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Optical phase conjugation in cooled organic dye film
Volume 80, number
3.4
OPTICS
Optical phase conjugation Hiroki Nakatsuka, Instiruie of&plied Received
COMMUNICATIONS
I January
1991
in cooled organic dye film
Dai Masuoka and Takeo Yamamoto
Physics, University of Tsukuba, Tsukuba Ibarakr 305, Japan
17 July 1990
Organic dye film is a good nonlinear medium that can be used to generate optical phase conjugation beams. It is very easy to prepare large films with high opttcal quality. One of the major disadvantages of the organic dye film. however, is that is is easily damaged by light irradiation. We demonstrated that by cooling an eosin yellow-doped polymer film to 80 K, the damage was greatly suppressed without a reduction of phase conjugation efficiency. The generatton efficiency of the phase conjugation stgnal agreed quite will with the result of a rate equation analysis based on a three-level model.
1. Introduction Optical phase conjugation is expected to be an important technology in the field of image processing. Many different kinds of materials have been proposed as the optical nonlinear medium, for example, highly nonlinear organic solvents, resonant vapors, photo-refractive crystals, semiconductors and organic dye films [ 11. However, if we need to make a sample of high optical quality with a large area, the organic due films are easiest to prepare [ 2,3 1. Since optical phase conjugation is a highly nonlinear process, rather strong irradiation light is necessary to obtain the signal. One of the major disadvantages of the organic dye films as a nonlinear medium is that the dye molecules in the films are easily bleached permanently by the light irradiation. By this bleaching a hologram is made in the film which lasts permanently even after the irradiation beams are removed. On the contrary, the phase conjugation component decays rapidly after the removal of the irradiation, because it originates from the transient population grating formed in the film [ 31. We show in this letter that by cooling an organic dye film to around liquid nitrogen temperature, the permanent bleaching was greatly suppressed without a significant reduction of optical phase conjugation efficiency. By further cooling the film to around liquid helium temperature, persistent holograms, which originate from persistent spectral hole burning 0030-4018/9
1603.50
0 199 I - Elsevter
Science Publishers
(PSHB) effect, are made by the irradiation of the beams [ 4 1. The persistent holograms based on PSHB can be erased either by raising the film temperature to around 100 K or by the irradiation of erasing light at a shorter wavelength [ 51. Although we do not consider the holograms based on PSHB in this letter, they have a big potentiality in the future practical application of the organic dye film.
2. Experiment The organic dye film we used was an eosin yellowdoped polyvinyl alcohol (PVA) film, and an argon laser at 5 14.5 nm was the excitation source to generate the optical phase conjugation signal. In the preparation of the sample film, both eosin yellow and PVA were dissolved in water ( -60”(I), cast on a sapphire plate, and dried for several days. For better cooling, the sample film was used without removing it from the sapphire substrate. The thickness of the film was about 100 urn and the optical density at 514.5 nm was about 1. The experimental setup is shown in fig. 1. The three incident beams E,, E2 and E, were about the same intensity ( 10 mW), and the spot size at the sample was 1 mm. The photodiode shown in fig. 1 detects both the phase conjugation and the holographic components of the signal. When the beam El is blocked only the holographic component remains. Therefore by intermittently block-
B.V. (North-Holland)
215
Volume 80, number
3.4
OPTICS
COMMUNICATIONS
Fig. I. Schematic diagram of the experiment. E,. Ez and I:‘, are the incident beams. and EI? is the phase conjugation beam of E,.
I January
I99
I
phase conjugation component [ 6 1. In the usual application of the optical phase conjugation, however, the build up of the holographic component must be suppressed. We tried to cool down the film in order to suppress the holographic component caused by the permanent bleaching of the dye molecules. The film on the sapphire plate was cooled down to around liquid nitrogen temperature in a gas flow type cryostat (Oxford CF1204). The excitation configuration and the beam intensities were the same as in fig. 2. Fig. 3 shows the intensity ratio between the phase conjugation component and the holographic component at 120 K (a) and at 80 K (b), where the thick solid line represents the sum of the two components and the thick dashed line represents only the holographic component. At around liquid nitrogen temperature. the thermal conductivity of sapphire is quite high,
1
IO 0
IO
20 Time
30
40
Time
Fig. 2. Temporal behavior of the signal intensity at room temperature. Beam E, was intermittently blocked (see lig. I ). The solid thick curve represents the sum of the phase conjugation and the holographic components, and the dashed thick curve represents only the holographic component.
ing the beam El we can measure the intensity ratio between the two components at room temperature. The result is shown in fig. 2, where the solid thick curve represents the sum of the two components and the thick dashed curve represents only the holographic component. We see that although the irradiation intensity was rather mild, the build up speed of the holographic component was quite high. Fujiwara et al. used the holographic component very effectively to obtain an interference pattern with the 216
(mln)
(mln)
( b)
Time
(mln)
Fig. 3. Temporal behavior of the signal mtensity at 120 K (a) and at 80 K (b). Beam E2 was mtermittently blocked (see fig. I ). The solid thick line represents the sum of the phase conjugation and the holographic components. and the dashed thick line represents only the holographic component.
Volume 80, number 3.4
I January I99 1
OPTICS COMMUNICATIONS
2
therefore the sapphire substrate (thickness 2 mm) acts as a heat sink for the film. The substrate was made to be in good thermal contact with the copper sample holder of the cryostat. At 80 K we could not see a significant holographic component even after a light irradiation of 20 min. Further suppression can be expected by more effective cooling of the film. There was no significant change of the phase conjugation efficiency in the temperature range from 80 K to room temperature.
I(r)=
j&
( f
{Em
,?I=1
exp[i(k,r-wt)]+c.c.}
>
,
(2) and (Tis the absorption cross section of the molecule at 5 14.5 nm, and r,, is the decay rate from i-level to j-level (i, j= 1, 2, 3). The intensity of the phase conjugation signal is obtained from the nonlinear polarization PN,_ induced in the dye film
X 4 i (
3. Analysis of phase conjugation efficiency
2
jE,,exp[i(k,r-oX)]+c.c.}
,n= I
>
,
(3)
and the Maxwell equation We consider the efficiency of the phase conjugation in the eosin yellow film by using a three level model of the dye molecule shown in fig. 4. The three levels I I), 13) and 12) represent the two singlet states So and S,, and the triplet state T, of the eosin yellow, respectively. The excitation beams at 514.5 nm are resonant with the I 1) to 13) transition. The rate equations for the populations N,, N, and N3 are given as
aE,-~Lta2E,/at’=~a2P,,lat2,
(4)
(Y is the polarizability of the dye molecule. In the analysis to follow, we assume that the intensities of the three incident beams are equal and that the main contribution of the intensity of the phase conjugation signal is given by the diffraction of the beam grating (AN(r) = the population R, by N, (r) - N3 (r) ) component with wave vector k2 - k4. Here the phase matching condition is satisfied as where
dN,(r)/df=-al(r)[N,(r)-N,(r)] +N,(r)r,,
+Nz(r)r:,
kl=(kz-kd)+k,,
.
where k3 is the wave vector of the phase conjugation beam. By a simple calculation a steady state solution is obtained from eqs. ( 1) as
dN~(r)ldt=N,(r)r,, --N2(r)rz1 , dN,(r)ldt=~~(r) [N(r) -Nj(r) 1 -h(r)
(r3, +r32) ,
AN(r)=N,
N,+Nz+-N,=No, where the photon expressed as
(1) flux of the incident
beams I(r)
is
(r) -N,(r)
x{a(2r2,
+r,,)
+r2,(r3,
+r,,)i-1
=r2, (r,,
[A+Bcos(k?
+rX,)NO -k4)r]
:
(6)
where A=ct
13)
(5)
N3
SI TI
Fig. 4. Schematic level diagram of the dye molecule.
(IE, I’+ IE2 (‘+ IEd 12)/4ho,
In order to compare the simulation with the present measurement, we used the values of the parameters asfo110ws:thedecayratesr7,=(5.3ms)-’,L’3,=(10 ns) - ’ and r,, = (20 ns) -I, and the absorption cross section 0=4x 10w20 ml. The simulation curve of the signal intensity is shown in fig. 5 together with the experimental results at 80 K. In the above analysis we did not consider the attenuation of the beam in217
Volume 80. number
3,4
OPTICS
COMMUNICATIONS
I January
1991
4. Conclusion
“G 0
A. 0. I
lncldent
J
/.
0.2
Beam
Intensity
0.3
(W/cm2)
Fig. 5. Phase conjugation beam intensity versus incident beam intensity at 80 K. The intensities of the three incident beams are equal. The solid curve represents the simulation based on the three-level model. and the filled circles represent the experimental results.
tensity by the absorption. so we left the signal intensity scale in arbitrary units. But the signal intensity measured (0.5 pW) for the maximum incident intensity (2 mW. spot size I mm) of an incident beam is within a difference of one order of magnitude from the calculated value (3.5 pW). The saturation behaviour i.e. from the deviation from the cubic dependence of the signal on the incident beam intensity is very well reproduced by the simple simulation.
218
By using an eosin yellow-doped PVA film we have demonstrated that the permanent bleaching of the dye film by light irradiation is greatly suppressed by cooling it to liquid nitrogen temperature. We have also shown that the saturation behaviour of phase conjugation in the dye film is very well reproduced by a rate equation analysis based on a three-level model of the dye molecule. The great suppression of the bleaching of the film at low temperatures implies that longer wavelength diode lasers can be used to obtain phase conjugation signal in some dye films by cooling them to low temperatures even if they arc very fragile at room temperature.
References
[ I ] R.A. Fisher, ed., Optical phase conjugation (Academic Press. New York. 1983). 121 Y. Silbcrberg and I. Bar-Joseph. Optics Comm. 39 (1981 ) 265. [ 31 H. FuJlwara and K. Nakagawa. Optics Comm. 55 ( 1985) 386. [ 41 A. Renn. R. Lochcr. A. Melxner and U.P. Wild. J. Lumin. 38 (1987) 37. [ 51 W.E. Moerner. cd., Persistent spectral hole-burning, Sclencc and applications (Springer-Vcrlag. Berlin. 1988). (61 K. Nakagawaand H. Fujiwara. OpucsComm. 70 ( 1989) 73.
3.4
OPTICS
Optical phase conjugation Hiroki Nakatsuka, Instiruie of&plied Received
COMMUNICATIONS
I January
1991
in cooled organic dye film
Dai Masuoka and Takeo Yamamoto
Physics, University of Tsukuba, Tsukuba Ibarakr 305, Japan
17 July 1990
Organic dye film is a good nonlinear medium that can be used to generate optical phase conjugation beams. It is very easy to prepare large films with high opttcal quality. One of the major disadvantages of the organic dye film. however, is that is is easily damaged by light irradiation. We demonstrated that by cooling an eosin yellow-doped polymer film to 80 K, the damage was greatly suppressed without a reduction of phase conjugation efficiency. The generatton efficiency of the phase conjugation stgnal agreed quite will with the result of a rate equation analysis based on a three-level model.
1. Introduction Optical phase conjugation is expected to be an important technology in the field of image processing. Many different kinds of materials have been proposed as the optical nonlinear medium, for example, highly nonlinear organic solvents, resonant vapors, photo-refractive crystals, semiconductors and organic dye films [ 11. However, if we need to make a sample of high optical quality with a large area, the organic due films are easiest to prepare [ 2,3 1. Since optical phase conjugation is a highly nonlinear process, rather strong irradiation light is necessary to obtain the signal. One of the major disadvantages of the organic dye films as a nonlinear medium is that the dye molecules in the films are easily bleached permanently by the light irradiation. By this bleaching a hologram is made in the film which lasts permanently even after the irradiation beams are removed. On the contrary, the phase conjugation component decays rapidly after the removal of the irradiation, because it originates from the transient population grating formed in the film [ 31. We show in this letter that by cooling an organic dye film to around liquid nitrogen temperature, the permanent bleaching was greatly suppressed without a significant reduction of optical phase conjugation efficiency. By further cooling the film to around liquid helium temperature, persistent holograms, which originate from persistent spectral hole burning 0030-4018/9
1603.50
0 199 I - Elsevter
Science Publishers
(PSHB) effect, are made by the irradiation of the beams [ 4 1. The persistent holograms based on PSHB can be erased either by raising the film temperature to around 100 K or by the irradiation of erasing light at a shorter wavelength [ 51. Although we do not consider the holograms based on PSHB in this letter, they have a big potentiality in the future practical application of the organic dye film.
2. Experiment The organic dye film we used was an eosin yellowdoped polyvinyl alcohol (PVA) film, and an argon laser at 5 14.5 nm was the excitation source to generate the optical phase conjugation signal. In the preparation of the sample film, both eosin yellow and PVA were dissolved in water ( -60”(I), cast on a sapphire plate, and dried for several days. For better cooling, the sample film was used without removing it from the sapphire substrate. The thickness of the film was about 100 urn and the optical density at 514.5 nm was about 1. The experimental setup is shown in fig. 1. The three incident beams E,, E2 and E, were about the same intensity ( 10 mW), and the spot size at the sample was 1 mm. The photodiode shown in fig. 1 detects both the phase conjugation and the holographic components of the signal. When the beam El is blocked only the holographic component remains. Therefore by intermittently block-
B.V. (North-Holland)
215
Volume 80, number
3.4
OPTICS
COMMUNICATIONS
Fig. I. Schematic diagram of the experiment. E,. Ez and I:‘, are the incident beams. and EI? is the phase conjugation beam of E,.
I January
I99
I
phase conjugation component [ 6 1. In the usual application of the optical phase conjugation, however, the build up of the holographic component must be suppressed. We tried to cool down the film in order to suppress the holographic component caused by the permanent bleaching of the dye molecules. The film on the sapphire plate was cooled down to around liquid nitrogen temperature in a gas flow type cryostat (Oxford CF1204). The excitation configuration and the beam intensities were the same as in fig. 2. Fig. 3 shows the intensity ratio between the phase conjugation component and the holographic component at 120 K (a) and at 80 K (b), where the thick solid line represents the sum of the two components and the thick dashed line represents only the holographic component. At around liquid nitrogen temperature. the thermal conductivity of sapphire is quite high,
1
IO 0
IO
20 Time
30
40
Time
Fig. 2. Temporal behavior of the signal intensity at room temperature. Beam E, was intermittently blocked (see lig. I ). The solid thick curve represents the sum of the phase conjugation and the holographic components, and the dashed thick curve represents only the holographic component.
ing the beam El we can measure the intensity ratio between the two components at room temperature. The result is shown in fig. 2, where the solid thick curve represents the sum of the two components and the thick dashed curve represents only the holographic component. We see that although the irradiation intensity was rather mild, the build up speed of the holographic component was quite high. Fujiwara et al. used the holographic component very effectively to obtain an interference pattern with the 216
(mln)
(mln)
( b)
Time
(mln)
Fig. 3. Temporal behavior of the signal mtensity at 120 K (a) and at 80 K (b). Beam E2 was mtermittently blocked (see fig. I ). The solid thick line represents the sum of the phase conjugation and the holographic components. and the dashed thick line represents only the holographic component.
Volume 80, number 3.4
I January I99 1
OPTICS COMMUNICATIONS
2
therefore the sapphire substrate (thickness 2 mm) acts as a heat sink for the film. The substrate was made to be in good thermal contact with the copper sample holder of the cryostat. At 80 K we could not see a significant holographic component even after a light irradiation of 20 min. Further suppression can be expected by more effective cooling of the film. There was no significant change of the phase conjugation efficiency in the temperature range from 80 K to room temperature.
I(r)=
j&
( f
{Em
,?I=1
exp[i(k,r-wt)]+c.c.}
>
,
(2) and (Tis the absorption cross section of the molecule at 5 14.5 nm, and r,, is the decay rate from i-level to j-level (i, j= 1, 2, 3). The intensity of the phase conjugation signal is obtained from the nonlinear polarization PN,_ induced in the dye film
X 4 i (
3. Analysis of phase conjugation efficiency
2
jE,,exp[i(k,r-oX)]+c.c.}
,n= I
>
,
(3)
and the Maxwell equation We consider the efficiency of the phase conjugation in the eosin yellow film by using a three level model of the dye molecule shown in fig. 4. The three levels I I), 13) and 12) represent the two singlet states So and S,, and the triplet state T, of the eosin yellow, respectively. The excitation beams at 514.5 nm are resonant with the I 1) to 13) transition. The rate equations for the populations N,, N, and N3 are given as
aE,-~Lta2E,/at’=~a2P,,lat2,
(4)
(Y is the polarizability of the dye molecule. In the analysis to follow, we assume that the intensities of the three incident beams are equal and that the main contribution of the intensity of the phase conjugation signal is given by the diffraction of the beam grating (AN(r) = the population R, by N, (r) - N3 (r) ) component with wave vector k2 - k4. Here the phase matching condition is satisfied as where
dN,(r)/df=-al(r)[N,(r)-N,(r)] +N,(r)r,,
+Nz(r)r:,
kl=(kz-kd)+k,,
.
where k3 is the wave vector of the phase conjugation beam. By a simple calculation a steady state solution is obtained from eqs. ( 1) as
dN~(r)ldt=N,(r)r,, --N2(r)rz1 , dN,(r)ldt=~~(r) [N(r) -Nj(r) 1 -h(r)
(r3, +r32) ,
AN(r)=N,
N,+Nz+-N,=No, where the photon expressed as
(1) flux of the incident
beams I(r)
is
(r) -N,(r)
x{a(2r2,
+r,,)
+r2,(r3,
+r,,)i-1
=r2, (r,,
[A+Bcos(k?
+rX,)NO -k4)r]
:
(6)
where A=ct
13)
(5)
N3
SI TI
Fig. 4. Schematic level diagram of the dye molecule.
(IE, I’+ IE2 (‘+ IEd 12)/4ho,
In order to compare the simulation with the present measurement, we used the values of the parameters asfo110ws:thedecayratesr7,=(5.3ms)-’,L’3,=(10 ns) - ’ and r,, = (20 ns) -I, and the absorption cross section 0=4x 10w20 ml. The simulation curve of the signal intensity is shown in fig. 5 together with the experimental results at 80 K. In the above analysis we did not consider the attenuation of the beam in217
Volume 80. number
3,4
OPTICS
COMMUNICATIONS
I January
1991
4. Conclusion
“G 0
A. 0. I
lncldent
J
/.
0.2
Beam
Intensity
0.3
(W/cm2)
Fig. 5. Phase conjugation beam intensity versus incident beam intensity at 80 K. The intensities of the three incident beams are equal. The solid curve represents the simulation based on the three-level model. and the filled circles represent the experimental results.
tensity by the absorption. so we left the signal intensity scale in arbitrary units. But the signal intensity measured (0.5 pW) for the maximum incident intensity (2 mW. spot size I mm) of an incident beam is within a difference of one order of magnitude from the calculated value (3.5 pW). The saturation behaviour i.e. from the deviation from the cubic dependence of the signal on the incident beam intensity is very well reproduced by the simple simulation.
218
By using an eosin yellow-doped PVA film we have demonstrated that the permanent bleaching of the dye film by light irradiation is greatly suppressed by cooling it to liquid nitrogen temperature. We have also shown that the saturation behaviour of phase conjugation in the dye film is very well reproduced by a rate equation analysis based on a three-level model of the dye molecule. The great suppression of the bleaching of the film at low temperatures implies that longer wavelength diode lasers can be used to obtain phase conjugation signal in some dye films by cooling them to low temperatures even if they arc very fragile at room temperature.
References
[ I ] R.A. Fisher, ed., Optical phase conjugation (Academic Press. New York. 1983). 121 Y. Silbcrberg and I. Bar-Joseph. Optics Comm. 39 (1981 ) 265. [ 31 H. FuJlwara and K. Nakagawa. Optics Comm. 55 ( 1985) 386. [ 41 A. Renn. R. Lochcr. A. Melxner and U.P. Wild. J. Lumin. 38 (1987) 37. [ 51 W.E. Moerner. cd., Persistent spectral hole-burning, Sclencc and applications (Springer-Vcrlag. Berlin. 1988). (61 K. Nakagawaand H. Fujiwara. OpucsComm. 70 ( 1989) 73.