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Two-photon absorption based optical limiting and stabilization in organic molecule-doped solid materials
15 May 1995
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 117 (1995) 133-136
Two-photon absorption based optical limiting and stabilization in organic molecule-doped solid materials Guang S. He a, Raz Gvishi a, Paras N. Prasad a, Bruce A. Reinhardt b aPhotonics Research Laboratory, Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260-3000, USA b Polymer Branch, Materials Directorate,
US Air Force Wright Laboratory,
Wright-Patterson
Air Force Base, OH 45433-7750,
USA
Received 28 November 1994
Abstract Optical limiting and stabilization via two-photon absorption (TF’A) in organic molecule-doped solid materials have been investigated. The nonlinear materials are epoxy rod and a composite glass rod doped with the same organic dopant (25
benzothiazole 3,4-didecyloxy thiophene). An ultrashort laser source with 0.5 ps pulsewidth and 602 nm wavelength was employed. The transmissivity of these two materials has been measured as a function of the input beam intensity. The measured results can be well fitted based on the assumption that TPA is the predominant mechanism producing the observed optical limiting behavior. Also, optical stabilization behavior is observed, and, at -930 MW/cm* input intensity level, the output intensity fluctuation is three times less than the input intensity fluctuation.
Optical limiting effects and devices are becoming more interesting in the area of nonlinear optics and opto-electronics because of their special application potential. There are several different mechanisms which could lead to optical limiting behavior, such as reverse saturable absorption (RSA) , two-photon absorption (TPA) , nonlinear refraction (including all types of beam induced refractive index changes), and optically induced scattering [ 11. A number of research studies of optical limiting effects related to TPA processes have been reported; most of them have focused on semiconductor materials [ 2-51. Recently, we reported four novel organic compounds that have shown strong two-photon absorption and consequently exhibit optical limiting behavior in tetrahydrofuran (THF) solutions [ 61. Among these four compounds, only compound I, i.e. 2,5benzothiazole 3,4-didecyloxy thiophene (or BBTDOT hereafter) , possesses both a large molecular TPS coefficient 0030-4018/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIOO30-4018(95)00097-6
and very high solubility in common organic solvents. For this reason BBTDOT can be further used to dope solid matrices for TPA purpose. In this letter, we present a preliminary experimental result on the optical limiting and stabilization behavior manifested by BBTDOT-doped solid materials. Two transparent materials are chosen as solid matrices: ( 1) optical grade epoxy and (2) composite glass. The epoxy used for our rod preparation is a commercial product (EPOTEK 301) that is transparent in the entire visible and near IR range. The composite glass in our case is a highly porous silica-gel bulk glass prepared by the solgel method and dried by slow heating from room temperature to 500°C. The dopant (BBTDOT) was dissolved in a methylmethacrylate (MMA) monomer, which was simultaneously catalyzed by the addition of 2% benzoyl peroxide, and then diffused into the silicagel pores and polymerized therein. After this process of dopant impregnation, the bulk was re-immersed in
134
G.S. He et al. /Optics
-
3.0
d d
2.5
Communications
0.0 400
600
Wavelength
d
800
(nm)
1.5
d
..._..
c,
:
(b) “--L_....
0.0
,
200
d I
400
600
Wavelength
800
1 1000
(nm)
Fig. 1. (a) One-photon absorption spectrum of a 40 km thick BBTDOT-doped epoxy film of 5 X 10V3 M/l concentration. The absorption of an undoped epoxy film of the same thickness is given by the dashed line, the molecular structure of the dopant is shown in the top-right comer. (b) One-photon absorption spectrum of a 2.5 mm thick BBTDOT-doped composite glass of - 1 X 10m4 M/l concentration. The absorption of an undoped composite glass of the same thickness is given by the dashed line.
a MMA-dopant solution, which at this stage was catalyzed for full polymerization by 0.5% benzoyl peroxide, and kept in a sealed container at 40°C until the polymerization process was completed [ 71. The linear one-photon absorption behavior of BBTDOT in the epoxy and the composite glass is nearly the same as that in a liquid (THF) solution [ 61. As an example, the linear absorption spectrum of a BBTDOT-doped epoxy film is shown in Fig. la by using a scanning spectrophotometer (W-3 1OlPC from Shimadzu) . The tested film is 40 km thick with a dopant concentration of 5 X lop3 M/l. The molecular structure of the dopant is shown in the top-right corner of Fig. la. For comparison, the absorption spectrum of an undoped
I17 (1995) 133-136
epoxy film with the same thickness is also shown in Fig. la by the dashed line. The additional influence from the substrate glass slides of the film samples was eliminated. Fig. lb shows the linear absorption spectrum of a 2.5 mm thick BBTDOT-doped composite glass of N 1 X 1O-4 M/l concentration. The absorption from an undoped composite glass of the same thickness is given by the dashed line. Both Fig. la and Fig. lb show that there is a stronger UV absorption in the spectral range of 300430 nm due to the dopant. Under illumination of an intense visible laser radiation both BBTDOT-doped epoxy and composite glass samples strongly emit frequency upconverted fluorescence. This implies a strong two-photon absorption process taking place in these solid samples. Therefore, they can manifest optical limiting behavior. In our experimental setup, the incident high peakpower visible laser beam was provided by an ultrashort laser oscillator-amplifier system as described in our earlier papers [ 8,9]. The wavelength, pulsewidth, spectral width, and repetition rate of the incident laser pulses were -602 nm, -0.5 ps, N 60 cm-‘, and 30 Hz, respectively. The input collimated laser beam of 5 mm size was focused onto the middle position of a doped solid rod sample via anf= 50 cm lens. A neutraldensity-filter set was placed in the optical path of the incident beam before the rod sample, so the local intensity (or irradiance) of the incident laser beam in the sample could be controlled by changing the combination of neutral density filters. Specifically, an intensity (irradiance) change ranging from 5 MW/cm2 to 1 GW/cm* was applied to the solid samples without any optical damage. To ensure that the observed intensitydependent nonlinear attenuation was only related with the induced absorption but not with the induced change of beam spatial structure, anf= 4 cm lens was used to refocus the output laser beam into a large-area photodiode detector in such a way that a precise object-image relation was fulfilled between the middle position of the sample rod and the detector surface; meanwhile, no aperture was used. Under the applied intensity levels, neither backward stimulated scattering nor forward continuum generation was observed from the sample rods. To monitor the incident beam intensity changes and measure the transmissivity for a given sample, the photodiode was employed in conjunction with a boxcar averager (Model 4420 from EG&G) .
G.S. He et al. /Optics
LX.4 d=0.09
cm epoxy M/I
Communications
fl= gzNO = ~N,dx
I
0.6
0.4
’
I
I
I
0.00
0.25
0.50
0.75
I 1 .oo
1.25
1 .oo
1.25
1 .o
0.6
0.4
0.2
1 0.00
1
I
/
0.25
0.50
0.75
Input
Intensity
133-136
(GW/cm2)
Fig. 2. Transmissivity as a function of input intensity levels for (a) anL=2.4cmlongepoxyrodwithadopantconcentrationof9 X lo-’ M/l, and (b) an L= 1.1 cm long composite glass rod with a dopant concentration of 1 X 10-r M/l. The solid line in (a) is a theoretical fit with uz=4.2X 10Vzo cm4/GW, and the dashed line in (b) is a fitting curve with cz=9.1 X lo-” cm4/GW.
The measured nonlinear absorption data as a function of the incident beam intensity for two solid rod samples are shown in Fig. 2. Fig. 2a is obtained horn a 2.4 cm long epoxy rod with a dopant concentration of 9 X lo-‘M/l, and Fig. 2b is obtained from a 1.1 cm long composite glass rod of 1 X 10-l M/l concentration. In both cases influences from surface-reflection and residual linear attenuation of the samples were already subtracted. According to the basic theoretical consideration, if the beam has a gaussian transverse distribution in the medium, the TPA induced decrease of transmissivity can be expressed as [ 1, lo] =ln( 1 +Z,L~)IZ,L~,
(1)
where I, is the incident intensity, L the thickness of a given sample, and /3 is the TPA coefficient of a given
1O-3 .
(2)
Here, N,, u2, and d are $e molecular density (in units of cme3), molecular TPA coefficient for cross section (in units of cm4/ GW) , and concentration (in units of M/l) of the dopant in a solid matrix, respectively. Finally, NA is the Avogadro constant. For a known experimental relationship between T(Z,) and Z,, the value of p or f12 can be easily estimated by fitting Eq. (1) with the measured results. The solid-line curve in Fig. 2a was given by Eq. ( 1) with a best fitting parameter of CT~ = 4.2 X 10e2’ cm4/GW, and the dashed-line curve in Fig. 2b was given by Eq. (1) with a fitting value of a2=9.1 X lo-*’ cm4/GW. These values are somewhat greater than the corresponding value of a, = 2.6 X 10p2’ cm4/GW based on the measurement of the BBTDOT solution in THF. We believe that variations of the a2 values are caused by matrix influences. The good agreement between the theoretical predictions of TPA [ 1,101 and the measured intensitydependent transmissivity changes shown in Fig. 2 lends support to the idea that TPA is the dominant mechanism causing the observed nonlinear absorption behavior. The measured values of a, for BBTDOT in a liquid solution and solid matrices are greater than the a2 values of rhodamine dyes by one order of magnitude [6,11,12]. r
I /
I
.oo
0.25
Input
T(Z,) =Z(L)lZ,
135
TPA medium. Furthermore, the TPA coefficient p (in units of cm/GW) of the sample is determined by
rod
crz=4.2x10%n’/GW
0.2
117(1995)
I
I
L=2.4
I
cm
epoxy
d=0.09
0.50
Intensity
0.75
rod
M/I
1.00
1.25
(GW/cm2)
Fig. 3. Transmitted intensity as a function of the input intensity for anL=2.4cmlongepoxyrodwithadopantconcentrationof9X lo-’ M/l. The long-dashed line represents the case of a transparent linear medium.
136
G.S. He et al. /Optics
/
1.4 x c 2 0, E WaI Eq E 2 z+ .-P -b a, 05
I
I
1
(a)
epoxy M/I
rod
1.0
o
0.8
a -- L
0.6
(b)
0.4
0.2
&$I
0.0
i
I I7 (1995) 133-136
case of Fig. 4b the transmissivity reduced to T= 0.36 and the relative transmitted intensity fluctuation also reduced to A= +3.8%. The influence from background fluctuation at the zero-line in Fig. 4 has been already subtracted for the above estimation of the relative intensity fluctuation. In conclusion, it has been experimentally demonstrated that the TPA based organic molecule-doped solid devices can be used as a novel approach to stabilizing optical power/intensity fluctuations. Further effort is to extend our current studies from solid rod to the waveguide and fiber configurations.
I
L=2.4cm d=0.09
1.2
Communications
.
I
0
5
10
15
Time
(min.)
20
25
30
Fig. 4. Transmitted intensity fluctuations at (a) a low input level (I,= 9.3 MW/cm*) and (b) a highinput level (I,=930 MW/cm*) through the L = 2.4 cm long epoxy rod with a dopant concentration of9XlO-‘M/l.
This work was supported by the Polymer Branch, Wright Laboratory and the Air Force Office of Scientific Research through contract number F4962093COOl-Z.
References The TPA induced nonlinear transmissivity change can be used not only for optical limiting but also for stabilizing peak-power fluctuations. This principle can be explained via Fig. 3, which displays the transmitted intensity as a function of the input intensity through the 2.4 cm long epoxy rod with a dopant concentration of 9X lOMa M/l. In Fig. 3 one can easily find that at a higher input level, a larger input intensity variation (A) will result in a much smaller output intensity variation (A’) due to the intrinsic property of TPA. This property can be demonstrated by monitoring the intensity variation for the input signal and output signal separately. As an example, Fig. 4 shows the relative transmitted intensity fluctuations at (a) a low input level (Z, = 9.3 MW/cm’) and (b) a high input level (IO = 930 MW/ cm*), respectively. The exposure time for both cases was about 3 minutes, and each data point was averaged over 30 laser pulses to achieve a higher signal/noise ratio. In case of Fig. 4a, the TPA was negligible and the relative transmitted intensity fluctuation, A = + 1 l%, was the same as the input. However, in
[l] L.W. Tuff and R.F. Boggess, Prog. Quantum Electron. [2 J [3 J [4]
[5] [6] [7] [8] [9] [lo] [ 1 l] [ 121
17 (1993) 299. A.C. Walker, A.K. Kar, Ji Wei, U. Keller and SD. Smith, Appl. Phys. Lett. 48 (1986) 683. Y.C. Chang, A.E. Chiou and M. Khoshnevissan, J. Appl. Phys. 71 (1992) 1349. E.W. Van Stryland, H. Vanherzeele, M.A. Woodall, M.J. Soileau, A.L. Smirl, S. Guha and T.F. Boggess, Opt. Eng. 24 (1985) 613. D.C. Hutchings and D.W. Van Stryland, J. Opt. Sot. Am. B 9 (1992) 2065. G.S. He, G.C. Xu, P.N. Prasad, B.A. Reinhardt, J.C. Bhatt and A.G. Dillard, Optics Lett. 20 (1995) 435. R. Gvishi, R. Reisfeld, E. Miron and Z. Brushtein, SPIE-Int. Sot. Opt. Eng. 1972 (1993) 390. G.S. He, G.C. Xu, Y. Pang and P.N. Prasad, J. Opt. Sot. Am. B 8 (1991) 1907. G.S. He, G.C. Xu, Y. Cui and P.N. Prasad, Appl. Optics 32 (1993) 4507. T.F. Boggess, K. Bohnert, K. Mansour, S.C. Moss, I.W. Boyd and A.L. Smirl, IEEE J. Quantum Electron. 22 ( 1986) 360. P. Qui and A. Penzkofer, Appl. Phys. B 48 ( 1989) 115. W. Bnmner, H. Duerr, E. Klose and H. Paul, Kvant. Elektron. (Moscow) 2 (1975) 832.
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 117 (1995) 133-136
Two-photon absorption based optical limiting and stabilization in organic molecule-doped solid materials Guang S. He a, Raz Gvishi a, Paras N. Prasad a, Bruce A. Reinhardt b aPhotonics Research Laboratory, Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260-3000, USA b Polymer Branch, Materials Directorate,
US Air Force Wright Laboratory,
Wright-Patterson
Air Force Base, OH 45433-7750,
USA
Received 28 November 1994
Abstract Optical limiting and stabilization via two-photon absorption (TF’A) in organic molecule-doped solid materials have been investigated. The nonlinear materials are epoxy rod and a composite glass rod doped with the same organic dopant (25
benzothiazole 3,4-didecyloxy thiophene). An ultrashort laser source with 0.5 ps pulsewidth and 602 nm wavelength was employed. The transmissivity of these two materials has been measured as a function of the input beam intensity. The measured results can be well fitted based on the assumption that TPA is the predominant mechanism producing the observed optical limiting behavior. Also, optical stabilization behavior is observed, and, at -930 MW/cm* input intensity level, the output intensity fluctuation is three times less than the input intensity fluctuation.
Optical limiting effects and devices are becoming more interesting in the area of nonlinear optics and opto-electronics because of their special application potential. There are several different mechanisms which could lead to optical limiting behavior, such as reverse saturable absorption (RSA) , two-photon absorption (TPA) , nonlinear refraction (including all types of beam induced refractive index changes), and optically induced scattering [ 11. A number of research studies of optical limiting effects related to TPA processes have been reported; most of them have focused on semiconductor materials [ 2-51. Recently, we reported four novel organic compounds that have shown strong two-photon absorption and consequently exhibit optical limiting behavior in tetrahydrofuran (THF) solutions [ 61. Among these four compounds, only compound I, i.e. 2,5benzothiazole 3,4-didecyloxy thiophene (or BBTDOT hereafter) , possesses both a large molecular TPS coefficient 0030-4018/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIOO30-4018(95)00097-6
and very high solubility in common organic solvents. For this reason BBTDOT can be further used to dope solid matrices for TPA purpose. In this letter, we present a preliminary experimental result on the optical limiting and stabilization behavior manifested by BBTDOT-doped solid materials. Two transparent materials are chosen as solid matrices: ( 1) optical grade epoxy and (2) composite glass. The epoxy used for our rod preparation is a commercial product (EPOTEK 301) that is transparent in the entire visible and near IR range. The composite glass in our case is a highly porous silica-gel bulk glass prepared by the solgel method and dried by slow heating from room temperature to 500°C. The dopant (BBTDOT) was dissolved in a methylmethacrylate (MMA) monomer, which was simultaneously catalyzed by the addition of 2% benzoyl peroxide, and then diffused into the silicagel pores and polymerized therein. After this process of dopant impregnation, the bulk was re-immersed in
134
G.S. He et al. /Optics
-
3.0
d d
2.5
Communications
0.0 400
600
Wavelength
d
800
(nm)
1.5
d
..._..
c,
:
(b) “--L_....
0.0
,
200
d I
400
600
Wavelength
800
1 1000
(nm)
Fig. 1. (a) One-photon absorption spectrum of a 40 km thick BBTDOT-doped epoxy film of 5 X 10V3 M/l concentration. The absorption of an undoped epoxy film of the same thickness is given by the dashed line, the molecular structure of the dopant is shown in the top-right comer. (b) One-photon absorption spectrum of a 2.5 mm thick BBTDOT-doped composite glass of - 1 X 10m4 M/l concentration. The absorption of an undoped composite glass of the same thickness is given by the dashed line.
a MMA-dopant solution, which at this stage was catalyzed for full polymerization by 0.5% benzoyl peroxide, and kept in a sealed container at 40°C until the polymerization process was completed [ 71. The linear one-photon absorption behavior of BBTDOT in the epoxy and the composite glass is nearly the same as that in a liquid (THF) solution [ 61. As an example, the linear absorption spectrum of a BBTDOT-doped epoxy film is shown in Fig. la by using a scanning spectrophotometer (W-3 1OlPC from Shimadzu) . The tested film is 40 km thick with a dopant concentration of 5 X lop3 M/l. The molecular structure of the dopant is shown in the top-right corner of Fig. la. For comparison, the absorption spectrum of an undoped
I17 (1995) 133-136
epoxy film with the same thickness is also shown in Fig. la by the dashed line. The additional influence from the substrate glass slides of the film samples was eliminated. Fig. lb shows the linear absorption spectrum of a 2.5 mm thick BBTDOT-doped composite glass of N 1 X 1O-4 M/l concentration. The absorption from an undoped composite glass of the same thickness is given by the dashed line. Both Fig. la and Fig. lb show that there is a stronger UV absorption in the spectral range of 300430 nm due to the dopant. Under illumination of an intense visible laser radiation both BBTDOT-doped epoxy and composite glass samples strongly emit frequency upconverted fluorescence. This implies a strong two-photon absorption process taking place in these solid samples. Therefore, they can manifest optical limiting behavior. In our experimental setup, the incident high peakpower visible laser beam was provided by an ultrashort laser oscillator-amplifier system as described in our earlier papers [ 8,9]. The wavelength, pulsewidth, spectral width, and repetition rate of the incident laser pulses were -602 nm, -0.5 ps, N 60 cm-‘, and 30 Hz, respectively. The input collimated laser beam of 5 mm size was focused onto the middle position of a doped solid rod sample via anf= 50 cm lens. A neutraldensity-filter set was placed in the optical path of the incident beam before the rod sample, so the local intensity (or irradiance) of the incident laser beam in the sample could be controlled by changing the combination of neutral density filters. Specifically, an intensity (irradiance) change ranging from 5 MW/cm2 to 1 GW/cm* was applied to the solid samples without any optical damage. To ensure that the observed intensitydependent nonlinear attenuation was only related with the induced absorption but not with the induced change of beam spatial structure, anf= 4 cm lens was used to refocus the output laser beam into a large-area photodiode detector in such a way that a precise object-image relation was fulfilled between the middle position of the sample rod and the detector surface; meanwhile, no aperture was used. Under the applied intensity levels, neither backward stimulated scattering nor forward continuum generation was observed from the sample rods. To monitor the incident beam intensity changes and measure the transmissivity for a given sample, the photodiode was employed in conjunction with a boxcar averager (Model 4420 from EG&G) .
G.S. He et al. /Optics
LX.4 d=0.09
cm epoxy M/I
Communications
fl= gzNO = ~N,dx
I
0.6
0.4
’
I
I
I
0.00
0.25
0.50
0.75
I 1 .oo
1.25
1 .oo
1.25
1 .o
0.6
0.4
0.2
1 0.00
1
I
/
0.25
0.50
0.75
Input
Intensity
133-136
(GW/cm2)
Fig. 2. Transmissivity as a function of input intensity levels for (a) anL=2.4cmlongepoxyrodwithadopantconcentrationof9 X lo-’ M/l, and (b) an L= 1.1 cm long composite glass rod with a dopant concentration of 1 X 10-r M/l. The solid line in (a) is a theoretical fit with uz=4.2X 10Vzo cm4/GW, and the dashed line in (b) is a fitting curve with cz=9.1 X lo-” cm4/GW.
The measured nonlinear absorption data as a function of the incident beam intensity for two solid rod samples are shown in Fig. 2. Fig. 2a is obtained horn a 2.4 cm long epoxy rod with a dopant concentration of 9 X lo-‘M/l, and Fig. 2b is obtained from a 1.1 cm long composite glass rod of 1 X 10-l M/l concentration. In both cases influences from surface-reflection and residual linear attenuation of the samples were already subtracted. According to the basic theoretical consideration, if the beam has a gaussian transverse distribution in the medium, the TPA induced decrease of transmissivity can be expressed as [ 1, lo] =ln( 1 +Z,L~)IZ,L~,
(1)
where I, is the incident intensity, L the thickness of a given sample, and /3 is the TPA coefficient of a given
1O-3 .
(2)
Here, N,, u2, and d are $e molecular density (in units of cme3), molecular TPA coefficient for cross section (in units of cm4/ GW) , and concentration (in units of M/l) of the dopant in a solid matrix, respectively. Finally, NA is the Avogadro constant. For a known experimental relationship between T(Z,) and Z,, the value of p or f12 can be easily estimated by fitting Eq. (1) with the measured results. The solid-line curve in Fig. 2a was given by Eq. ( 1) with a best fitting parameter of CT~ = 4.2 X 10e2’ cm4/GW, and the dashed-line curve in Fig. 2b was given by Eq. (1) with a fitting value of a2=9.1 X lo-*’ cm4/GW. These values are somewhat greater than the corresponding value of a, = 2.6 X 10p2’ cm4/GW based on the measurement of the BBTDOT solution in THF. We believe that variations of the a2 values are caused by matrix influences. The good agreement between the theoretical predictions of TPA [ 1,101 and the measured intensitydependent transmissivity changes shown in Fig. 2 lends support to the idea that TPA is the dominant mechanism causing the observed nonlinear absorption behavior. The measured values of a, for BBTDOT in a liquid solution and solid matrices are greater than the a2 values of rhodamine dyes by one order of magnitude [6,11,12]. r
I /
I
.oo
0.25
Input
T(Z,) =Z(L)lZ,
135
TPA medium. Furthermore, the TPA coefficient p (in units of cm/GW) of the sample is determined by
rod
crz=4.2x10%n’/GW
0.2
117(1995)
I
I
L=2.4
I
cm
epoxy
d=0.09
0.50
Intensity
0.75
rod
M/I
1.00
1.25
(GW/cm2)
Fig. 3. Transmitted intensity as a function of the input intensity for anL=2.4cmlongepoxyrodwithadopantconcentrationof9X lo-’ M/l. The long-dashed line represents the case of a transparent linear medium.
136
G.S. He et al. /Optics
/
1.4 x c 2 0, E WaI Eq E 2 z+ .-P -b a, 05
I
I
1
(a)
epoxy M/I
rod
1.0
o
0.8
a -- L
0.6
(b)
0.4
0.2
&$I
0.0
i
I I7 (1995) 133-136
case of Fig. 4b the transmissivity reduced to T= 0.36 and the relative transmitted intensity fluctuation also reduced to A= +3.8%. The influence from background fluctuation at the zero-line in Fig. 4 has been already subtracted for the above estimation of the relative intensity fluctuation. In conclusion, it has been experimentally demonstrated that the TPA based organic molecule-doped solid devices can be used as a novel approach to stabilizing optical power/intensity fluctuations. Further effort is to extend our current studies from solid rod to the waveguide and fiber configurations.
I
L=2.4cm d=0.09
1.2
Communications
.
I
0
5
10
15
Time
(min.)
20
25
30
Fig. 4. Transmitted intensity fluctuations at (a) a low input level (I,= 9.3 MW/cm*) and (b) a highinput level (I,=930 MW/cm*) through the L = 2.4 cm long epoxy rod with a dopant concentration of9XlO-‘M/l.
This work was supported by the Polymer Branch, Wright Laboratory and the Air Force Office of Scientific Research through contract number F4962093COOl-Z.
References The TPA induced nonlinear transmissivity change can be used not only for optical limiting but also for stabilizing peak-power fluctuations. This principle can be explained via Fig. 3, which displays the transmitted intensity as a function of the input intensity through the 2.4 cm long epoxy rod with a dopant concentration of 9X lOMa M/l. In Fig. 3 one can easily find that at a higher input level, a larger input intensity variation (A) will result in a much smaller output intensity variation (A’) due to the intrinsic property of TPA. This property can be demonstrated by monitoring the intensity variation for the input signal and output signal separately. As an example, Fig. 4 shows the relative transmitted intensity fluctuations at (a) a low input level (Z, = 9.3 MW/cm’) and (b) a high input level (IO = 930 MW/ cm*), respectively. The exposure time for both cases was about 3 minutes, and each data point was averaged over 30 laser pulses to achieve a higher signal/noise ratio. In case of Fig. 4a, the TPA was negligible and the relative transmitted intensity fluctuation, A = + 1 l%, was the same as the input. However, in
[l] L.W. Tuff and R.F. Boggess, Prog. Quantum Electron. [2 J [3 J [4]
[5] [6] [7] [8] [9] [lo] [ 1 l] [ 121
17 (1993) 299. A.C. Walker, A.K. Kar, Ji Wei, U. Keller and SD. Smith, Appl. Phys. Lett. 48 (1986) 683. Y.C. Chang, A.E. Chiou and M. Khoshnevissan, J. Appl. Phys. 71 (1992) 1349. E.W. Van Stryland, H. Vanherzeele, M.A. Woodall, M.J. Soileau, A.L. Smirl, S. Guha and T.F. Boggess, Opt. Eng. 24 (1985) 613. D.C. Hutchings and D.W. Van Stryland, J. Opt. Sot. Am. B 9 (1992) 2065. G.S. He, G.C. Xu, P.N. Prasad, B.A. Reinhardt, J.C. Bhatt and A.G. Dillard, Optics Lett. 20 (1995) 435. R. Gvishi, R. Reisfeld, E. Miron and Z. Brushtein, SPIE-Int. Sot. Opt. Eng. 1972 (1993) 390. G.S. He, G.C. Xu, Y. Pang and P.N. Prasad, J. Opt. Sot. Am. B 8 (1991) 1907. G.S. He, G.C. Xu, Y. Cui and P.N. Prasad, Appl. Optics 32 (1993) 4507. T.F. Boggess, K. Bohnert, K. Mansour, S.C. Moss, I.W. Boyd and A.L. Smirl, IEEE J. Quantum Electron. 22 ( 1986) 360. P. Qui and A. Penzkofer, Appl. Phys. B 48 ( 1989) 115. W. Bnmner, H. Duerr, E. Klose and H. Paul, Kvant. Elektron. (Moscow) 2 (1975) 832.