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Fast optical switching using cyanine dye-doped nematic liquid crystal
ARTICLE IN PRESS
Journal of Luminescence 122-123 (2007) 427–429 www.elsevier.com/locate/jlumin
Fast optical switching using cyanine dye-doped nematic liquid crystal Zhao-hui Jin, Zhongyu Li, Kazuo Kasatani, Hiroaki Okamoto Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube 755-8611, Japan Available online 9 March 2006
Abstract A cyanine dye, 1,10 ,3,3,30 ,30 -hexamethyl-4,40 ,5,50 -dibenzo-2,20 -indotricarbocyanine perchlorate (NK-2014) was doped in 4-cyano-40 -npentylbiphenyl (5CB), and the mixture was sandwiched between two pieces of rubbed glass plates. The third-order nonlinear optical responses of oriented NK-2014-doped 5CB were measured by the resonant femtosecond degenerate four-wave mixing (DFWM) technique at 820 nm. The time resolution of the system was ca. 0.3 ps (FWHM). The electronic component of third-order nonlinear optical susceptibility of one of the present samples was 1.3 108 esu (NK-2014 concentration: 1.0 wt%). The slow DFWM response of NK-2014-doped 5CB was accelerated with increasing laser power due to amplified spontaneous emission (ASE). On the other hand, we did not observe a similar phenomenon for NK-2014-doped polyethylene glycol (PEG-400). Oriented NK-2014 molecules in nematic liquid crystal must have very high ASE efficiency. Hence the population grating disappears in ca. 3 ps. NK-2014-doped 5CB can be used as a material for very fast all-optical switching. r 2006 Elsevier B.V. All rights reserved. Keywords: Optical switching; Amplified spontaneous emission (ASE); Degenerate four-wave mixing (DFWM); Cyanine dye; Nematic liquid crystal
Organic compounds have been attracting much attention for their applications in all-optical devices because they possess large optical nonlinearities and fast response [1]. All-optical switches are important components of highspeed optical communication networks [2], and their response is expected to be much faster than that of electrical switches. However, the third-order optical nonlinearities of organic materials reported so far have been several orders of magnitude smaller than the criterion for practical use. Nonlinear optical materials can be used under resonant conditions in order to obtain large optical nonlinearity. However, their response could be very slow. Therefore, third-order optical nonlinearities under resonant conditions have not been paid much attention for a long time [3]. In this study, we propose a new method to obtain large third-order optical nonlinearities and fast response for optical switching at the same time using cyanine dye-doped nematic liquid crystal. 1,10 ,3,3,30 ,30 -hexamethyl-4,40 ,5,50 -dibenzo-2,20 -indotricarbocyanine perchlorate (NK-2014) was purchased from Corresponding author. Tel.: +81 836 85 9641; fax: +81 836 85 9601.
E-mail address: [email protected] (K. Kasatani). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.180
Hayashibara Biochemical Laboratories, Inc., 4-cyano-40 n-pentylbiphenyl (5CB) was purchased from Tokyo Kasei Kogyo Co., Ltd., and polyethylene glycol (PEG-400) (molecular weight is ca. 400) was purchased from Ishizu Seiyaku Ltd., and they were used without further purification. While 5CB shows nematic phase at room temperature, PEG-400 is liquid. The structural formulas of NK2014 and 5CB are shown in Fig. 1. Oriented samples of NK-2014-doped 5CB were fabricated by the following way. The appropriate amount of NK-2014 was dissolved in 5CB at 60 1C and then cooled to room temperature. One drop of mixture was sandwiched between two pieces of rubbed glass plates. NK-2014 concentration was varied between 0.2 and 1.0 wt%. The system used in our experimental study of the degenerate four-wave mixing (DFWM) is same as that described in previous publications [3]. The folded boxCARS type geometry with the three linearly polarized beams of fundamental output of a Ti: sapphire regenerative amplifier laser (Spectra-Physics, Spitfire, 130 fs, 1 kHz, 1 W) was used for the DFWM experiment. The laser beam was attenuated by neutral density (ND) filters in order to avoid saturation; the energies of beams 1 and 2 were less than 4 mJ/pulse, and that of beam 3 was less than
ARTICLE IN PRESS Z.-h. Jin et al. / Journal of Luminescence 122-123 (2007) 427–429
428
1.2 ClO41 N+
0.5 GW/cm2 0.9 GW/cm2 2.5 GW/cm2 CCl4
IDFWM / Arb. Units
N NK-2014
C5H11
CN 5CB
Fig. 1. The structural formulas of NK-2014 and 5CB.
0.8
0.6
0.4
0.2 1
0.8
Absorbance
0
NK-2014-doped 5CB NK-2014-doped PEG
-5
0
5 10 Delay Time / ps
15
20
Fig. 3. Temporal profiles of a DFWM signal of oriented NK-2014-doped 5CB layer as functions of beam 3 delay time. NK-2014 concentration: 1.0 wt%, laser wavelength: 820 nm.
0.6
0.4 1.2 0.2
1 0.21 GW/cm2
550
600
650 700 Wavelength / nm
750
800
850
Fig. 2. Absorption spectra of NK-2014-doped 5CB and NK-2014-doped PEG-400. NK-2014 concentration: 1.0 wt%.
400 nJ/pulse. The sample was simultaneously irradiated with beams 1 and 2, and then irradiated with beam 3 after a suitable delay time. Beam 4, the produced DFWM signal, was detected by a charge-coupled device (CCD) after passing through a monochromator. The output of the CCD was acquired by a personal computer. The time resolution of the system was ca. 0.3 ps (FWHM). The DFWM signals of NK-2014-doped 5CB were measured at 820 nm. The intensity of the DFWM signal due to the glass substrate was completely negligible for each sample. The absorption spectra of NK-2014-doped 5CB and NK-2014-doped PEG-400 were measured using a spectrophotometer (Jasco, V550), and are shown in Fig. 2. NK2014 in PEG-400 and 5CB showed absorption maximum at 787 and 793 nm, respectively. The laser power dependence of the DFWM optical nonlinear response for NK-2014-doped 5CB is shown in Fig. 3. At a low laser power, the DFWM response has a spontaneous electronic component and a slow response due to population gratings of the S1 state. The decay of the latter was accelerated with increasing laser power. Very fast nonlinear optical response was obtained at a laser power greater than ca. 1 GW/cm2. The laser power dependence of the DFWM optical nonlinear response for NK-2014-doped
IDFWM / Arb. Units
0.42 GW/cm2 0 500
0.8
0.6
0.4
0.2
0 -5
0
5 10 Delay Time / ps
15
20
Fig. 4. Temporal profiles of a DFWM signal of NK-2014-doped PEG-400 layer as functions of beam 3 delay time. NK-2014 concentration: 1.0 wt%, laser wavelength: 820 nm.
PEG-400 is shown in Fig. 4. In this case, we did not observe such a phenomenon. NK-2014 in PEG-400 was decomposed at a laser power greater than 0.5 GW/cm2. Temporal profiles of transient absorption of NK-2014doped 5CB and NK-2014-doped PEG-400 are shown in Fig. 5. We found that the ground state recovery of transient absorption of NK-2014 in 5CB was finished at ca. 3 ps after the laser pulse of 7.7 GW/cm2, and it was much faster than that in PEG-400. Amplified spontaneous emission (ASE) spectra of NK2014-doped 5CB were measured using the fundamental output of a Ti: sapphire regenerative amplifier laser as a light source at 788 nm. The laser intensity was controlled
ARTICLE IN PRESS Z.-h. Jin et al. / Journal of Luminescence 122-123 (2007) 427–429
0.1
Absorbance Change / Arb. Units
NK-2014-doped 5CB NK-2014-doped PEG 0
-0.1
-0.2 -3
-2
-1
0
1
2
3
4
5
Time / ps Fig. 5. Temporal profiles of transient absorption of NK-2014 in 5CB and in PEG-400. The intensity of laser power: 7.7 GW/cm2, laser wavelength: 800 nm.
Emission Intensity / Arb. Units
1
0.8
0.6
0.04 GW/cm2 0.08 GW/cm2 0.21 GW/cm2 0.82 GW/cm2
0.4
0.2
0 600
700
800 Wavelength / nm
900
1000
Fig. 6. Normalized emission spectra of NK-2014-doped 5CB at different excitation power. Laser wavelength: 788 nm.
using ND filters. The pump beam was softly focused perpendicularly to the glass plate of the sample. The emission propagated along the direction of the sample layer, and perpendicular to the rubbing direction was
429
collected and detected by a CCD. Fig. 6 shows normalized emission spectra from the NK-2014-doped 5CB layer under various excitation laser power at 788 nm at 24 1C. For low excitation energies, the spectra are dominated by broad fluorescence. With increasing the pump energy the emission intensity increases and finally emission appears as a narrow band. A blue-shift of the emission was observed at a high laser power. While ASE of oriented NK-2014doped 5CB layer occurred easily, no ASE was observed for NK-2014-doped PEG-400 up to a laser power of 0.82 GW/cm2. The values of the third-order nonlinear susceptibility, w(3), were obtained from the DFWM signal intensity by comparison with that of the reference sample CCl4 (w(3) ¼ 4.01 1014 esu) [4] measured under the same conditions. w(3) for oriented samples of NK-2014-doped 5CB were calculated using the following equation: 2 1=2 ðLref =Lsample ÞaLsample wð3Þ sample ¼ ðnsample =nref Þ ðI sample =I ref Þ
expðaLsample =2Þ½1 expðaLsample Þ1 wð3Þ ref ,
ð1Þ
where I is the DFWM signal intensity, a is the linear absorption coefficient, n is the linear refractive index, and L is the sample length. The thickness of the samples was ca. 1–3 mm. The value of 1.7165 for 5CB was used for the linear refractive index of the samples, nsample. The third-order nonlinear susceptibilities of oriented samples of NK-2014-doped 5CB were determined using Eq. (1). The largest value of w(3) of our samples was ca. 1.3 108 esu (NK-2014 concentration: 1.0 wt%). All of the measurements with the Ti: sapphire lasers were carried out in the Venture Business Laboratory, Yamaguchi University. This work was partly supported by a Grant-in-Aid for Science Research (no.1556033) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] H.S. Nalwa, Adv. Mater. 5 (1993) 341. [2] A.M.C. Dawes, L. Illing, S.M. Clark, D.J. Gauthier, Science 308 (2005) 672. [3] K. Kasatani, Opt. Mater. 21 (2003) 93. [4] K. Kamada, M. Ueda, T. Sakaguchi, K. Ohta, T. Fukumi, Chem. Phys. Lett. 263 (1996) 215 (Errata; 267 (1997) 402).
Journal of Luminescence 122-123 (2007) 427–429 www.elsevier.com/locate/jlumin
Fast optical switching using cyanine dye-doped nematic liquid crystal Zhao-hui Jin, Zhongyu Li, Kazuo Kasatani, Hiroaki Okamoto Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube 755-8611, Japan Available online 9 March 2006
Abstract A cyanine dye, 1,10 ,3,3,30 ,30 -hexamethyl-4,40 ,5,50 -dibenzo-2,20 -indotricarbocyanine perchlorate (NK-2014) was doped in 4-cyano-40 -npentylbiphenyl (5CB), and the mixture was sandwiched between two pieces of rubbed glass plates. The third-order nonlinear optical responses of oriented NK-2014-doped 5CB were measured by the resonant femtosecond degenerate four-wave mixing (DFWM) technique at 820 nm. The time resolution of the system was ca. 0.3 ps (FWHM). The electronic component of third-order nonlinear optical susceptibility of one of the present samples was 1.3 108 esu (NK-2014 concentration: 1.0 wt%). The slow DFWM response of NK-2014-doped 5CB was accelerated with increasing laser power due to amplified spontaneous emission (ASE). On the other hand, we did not observe a similar phenomenon for NK-2014-doped polyethylene glycol (PEG-400). Oriented NK-2014 molecules in nematic liquid crystal must have very high ASE efficiency. Hence the population grating disappears in ca. 3 ps. NK-2014-doped 5CB can be used as a material for very fast all-optical switching. r 2006 Elsevier B.V. All rights reserved. Keywords: Optical switching; Amplified spontaneous emission (ASE); Degenerate four-wave mixing (DFWM); Cyanine dye; Nematic liquid crystal
Organic compounds have been attracting much attention for their applications in all-optical devices because they possess large optical nonlinearities and fast response [1]. All-optical switches are important components of highspeed optical communication networks [2], and their response is expected to be much faster than that of electrical switches. However, the third-order optical nonlinearities of organic materials reported so far have been several orders of magnitude smaller than the criterion for practical use. Nonlinear optical materials can be used under resonant conditions in order to obtain large optical nonlinearity. However, their response could be very slow. Therefore, third-order optical nonlinearities under resonant conditions have not been paid much attention for a long time [3]. In this study, we propose a new method to obtain large third-order optical nonlinearities and fast response for optical switching at the same time using cyanine dye-doped nematic liquid crystal. 1,10 ,3,3,30 ,30 -hexamethyl-4,40 ,5,50 -dibenzo-2,20 -indotricarbocyanine perchlorate (NK-2014) was purchased from Corresponding author. Tel.: +81 836 85 9641; fax: +81 836 85 9601.
E-mail address: [email protected] (K. Kasatani). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.180
Hayashibara Biochemical Laboratories, Inc., 4-cyano-40 n-pentylbiphenyl (5CB) was purchased from Tokyo Kasei Kogyo Co., Ltd., and polyethylene glycol (PEG-400) (molecular weight is ca. 400) was purchased from Ishizu Seiyaku Ltd., and they were used without further purification. While 5CB shows nematic phase at room temperature, PEG-400 is liquid. The structural formulas of NK2014 and 5CB are shown in Fig. 1. Oriented samples of NK-2014-doped 5CB were fabricated by the following way. The appropriate amount of NK-2014 was dissolved in 5CB at 60 1C and then cooled to room temperature. One drop of mixture was sandwiched between two pieces of rubbed glass plates. NK-2014 concentration was varied between 0.2 and 1.0 wt%. The system used in our experimental study of the degenerate four-wave mixing (DFWM) is same as that described in previous publications [3]. The folded boxCARS type geometry with the three linearly polarized beams of fundamental output of a Ti: sapphire regenerative amplifier laser (Spectra-Physics, Spitfire, 130 fs, 1 kHz, 1 W) was used for the DFWM experiment. The laser beam was attenuated by neutral density (ND) filters in order to avoid saturation; the energies of beams 1 and 2 were less than 4 mJ/pulse, and that of beam 3 was less than
ARTICLE IN PRESS Z.-h. Jin et al. / Journal of Luminescence 122-123 (2007) 427–429
428
1.2 ClO41 N+
0.5 GW/cm2 0.9 GW/cm2 2.5 GW/cm2 CCl4
IDFWM / Arb. Units
N NK-2014
C5H11
CN 5CB
Fig. 1. The structural formulas of NK-2014 and 5CB.
0.8
0.6
0.4
0.2 1
0.8
Absorbance
0
NK-2014-doped 5CB NK-2014-doped PEG
-5
0
5 10 Delay Time / ps
15
20
Fig. 3. Temporal profiles of a DFWM signal of oriented NK-2014-doped 5CB layer as functions of beam 3 delay time. NK-2014 concentration: 1.0 wt%, laser wavelength: 820 nm.
0.6
0.4 1.2 0.2
1 0.21 GW/cm2
550
600
650 700 Wavelength / nm
750
800
850
Fig. 2. Absorption spectra of NK-2014-doped 5CB and NK-2014-doped PEG-400. NK-2014 concentration: 1.0 wt%.
400 nJ/pulse. The sample was simultaneously irradiated with beams 1 and 2, and then irradiated with beam 3 after a suitable delay time. Beam 4, the produced DFWM signal, was detected by a charge-coupled device (CCD) after passing through a monochromator. The output of the CCD was acquired by a personal computer. The time resolution of the system was ca. 0.3 ps (FWHM). The DFWM signals of NK-2014-doped 5CB were measured at 820 nm. The intensity of the DFWM signal due to the glass substrate was completely negligible for each sample. The absorption spectra of NK-2014-doped 5CB and NK-2014-doped PEG-400 were measured using a spectrophotometer (Jasco, V550), and are shown in Fig. 2. NK2014 in PEG-400 and 5CB showed absorption maximum at 787 and 793 nm, respectively. The laser power dependence of the DFWM optical nonlinear response for NK-2014-doped 5CB is shown in Fig. 3. At a low laser power, the DFWM response has a spontaneous electronic component and a slow response due to population gratings of the S1 state. The decay of the latter was accelerated with increasing laser power. Very fast nonlinear optical response was obtained at a laser power greater than ca. 1 GW/cm2. The laser power dependence of the DFWM optical nonlinear response for NK-2014-doped
IDFWM / Arb. Units
0.42 GW/cm2 0 500
0.8
0.6
0.4
0.2
0 -5
0
5 10 Delay Time / ps
15
20
Fig. 4. Temporal profiles of a DFWM signal of NK-2014-doped PEG-400 layer as functions of beam 3 delay time. NK-2014 concentration: 1.0 wt%, laser wavelength: 820 nm.
PEG-400 is shown in Fig. 4. In this case, we did not observe such a phenomenon. NK-2014 in PEG-400 was decomposed at a laser power greater than 0.5 GW/cm2. Temporal profiles of transient absorption of NK-2014doped 5CB and NK-2014-doped PEG-400 are shown in Fig. 5. We found that the ground state recovery of transient absorption of NK-2014 in 5CB was finished at ca. 3 ps after the laser pulse of 7.7 GW/cm2, and it was much faster than that in PEG-400. Amplified spontaneous emission (ASE) spectra of NK2014-doped 5CB were measured using the fundamental output of a Ti: sapphire regenerative amplifier laser as a light source at 788 nm. The laser intensity was controlled
ARTICLE IN PRESS Z.-h. Jin et al. / Journal of Luminescence 122-123 (2007) 427–429
0.1
Absorbance Change / Arb. Units
NK-2014-doped 5CB NK-2014-doped PEG 0
-0.1
-0.2 -3
-2
-1
0
1
2
3
4
5
Time / ps Fig. 5. Temporal profiles of transient absorption of NK-2014 in 5CB and in PEG-400. The intensity of laser power: 7.7 GW/cm2, laser wavelength: 800 nm.
Emission Intensity / Arb. Units
1
0.8
0.6
0.04 GW/cm2 0.08 GW/cm2 0.21 GW/cm2 0.82 GW/cm2
0.4
0.2
0 600
700
800 Wavelength / nm
900
1000
Fig. 6. Normalized emission spectra of NK-2014-doped 5CB at different excitation power. Laser wavelength: 788 nm.
using ND filters. The pump beam was softly focused perpendicularly to the glass plate of the sample. The emission propagated along the direction of the sample layer, and perpendicular to the rubbing direction was
429
collected and detected by a CCD. Fig. 6 shows normalized emission spectra from the NK-2014-doped 5CB layer under various excitation laser power at 788 nm at 24 1C. For low excitation energies, the spectra are dominated by broad fluorescence. With increasing the pump energy the emission intensity increases and finally emission appears as a narrow band. A blue-shift of the emission was observed at a high laser power. While ASE of oriented NK-2014doped 5CB layer occurred easily, no ASE was observed for NK-2014-doped PEG-400 up to a laser power of 0.82 GW/cm2. The values of the third-order nonlinear susceptibility, w(3), were obtained from the DFWM signal intensity by comparison with that of the reference sample CCl4 (w(3) ¼ 4.01 1014 esu) [4] measured under the same conditions. w(3) for oriented samples of NK-2014-doped 5CB were calculated using the following equation: 2 1=2 ðLref =Lsample ÞaLsample wð3Þ sample ¼ ðnsample =nref Þ ðI sample =I ref Þ
expðaLsample =2Þ½1 expðaLsample Þ1 wð3Þ ref ,
ð1Þ
where I is the DFWM signal intensity, a is the linear absorption coefficient, n is the linear refractive index, and L is the sample length. The thickness of the samples was ca. 1–3 mm. The value of 1.7165 for 5CB was used for the linear refractive index of the samples, nsample. The third-order nonlinear susceptibilities of oriented samples of NK-2014-doped 5CB were determined using Eq. (1). The largest value of w(3) of our samples was ca. 1.3 108 esu (NK-2014 concentration: 1.0 wt%). All of the measurements with the Ti: sapphire lasers were carried out in the Venture Business Laboratory, Yamaguchi University. This work was partly supported by a Grant-in-Aid for Science Research (no.1556033) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] H.S. Nalwa, Adv. Mater. 5 (1993) 341. [2] A.M.C. Dawes, L. Illing, S.M. Clark, D.J. Gauthier, Science 308 (2005) 672. [3] K. Kasatani, Opt. Mater. 21 (2003) 93. [4] K. Kamada, M. Ueda, T. Sakaguchi, K. Ohta, T. Fukumi, Chem. Phys. Lett. 263 (1996) 215 (Errata; 267 (1997) 402).