Frequency doubling of femtosecond pulses in walk-off compensated npp

16 / QELS 2001 / MONDAY MORNING Energy (eV) 0.8 1.2 1.6 0.8 1.2 1.6 Retardance A (A) QME3 Fig. 2. (a) THG intensity 13mm and (open and filled circles) as a function of ellipticity of the incident photons. From this one obtains (b) the phase difference and (c) the amplitude ratio of the underlying independent elements o~x''). nonzero components are x'2 = K= ei'oxx and x'& = 'cXY e'+q for the applicable 4/mmm symmetry. As shown in Fig. 2a, we measure the corresponding THG intensities I2 and 1: as a function of ellipticity of the incident pulses (retardance A away from linear polarization). From this, one obtains the phase difference 6 = 9, - 9, and amplitude ratio p = ".pa. These values shown in Figs. 2(b,c) exhibit step-like changes concurrent with the appearance of the resonance (Fig. 1). The resonance thus affects differently the products of matrix elements parallel to the same axis, ~(2, and the mixed products, x(&,, implying at least a significant contribution of a parity forbidden transition active at 2w. Furthermore, it can be shown that lzXy = 0 occurs only for p = 3, a value associated with an overall spherical electronic distribution. As shown in Fig. 2, while p = 3 at the resonance, the transition through the resonance is associated with a change of symmetry of the THG tensor. More detailed calculations are underway to link these novel results to the symmetries of the underlying wavefunctions in the Sr,CuO,CI, bandstructure. References 1. 2. 3. M.Z. Hasan, et al. Science 288, 1811 (2000). Y.Y. Wang, et al. Phys. Rev. Lett. 77, 1809 (1996). H. Kishida, et al. Nature 405,929 (2000). ~~~~~ Nonlocal effects in the nonlinear optlcal response of chiral molecules: Third-order nonllnear circular dichroism H. Mesnil, M. C. Schanne-Klein and E Hache, Optical Bioscience Laboratory Ecole Polytechnique F-91128 Palaiseau-France; Ernail: [email protected] Although it was predicted as soon as 1967,' thirdorder nonlinear circular dichroism has never been experimentally observed. In this paper, we investigate this phenomenon in a liquid of chiral molecules. We first perform a theoretical calculation including nonlocal effects in the nonlinear light-matter interaction that allows to get insight into the process and to obtain its order of magnitude? We then demonstrate experimentally the existence of such a third-order nonlinear circular dichroism with a Ruthenium-tris(bipyridy1) salt that we have under its two enantiomeric (A and A) forms. Several experiments are carried out with a 1 kHz, 180 fs laser source: a one beam saturation experiment where a saturation of the circular dichroism is clearly observed3 and a pumpprobe experiment. In the latter case, by modulating the polarization of the probe or of the pump from a left to a right circular one, we observe an intensity-dependent circular dichroism (fig. 1). All these experiments are shown to derive from the same nonlinear mechanism, namely a chiral signature in the optical Kerr effect. We furthermore show that our results are in good agreement with our theoretical calculation.* Extension of this technique to time-resolved circular dichroism measurement is discussed. 1. S.A. Akhmanov andV.1. Zharikov, JETP Lett. 6,137 (1967). 2. F. Hache, H. Mesnil and M.C. Schanne-Klein, Phys. Rev. B 60,6405 (1999). 3. H. Mesnil and F. Hache, Phys. Rev. Lett. 85, 4257 (2000). QME5 1115 am Frequencydoubllng of femtosecond pulses in walk-off compensated N-( 4nitrophenyl)-L-prolinol Juan P. Torres, Silvia Carrasco and Lluis Torner, Lab. ofPhotonics, Polytechnic Univ. ofcatalonia, UPC 03, Barcelona 08034, Spain Eric W. VanStryland, CREOL, University of Central Florida, Orlando, Florida 32826, USA N-(4-nitrophenyl)-L-prolinol (NPP) is an organic molecular crystal developped by molecular I .8 . I .6 1 .4 I .I I .o 0.8 0.6 0.4 0.2 0 .O .0.2 -0.4 -0.6 .0.8 .I .o -1.2 Ill, QME4 1100 am -0.8 0.0 0.2 0.4 0.6 0.8 1.0 111, QME4 Fig. 1. Pump induced circular dichroism for a polarization-modulated pump (a) or probe (b) vs pump intensity. Squares and dots: A and A enantiomers, triangles: racemic mixture. engineering,' that exhibits one of the highest phase-matchable second-order susceptibilities reported so far in the near-infrared spectral range (deE = 56 pm/V). However, the large spatial and temporal walk-off existing in NPP can limit severely the usefulness of the material away from the noncritical phase-matching (ncpm) wavelength and for shorter pulses.2 Here we show that subpicosecond pulses can be efficiently frequency-doubled and mixed in NPP with moderate pump intensities, by employing tilted pulse techniques.' which makes use of the large Poynting vector walk-off exhibited by NPP crystals outside the ncpm. Such techniques are based on the diffraction of the input pump wave by a grating so that each spectral component is dispersed in a different direction, thus the resulting signal is a tilted pulse. When the beam width of the incident signal on the grating is large (i.e., a few mm), the effect of diffraction can be accounted for by considering temporal evolution only but with new effective inverse group velocities uv = k', + tan(yr) tan(pv)/c and group-velocity dispersions g, = V'" - (tan(V)/c)'/k,,. Here, \v is the tilt angle, c is the velocity of light in vacuum, k, are the linear wavenumbers, k', are the inverse group velocities, IC', are the group velocity dispersion (GVD) and pv are the Poynting-vector wakoff angles. We proceed as follows. First, we calculate the phase-matching angle €Iprn at the pump wavelength using the appropriate Sellmeier equations. Second, we find the value of the tilt yr,, required for group velocity mismatch (GVM) cancelation (U' = u2). All this then gives the effective GVDs, g,,. Figure 1 shows that a tilt-acceptance of several degrees is obtained with walk-off lengths in excess of 5 mm. Similarly, for a fixed tilt a wave- -1 I v E 64 II II SH I . -,- - - - 10 20 30 40 Tilt (degrees) 0 ----- ~ 1.5 1.55 1.6 1.65 1.7 Wavelength@rn) QME5 Fig. 1. In (a): Effective dispersion lengths, L,, at the fundamental (FF) and secondharmonic (SH) frequencies (solid lines), and temporal walk-off length, L,, (dashed lines), as a function of the tilt angle for a fixed pump wavelength h = 1.6 pm. In (b): Dispersion and walkoff lengths as a function of pump wavelength for a fixed tilt angle yro = 24.9'. In both cases: €I = €Ipm, T = 100 fs (full width half maximum at intensity). MONDAY MORNING / QELS2001 / 17 t P. A technique for determining rotational invariants of p by means of Kleinman disallowed hyper-Rayleigh scattering was designed and reported re~ently.~ A quantum mechanical sumover-states theory of the molecular hyperpolarizability indicated that low-lying states with transition dipole moments perpendicular to the molecule axis (i.e. B-states) are responsible for nonzero Kleinman-disallowed (including the second-rank) part of p. Molecules of C,, or similar symmetry are common in organic nonlinear optics as they do not have the inversion center (a major requirement for NLO materials), and they also often have a large permanent dipole moment that made them suitable for electric field poled materials. On the other hand, this symmetry allows the existence of electronic states that are odd with respect to two-fold rotation (corresponding to B-representations), which, in turn, results in a second-rank tensor contribution to p. Early studies showed that A-shaped chromophores, i.e. those having an electron acceptor (donor) connected to two donor (acceptor) groups with nconjugated bridges to form a A shape, may be of interest as they may have a large second-rank component of p that can be employed in alignment schemes as mentioned above. Here we present systematic measurements of the rotational invariant figures of merit of several A-shaped chromophores (Figure 1). Table 1 summarizes the data for the studied materials. One can notice that the figure of merit of the second-rank component for all materials reaches, and in some E- =*' *- = m QME5 Fig. 2. Detailed evolution of the SH pulse. (a): With GVM compensation; (b): without GVM compensation. Conditions: h = 1.6 pm, 8 = epm, 7 = 100 fs, I,, = 10 MW/cmZ (peak intensity) and a,, = 3 mm (beam waist). No SH pulse at the imput. length bandwidth of several tens of nanometers is obtained. Figure 2 shows the detailed SH pulse evolution whether there is walk-off compensation or not, by solving numerically the evolution equations. The difference is clearly visible: while without walk-off compensation no useful second-harmonic (SH) is generated, a clean, narrow high intensity SH pulse is obtained with the tilted pulse. Losses would reduce the efficiency of frequency doubling in thick crystals, but a high quality output second harmonic pulse is always obtained. In conclusion, we predict that highly efficient frequency doubling of subpicosecond pulses can be accomplished in walk-off compensated NPP with peak pump intensities in the MW/cm2 range, in a wide wavelength band centered around the third telecommunication window. Results are believed to greatly expand the potential applications of NPP to the implementation of parametric devices and cascading phenomena in general. References 1. 2. 7. Zyss, J.F. Nicoud, and M. Coquillay, J. Chem. Phys. 81,4160 (1984). 2. Wang et al., J. Opt. Soc. Am. B 14, 76 (1997); G.P. Banfi at al., Opt. Lett. 23, 439 (1998). V.D.Volosov et al., Sov. J. Quantum Electron. 4, 1090 (1975); G. Szabo and 2s. Bor, Appl. Phys. 58,237 (1994).; R. Danielius et al., Opt. Lett. 21,973 (1996). 3. QME6 W30 am Optimization of A-shaped molecules for second harmonic generation in media with nonpolar alignment V. Ostroverkhov, R. G. Petschek, K. D. Singer, Department of Physics, Case Western Reserve University, Cleveland, Ohio, USA; Email: [email protected] M. He and R. I, Twieg, Department of Chemistry, Kent State University, Kent, Ohio, USA Considerable effort has been directed towards creating organic nonlinear materials for second harmonic generation electro-optic control that would implement the non-vector irreducible components of the first hyperpolarizability tensor.',' In particular, the second-rank tensor component can give rise to macroscopic nonlinearity x(') in systems having chiral nonpolar symmetry of the bulk, such as 0, or D2. These types of alignment can be achieved in uniaxially or biaxially stretched chiral polymers, or in various uniaxial (N, SmA) and biaxial (SmC) liquid crystal phases. To make an efficient frequency conversion material, one has to optimize both the molecular response of the active chromophore and the macroscopic alignment scheme. The microscopic aspect of the problem is addressed by the quantum mechanical theory of the first hyperpolarizability combined with measurements of the rotationally invariant figures of merit of the relevant irreducible components of RT9090 (480nm) cv: (588Nn) MG: (615nm) BG: (625~11) 1955-49 (498nm) QME6 Fig. 1. Molecular structures and maximum absorption wavelengths of studies materials. QME6 Table 1. Rotational invariant for measured A-shaped chromophores Material (h, nm) I I PISS I I I I PImm I I I I Pzmm I I I I Pfss I I (esu X~O-~~) pNA (reference) cv MG BG pNA (reference) RT9090 1955-49 pNA (reference) cv pNA (reference) cv MG BG RT9090 1955-49 1560 nm 7.12 2 0.16 83.5 t 2.1 69.8 t 1.8 92.7 t 2.1 1340 8.09 2 0.09 266.8 2 5.5 316.0 2 9.4 1064 11.2 t 1.6 305 I 58 780 nm 56.4 I 5.9 552 t 42 122 t 10 1422 11 278 2 21 289 t 22 0.0 t 2.1 72.2 2 2.1 14 t 30 24 t 24 3.0 I 2.6 84.1 t 6.5 38.6 t 9.5 57.6 I 6.1 0.0 t 2.4 3.6 2 1.1 15 I41 128.8 t 4.9 0 t 98 4.6 t 1.2 341 t 72 2 I 18 162 I 18 186 t 16 112 t 27 119 t 25 180 t 35 2.1 t 2.6 276 t 52 23.4 I 3.8 353 t 34 309 t 36 120 I 13 136 2 12 135 I 19 167 t 16 ~~ 4.95 t 0.23 76.0 t 4.2 54.0 t 5.0 68.0 t 3.2 5.40 2 0.19 201.4 t 4.6 215 2 11 8.2 t 1.2 398 t 81 38.3 -t 4.1 287 t 31 131 2 14 153 t 13 198 t 16 200 t 16