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Efficient near-IR second harmonic generation in II–VI semiconductors
Synthetic Metals 124 (2001) 261±263
Ef®cient near-IR second harmonic generation in II±VI semiconductors Andrea Zappettinia,*, Silvia M. Pietralungaa, Antonella Milania, Mario Martinellia, Andrzej Mycielskib a
CORECOM, Consorzio Elaborazione Commutazione Ottica Milano, via AmpeÁre, 30 20131 Milano, Italy b Institute of Physics, Polish Academy of Sciences, 02-668 Al. lotnikow, 32/46 Warsaw, Poland
Abstract We have measured the absolute value of the second-order non-linear optical coef®cient (d41) at l 1:5 mm in the bulk wide-bandgap II± VI semiconductors ZnTe, Cd0.8Zn0.2Te, and Cd0.78Mn0.22Te, by evaluating the second harmonic generation (SHG) ef®ciency using a femtosecond laser source in a spectral Maker-like fringes technique. Values d41 59 pm/V, d41 78 pm/V and d41 71 pm/V have been obtained, respectively for ZnTe, CdZnTe, and CdMnTe. High non-linearity and transparency of the ternary alloys characterize them as promising materials for operation in the spectral range for optical communications. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Second harmonic generation; Te-based II±VI semiconductors; ZnTe; CdZnTe; CdMnTe; Near-IR non-linear optical properties
1. Introduction
2. Experimental
In recent years, many wide-bandgap II±VI semiconductors have been characterized, as basic materials for nonlinear optical applications and wavelength conversion [1±5]. In particular, the binary compound ZnTe displays a high second-order non-linear d41 coef®cient [1±6]. In order to ef®ciently exploit the strong non-linearity, operation in the spectral region of transparency of materials is required, so as to minimize transmission losses. This implies that, once the optical range of interest has been de®ned, the choice of a suitable basic material is mandatory to realize ef®cient devices. Thus, the non-linear optical characterization of materials of good transparency around l 1:5 mm is potentially of high practical interest, due to the quest for all-optical wavelength conversion mechanisms for signals for optical communications in the third ®ber optic transmission window [2,7]. In this work, we report on the second-order non-linear characterization, performed at l 1:5 mm, of three telluride-based wide-bandgap II±VI semiconductors, namely, ZnTe and the ternary alloys Cd0.8Zn0.2Te and Cd0.78Mn0.22Te. We have measured the ef®ciency of second harmonic generation (SHG), evaluating the d41 coef®cients and comparing the performances.
The second-order non-linear coef®cient has been evaluated using a phase-mismatched SHG technique, namely, the spectrally resolved Maker fringes method [3±8]. The experimental setup is schematically represented in Fig. 1. The beam at the fundamental frequency (FF) is provided by a tunable optical parametric ampli®er, pumped by a Ti:Sapphire laser, delivering pulses of 110 fs duration, at 80 MHz repetition rate and 9.3 kW peak pulse power. The spectral bandwidth of FF pulses measures Dl 25 nm, so that interference effects between SH bound and free waves occur for normal incidence. Measurements have been performed on vapor-phase grown ZnTe and Cd0.8Zn0.2Te samples, 780 mm thick, and Bridgman-grown samples of Cd0.78Mn0.22Te, 1080 mm thick. All samples are cut and oriented as shown in the insert of Fig. 1. FF beam is normally incident on the crystal sample in the [1 1 0] direction, linearly polarized in the plane of incidence and focused to a waist of o0 32 mm. The second harmonic (SH) beam is orthogonally polarized and selected in detection by the polarization analyzer A. The interference between SH bound and free waves is spectrally resolved in multichannel detection by a monochromator equipped with a grating at 600 grooves/mm and displayed by a streak camera. By measuring SHG power without spectral resolution, the absolute magnitude of the d 1=2w 2 coef®cient can be evaluated. This has been carried on by substituting the
* Corresponding author. Tel.: 39-2-2369-131; fax: 39-2-2369-1322. E-mail address: [email protected] (A. Zappettini).
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 4 6 9 - 6
262
A. Zappettini et al. / Synthetic Metals 124 (2001) 261±263
Fig. 1. Schematic representation of the experimental setup. The insert shows in detail the cutting directions of the crystalline samples and the relative orientations of the samples and polarization of the propagating light beams.
multichannel detection setup with a CW power meter, as shown in Fig. 1, where the dashed frame indicates the alternative detection con®gurations. 3. Discussion of results Measured spectral interference patterns are shown in Fig. 2. Results have been interpreted using the theory outlined in [3,8]. The experimentally obtained SH spectrum has been ®tted using the following relation: Po2
C o0 4g Z 1 g o1 o2 =2
o0 g o2
o1
o0 1
gei DbL do1 (1)
where the constant C(o0) depends on the SHG efficiency, o0 is the angular frequency corresponding to the center wavelength of FF pulse, g o o0 the Fourier transform of a sech2(t) pulse temporal function, L the sample length and the parameter g is related to the phase-mismatch Db between FF and SH beams.
The chromatic dispersion of the wide spectral bandwidth of the pulses has been modeled using the Sellmeier equation: n l2 A B
l2 l
2
C
(2)
The Sellmeier coefficients that correctly fit experimental patterns are given in Table 1. A 1% tolerance is estimated on both sample thickness measurement and refractive index values. The experimental non-zero minima in Fig. 2 must be attributed to the spectral resolution of the monochromator, since multiple path effects and beam divergence can be considered as negligible, under present setup conditions. The magnitude of the d41 coef®cient is derived from measured SH power [8]: s p e0 cpo20 n2o n22o P2o 2 d41 (3) 8t4 TP2o R In Eq. (3), o0 is the beam waist at FF, t and T are transmission factors at FF and SH, respectively, R is a correction factor for multiple reflections. Beam size effects have been neglected, and a plane-wave approximation can be introduced. Measured values for the d41 coef®cient are listed in Table 1.
Fig. 2. Measured spectral interference patterns.
A. Zappettini et al. / Synthetic Metals 124 (2001) 261±263
263
Table 1 SHG measurement in ZnTe, Cd0.8Zn0.2Te and Cd0.78Mn0.22Te samples: parameters inserted in the fitting procedure and experimentally obtained non-linear coefficient d14 Sample
Eg (eV)
L (nm)
Sellmeier parameters
d41 (pm/V)
ZnTe Cd0.8Zn0.2Te Cd0.78Mn0.22Te
2.28 1.6 1.9
780 780 1080
A 4.04, B 3.22, C 0.149 [9] A 5.88, B 1.33, C 0.345 [9] A 5.14, B 1.77, C 0.27 [10]
59 78 71
The result for ZnTe agrees with data previously reported at l 1:3 mm [3]. No values for d41 parameter have ever been reported so far for Cd1 xZnxTe. As for Cd1 xMnxTe, this is the ®rst time that second-order non-linearity is measured at l 1:5 mm. Available data, measured at l 10 mm, were relative to CdTe-values [10]. Unfortunately, the strong absorption of CdTe at l 750 nm prevents a direct comparison in this case. When comparing the Te-based ternary alloys and ZnTe, the former show higher conversion ef®ciency for operation in the third ®ber optic window, while keeping a good transparency level. They can, therefore, be considered as interesting candidates as basic materials to implement alloptical devices for processing of communication signals.
References [1] I. Shoji, T. Kondo, A. Kitamoto, M. Shirane, R. Ito, J. Opt. Soc. Am. B 14 (1997) 2268. [2] H.P. Wagner, S. Wittman, H. Schmitzer, H. Stanzi, J. Appl. Phys. 77 (1995) 3637. [3] H.P. Wagner, M. KuÈhnelt, W. Langbein, J.M. Hvam, Phys. Rev. B 58 (1998) 10494. [4] M. KuÈhnelt, H.P. Wagner, J. Non-Linear Opt. Phys. Mater. 7 (1998) 553. [5] H.P. Wagner, Phys. Stat. Sol. (b) 187 (1995) 363. [6] L. Kowalczyk, J. Crystallogr. 72 (1985) 389. [7] S.J.B. Yoo, IEEE J. Lightweight Technol. JLT-14 (1996) 955. [8] J. Jerphagnon, S.K. Kurtz, J. Appl. Phys. 41 (1970) 1167. [9] S. Adachi, T. Kimura, Jpn. J. Appl. Phys. 32 (1993) 3866. [10] M. Luttmann, Ph.D. Thesis (Physics), Univ. J. Fourier Grenoble 1 (1994) 144.
Ef®cient near-IR second harmonic generation in II±VI semiconductors Andrea Zappettinia,*, Silvia M. Pietralungaa, Antonella Milania, Mario Martinellia, Andrzej Mycielskib a
CORECOM, Consorzio Elaborazione Commutazione Ottica Milano, via AmpeÁre, 30 20131 Milano, Italy b Institute of Physics, Polish Academy of Sciences, 02-668 Al. lotnikow, 32/46 Warsaw, Poland
Abstract We have measured the absolute value of the second-order non-linear optical coef®cient (d41) at l 1:5 mm in the bulk wide-bandgap II± VI semiconductors ZnTe, Cd0.8Zn0.2Te, and Cd0.78Mn0.22Te, by evaluating the second harmonic generation (SHG) ef®ciency using a femtosecond laser source in a spectral Maker-like fringes technique. Values d41 59 pm/V, d41 78 pm/V and d41 71 pm/V have been obtained, respectively for ZnTe, CdZnTe, and CdMnTe. High non-linearity and transparency of the ternary alloys characterize them as promising materials for operation in the spectral range for optical communications. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Second harmonic generation; Te-based II±VI semiconductors; ZnTe; CdZnTe; CdMnTe; Near-IR non-linear optical properties
1. Introduction
2. Experimental
In recent years, many wide-bandgap II±VI semiconductors have been characterized, as basic materials for nonlinear optical applications and wavelength conversion [1±5]. In particular, the binary compound ZnTe displays a high second-order non-linear d41 coef®cient [1±6]. In order to ef®ciently exploit the strong non-linearity, operation in the spectral region of transparency of materials is required, so as to minimize transmission losses. This implies that, once the optical range of interest has been de®ned, the choice of a suitable basic material is mandatory to realize ef®cient devices. Thus, the non-linear optical characterization of materials of good transparency around l 1:5 mm is potentially of high practical interest, due to the quest for all-optical wavelength conversion mechanisms for signals for optical communications in the third ®ber optic transmission window [2,7]. In this work, we report on the second-order non-linear characterization, performed at l 1:5 mm, of three telluride-based wide-bandgap II±VI semiconductors, namely, ZnTe and the ternary alloys Cd0.8Zn0.2Te and Cd0.78Mn0.22Te. We have measured the ef®ciency of second harmonic generation (SHG), evaluating the d41 coef®cients and comparing the performances.
The second-order non-linear coef®cient has been evaluated using a phase-mismatched SHG technique, namely, the spectrally resolved Maker fringes method [3±8]. The experimental setup is schematically represented in Fig. 1. The beam at the fundamental frequency (FF) is provided by a tunable optical parametric ampli®er, pumped by a Ti:Sapphire laser, delivering pulses of 110 fs duration, at 80 MHz repetition rate and 9.3 kW peak pulse power. The spectral bandwidth of FF pulses measures Dl 25 nm, so that interference effects between SH bound and free waves occur for normal incidence. Measurements have been performed on vapor-phase grown ZnTe and Cd0.8Zn0.2Te samples, 780 mm thick, and Bridgman-grown samples of Cd0.78Mn0.22Te, 1080 mm thick. All samples are cut and oriented as shown in the insert of Fig. 1. FF beam is normally incident on the crystal sample in the [1 1 0] direction, linearly polarized in the plane of incidence and focused to a waist of o0 32 mm. The second harmonic (SH) beam is orthogonally polarized and selected in detection by the polarization analyzer A. The interference between SH bound and free waves is spectrally resolved in multichannel detection by a monochromator equipped with a grating at 600 grooves/mm and displayed by a streak camera. By measuring SHG power without spectral resolution, the absolute magnitude of the d 1=2w 2 coef®cient can be evaluated. This has been carried on by substituting the
* Corresponding author. Tel.: 39-2-2369-131; fax: 39-2-2369-1322. E-mail address: [email protected] (A. Zappettini).
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 4 6 9 - 6
262
A. Zappettini et al. / Synthetic Metals 124 (2001) 261±263
Fig. 1. Schematic representation of the experimental setup. The insert shows in detail the cutting directions of the crystalline samples and the relative orientations of the samples and polarization of the propagating light beams.
multichannel detection setup with a CW power meter, as shown in Fig. 1, where the dashed frame indicates the alternative detection con®gurations. 3. Discussion of results Measured spectral interference patterns are shown in Fig. 2. Results have been interpreted using the theory outlined in [3,8]. The experimentally obtained SH spectrum has been ®tted using the following relation: Po2
C o0 4g Z 1 g o1 o2 =2
o0 g o2
o1
o0 1
gei DbL do1 (1)
where the constant C(o0) depends on the SHG efficiency, o0 is the angular frequency corresponding to the center wavelength of FF pulse, g o o0 the Fourier transform of a sech2(t) pulse temporal function, L the sample length and the parameter g is related to the phase-mismatch Db between FF and SH beams.
The chromatic dispersion of the wide spectral bandwidth of the pulses has been modeled using the Sellmeier equation: n l2 A B
l2 l
2
C
(2)
The Sellmeier coefficients that correctly fit experimental patterns are given in Table 1. A 1% tolerance is estimated on both sample thickness measurement and refractive index values. The experimental non-zero minima in Fig. 2 must be attributed to the spectral resolution of the monochromator, since multiple path effects and beam divergence can be considered as negligible, under present setup conditions. The magnitude of the d41 coef®cient is derived from measured SH power [8]: s p e0 cpo20 n2o n22o P2o 2 d41 (3) 8t4 TP2o R In Eq. (3), o0 is the beam waist at FF, t and T are transmission factors at FF and SH, respectively, R is a correction factor for multiple reflections. Beam size effects have been neglected, and a plane-wave approximation can be introduced. Measured values for the d41 coef®cient are listed in Table 1.
Fig. 2. Measured spectral interference patterns.
A. Zappettini et al. / Synthetic Metals 124 (2001) 261±263
263
Table 1 SHG measurement in ZnTe, Cd0.8Zn0.2Te and Cd0.78Mn0.22Te samples: parameters inserted in the fitting procedure and experimentally obtained non-linear coefficient d14 Sample
Eg (eV)
L (nm)
Sellmeier parameters
d41 (pm/V)
ZnTe Cd0.8Zn0.2Te Cd0.78Mn0.22Te
2.28 1.6 1.9
780 780 1080
A 4.04, B 3.22, C 0.149 [9] A 5.88, B 1.33, C 0.345 [9] A 5.14, B 1.77, C 0.27 [10]
59 78 71
The result for ZnTe agrees with data previously reported at l 1:3 mm [3]. No values for d41 parameter have ever been reported so far for Cd1 xZnxTe. As for Cd1 xMnxTe, this is the ®rst time that second-order non-linearity is measured at l 1:5 mm. Available data, measured at l 10 mm, were relative to CdTe-values [10]. Unfortunately, the strong absorption of CdTe at l 750 nm prevents a direct comparison in this case. When comparing the Te-based ternary alloys and ZnTe, the former show higher conversion ef®ciency for operation in the third ®ber optic window, while keeping a good transparency level. They can, therefore, be considered as interesting candidates as basic materials to implement alloptical devices for processing of communication signals.
References [1] I. Shoji, T. Kondo, A. Kitamoto, M. Shirane, R. Ito, J. Opt. Soc. Am. B 14 (1997) 2268. [2] H.P. Wagner, S. Wittman, H. Schmitzer, H. Stanzi, J. Appl. Phys. 77 (1995) 3637. [3] H.P. Wagner, M. KuÈhnelt, W. Langbein, J.M. Hvam, Phys. Rev. B 58 (1998) 10494. [4] M. KuÈhnelt, H.P. Wagner, J. Non-Linear Opt. Phys. Mater. 7 (1998) 553. [5] H.P. Wagner, Phys. Stat. Sol. (b) 187 (1995) 363. [6] L. Kowalczyk, J. Crystallogr. 72 (1985) 389. [7] S.J.B. Yoo, IEEE J. Lightweight Technol. JLT-14 (1996) 955. [8] J. Jerphagnon, S.K. Kurtz, J. Appl. Phys. 41 (1970) 1167. [9] S. Adachi, T. Kimura, Jpn. J. Appl. Phys. 32 (1993) 3866. [10] M. Luttmann, Ph.D. Thesis (Physics), Univ. J. Fourier Grenoble 1 (1994) 144.