-10
-5
Fig. 2. (a) Z-scan closed aperture and (b) Z-scan open aperture.
15
T.C. Sabari Girisun, S. Dhanuskodi / Chemical Physics Letters 491 (2010) 248–253
Normalised Transmittance
1.4
7 6
Output Intensity (mW)
the on axis distance Z measured from the focal point. The pattern refers to saturable absorption and the b is 7.27 103 cm W1. Since the closed aperture transmittance is affected by the nonlinear refraction and absorption, the determination of n2 is less straight forward. Hence to obtain the pure effective n2 is to divide the closed aperture transmittance with its corresponding open aperture transmittance and the obtained graph is shown in Fig. 3. A pre-focus peak followed by a post-focus valley indicates that the nonlinear property of the sample is negative and the lensing effect is defocusing. The value of n2 obtained for the sample DGT is 4.93 108 cm2 W1, the value is lager than its parent molecule thiourea (3.61 108 cm2 W1) and glycine (3.48 108 cm2 W1). This increase in nonlinear response clearly picturizes the increase in the delocalization due to the complex formation. Thus it is worth while to note here that not only the metal complex formation increases the nonlinear response, but also the combination of organic NLO chromophore will enhance the nonlinearity. The variation in n2 can be due to different mechanisms depending on the nature of the medium. In this case with CW laser irradiation, it is attributed to the thermally induced variation in the nonlinear refractive index [14]. Table 2 summarizes the nonlinear optical parameters evaluated for DGT from the Z-scan patterns. v(3) of DGT is found to be larger than the well known inorganic compounds. This is due to the p electron cloud movement from the donor to the acceptor, which makes the molecule highly polarizable. Fig. 4 shows the optical limiting behavior of DGT at 0.1 M concentration of the DGT solution. For incident energy less than 18.6 mW, the output varies linearly according to Beer’s law and above this, the output saturates and becomes a plateau. The
251
5 4 3 2 1 0 0
10
20 30 Input Intensity (mW)
40
Fig. 4. Optical limiting behavior of DGT.
threshold value is 18.6 mW and the corresponding output value gets clamped at 5.2 mW. This verifies that the samples are good candidates for optical limiting at 532 nm CW lasers. Thus DGT with strong nonlinear response and self-defocusing behavior is a potential candidate for the protection of optical sensors such as night vision devices [15]. The nonlinearity in this case being of thermal origin, the value of the nonlinear refractive index depends on the sample concentration. The optical limiting threshold can be improved by changing the operational parameters such as sample concentration, aperture size and the focal length of the lens. Thus the optical arrangement can be modified according to the requirements of the dynamic range and field of view of the sensor. Numerically the response time of optical limiting action for the samples is estimated using the relation [16]
1.2 1
s
0.8 0.6 0.4 0.2 0 -15
-10
-5
0 z (mm)
5
10
15
Fig. 3. Z-scan ratio of closed and open aperture.
Table 2 Comparison of NLO parameters of TU and DGT. Parameters
Thiourea
DGT
Nonlinear refractive index (n2) 108 cm2/W Nonlinear absorption coefficient (b) 103 cm/W Real part of the third-order susceptibility [Re(v(3))] 106 esu Imaginary part of the third-order susceptibility [Im(v(3))] 106 esu Third-order nonlinear optical susceptibility (v(3)) 106 esu Second-order hyperpolarizability (ch) 106 esu Optical limiting threshold value (mW)
3.61 7.27
4.93 7.27
4.44
1.96
3.78
2.63
5.8
3.7
1.84
7.92
18.6/1.72
18.6/6
ðqCÞR2
j
ð9Þ
For a tightly collimated beam with radius R = 24 lm, the response time is approximately 3 ms for the samples. This shows that the heat load equilibrates in a characteristic time of milliseconds and does not change much with matrix composition. A relatively rapid and simple method of screening the materials for frequency conversion applications prior to crystal growth is needed. Kurtz and Perry were the first to develop such a method [12]. The advantage of this method is that it is inexpensive and not only provides SHG efficiency of a material but also determines whether the material is phase matchable or not [16]. The SHG intensity from the material is measured as a function of particle size. For a crystalline material to be of practical significance as a second harmonic generator, bulk crystals of the compound must be phase matchable, allowing the second harmonic generation to increase monotonically as it passes through the crystal. For a nonphase matchable material, once the particle size becomes greater than the average coherence length lc (so that for most or all orientations phase – mismatch become apparent), the SHG varies inversely as the particle size. However, for a material that is phase matchable, once the particle size matches the average coherence length, the gain in SHG becomes a constant. The net result is that the overall SHG remains a constant. In this method for a phase matchable crystals like KDP, the SH intensity exhibits nearly a linear increase with particle size up to the average coherence length, at which the intensity levels off and is independent of particle size [17,18]. For a nonphase matchable material like quartz (SiO2), the
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T.C. Sabari Girisun, S. Dhanuskodi / Chemical Physics Letters 491 (2010) 248–253
250
DGT
KDP
SHG Output (mV)
200
150
100
50
0 <53
53- 106
106- 126
126- 150
150- 250
>250
Particle size variation (um) Fig. 5. SHG vs. particle size variation.
4. Conclusions
Table 3 Average powder SHG efficiency of DGT and other thiourea complexes. Sample
Efficiency
KDP (reference) Bisthiourea zinc chloride (BTZC) Tristhiourea zinc sulphate (ZTS) DGT Bisthiourea cadmium chloride (BTCC)
1 0.65 1.2 2.5 4.77
SH intensity shows an increase followed by a slow decrease for increase in the average particle size. The SHG efficiency of DGT increases with particle size up to >150 lm, and the increase is slower for larger particles. Thus Fig. 5 gives a solid proof for the phase matching property of DGT. The experimental results for DGT are compared with pulverized KDP and are shown in Fig. 5. From the figure, the SHG output P (2x) increases with respect to the range of particle sizes (r), since all the particles in the light path decreases inversely with (r). As (r) increases larger than the average coherence length, the output reaches saturation, indicating the phase matchable character of DGT. This is because some particles in the light path should have the correct orientation for phase matching. This suggests that the material is phase matchable with a coherence length of a few lm. Single crystals of DGT crystallize in a noncentrosymmetric space group, i.e. lacking an inversion center which satisfies the essential criteria for second order NLO material. The SHG efficiency as a function of particle size of DGT is compared with standard inorganic NLO material KDP (Fig. 3). DGT shows the SHG output 2.5 times (average of all particle sizes) that of the standard inorganic KDP. The powder SHG efficiency as function of particle size is measured for few of the known metal complexes of thiourea and the average powder SHG efficiency (average of all particle sizes) are given in Table 3. Thus on the basis of semiquantitative approach, it can be concluded that DGT can be used as an alternate for KDP for the frequency conversion NLO applications.
An organic material for second order NLO applications was synthesized and single crystals were grown by slow evaporation technique. The material was characterized by powder XRD to deduce the unit cell parameters. The close agreement with the reported values confirms the identity of the compound formed. DGT was characterized with negative nonlinear refraction and saturable absorption behavior by employing the Z-scan technique using CW excitation at 532 nm. They possessed high nonlinearities which are primarily thermal in nature. In solution form, its nonlinear refractive index, nonlinear absorption coefficient and third-order nonlinear optical susceptibility were recorded as – 4.93 108 cm2 W1, 7.27 103 cm W1 and 3.4 106 esu, respectively. DGT possess low optical limiting thresholds in the range of 18 mW. These could be efficiently used as optical limiters by utilizing their high refractive nonlinearity and judicious aperture based design in low power CW regime. By Kurtz powder technique, the efficiency of DGT was found to be 2.5 times that of KDP and is found to be phase matchable. Thus DGT will be an excellent NLO material which can be used as an alternate for inorganic materials that are used in NLO applications.
References [1] Y. Mori, Y.K. Yap, T. Kamimura, M. Yoshimura, T. Sasaki, Opt. Mater. 19 (2002) 1. [2] M. Ushasree, R. Muralidharan, R. Jayavel, P. Ramasamy, J. Crystal Growth 218 (2000) 365. [3] Zhengdong Li, Kechen Wu, Genbo Su, Youing He, Opt. Mater. 20 (2002) 295. [4] A.M. Petrosyan, R.P. Sukiasyan, H.A. Karapetyan, S.S. Terzyan, K.S. Feigelson, J. Crystal Growth 213 (2000) 103. [5] H. Reyes, B.M. Munoz, N.F.R. Santillan, S. Rojas-Lima, P.G. Lacroix, K. Nakatani, Mater. Chem. 12 (2002) 2898. [6] A.K. Batra, Tesfaye Gebre, K. Bhat, M.D. Aggarwal, B. Peterson, S. Sarkisov, R.B. Lal, Proc. SPIE 5351 (2004) 57. [7] M. Lenin, G. Bhavannarayana, P. Ramasamy, Opt. Commun. 282 (2009) 1202. [8] Min-hua Jing, Qi Fang, Adv. Mater. 13 (1999) 11.
T.C. Sabari Girisun, S. Dhanuskodi / Chemical Physics Letters 491 (2010) 248–253 [9] R. Ezhil Vizhi, S. Kalainathan, Mater. Lett. 62 (2008) 791. [10] V. Krishnakumar, S. Kalyaranaman, M. Piasecki, I.V. Kityk, P. Bragie, J. Raman Spectro. 39 (2008) 1450. [11] V. Krishnakumar, R. Nagalakshmi, S. Manohar, M. Piasecki, I.V. Kityk, P. Bragiel, Physica B 405 (2010) 839. [12] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798. [13] M. Sheik-Bahae, A.A. Said, T. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quant. Electron. QE 26 (1990) 760.
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[14] K. Sathiyamoorthy, C. Vijayan, M.P. Kothiyal, J. Phys. D: Appl. Phys. 40 (2007) 6121. [15] T.C. Sabari Girisun, S. Dhanuskodi, Crystal Res. Technol. 44 (2009) 1297. [16] R.W. Boyd, Nonlinear Optics, Academic, New York, 2003. [17] S. Manivannan, S.K. Tiwari, S. Dhanuskodi, Solid State Commun. 132 (2004) 123. [18] H.S. Nalwa, S. Miyatta, Nonlinear Optics of Organic Molecules and Polymers, CRC Press, Boca Raton, New York, 1997.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Nonlinear optical susceptibilities of diglycinyl thiourea for frequency conversion and optical limiting applications T.C. Sabari Girisun a,b, S. Dhanuskodi a,* a b
School of Physics, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India Department of Physics, Bishop Heber College, Tiruchirappalli 620 017, India
a r t i c l e
i n f o
Article history: Received 11 February 2010 In final form 7 April 2010 Available online 10 April 2010
a b s t r a c t Organic nonlinear optical material diglycinyl thiourea (DGT) has been synthesized and single crystals have been grown from the aqueous solution by solvent evaporation technique. Powder second harmonic generation (SHG) study shows that DGT is 2.5 times efficient than KDP and is phase matchable. The thirdorder nonlinear optical and optical limiting properties of DGT have been investigated using a 532 nm second harmonics of diode-pumped Nd:YAG laser (1064 nm, 50 mW). The magnitude of nonlinear refractive index, nonlinear absorption coefficient and third-order susceptibility was found to be in the order of 108 cm2/W, 103 cm/W and 106 esu, respectively. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Extensive research in organic functional group materials is an expanding area of research because of their interesting nonlinear optical effects extended to optical parametric amplifiers, optical parametric oscillators, Q-switched intracavity second harmonic devices, high optical damage threshold and other electro-optical applications [1–5]. Even though intensive efforts in the field have been made for about 30 years, it is still very attractive to search for new nonlinear optical materials with various practical interests. At present, inorganic crystals such as lithium niobate and potassium dihydrogen phosphate (KDP) are generally used in nonlinear optical devices. The search is for new nonlinear optical materials applied to ultraviolet (UV), even vacuum ultraviolet (VUV) and far infrared (FIR) regions, as well as direct second harmonic generation (SHG) of diode laser [6]. Optical limiters for low power CW lasers are also very important in applications such as prevention of optical damage to very sensitive sensors. For example, the human eye will be damaged permanently if exposed to a laser beam of power 1–5 mW for a few seconds. Information about the damage level of the sensor is necessary to determine the required limiting level of the device. Amino acid family crystals have over the years been subjected to extensive investigations by several researchers for their nonlinear optical properties. Although some of the amino acids are already reported to have NLO activity, the amino acid glycine based crystals are now a days shown greater interest for NLO applications [7]. Glycine is the simplest amino acid. Unlike other amino acids, it has no asymmetric carbon atom and is optically inactive. * Corresponding author. Fax: +91 0431 2407045. E-mail address: [email protected] (S. Dhanuskodi). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.04.014
The organic ligands such as thiocyanate SCN, urea CO(NH2)2 and thiourea CS(NH2)2 are more dominant in NLO effect. Thiourea molecule is an interesting matrix modifier due to its large dipole moment and its ability to form an extensive network of hydrogen bonds. Thiourea crystallizes in the centrosymmetric space group Pbnm and SHG inactive [8]. Combinations of amino acids with organic ligands are promising materials for optical second harmonic generation. The only work reported along these lines was that giving preliminary data for the diglycinyl thiourea (DGT) as an efficient NLO material [9]. Recently, a similar type of organic complex of resorcinol and urea- was identified to be an excellent second order NLO material. Such type of materials is extremely promising for NLO applications and in particular thiourea-based materials are already available in literature [10,11]. In continuation of our work on thiourea-based compounds, in the present study thiourea has been coordinated with glycine, with an idea of improving the nonlinear response. This Letter reports the nonlinear optical properties of the DGT by SHG Kurtz powder technique and Z-scan experiment. The SHG efficiency, SHG particle variation, the nonlinear refractive index, nonlinear absorption coefficient and third-order nonlinear susceptibility were reported. The optical limiting behavior of the sample was discussed in detail. The present work is on the design of a novel optical limiter for the low power regime. 2. Experimental 2.1. Synthesis and crystal growth The DGT was synthesized by taking glycine and thiourea in the molar ratio 2:1 according to the following reaction,
T.C. Sabari Girisun, S. Dhanuskodi / Chemical Physics Letters 491 (2010) 248–253
249
Fig. 1. (a) Solubility curve and (b) Single crystal of DGT.
2C2 H5 NO2 þ CH4 N2 S ! C5 H10 N4 S þ 2H2 O The prepared thiourea solution is mixed with glycine solution using a magnetic stirrer and is stirred continuously for 2 h at 45 °C. This saturated solution is taken in a clean beaker and left for evaporation. The material is further purified by the repeated recrystallization process. Solubility is one of the most promising factors which influence the growth of a crystal. The solubility of the DGT is determined with water as the solvent. On reaching saturation, the equilibrium concentration of the material is determined gravimetrically. The above procedure is repeated for every 5 °C in water, methanol and ethanol from 25 to 45 °C and the solubility curve for DGT is plotted (Fig. 1a). From the solubility, it can be inferred that the material has a moderate solubility at 30 °C. By slow evaporation technique, with water as a solvent, optically good quality single crystals (12 4 3 mm3) were obtained in a period of 28 days (Fig. 1b).
2.2. Structural and nonlinear optical studies The grown crystal have been crushed to a uniform fine powder and subjected to powder X-ray diffraction using a Bruker AXS D8 Advance powder X-ray diffractometer, (CuKa, 1.5406 Å) and was scanned in the reflection mode in the 2h range 10–80°.
The first and the most widely used technique for confirming the SHG from prospective second order NLO materials is the Kurtz and Perry powder technique [12] to identify the materials with noncentrosymmetric crystal structures. A preliminary study of the powder SHG measurements were performed using a modified Kurtz technique (Nd:YAG 1064 nm, 8 ns, 10 Hz). The crystalline sample was powdered to different particle sizes in the range 53– 250 lm. To make relevant comparisons with known SHG materials, KDP was also ground and sieved into the same particle size range. The powdered samples were filled air-tight in separate micro-capillary tubes of uniform bore of about 1.5 mm diameter. The input laser energy incident on the capillary tube was chosen to be 3 mJ, an energy level optimized not to cause any chemical decomposition of the sample. The SHG output (532 nm) in each case was measured as the average of a few pulses to eliminate slight variations of input power. The transmittance through the aperture as a function of the distance between the sample and the beam waist is called Z-scan curve. The Z-scan technique offers a useful path to probe both the nonlinear refractive index and the nonlinear absorption coefficient of a sample. The nonlinear refractive index of DGT was determined by the closed Z-scan method developed by Sheik-Bahae et al. [13] where a linearly polarized GAUSSIAN beam from a diodepumped Nd:YAG laser of wavelength 532 nm was used as the excitation source for the Z-scan technique. The GAUSSIAN profiled laser
250
T.C. Sabari Girisun, S. Dhanuskodi / Chemical Physics Letters 491 (2010) 248–253
beam was focused on a 1 mm quevette containing the 1 mM concentration DGT solution by lens, f = 3.5 cm to produce a beam waist x0 of 15.35 lm. As the sample is scanned through the beam, the far field profile shows intensity variation across the beam profile, which is recorded through an aperture. Open aperture Z-scan was performed on the material in order to probe the nonlinear absorption mechanisms. Also, the experiment was repeated with pure solvent (water) to measure its contribution and no significant feature was observed in either the open or the closed Z-scan traces. The experimental set-up for the demonstration of optical limiting of the laser beam by DGT under CW laser illumination is very similar to the standard Z-scan geometry. Additionally a polarizer analyzer combination (PA) was used to vary the input power. The cuvette containing the nonlinear medium is placed just after the focal point. This beam is then made to fall on the optical sensor, a photo detector in this case. The input laser intensity is varied systematically and the corresponding output intensity values were measured by the photo detector.
3. Results and discussion
pffiffiffi 2 2 DT b¼ I0 Leff
where DT is the one valley value at the open aperture Z-scan curve. The value of b will be negative for saturable absorption and positive for two photon absorption. The real and imaginary parts of the third-order nonlinear optical susceptibility v(3) are defined as
Revð3Þ ðesuÞ ¼ 104 ðe0 C 2 n20 n2 Þ=p ðcm2 =WÞ
ð5Þ
Imvð3Þ ðesuÞ ¼ 102 ðe0 C 2 n20 kbÞ=4p2 ðcm=WÞ
ð6Þ
where e0 is the vacuum permittivity, n0 is the linear refractive index of the sample and c is the velocity of light in vacuum. The third-order nonlinear optical susceptibility is thus
vð3Þ ¼
ð1Þ
where S is the aperture linear transmittance and is calculated using the relation
S ¼ 1 expð2r 2a =x2a Þ
ð2Þ
where ra is the aperture and xa is the beam radius at the aperture. The nonlinear refractive index is given
ð7Þ
vð3Þ
ð8Þ
L4 N
where N is the density of molecules. The term L is the local field factor which in the Lorentz approximation is given by L ¼ ðn20 þ 2Þ=3 where n0 is linear refractive index of the medium. The normalized transmittance obtained from the closed (Fig. 2a) and open (Fig. 2b) Z-scan experiment as a function of
a Normalised Transmittance
DT p—v ¼ 0:406ð1 SÞ0:25 jD/j
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðReðvð3Þ ÞÞ2 ðImðvð3Þ ÞÞ2 :
Also the second order hyperpolarizability which describes the nonlinear induced polarization per molecule, is related to the third-order bulk susceptibility as [12]
ch ¼
From the powder X-ray diffraction data, the lattice parameters have been calculated (Table 1). There is a close agreement with the reported values from single crystal X-ray diffraction. The effective nonlinear refractive coefficient n2 is calculated by n = no + Dn = no + n2I, where n is the total refractive index, no is the linear refractive index without illumination, Dn is the total nonlinear refractive index change induced by illumination light, and I is the intensity of the GAUSSIAN beam inside the sample. For the calculation of the nonlinear refractive index the following standard relations are utilized [13]. The difference between the normalized peak and valley transmission (DTp–v) is written in terms of the on axis phase shift |D/| at the focus as,
ð4Þ
1.4 1.2 1 0.8 0.6 0.4 0.2 0
D/ n2 ¼ KI0 Leff
-15
ð3Þ
Table 1 Cell parameters of DGT. Data
DGT
System a (Å) b (Å) c (Å) a (deg) b (deg) c (deg)
Present Monoclinic 5.113 (2) 11.988 (4) 5.564 (3) 90 112 90
[9] Monoclinic 5.091 11.980 5.45 90 111.92 90
b Normalised Transmittance
where K = 2p/k (k is the laser wavelength), I0 is the intensity of the laser beam at focus (Z = 0), Leff is the effective thickness of the sample, [Leff = [1 exp(aL)]/a], L is the thickness of the sample and a is the linear absorption coefficient. From the open aperture Z-scan data, the nonlinear absorption coefficient is estimated
-10
-5
0 z (mm)
5
10
0 z (mm)
5
10
15
1.14 1.12 1.1 1.08 1.06 1.04 1.02 1 0.98 -15
-10
-5
Fig. 2. (a) Z-scan closed aperture and (b) Z-scan open aperture.
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Normalised Transmittance
1.4
7 6
Output Intensity (mW)
the on axis distance Z measured from the focal point. The pattern refers to saturable absorption and the b is 7.27 103 cm W1. Since the closed aperture transmittance is affected by the nonlinear refraction and absorption, the determination of n2 is less straight forward. Hence to obtain the pure effective n2 is to divide the closed aperture transmittance with its corresponding open aperture transmittance and the obtained graph is shown in Fig. 3. A pre-focus peak followed by a post-focus valley indicates that the nonlinear property of the sample is negative and the lensing effect is defocusing. The value of n2 obtained for the sample DGT is 4.93 108 cm2 W1, the value is lager than its parent molecule thiourea (3.61 108 cm2 W1) and glycine (3.48 108 cm2 W1). This increase in nonlinear response clearly picturizes the increase in the delocalization due to the complex formation. Thus it is worth while to note here that not only the metal complex formation increases the nonlinear response, but also the combination of organic NLO chromophore will enhance the nonlinearity. The variation in n2 can be due to different mechanisms depending on the nature of the medium. In this case with CW laser irradiation, it is attributed to the thermally induced variation in the nonlinear refractive index [14]. Table 2 summarizes the nonlinear optical parameters evaluated for DGT from the Z-scan patterns. v(3) of DGT is found to be larger than the well known inorganic compounds. This is due to the p electron cloud movement from the donor to the acceptor, which makes the molecule highly polarizable. Fig. 4 shows the optical limiting behavior of DGT at 0.1 M concentration of the DGT solution. For incident energy less than 18.6 mW, the output varies linearly according to Beer’s law and above this, the output saturates and becomes a plateau. The
251
5 4 3 2 1 0 0
10
20 30 Input Intensity (mW)
40
Fig. 4. Optical limiting behavior of DGT.
threshold value is 18.6 mW and the corresponding output value gets clamped at 5.2 mW. This verifies that the samples are good candidates for optical limiting at 532 nm CW lasers. Thus DGT with strong nonlinear response and self-defocusing behavior is a potential candidate for the protection of optical sensors such as night vision devices [15]. The nonlinearity in this case being of thermal origin, the value of the nonlinear refractive index depends on the sample concentration. The optical limiting threshold can be improved by changing the operational parameters such as sample concentration, aperture size and the focal length of the lens. Thus the optical arrangement can be modified according to the requirements of the dynamic range and field of view of the sensor. Numerically the response time of optical limiting action for the samples is estimated using the relation [16]
1.2 1
s
0.8 0.6 0.4 0.2 0 -15
-10
-5
0 z (mm)
5
10
15
Fig. 3. Z-scan ratio of closed and open aperture.
Table 2 Comparison of NLO parameters of TU and DGT. Parameters
Thiourea
DGT
Nonlinear refractive index (n2) 108 cm2/W Nonlinear absorption coefficient (b) 103 cm/W Real part of the third-order susceptibility [Re(v(3))] 106 esu Imaginary part of the third-order susceptibility [Im(v(3))] 106 esu Third-order nonlinear optical susceptibility (v(3)) 106 esu Second-order hyperpolarizability (ch) 106 esu Optical limiting threshold value (mW)
3.61 7.27
4.93 7.27
4.44
1.96
3.78
2.63
5.8
3.7
1.84
7.92
18.6/1.72
18.6/6
ðqCÞR2
j
ð9Þ
For a tightly collimated beam with radius R = 24 lm, the response time is approximately 3 ms for the samples. This shows that the heat load equilibrates in a characteristic time of milliseconds and does not change much with matrix composition. A relatively rapid and simple method of screening the materials for frequency conversion applications prior to crystal growth is needed. Kurtz and Perry were the first to develop such a method [12]. The advantage of this method is that it is inexpensive and not only provides SHG efficiency of a material but also determines whether the material is phase matchable or not [16]. The SHG intensity from the material is measured as a function of particle size. For a crystalline material to be of practical significance as a second harmonic generator, bulk crystals of the compound must be phase matchable, allowing the second harmonic generation to increase monotonically as it passes through the crystal. For a nonphase matchable material, once the particle size becomes greater than the average coherence length lc (so that for most or all orientations phase – mismatch become apparent), the SHG varies inversely as the particle size. However, for a material that is phase matchable, once the particle size matches the average coherence length, the gain in SHG becomes a constant. The net result is that the overall SHG remains a constant. In this method for a phase matchable crystals like KDP, the SH intensity exhibits nearly a linear increase with particle size up to the average coherence length, at which the intensity levels off and is independent of particle size [17,18]. For a nonphase matchable material like quartz (SiO2), the
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250
DGT
KDP
SHG Output (mV)
200
150
100
50
0 <53
53- 106
106- 126
126- 150
150- 250
>250
Particle size variation (um) Fig. 5. SHG vs. particle size variation.
4. Conclusions
Table 3 Average powder SHG efficiency of DGT and other thiourea complexes. Sample
Efficiency
KDP (reference) Bisthiourea zinc chloride (BTZC) Tristhiourea zinc sulphate (ZTS) DGT Bisthiourea cadmium chloride (BTCC)
1 0.65 1.2 2.5 4.77
SH intensity shows an increase followed by a slow decrease for increase in the average particle size. The SHG efficiency of DGT increases with particle size up to >150 lm, and the increase is slower for larger particles. Thus Fig. 5 gives a solid proof for the phase matching property of DGT. The experimental results for DGT are compared with pulverized KDP and are shown in Fig. 5. From the figure, the SHG output P (2x) increases with respect to the range of particle sizes (r), since all the particles in the light path decreases inversely with (r). As (r) increases larger than the average coherence length, the output reaches saturation, indicating the phase matchable character of DGT. This is because some particles in the light path should have the correct orientation for phase matching. This suggests that the material is phase matchable with a coherence length of a few lm. Single crystals of DGT crystallize in a noncentrosymmetric space group, i.e. lacking an inversion center which satisfies the essential criteria for second order NLO material. The SHG efficiency as a function of particle size of DGT is compared with standard inorganic NLO material KDP (Fig. 3). DGT shows the SHG output 2.5 times (average of all particle sizes) that of the standard inorganic KDP. The powder SHG efficiency as function of particle size is measured for few of the known metal complexes of thiourea and the average powder SHG efficiency (average of all particle sizes) are given in Table 3. Thus on the basis of semiquantitative approach, it can be concluded that DGT can be used as an alternate for KDP for the frequency conversion NLO applications.
An organic material for second order NLO applications was synthesized and single crystals were grown by slow evaporation technique. The material was characterized by powder XRD to deduce the unit cell parameters. The close agreement with the reported values confirms the identity of the compound formed. DGT was characterized with negative nonlinear refraction and saturable absorption behavior by employing the Z-scan technique using CW excitation at 532 nm. They possessed high nonlinearities which are primarily thermal in nature. In solution form, its nonlinear refractive index, nonlinear absorption coefficient and third-order nonlinear optical susceptibility were recorded as – 4.93 108 cm2 W1, 7.27 103 cm W1 and 3.4 106 esu, respectively. DGT possess low optical limiting thresholds in the range of 18 mW. These could be efficiently used as optical limiters by utilizing their high refractive nonlinearity and judicious aperture based design in low power CW regime. By Kurtz powder technique, the efficiency of DGT was found to be 2.5 times that of KDP and is found to be phase matchable. Thus DGT will be an excellent NLO material which can be used as an alternate for inorganic materials that are used in NLO applications.
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