JW2A.54.pdf
CLEO:2016 © OSA 2016
Measurement of Nonlinear Optical Response Functions of Common Organic Solvents Peng Zhao,1 Matthew Reichert,1,2 David J. Hagan1 and Eric W. Van Stryland1,*
2
1 CREOL, College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA Current address: Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08455, USA *
[email protected]
Abstract: The nonlinear optical (NLO) response functions of 24 widely used organic solvents are measured, including both bound-electronic and nuclear contributions. The response function establishes a self-consistent reference for predicting the outcomes of various NLO experiments. OCIS codes: (190.4400) Nonlinear optics, materials; (190.7110) Ultrafast nonlinear optics
1. Introduction The nonlinear optical (NLO) properties of organic solvents have long been studied [1].Knowledge of the magnitudes and temporal dynamics of their nonlinear refraction (NLR) helps to understand the physical origins of solvent nonlinearities, and is also useful for interpreting results performed on solutions of organic dyes in solvents, i.e. to of commonly used solvents has been measured separate the response of the solute. The nonlinear refractive index in numerous studies, but results often conflict in value depending on the experimental methods, pulsewidths used or interpretations [2]. One important reason is the noninstantaneous nuclear nonlinearity of organic solvents complicates the NLO response function making the effective nonlinear refractive index , pulsewidth dependent [1]. Recently, using carbon disulfide (CS2), we developed a methodology to experimentally determine the NLO response function with our ultrafast beam deflection (BD) technique [3, 4], from which we can predict the outcomes measured by Z-scan [3]. In this model, the of other NLO experiments on CS2 such as the pulsewidth dependent , overall NLO response function is attributed to an instantaneous , , which originates from the purely boundelectronic hyperpolarizability, and several strong noninstantaneous mechanisms due to nuclear motions. Together these lead to an irradiance- and time-dependent third-order response giving a change of refractive index linear in the pump irradiance as; Δ
2
,
,
,
(1)
where , and are the magnitude and normalized temporal response of the th nuclear mechanism. These mechanisms include a small but fast exponential component due to inter-molecular collisions ( , ), a quickly damped oscillatory response driven by the molecular librational motion ( , ) [1, 3], and a slower exponential decaying response governed by diffusive reorientation ( , ), as fully discussed in [3]. Following this methodology, we have measured the NLO response functions of other widely used organic solvents, including benzene, nitrobenzene, toluene, dichlorobenzene, xylene, dichloromethane, chloroform, carbon tetrachloride, hexane, cyclohexane, methanol, ethanol, butanol, octanol, acetone, acetonitrile, butyl salicylate, tetrahydrofuran, pyridine, dimethyl sulfoxide, dimethylformamide, water and heavy water. The pulsewidth dependent , is predicted for each solvent using the measured response functions. In this paper, the results for toluene, chloroform and octanol are discussed as examples. 2. Experiment and discussion Beam deflection (BD) is an excite-probe technique where the probe beam is focused ~5× smaller in spatial extent than the excitation beam .By translating the probe to the wings of the excitation, the transient refractive index gradient induced by the excitation pulse acts as a prism deflecting the probe beam by a small angle. The probe deflection can be measured using a quad-segmented detector by taking the difference of the energy falling on the left and right halves ΔE = Eleft - Eright. The normalized BD signal ΔE/E is directly proportional to Δ [4]. To explicitly separate each contributing mechanism, BD data is taken for three polarization combinations of excitation and probe to yield the tensor symmetries. , and , follow isotropic symmetry Δ Δ ∥ /3 and give all positive contributions to NLR, while , and , follow reorientational symmetry Δ Δ ∥ /2 which becomes negative for the perpendicular polarized case and goes to zero at the magic angle (θ = 54.7°). In this work, the wavelength of excitation and probe are 800 nm and 700 nm, respectively, which are chosen to be slightly nondegenerate within the transparency window of most solvents to avoid artifacts. All samples are measured under identical experimental conditions, where contributions from cuvette are measured separately and subtracted.
JW2A.54.pdf
Toluene
6 4 2
4
0
3
Chloroform
2 1
0.0
0.5
1.0
1.5
-1 2.0 -0.5
,
0.0
0.5
1.0
1.5
,
,
150±50 Toluene 0.58 0.12 0.9 150±50 100±50 Chloroform 0.41 0.08 0.4 100±50 200±50 Octanol 0.4 0.06 0.03 100±50
,
,
, ,
11±2 250±50 250±50 2.8 6±2 1700±100 5±2 500±50 250±50 0.75 2±1 1800±300 2±1 N/A 150±50 0 2±1 N/A
Table 1. Fit parameters of response functions of solvents. , are given in units of 10m2/W; , and , are given in units of fs; and are given in units of ps-1.
19
2.0
Octanol 2
1
-0.5
0.0
0.5
1.0
1.5
2.0
Delay (ps)
Delay (ps) ,
parallel perpendicular magic angle
0
Delay (ps)
,
(c)
3
0
10
n2,eff (10-19 m2/W)
-2 -0.5
4
parallel perpendicular magic angle
(b)
E/E (%)
8
E/E (%)
5
parallel perpendicular magic angle
(a)
E/E (%)
10
CLEO:2016 © OSA 2016
(d)
1
0.1 0.01
Toluene Chloroform Octanol
0.1 1 10 Pulse Width (ps, FWHM)
100
Fig. 1. (a-c) Measured (circles) and fit (curves) of BD signals for (a) toluene, (b) chloroform and (c) octanol with parallel (black), perpendicular with parameters given in Table 1. (red) and magic angle (blue) polarization combinations; (d) calculated pulsewidth dependence of ,
The time-resolved BD signal of a few selected solvents are shown in Fig 1 (a-c) with parallel, perpendicular and magic angle polarization combinations. The signal at the magic angle is used first to fit , and the collisional response with rising and falling time constants , and , . Considering the tensor symmetries, the parallel and perpendicular results can then be used together to fit the librational and reorientational components, where a quantum harmonic oscillator model [5] is used to describe inhomogeneously broadened librational motions by considering an antisymmetrized Gaussian distribution with center frequency and bandwidth . Errors in , can be estimated as ~20% from the irradiance uncertainty. Large librational and reorientational responses are resolved from toluene as well as other benzene-derivative solvents, resulting in universal temporal dynamics belonging to anisotropic molecules such as CS2. For example, this is observed in less symmetrical chloroform, but not in highly symmetrical carbon tetrachloride due to the absence of a polarizability anisotropy. Exceptions are alcohols such as octanol shown in Fig. (c), which, as an anisotropic molecule, does not show an obvious reorientational response within our temporal resolution. Only a small librational response is resolved, owing to the large impact of reorientational symmetry that gives the opposite sign for parallel and perpendicular results. Finally, using the NLO response functions with the parameters tabulated in Table 1, the pulsewidth dependence of NLR, , , is calculated [3]. As shown in Fig. 1 (d), the bound-electronic nonlinearity dominates in the short pulse regime (< 50 fs) due to its nearly instantaneous nature. Noninstantaneous nuclear nonlinearities will significantly increase , for longer pulsewidths, as seen in toluene and chloroform. Octanol shows less pulsewidth dependence due to the small , and negligible , . Other experiments such as Zscan will be performed in the future to verify such predictions based on the measured NLO response functions. 3. Conclusions Using the polarization resolved BD technique, the total NLO response function of 24 organic solvents are determined, including both bound-electronic and nuclear mechanisms. We believe this study will establish self-consistent references for various NLO applications. 4. References 1. 2. 3. 4. 5.
McMorrow, D., W.T. Lotshaw, and G.A. Kenney-Wallace, IEEE Journal of, 1988. 24(2): p. 443-454. Iliopoulos, K., et al., Optics Express, 2015. 23(19): p. 24171-24176. Reichert, M., et al., Optica, 2014. 1(6): p. 436-445. Ferdinandus, M.R., et al., Optics Letters, 2013. 38(18): p. 3518-3521. McMorrow, D., et al., The Journal of Physical Chemistry A, 2001. 105(34): p. 7960-7972.
