NTu1B.2.pdf
Nonlinear Optics Technical Digest © OSA 2013
Beam Deflection for Time-resolved Measurement of Nonlinear Refraction Manuel R. Ferdinandus1, Honghua Hu1, Matthew Reichert1, David J. Hagan1,2, and Eric W. Van Stryland1,2 1
CREOL, the College of Optics and Photonics, University of Central Florida, Orlando, FL, 32816 2 Department of Physics, University of Central Florida, Orlando, FL 32816 Corresponding email:
[email protected]
Abstract: We modify the photothermal beam deflection technique for the sensitive measurement of ultrafast nonlinear refraction (λ/5000 sensitivity to nonlinearly-induced phase changes). This allows direct measurement of the temporal dynamics and polarization dependence of nonlinear refraction. OCIS codes: (190.4400) Nonlinear optics, materials, (190.3270) Kerr effect
1. Introduction Z-scan [1] is a simple and sensitive measurement technique for nonlinear refraction and absorption; however, it is a single-beam technique and cannot determine the temporal dynamics of the nonlinearities. The understanding of the physical mechanisms underlying the nonlinear response requires knowledge of the magnitude, temporal response and polarization dependence of the nonlinear refraction, which is usually obtained by two-beam techniques, e.g. excite-probe technique. Photothermal beam deflection is a well-known and sensitive method for measuring small absorption from the induced thermal lensing caused by an excitation beam [2]. Here we modify this method using ultrashort pulses to measure the nonlinear refraction of a medium. An excite beam induces index changes that in turn deflect a probe beam. Using standard excite-probe relative temporal delays the ultrafast dynamics of the nonlinearly -induced phase shift can be determined as well as the relative polarization dependencies of these temporal dynamics. The sensitivity of this technique can be made extremely high since modulation techniques and phase sensitive detection are possible with two-beam experiments. Here with relatively preliminary measurements we can demonstrate λ/5000 sensitivity. This method joins the other techniques employed in characterizing the nonlinear response of materials such as Z-scan (absolute measurement), excite-probe (dynamics of nonlinear absorption), and Optical Kerr Experiments (dynamics of nonlinear refraction) [3]. Here we give preliminary results demonstrating the sensitivity of the ultrafast beam-deflection technique in quartz and separation of nuclear re-orientational contributions of nonlinear refraction in CS2. This method measures the absolute magnitude of the nonlinear refraction unlike the Optical Kerr effect method [3], which measures induced birefringence. 2. Experiments and Results The experimental apparatus involves a very simple modification to a standard excite-probe setup (Fig.1a). The excitation pulse is generated from a Ti:Sapphire system producing 240-270 fs (FWHM) pulses at 780 nm at a 1 kHz repetition rate, which pumps an optical parametric generator/amplifier giving a 650 nm probe (170-193 fs, FWHM). The beam waist of the excitation is approximately 4-5 times larger than that of the probe. As shown in Fig. 1b, the probe is offset from the center of the excitation beam. A quad-segmented photodiode shown in Fig 1c, is used to observe the deflected beam. The index change, ∆n, induced in the sample is calculated from the ratio of the differential signal ∆E = Eleft - Eright = (E1+E3) - (E2+E4) to the total pulse energy, S = ∆E/E. Fig. 1d shows the nonlinear response of 1 mm of fused silica, here quantified as ∆n, under the peak excitation irradiance of 51 GW/cm2, where the excitation and probe are set co-polarized (0º) or cross-polarized (90º), respectively. The induced index change, ∆n, for fused silica as a function of delay, τd, follows the cross-correlation between excitation and probe pulse, suggesting it is electronic in origin. This is also confirmed by the factor of ~3 difference in ∆n between the co-polarized to cross-polarized cases (χ1122/χ1111 = 3 for isotropic media) [4]. To demonstrate the sensitivity of this technique, we reduced the excitation peak irradiance to 0.35 GW/cm 2, as the ∆n
NTu1B.2.pdf
Nonlinear Optics Technical Digest © OSA 2013
signal shown in the inset of Fig. 1d. Assuming a Signal-to-Noise Ratio (SNR) of unity for the minimum detectable signal, the minimum detectable ∆nmin = 2.23 × 10-8, which corresponds to a phase change of approximately λ/5000. In all experiments, the nonlinear refractive index (n2) of quartz is estimated to be ~0.25×10 -15 cm2/W, consistent with literature [5] and also verified in our Z-scan experiments. Excite
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Fig. 1: (a) Beam deflection schematic, (b) overlap of excitation and probe beam,(c) quad cell detector diagram, (d) Index change Δn as a function of delay τd for 1mm quartz with peak excitation irradiance of 51 GW/cm2 in co-polarized (1) and cross-polarized case (2). Curve 3 of inset shows Δn at peak excitation irradiance of 0.35 GW/cm2. The excite beam radius is 170 μm, HW1/e2M. (e) Index change Δn as a function of delay τd for 1 mm CS2 with peak excitation irradiance of 37 GW/cm2 in co-polarized (1), cross-polarized (2), and “magic angle” case (3). The excite beam radius is 140 μm, HW1/e2M.
We also performed beam deflection experiments on CS2, as shown in Fig. 1e for co-, cross-polarized and magic angles. These results are consistent with what is expected for combinations of electronic (shown with magic angle) and nuclear contributions. At delays after 1ps, where orientational relaxation dominates, |∆n | in the co-polarized case is 2× that of the cross-polarized case. This is consistent with the tensor elements of the re-orientational nonlinearity in isotropic media (i.e. χ1111/ χ1122= - 2) [4]. Note the small dip observed pre-zero delay is probably due to stimulated Rayleigh-wing scattering [6]. Using beam deflection allows the polarizations of excite and probe to be set at the magic angle (54.7º) to eliminate the effects of molecular reorientation (this cannot be done in OKE experiments). Hence, beam deflection can be used to separate nonlinear mechanism from re-orientational related contributions. More materials are being investigated using this technique. 3. Conclusions A modified photothermal beam deflection technique has been adopted to measure ultrafast nonlinear refraction of materials with excellent sensitivity (λ/5000). This allows direct measurement of the temporal dynamics and polarization dependence of nonlinear refraction for determining the origins of these nonlinearities. Acknowledgments: This work is supported in part by AFOSR MURI grant FA9550-10-1-0558.
4. References [1]. M. Sheik-Bahae et al, IEEE J. Quantum. Elect., 26, 760-769 (1990). [2]. W. B. Jackson et al, Appl. Opt., 20, 1333-1344 (1981). [3]. D. McMorrow et al, IEEE J. Quantum. Elect., 24, 443-454 (1988). [4]. G. I. Stegeman et al, Nonlinear Optics: Phenomena, Materials and Devices; John Wiley & Sons, Inc: Hoboken, NJ, 2012. [5]. D. Milam, Appl. Opt., 37, 546-550 (1998). [6]. A. Dogariu et al, J. Opt. Soc. Am. B, 14, 796-803 (1997).
