Appl. Phys. B 32,137-143 (1983) - r ^ ! _ - i MppilöUi physics Physics B S Ä © Springer-Verlag 1983 Measurement of Third-Order Nonlinear Susceptibilities by Non-Phase Matched Third-Harmonic Generation M . Thalhammer and A . Penzkofer Naturwissenschaftliche Fakultät II - Physik, Universität, D-8400 Regensburg, Fed. Rep. Germany Received 1 June 1983/Accepted 5 August 1983 Abstract. The third-order susceptibilities x xxx (~ 3 > i » v 1) °^ liquids, solids and air are determined by third-harmonic generation. The samples are placed behind the focal region of a laser beam in an evacuated environment to avoid third-harmonic generation of the surroundings. F o r fluid media the sample cell is made out of two thin fused quartz plates and oriented to an angle of zero net third-harmonic production in each window (minimum Maker fringe position). 3 co w : 0 3 03 s o m e x P A C S : 42.65 The measurement of third-order nonlinear susceptibilities x ( — co ; co co coJ responsible for thirdharmonic generation is aggravated by disturbing light production in surrounding media (sample cell, air). Avoiding the influence of the surrounding substances in the light path by tight focussing (sample length longer than confocal parameter) results in zero thirdharmonic generation under phase-matched condition and for positive phase-mismatch Ak = k — 3k >0 (normal dispersive media). Only in anomalous dispersive matter light generation at frequency co occurs [1, 2]. In a collimated pump beam (frequency c o j the third-harmonic light is periodically generated and annihilated for Ak + 0 and efficient light generation requires Ak = 0. Circularly polarized laser light does not generate third-harmonic light in transparent media at all [6]. (3) 3 v v 3 x 3 In this paper we report on an accurate method to determine the third-order nonlinear susceptibilities X ( — co ; co co co ) of solids, liquids and gases. Third-harmonic generation in a slightly diverging beam (thin sample behind focal plane of a lens) is studied. Third-harmonic generation i n air along the path of the laser beam is avoided by putting the sample into a vacuum chamber. The entrance and exit windows of the sample cell for liquids and gases are made out of thin plane fused silica plates (thickness: « 0 . 2 mm). By tilting the cell to a Maker fringe minimum position for each window [11] no thirdharmonic light is produced in each of the windows. The cell is slightly wedged in order to record a Maker fringe curve when the filled cell is laterally shifted relative to the light path. (3) 3 v v t (3) Third-order nonlinear susceptibility values # ( — co ; co co ,co )of gases were obtained by putting a gas cell with its entrance window (absorbing at co ) in the focal plane of a laser [1], The susceptibilities of some solids were measured in reflection [3]. A comparison of third-harmonic generation with cascading secondorder processes was used to derive x ( — co ; co co co ) values for liquids and solids [4, 5], In mixtures of substances with normal and anomalous dispersion optimum third-harmonic conditions were achieved [6-10] and nonlinear susceptibility values could be obtained. 3 v 1 1 1. Theory 3 (3) 3 x v v The calculation of third-harmonic light generation starts from the wave equation 2 d 2 2 n d r E E an d ä? "?^ "7ä7 E = / l o 2 d ö? P N L ( 1 ) ' where E is the electrical field strength and P the nonlinear polarization; n and a are the refractive index and the linear absorption coefficient, respectively. The N L ® Fig. 1. (a) Experimental setup for third-harmonic generation. (L1-L3: lenses, PD1, PD2: photodetectors, D C : dye cell for intensity detection, V C : vacuum chamber, S: sample. F l , F2: filters, P M : photomultiplier.) (b) Schematic light path through sample cell S. (A: entrance window, B: inner space, C : exit window, 6: tilting angle, Ö: wedge angle) nonlinear polarization which is responsible for thirdharmonic generation is [12] 3 P =4£ < >;EEE nl (2) Z Insertion of (5) and (6) into (4) gives (E =E 30x d a 17 3 0 + 30 \E 3 0 y 3 3 0 Y (3) with x being the third-order nonlinear susceptibility tensor. Equation (1) is solved with the plane-wave ansatz for the third-harmonic generation process <JO +CD 1 3a (z'-z ) 1 1 0 -\iAk{z'-z ) •exp A = A +A =^{A 1 +A 3 3 0 exp [ i ^ z - co tj] 1 0 exp [i(/c z - co ty] + c.c.}, 3 .exp[- -^(z'-zj] (3) E (z') = where A stands for E and P . A and A are amplitudes at co and co , respectively. Using slowly varying amplitude approximation and transformation z' = z and t' = t — nz/c leads to (ß = 1 0 3 0 3 the the the 1, 3) 3 E (z lC^X E (z ) ln c 30 x 30 0 10 0 3 •exp y(z'-Z )-i<p(z ) 0 0 exp {[(a - 3a, )/2 - i A k] (z' - z )} - 1 3 0 (a — 3a )/2 — \Ak 3 dz' F o r transparent media (oc = a =0) Eq. (8) simplifies to R 1 Neglecting the effect of P field amplitude is given by N L 1 0 E , o(z') = E ( z ) exp [ - utf 1 0 on E 1 0 (5) 0 In the experiments linearly polarized laser light is used, i.e. E =E , E y =E = 0. F o r this situation the nonlinear polarization P reduces to 1 0 x 10 10 t 10 3 , the pump laser - z )/2]. 0 + (p(z ) •exp 0 •sin[\d/c(z'-z )/2]. (9) 0 > z N L > 3 0 Equation (9) indicates that the contribution to thirdharmonic light generation E (z') — E (z ) in a transparent medium of length z' —z is zero if z' — z = 2ml , where l = n/Ak is the coherence length and m is an integer. The thicknesses of our windows are adjusted to this length condition (entrance window ^ 34fcoh> ^ window / = 32^ , Fig. lb) in order to avoid contributions of the cell windows to the thirdharmonic light generated in the sample. The light path through the sample cell is shown schematically in F i g . l b . The cell has a wedge d. Its entrance face is tilted to an angle 0 to the input light 30 ^ . 3 o . ^ ) = ß o^(- ö 3^i> ö i> c o i) ?o(^) £ 0 (6) 0 and ^NL,3O,>; ^ N L , 3 O , z 0 for isotropic media and all crystalline classes except triclinic and monoclinic. Ak = k — 3k = (n co — 3n co )/c = 3co (n — nj/c is the wave vector mismatch. cp(z ) is the phase of the nonlinear polarization at the initial position z . In the further discussion X xxx(~^3 > v v ^ I ) * abbreviated by x \ 3 x = 3 3 1 1 1 3 0 0 3 x {3 00 00 30 0 • exp { - i[A k(z' - z ) + cp(z )-]} = (8) 1 (4) NL,0O- 2n (7) (3) 3 NL l ou The solution of (7) is x + (p{z 0 s coh = coh e x c oh 0 0 r {x) is the reflectivity of light of frequency co at the interface x( = a,b,c, d). [r = — sin(0. — 0 )/sin (0. + 0 ), where 0. and 6 are the angles of incidence and refraction]. M(a) and M(b) denote the magnification of the beam diameter in the plane of refraction ( M = cos0 /cos0 ). \x \ is determined from (13) by measuring all quantities entering (11) and (13). Using a cell with a slight wedge the cos(Akl ) dependence of //, (11), is observed by transverse displacement of the sample. In case of strong dispersion cos(Akl ) averages already across the beam diameter [<cos(4fc/ )>=0]. direction. The light is polarized perpendicular to the plane of incidence. The input light amplitude E (a_) before the entrance window is made negligibly small by focusing the pump laser into a long vacuum chamber and positioning the sample in the focal region behind the focal plane (Fig. la). (Subscripts — and + indicate positions before and after an interface, respectively.) A t the end of the transparent entrance window A at position b_ it is E {b_) = E (l ) = 0, since £ ( z = 0) = £ ( a ) = 0 and l = 2m l with m = 17, (9). A t the end c_ of the inner space of the cell B of thickness l the amplitude E (c_) = E {l +,l ) is given by (8) with £ ( z = /^) = 0 and z' — z = / . The exit window C transfers the generated signal E (l + l ) unchanged to the outside (except reflection losses, see below) since its thickness is an integer multiple of 2l [see (9), E tf = l + l + l ) = E {z = l + l \ since sin[J/c(z' - z ) / 2 ] = sin(zl/c/ /2) = 0]. The generated third-harmonic intensity is calculated from (8) by using the relation I = (nce /2)\E\ . F o r the conditions stated above [ £ ( a ) = 0, l = 2m l ] the intensity at interface c is ±i 3O 3 O o 3 O + 3O A A 30 3 0 30 A 0 0 B B B B 2. Experimental B coh A 30 B A B c 0 30 A passively mode-locked Nd-phosphate glass laser (v = 9480cm" ) is used in the experiments. A single transform limited pulse is selected in the early part of the pulse train. Broadened spectra as they occur in later parts of the pulse train cause a spreading of Ak due to the dispersion of the refractive index and wash out the Maker fringes. The selected pulse is increased in energy by double passage through a Nd-glass amplifier. t 0 1 c 2 0 3 0 h(c-) + A A coh = K\ \ ll(b ) (3) 2 X (10) + with K = I B A A co 3 {exp( — 3a l ) + exp( — oc l ) — 2 exp [ — (a + 3a )/ /2] cos(Akl )} 1 B 3 B 3 2 ~~ 2 1 B B 2 n n\c*s \_{a - 3a ) /4 + Ak ~\ 3 3 t {3) f coh 30 r t A B i L 30 (11) x The set-up for x -measurement i displayed in Fig. l a . The input pulse energy W is registered with photodetector P D 1 . The input peak intensity I is determined from energy-transmission measurements through a saturable absorber with detectors P D 2 and P D 1 [13]. The laser pulse is focused with a lens L 2 of / = 30cm focal length. This lens is placed in front of a 80 cm long vacuum chamber (pressure: ^0.1 mbar). The sample is positioned 36 cm behind the lens L 2 . The laser peak intensity at the sample is in the range between 2 x 10 and 8 x 10 W / c m . In this intensity region light generation at co by cascading parametric processes (parametric interaction co +co -+co + co and frequency conversion co -\-co + co ^co ) is negligible [14]. Behind the vacuum chamber the fundamental laser is filtered out with filters F l . The third-harmonic light is focused with lens L 3 to the photomultiplier P M which measures the generated light energy W . Detectors P D 1 and P M were cross-calibrated by detecting the same second harmonic signal with both detectors and using spectral sensitivity curves for their response to the fundamental (PD1) and thirdharmonic frequency (PM). The sample cell for liquids has a length of l = 2 mm. Its opening is rectangular having a width of 9 mm and a height of 3 m m (small opening to avoid distortions in vacuum chamber). The wedge of the cell along the (3) For transparent media K simplifies to s l K = 2 col sin {Akl /2) B A 2 2 n n\c z (Ak/2) 3 (12) ' In the experiments the energy conversion ratio rj = W {d )/W {a_) of third-harmonic pulse energy W (d ) after the end of the sample to incident fundamental pulse energy W (a_) before the entrance window is measured. F r o m (10) the energy conversion rj = W (c^)/W (b ) may be calculated. F o r a Gaussian temporal and spatial pulse shape one obtains exp 3 3 + 1 + x 3 1 + 3 x + 3 / 2 lx l / (M. (3) 2 2 (13) 0 I {b ) is the input peak intensity at the beginning of the inner space of the cell. The energies W (c_), W (f> ) and the intensity I (b ) are related to the measurable quantities W {d ), W^(a_) and / ( a _ ) by 10 + 3 10 3 + + (14) M{a)M{b) 2 W,{b ) = W {a-) [1 - r W,(c_) = x ±1 (a)] [1 -r W (d ) 3 + [l-ri (c)][l-ri (d)]' 3 + 10 1 + 1 + / (fe ) = / (a_) 1 9 9 2 3 l W (c_) _ K W {b ) " 3 10 3 2 ±1 (b)], (15) (16) 1 1 s s I I 3 3 B of various solids, liquids and air have been measured. The results are summarized in Table 1. The normalized signal S = fj/Il (b ) averaged over a modulation period changes only slightly for the various substances. As an example at 5 x 10 W / c m an energy conversion of 7 7 ~ 5 x l O is obtained i n water. The |% |-values are presented in SI units. They are transformed to esu-units by multiplying the reported values with 9 x 10 /(47i) [12]. The relative error of the presented data is estimated to be zlx /x — ± 0 . 2 . The lx |-value of air is small because of the reduced density of molecules in gaseous state. The nonlinear susceptibilities of the liquids and solids vary over three orders of magnitude. They are related to the electronic transition frequencies and transition strengths. A n increase of x is observed when co approaches an absorption band (SF10, SF59, rutile, nitrobenzene, 2-nitrotoluene, carbon disulfide, allo-ocimene). Correspondingly, x increases with growing refractive indices. Instead of x the third-order hyperpolarizability y may be used to characterize the media. It describes the nonlinearity per molecule and is given by [26] 0 E + 9 2 - 1 1 (3) 8 (3) (3) (3) er. z o >er {3) 3 Q UJ M i3) < et o {3) 1 LATERAL 2 3 DISPLACEMENT xlmml (3) { 24s NZ „(3)- Fig. 2. (a) Third-harmonic generation in empty sample cell versus tilting angle 0. Dashed curve (O), entrance window (fused silica Suprasill, thickness d — 0.222 mm). Solid curve (•), empty cell consisting of entrance plate with 9 = 6° and exit plate with 0 = O°. [Curves calculated, (12) and (13).] (b) Third-harmonic generation versus lateral displacement x of wedged cell. Tilting angle 9 = 9 = 5°. Dotted curve (•), empty cell. Solid curve (A), methanol. Dashed curve (O), benzene. (Curves are fitted to experimental points, not calculated) min min min (3) oX (17) 4 is the number density of molecules. N = 6.022169 x 1 0 m o l " is the Avogadro number, Q the density and M the molar mass. L = (n\ + 2) (rc + 2)/81 is the Lorentz local field correction factor. (Other definitions of y without factor 24 and £ are sometimes used [1, 34, 36].) The hyperpolarizability values y are listed in Table 1 (1 C m / V = 8.0888 x 10 esu). They exhibit a similar dependence as % . In case of rutile y is reduced compared to x due to the large local field correction term L . The hyperpolarizability of air has about the same value as the liquid water. According to the anharmonic oscillator model [25, 26] the hyperpolarizability y is related to the linear polarizabilities ^ = (3e /iV) {nf- l)/(n + 2) (i = l,3) by 6m «(3),' ( - co ; co co co J = £ (18) y[ N = N Q/M A A 23 1 4 3 2 ( 3 ) 0 (3) 4 9 mm width is 5 = 0.46°. The Suprasil windows (microscope cover plates) have thicknesses of 0.222 and 0.209 mm. They are pressed to the brass cell with brass fittings. Teflon washers are used for tightening. A cell of 12 mm length was used for the investigation of air. Figure 2a depicts the normalized third-harmonic signal S = t]/ll (b ) versus tilting angle 6 for the entrance plate (dashed curve) and the empty cell (both windows, solid curve). Figure 2b shows S versus displacement x at a tilting angle 6 = 0 = 5° for the empty cell (dotted line), for methanol (full line) and benzene (dashed line). In case of methanol the modulation of S{x) due to the cos(zi/d )-dependence of (11) is clearly seen. F o r benzene the signal averages already within the laser diameter. 0 + m i n B 3 24 (3) ( 3 ) (3) 4 ( 3 ) 1 } 2 0 1)3 3 v 19 where £ is the anharmonic coupling constant, m the electron mass, e the electron charge and / the oscillator strength. Equation (18) leads to X o c ( n - l ) ( n - l ) (Miller's rule [25]). The effective anharmonic coupling constant £/f is listed in Table 1. It is proportional to y /[y ^ y ]. The data indicate strong anharmonic coupling constants for substance with small y -values (LiF, water, air) while for substances with large nonlinearity y grows less rapidly ( 3 ) 2 3 2 2 3. Results X3) ( )3 ( 1} 3 The magnitude of the third-order nonlinear susceptibility Xxxxx(-^3'^u (<*>i =2ncv x l9 v =9480 c m " ) x (3) ( 3 ) Table 1. Third-order nonlinear susceptibilities and data relevant for their calculation (temperature 23 °C) a Substances n, a, a 2 3 (3) (3) 2 fj/I (b ) [10- cm /W ] |* | [10~ m /V ] |y | [ 10 ~ 3.2 3.2 4.5 6.2 1.0 3.7 1.1 30 34 3.6 4.7 32 87 1.2 3.6 5.2 2800 2000 0.97 l.l 5.6 5.9 0.14 1.2 1.1 8.8 12.9 1.01 0.70 0.34 0.56 14.8 0.32 0.11 0.82 1.6 0.57 23.5 0.93 1.95 3.2 5.9 14.8 7.1 14.9 6.7 11.8 15.1 7.1 14.4 10.4 21.6 9.7 33.7 0.65 0.23 0.15 0.038 1.39 0.023 0.042 0.024 0.019 0.23 0.033 0.052 0.046 0.012 l0 + 30 4 2 23 2 2 6 1 c/f C m / V ] [10 m 4 3 51 - 2 - 2 s ] 1 [cm' ] Solids Fused silica Glass Bk7 Glass SF10 Glass SF59 LiF Calcite E J c Calcite E , I f , k , l c Rutile Ej \\c Rutile E i l e , k 1c c c 1.4501 1.5070 1.7024 1.9085 1.3870 1.4800 1.6429 2.7447 2.4823 b 1.4769 1.5392 1.8144 2.1233 1.4032 1.5040 1.6968 4.25 3.69 d b d d b d d d c c c c c c c e c t e 0 0 0 0 0 0 0 0 0 0 0.053 22.6 45.6 0 0 0 2x 10 4x 10 0.172 0.144 0.112 0.031 0 0 0 0 0 0.01 0 0 0.43 0.2 0 0 2.3xl0 0.2 0 0 0 0 0 2.48 2.25" 0 5900° 175 0.031 1.6 0.74 133 1.9 4.8 1.6 2.7 3.5 5.4 3.3 1.3 2.2 2.0 1.3 4.8 3.1 3.5 2.4 3.6 2.2 11.1 1.6 2.6 3.3 6.2 17.0 8.5 19 7.0 13.0 41.1 5.2 12.7 8.6 14.7 0 1.5 5.6 x 10- 4e 4e q r s q r s Liquids Water Water + 6mol/l KI Methanol Ethanol Acetone CC1 Benzene Benzonitrile Nitrobenzene Tuoluene 2-nitrotoluene f f 1.3252 1.4391 1.3225 1.3547 1.3511 1.4498 1.4286 1.5080 1.5258 1.4774 1.5236 1.5928 1.3915 1.4345 1.4187 1.4799 1.3487 1.5034* 1.3416 1.3778 1.3771 V 1.4848 1.5518 1.5861 1.6371 1.5402 1.6195* 1.7395 1.4294 1.5120 1.4896 1.5494 1.0002490™ 1.0002598 g h h 1 1 1 f 4 f h h h h h h h h 1 cs f 2 1,5-hexadiene c, t 2,4-hexadiene f, t 2,4-hexadiene Allo-ocimene f h h j k 1 1 j k -3 p 1 U Gases Air (0.9 bar) ' Absolute refractive index From Glaswerke Schott, Mainz Linear interpolated from [15] Schott data sheet [16] Linear interpolated from [17] [18] m 0 h Extrapolated from [17] using single oscillator model [18] Extrapolated from [19] using single oscillator model [18] Extrapolated from own measurements [20] using single oscillator model [18] Own measurements [20] [21] [15] " [22] 1 J k 1 m 3 x) 2 than y ^ y (£/f decreases). A similar behaviour was reported in [28]. In Table 2 the measured X ( — co ; co co co^ values are compared with reported electronic thirdorder nonlinear susceptibility contributions X xxx,ei( — ^ <°v 2» 3)' The third-harmonic generation is caused by electronic nonlinearity. Our data are in reasonable agreement with most reported numbers (good agreement with [34]). In case of 6 molar K I dissolved i n water x ^ (— co ; a> co coj is about a half of ^ ( - c o ; co , o , - c ^ ) with co = 2co The enlargement of x (~"^3> ^ 2 » 1> ~ i) responsible for four wave mixing seems to be due to a two-photon 3 ( 3) X 0 3 3 XJC 3 19 v 03 a) x x x 3 xx 2 (3) 3 v v 2 2 03 CJO v 4 0.54 4.0 ° [23] " [24] Average molar mass of M = 70 g/mol is assumed [17] M= 162 g/mol is assumed [17] M= 175 g/mol is assumed [17] Apparent hyperpolarizability of KI-ions [26] For KI-ions [26] q r s 1 u resonance contribution (2co near to electronic transition in K I solution). The nonlinear "electronic" susceptibilities obtained from optical Kerr effect measurements with picosecond lasers [37-39] are about an order of magnitude larger than the electronic nonlinear susceptibilities obtained from third harmonic and four-photon frequency mixing experiments. This fact indicates that the main contribution to the fast part of the Kerr constant is still due to molecular effects of short (subpicosecond) response time and not of pure electronic origin. Nonlinear susceptibility contributions with subpicosecond response times have been observed recently [40, 41]. 2 Table 2. Electronic third-order nonlinear susceptibilities zS.x.v.ei ( — Substance w i co co i* i) « ( ] x\ Wavelength A,- = 27rc/co - x xx ( e l 1 ( - w ; w , , co , 4 2 15 [10 * esu] [nm] Solids Fused silica Fused silica Fused silica Glass Bk7 Glass (n = 1.516) Glass BSC Glass BSC LiF LiF LiF LiF Calcite Calcite n 1055 694.3 557 1055 1055 1060 557 1055 694.3 1890 557 1055 1055 694.3 557 1055 1055 1060 557 1055 694.3 1890 557 1055 - - 1055 527.5 1055 527.5 1055 555 1060 1055 1890 570 1060 555 1055 527.5 1055 527.5 1055 555 1060 1055 1890 532 1060 555 - - - - - 1055 745.7 594 1055 0 1060 594 1055 694.3 1890 594 1055 a 2.6 10 1\ 3.8 3.4 3.8 8 9.2 , 6.2 0.9 ,3 ,3.6 2 ~\J 3.4 , 1.9 2.6 ,3.7 b d a e f c d a f g b h 1 4 - c C d a a yd 4 d Liquids Water Water 6 mol KI in water 6 mol KI in water CC1 CC1 CC1 Benzene Benzene Benzene Benzene Benzene Benzene Nitrobenzene Nitrobenzene Nitrobenzene Nitrobenzene 2-nitrotoluene 2-nitrotoluene CS CS CS CS CS Allo-ocimene Allo-ocimene 4 4 4 2 2 2 2 2 1055 -1055 1055 -1055 1055 - 587 0 1055 1890 - 532 0 - 587 1.6" 1.5 8.0 19 4.5 11 8.7 12.2 15.8 20 10.6 17.5 , 18.4 14 14 40 16.7 300 9.3 330 30 86.5 87 30 120 ll 35 1 a 1 a j e a h k e j 1055 555 1060 1060 1055 1060 1055 570 555 1060 1060 1055 1890 1055 555 1060 -1060 1055 -1060 1055 532 555 1060 -1060 1055 1890 1055 1055 1 d - 1055 587 0 530 1055 530 1055 - 532 - 587 0 530 1055 1890 a - j e p a p a k j e q a m Gases Air (p = 1 bar, T =m K ) Air (p = 1 bar, T = 273 K ) 0 4a 4.9 x 10~ , 4.4 1055 n 0 694.3 0 3 694.3 694.3 2x 10~ or 7.9 n 0 a This work b [ii] c [29] [30] e [34] f [3] g [4] h [4] i [26] j [32] k 1 m [33] [35] [36] d m " x (air, Po, r ) = £f £ (3 0 Z » (air, p, T) P -»o ° [1] From optical Kerr effect (x xx ,ei ( « , CÜJ, —co co ) = i n, refractive index at a> '•> i nonlinear electronic optical Kerr index [12] [37] (see text) From optical Kerr effect [39] (see text) p l — XX n 2 q 2 lt 2 co ) 3 4. 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