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Spectral dependence of holographic grating formation in electrolytically coloured potassium bromide crystals
May 1996
ELSBVIER
Optical Materials 5 (1996) 293-299
Spectral dependence of holographic grating formation in electrolytically coloured potassium bromide crystals P. Riihola a, A. 0~01s b, 0. Salminen a, P. Ketolainen
a
a ViiisiiliiLaboratory, Department of Physics, Unioersity of Joensuu, FIN-80101, Finland b Institute of Solid State Physics, Latoian Universi@, Kengaraga iel8, LV-1063, Riga, L.att,ia Received 27 November 1995; accepted 27 November
1995
Abstract Electrolytically coloured KBr crystals were exposed at elevated temperature by irradiating them with a single Gaussian laser beam and by recording volume holographic gratings. Bleaching took place through F-X colour centre conversion when wavelengths 514.5, 647.1, and 676.4 nm were used. A different bleaching mechanism was found in most cases when HeNe laser line 632.8 nm was used. F centre electrons were captured by traps which were formed during heat treatments of the sample. Efficient nonlinear absorption gratings could be created using F-D-UP conversion.
1. Introduction
the concentration of dislocations [6,7]. Equations describing the simultaneous processes are
The structure of the F centre is an electron trapped by an anion vacancy. X centres are assumed to be large groups of F centres. The coagulation of F centres to X centres may be thermal in nature or it may be induced by light. The F-X conversion in alkali halide crystals has been investigated since 1960 [l] and various results using different colouration processes have been presented for this conversion in KBr crystals [2-51. A model for the F-X conversion is presented in Ref. [6]. The model is based on the assumption that the F + X and X + F photothermal reactions take place simultaneously. The equilibrium between these two reactions determines the saturation state. X centres are assumed to grow by joining more and more F centres in each X centre. The number of F centres in one X centre is of X centres N,, remains N Fx. The concentration constant and it is assumed that N,, is determined by
dN,/dt=
-NF(
a,,Z+
W,,)
+Nx0N,x(%x~+
WTFX)’
(1)
NF + N,, NFx = NFO = constant,
(2)
OD = ( NF~F + N,,N,,o,)d,
(3)
where NF is concentration of F centres, OD is optical density, I is the intensity of the light inside the sample, aOF I and W,, are probabilities of photothermal and thermal coagulation of F centres, respectively. Further, cxOFxI and W,,, probabilities of photothermal and thermal outcome of F centre from X centre. Absorption cross sections of F and X centres are crF and (TV, respectively, and d is the path length of the light in the sample. These equations are valid if the gradients of the light intensity distribution and temperature are small enough [8]. In this work the bleaching behaviour of electrolyt-
00925-3467/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SO925-3467(96)00002-X
294
P. Riihola
et al. /Oprical
ically coloured KBr crystals has been studied at four different wavelengths. The shortest wavelength used was the 514.5 nm Ar laser line. Although it lies far from the F band maximum it has been shown to stimulate the F -+ X conversion. The longest wavelength used in this work was the 676.4 nm KC laser line. The efficiency of the bleaching has been estimated calculating the relative bleaching parameter. The purpose of this work is to shed more light to the spectral dependence of the holographic grating formation in electrolytically coloured KBr crystals. The spatial and temporal dependencies of holographic grating formation and the stability of the gratings are already reported elsewhere [8-l 11.
2. Experimental details KBr crystals were grown from Merck’s 4904 Suprapur@ KBr salt in the argon atmosphere by a combined Czochralski-Kyropoulos method. The crystals were coloured electrolytically by multielectrode colouration method at 620°C [12]. The current between the electrodes was gradually (step by step) increased. F centre concentrations of the samples and the thicknesses of the were - 1.5 X 10” cme3 samples were - 1.5 mm, which correspond to optical density around 3. To obtain a pure F band aggregate centres were removed by quenching the samples rapidly from 450°C to room temperature (RT = 20°C). In order to reuse the samples the quenching process was repeated. In practice one sample can be reused only about six times because successive heatings and toolings make it fragile. The existence of pure F band spectra was confirmed by recording the absorption spectra after the quenching. Because the F + X reaction takes place at elevated temperatures the samples were positioned in an oven type sample holder during the exposure. The bleaching temperature was - 200°C and it was measured with a chrome1 alumel thermocouple. KBr crystals were bleached in two different ways. They were either irradiated directly with one single Gaussian laser beam or two equal beams were used to record a transmission grating with grating period I A = 1.5 urn. The average bleaching light intensitv in
Materials
5 (1996) 293-299
ing the diffracted beam was measured every 30 seconds by blocking one recording beam for a short period. For the bleaching four different wavelengths were used: 514.5 nm Ar+ laser line, 632.8 nm HeNe laser line and 647.1 and 676.4 nm Kr’ laser lines. The I /e’ beam diameter was 1.1 mm for the HeNe laser and 1.5 mm for the Ar+ and Kr’ lasers. After the bleaching the absorption spectra in wavelength region 200-850 nm were measured at room temperature. An anomalous bleaching behaviour was observed when the samples were bleached at the wavelength of 632.8 nm. The absorption spectra of these samples were also measured in the infrared region (850 nm-50 km). Measurements over the visible region of the spectrum were also then done at liquid nitrogen temperature &NT = 196°C).
3. Results and discussion 3.1. Absorption
spectra
The F bands of one of our samples measured at RT and at the recording temperature are seen in Fig. 1. The positions of each laser wavelength are also displayed. Among these wavelengths the HeNe laser wavelength 632.8 nm is the closest one to the F band maximum at RT (h, = 625 nm) whereas the 647.1
’
-FBANDATRT --.FBANDAT180OC XBANDATRT
632.8
nm
: 647.1
-
11
400
500
600
nm
1
I
700
800
WAVELENGTH (nm)
I F band at RT and at the recording temperature. The X band ir mP.,rnm~.,t PI- nLn~l,inn ,.>.,En~.-fr\rmJ .rri,b tk~ &?4-~ Q nm
Fig.
295
P. Riihola et al. /Optical Materials 5 (1996) 293-299 4
,,.I”‘#*““‘.”
.
2.01
-FBANDATRT - - - ANOMALOUS SPECTRUM AT RT
t 1 .5
’
4
r-
**
__----_
‘0.
‘\
d
1 .o
0.5’ 400
500
600 WAVELENGTH
700 (nm)
800
Fig. 2. Anomalous bleaching behaviour. Sample has been exposed with the 632.8 nm line. The initial F band at RT and the anomalous band at RT and LNT are also shown.
nm Kr+ laser line is the closest one at the recording temperature (A, = 645 nm>. Our measurements showed that some samples did not have typical absorption spectra of the X centres after the bleaching (Fig. 2). These spectra appeared only when the 632.8 nm line was used for the bleaching. Hereafter we will call this type of absorption as “anomalous” although for 632.8 nm bleaching it was observed almost in every case. In some cases we have observed both normal and anomalous spectra for the same sample but at different reusing cycles. The integrated absorption (sum of the F and X bands) should stay constant in the F-X conversion [ 131 but according to the Fig. 2 the integrated absorption after the HeNe line bleaching is smaller than the integrated absorption of the initial F band. The situation is displayed in Fig. 3 where relative integrated absorption RA is presented as a function of the bleaching temperature. Here, RA is defined as the ratio of integrated absorptions before and after the bleaching. The exposures were done with a single Gaussian beam.
3.2. A model for anomalous
\
\
‘\ 0
-
180
1
1
0 GAUSSIAN, NORMAL 0 GAUSSIAN, ANOMALOUS
‘.
---@-_ *
190 TEMPERATURE
200 (OC)
--.?
i '
210
Fig. 3. Relative integrated absorption RA (the ratio of integrated absorptions before and after bleaching) as a function of bleaching temperature.
sured at RT in the far infrared region up to wavelength 50 pm, but no other absorption bands were found. This means that there are no large colloids (diameter > 10 nm) in the crystal [ 14,151, which could account for the low number of the F and X centres. Furthermore, when the sample was quenched after the bleaching process, the F centres were recovered as expected. We suggest that the absorption in the anomalous case (peak around 600 nm at LNT) is not solely caused by F centres or perturbed F centres. Rather it must be caused partly by some traps which capture the F electrons. The traps are most likely created by thermal erasure. The traps are located near the F centres spatially and spectrally. We propose an energy band diagram as seen in Fig. 4 at the recording
CB
absorption
According to Fig. 2 the anomalous spectrum at LNT is resolved into two separate bands whose maxima are situated around 600 and 760 nm. The 760 nm band is the X band. The spectra of these anomalously behaving KBr crystals were also mea-
VB Fig. 4. Energy level diagram for the trap model before bleaching at the recording temperature. CB is the conduction band and VB is the valence band. The F centre ground state 1s and the first excited state 2p are also labelled.
I’. Riihola et al. /Optical Materials 5 (1996) 293-299
296
w L
trap
level 2p
IS
3.3. Bleaching eficiency
CB
We have defined relative
~Zl?V
_..
*4l?V 42eV
i
1
/ Fig. 5. Energy level diagram for the trap model LNT. CB is the conduction band and VB is the F centre ground state Is and the first excited labelled. L, is one of the higher excited states
after bleaching at valence band. The state 2p are also of the F centre.
temperature before the bleaching and in Fig. 5 at LNT after the bleaching. When the samples are bleached with HeNe laser the F electrons are captured by the traps (Fig. 4.), which are in resonance with the 632.8 nm line (1.96 eV). The energy for the F transition from 1s to 2p state corresponds the wavelength A, = 645 nm (1.92 eV>. The proposed model is supported by the fact that other laser lines (Ar+ laser; 514.5 nm, Kr+ laser; 647.1 and 676.4 nm) did not give rise to the anomalous effect. It is therefore suggested that the trap level lies approximately 2 eV above the 1s state. In Fig. 2 the number of F electrons at the 2p state is low compared to the number of electrons captured by the traps. In Fig. 5 it is seen that at LNT the energy to lift an F electron from 1s state to the conduction band (CB) is about 4 eV [l], while the transition energy from the 1s state to the trap level is about 2 eV. So 2 eV in energy is required to shift the trapped electron to the conduction band. We suggest that it is this trap to conduction band transition together with F centre absorption that is responsible for the broad 600 nm band at LNT. The oscillator strength of the transition from the trap state to the conduction band is less than the oscillator strength of the transition from state 1s to 2p. This explains the reduced area of the anomalous absorption band (the area is directly proportional to the oscillator strength).
bleaching
parameter
mF = 1 - OD/OD,,
as (4)
where OD, is the initial optical density before the bleaching at the wavelength involved and OD is the optical density after the bleaching at the same wavelength. The parameter mF has an advantage that it does not depend on the thickness of the sample. Fig. 6 presents this factor for Ar+ and Kr+ laser wavelengths. Also HeNe laser wavelength is included when the normal bleaching process is involved. According to the figure there is no difference in bleaching whether the samples have been exposed with one single beam (Gaussian intensity distribution) or with two interfering beams (sinusoidal intensity distribution). To compare the Gaussian and sinusoidally periodical exposures we have defined the total average bleaching intensity I,” on the surface of the sample as I,, = 4P/rrd,2.
(5)
Here P is the total average incident light power and d, is the l/e’ diameter of the laser beam. Neglecting small elipticity of the beams (the angle of incidence N 10’) we can write the general expression describing intensity distributions in both cases: Z=Z,exp[-a2(..?+~2)](1
+McosKx),
where M is the fringe visibility,
0.7
- 0
is the
_o_____@-----Q
GAUSSIAN 0 SINUSOIDAL
0.6
(6)
K = 2r/A
___Q_---
,Q-=
0.5 -
, a’
i
o.ou. 0
40 60 EXPOSURE ENERGY (J/cm*)
20
60
’
100
Fig. 6. Relative bleaching as a function of exposing energy for Gaussian and sinusoidal light intensity distributions. The sample has been exposed with the 632.8 nm HeNe laser line and the type of the absorption is anomalous.
P. Riihola et al. /Optical
modulus of the grating vector and a = 2fi/d,,. For a single beam M = 0 while for the periodic distribution created by two interfering beams with total maximum intensity I,, 0 < A4 I 1. Let us show that the average bleaching intensities for the Gaussian and sinusoidal intensity distributions are equal. The total average power over the intensity distribution is
Materials
297
5 (I 9%) 293-299
not influence the saturation state of the bleaching. Therefore the most important parameter must be VI 2 al/ax. The derivation results to the equation aZ/ax=I,exp[-a’(x*+Y*)] X[-2x(w2(l
+McosKx)
-MKsinfi]. (8)
P=Io/_“,/”exp[-cz’(x*+$)] -r
x(1 =
Because d, s A, 1XI I d,/2 and K z=- LYwe have For the Gaussian beam M = 0 and for the periodical distribution we suppose M = 1. Now it can be easily be easily seen from Eq. (8) that (CU/&x),M,‘,‘/(aZ/axl,M,“,’ = IOK/(I,cxfi) = s-d,&/(2A> 2 1900 Accordingly the intensity gradient for the grating is more than 1900 times larger than for Gaussian beam. To show theoretically that VI is responsible for the differences of the bleaching parameters in Fig. 6 and 7 we apply the F-X bleaching model of 0~01s [6] to calculate m,(t). For the initial bleaching stage according to Eqs. (l)-(3) and (6) we obtain the relative bleaching parameter mF MK sin Kx z=- 2 X(Y2( 1 + M cos Kx).
+McosKx)dxdv
(al,/a*)[
I + Mexp(
-K’/4a’)].
Typically K’/(4a’> s 1 (in our case > 105), Mexp[ - K/(4a2>] = 0 and the equation simplifies I,, = (Y*P/T = to P = aI,/a *. Rearranging 8 P/(s-di) = 2 I,“. i.e. we have for both the Gaussian and periodical intensity distributions I,” = 4P/(rdi) if d, B- A. Fig. 6 shows the bleaching efficiency for the 632.8 nm line. This is the case of the anomalous absorption. According to the figure the sinusoidal exposure bleaches the samples better than the exposure with two beams. The behaviour can be explained if the bleaching process is shown to be dependent on the intensity gradient and/or local intensity maxima. The value of the local intensity maximum is I, for a Gaussian beam and (1 + M)I, I 2 I,, for the sinusoidal intensity distribution. However, our measurements for normal F-X reaction (Fig. 7) show that differing values of the local intensity maxima do
0.7 0 SINUSOIDAL
AOD mF =
OD
=
[
WTFt + 0.2171nlO
czoFlot]
x (1 - ax/a,>.
(9)
We see that mF does not depend on spatial intensity distribution given by the parameter M. This is the case (within experimental error) for the bleaching by 514.5, 632.8 (normal), 647.1 and 676.4 nm light. The reason for the anomalous bleaching differences of the two intensity distributions at 632.8 nm (Fig. 6) is most probably in AI affecting anomalous bleaching. This is not taken into account in the derivation of Eqs. (l)-(3).
0.6 0.5
3.4. Diffraction t$iciency
IL 0.4 E 0.3 0.2 0.1 Al-l
U.”
0
10
20
30
40
EXPOSURE Fig. 7. Relative
50
60
70
a0
90
100
ENERGY (J/cmz)
bleaching as a function of exposing energy different wavelengths and spatial light intensity distributions normal F-X reactions.
for for
Fig. 8 displays diffraction efficiencies for five different holographic gratings. The grating recorded with the green Ar’ laser line (514.5 nm) represents mainly phase grating [ 161. Therefore the highest diffraction efficiency of 5% have been obtained at this wavelength. Its behaviour is analysed in detailed in one of our studies [8]. According to the calculations [17] the diffraction efficiency can be much higher upto 70%. In additively coloured KBr crystals
298
P. Riihola et al. / Optical Materials
- -.
“‘.
- -
514.5nm 632.6 nm. ~nomolous recording 632.6 nm, normal recording 647.1 nm 676.4 nm ,----_-
EXPOSURE
ENERGY
_*
**
5 (1996) 293-299
dently the recording nonlinearity arises from the “anomalous” recording mechanism which we have found in our experiments. The reason why anomalous recording takes only in electrolytically coloured KBr crystals remains unclear. It may be connected with larger non-homogeneity in the F centre distribution compared to the additively coloured crystals. The diffraction efficiency can be substantially improved if homogenous plane waves instead of Gaussian shaped beams were used [8,17].
(J/m-12)
Fig. 8. Diffraction efficiency of volume holographic gratings for five different recording wavelengths as a function of exposure energy. The right v-axis is for the 632.8 nm normal HeNe laser recording.
diffraction efficiency of 12.1% is achieved [ 161. However, in these kind of samples the light scattering from the crystal imperfections, the inhomogeneity of the spatial F centre and temperature distributions as well as the Gaussian shaped intensity profile decrease the efficiency. Gaussian shaped beam profiles induce F centres to migrate towards the centre of the light intensity distribution thus increasing the average absorption and decreasing the grating contrast [S]. But the change of absorption spectra without any doubt show that the information storage at 514.5 nm is based on the F X colour centre conversion. The F + X colour centre conversion takes place also when Kr+ laser 647.1 and 676.4 nm lines are used for the recording (Fig. 8) and in some cases (as explained above) also when HeNe laser 632.8 nm is used, The diffraction efficiency of these gratings is low because they are predominantly absorption gratings [16,17]. Instead, the gratings recorded by 632.8 nm line behave differently when anomalous bleaching takes place. The anomalous bleaching is the most common case for 632.8 nm recording in electrolytically coloured KBr crystals but phenomenon has never been observed in additively coloured KBr crystals [6,11]. In anomalous bleaching process the diffraction efficiency is high and exceeds the maximum value of 3.7% calculated for the linear absorption grating [18]. High diffraction efficiencies are possible only for nonlinear absorption gratings [ 111. Evi-
4. Conclusions Spectral dependence of the holographic recording in electrolytically coloured KBr crystals has been experimentally studied. It is found that the holographic recording at 514.5, 647.1, 676.4 nm and in some cases at 632.8 nm laser lines is based on the F -+ X colour centre conversion. In these cases there is no difference in the bleaching parameters when exposures with the Gaussian and sinusoidal light intensity distributions are compared. In most cases when bleaching was performed by the 632.8 nm HeNe line a different (anomalous) type of absorption spectra changes take place. For this type of photoreaction bleaching by the sinusoidal intensity distribution is much more efficient than the Gaussian one. We believe that this difference is caused by the intensity gradient dependence of anomalous photoreaction. A model is proposed to explain this anomalous behaviour. Accordingly the photoexcited F electrons are captured by traps which are in resonance with 632.8 nm line and which are created during the thermal treatment of the crystals. The 632.8 nm holographic recording is much efficient for the anomalous photoreaction than for the normal F -+ X colour centre conversion. The maximum diffraction efficiency of the normal recording (514.5 nm line) did not exceed 5%. The reason for the low diffraction efficiency values is mainly due to the Gaussian beam profile which induces undesirable F centre diffusion. This obstacle can be removed by using plane waves for the recording instead of Gaussian beams. We have demonstrated that electrolytically coloured KBr crystals can be used as moderately recyclable holographic recording material in the wide
P. Riihola et al. /Optical Materials 5 (19%) 293-299
spectral range. They do not need any postprocessing and the readout is nondestructive. The possible applications include high density optical information storage and cheap holographic optical elements. These crystals can be applied as a model of the holographic photochromic materials and used also for demonstration purposes.
References [I] J.H. Schulman and W.D. Compton, Colour Centres in Solids (Pergamon, Oxford, 1963). [2] B. Stadnik and Z. Tronner, Nouvelle Rev. Opt. Appl. 3 (1972) 347. [3] G.E. Scrivener and M.R. Tubbs, Optics Commun. 6 (1972) 242. [4] B. Stadnik and Z. Tronner, Optics Commun. 6 (1972) 122. [5] B. Stadnik and Z. Tronner, Opt. Spectrosc. 41 (1976) 66.
299
[6] A. 0~01s. Latvijas PSR ZinBtnu Akadgmijas VEstis N 4 (1978) 28. [7] A. Hughes and S.C. Jain, Adv. Phys. 28 (1979) 717. [8] P. Riihola, 0. Salminen, A. 0~01s and T. Viitala, Spatially Recorded F ++ X Colour Centre Conversion Study in Electrolytically Coloured KBr Crystals, to be published. [9] A. Ozols, 0. Salminen, P. Raerinne and P. Ketolainen, J. Mod. Optics 40 (1993) 707. [lo] 0. Salminen, A. Ozols and P. Riihola, J. Mod. Optics 41 (1994) 1507. [l II E. Raita, A. 0~01s and 0. Salminen, Appl. Optics 34 (1995) 838. 1121 P. Raerinne, Meas. Sci. Technol. 3 (1992) 75. [I31 W.T. Doyle, Phys. Rev. 111 (1958) 1067. [14] M.Z. Savostianova, Physik 64 (1930) 262. [I51 C.J. Penley and R.S. Witte, J. Appl. Phys. 33 (1962) 2875. [I61 A. Ozols, Latvijas PSR Zinatnu AkadEmijas VFstis N 3 (1979) 138. 1171 A. Ozols, Latvija PSR Zinatnu Akademijas VEstis. Fiz. Tehn. Zin. SErija N 5 (1978) 16. [I81 H. Kogelnik. Bell Syst. Tech. J. 48 (1969) 2909.
ELSBVIER
Optical Materials 5 (1996) 293-299
Spectral dependence of holographic grating formation in electrolytically coloured potassium bromide crystals P. Riihola a, A. 0~01s b, 0. Salminen a, P. Ketolainen
a
a ViiisiiliiLaboratory, Department of Physics, Unioersity of Joensuu, FIN-80101, Finland b Institute of Solid State Physics, Latoian Universi@, Kengaraga iel8, LV-1063, Riga, L.att,ia Received 27 November 1995; accepted 27 November
1995
Abstract Electrolytically coloured KBr crystals were exposed at elevated temperature by irradiating them with a single Gaussian laser beam and by recording volume holographic gratings. Bleaching took place through F-X colour centre conversion when wavelengths 514.5, 647.1, and 676.4 nm were used. A different bleaching mechanism was found in most cases when HeNe laser line 632.8 nm was used. F centre electrons were captured by traps which were formed during heat treatments of the sample. Efficient nonlinear absorption gratings could be created using F-D-UP conversion.
1. Introduction
the concentration of dislocations [6,7]. Equations describing the simultaneous processes are
The structure of the F centre is an electron trapped by an anion vacancy. X centres are assumed to be large groups of F centres. The coagulation of F centres to X centres may be thermal in nature or it may be induced by light. The F-X conversion in alkali halide crystals has been investigated since 1960 [l] and various results using different colouration processes have been presented for this conversion in KBr crystals [2-51. A model for the F-X conversion is presented in Ref. [6]. The model is based on the assumption that the F + X and X + F photothermal reactions take place simultaneously. The equilibrium between these two reactions determines the saturation state. X centres are assumed to grow by joining more and more F centres in each X centre. The number of F centres in one X centre is of X centres N,, remains N Fx. The concentration constant and it is assumed that N,, is determined by
dN,/dt=
-NF(
a,,Z+
W,,)
+Nx0N,x(%x~+
WTFX)’
(1)
NF + N,, NFx = NFO = constant,
(2)
OD = ( NF~F + N,,N,,o,)d,
(3)
where NF is concentration of F centres, OD is optical density, I is the intensity of the light inside the sample, aOF I and W,, are probabilities of photothermal and thermal coagulation of F centres, respectively. Further, cxOFxI and W,,, probabilities of photothermal and thermal outcome of F centre from X centre. Absorption cross sections of F and X centres are crF and (TV, respectively, and d is the path length of the light in the sample. These equations are valid if the gradients of the light intensity distribution and temperature are small enough [8]. In this work the bleaching behaviour of electrolyt-
00925-3467/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SO925-3467(96)00002-X
294
P. Riihola
et al. /Oprical
ically coloured KBr crystals has been studied at four different wavelengths. The shortest wavelength used was the 514.5 nm Ar laser line. Although it lies far from the F band maximum it has been shown to stimulate the F -+ X conversion. The longest wavelength used in this work was the 676.4 nm KC laser line. The efficiency of the bleaching has been estimated calculating the relative bleaching parameter. The purpose of this work is to shed more light to the spectral dependence of the holographic grating formation in electrolytically coloured KBr crystals. The spatial and temporal dependencies of holographic grating formation and the stability of the gratings are already reported elsewhere [8-l 11.
2. Experimental details KBr crystals were grown from Merck’s 4904 Suprapur@ KBr salt in the argon atmosphere by a combined Czochralski-Kyropoulos method. The crystals were coloured electrolytically by multielectrode colouration method at 620°C [12]. The current between the electrodes was gradually (step by step) increased. F centre concentrations of the samples and the thicknesses of the were - 1.5 X 10” cme3 samples were - 1.5 mm, which correspond to optical density around 3. To obtain a pure F band aggregate centres were removed by quenching the samples rapidly from 450°C to room temperature (RT = 20°C). In order to reuse the samples the quenching process was repeated. In practice one sample can be reused only about six times because successive heatings and toolings make it fragile. The existence of pure F band spectra was confirmed by recording the absorption spectra after the quenching. Because the F + X reaction takes place at elevated temperatures the samples were positioned in an oven type sample holder during the exposure. The bleaching temperature was - 200°C and it was measured with a chrome1 alumel thermocouple. KBr crystals were bleached in two different ways. They were either irradiated directly with one single Gaussian laser beam or two equal beams were used to record a transmission grating with grating period I A = 1.5 urn. The average bleaching light intensitv in
Materials
5 (1996) 293-299
ing the diffracted beam was measured every 30 seconds by blocking one recording beam for a short period. For the bleaching four different wavelengths were used: 514.5 nm Ar+ laser line, 632.8 nm HeNe laser line and 647.1 and 676.4 nm Kr’ laser lines. The I /e’ beam diameter was 1.1 mm for the HeNe laser and 1.5 mm for the Ar+ and Kr’ lasers. After the bleaching the absorption spectra in wavelength region 200-850 nm were measured at room temperature. An anomalous bleaching behaviour was observed when the samples were bleached at the wavelength of 632.8 nm. The absorption spectra of these samples were also measured in the infrared region (850 nm-50 km). Measurements over the visible region of the spectrum were also then done at liquid nitrogen temperature &NT = 196°C).
3. Results and discussion 3.1. Absorption
spectra
The F bands of one of our samples measured at RT and at the recording temperature are seen in Fig. 1. The positions of each laser wavelength are also displayed. Among these wavelengths the HeNe laser wavelength 632.8 nm is the closest one to the F band maximum at RT (h, = 625 nm) whereas the 647.1
’
-FBANDATRT --.FBANDAT180OC XBANDATRT
632.8
nm
: 647.1
-
11
400
500
600
nm
1
I
700
800
WAVELENGTH (nm)
I F band at RT and at the recording temperature. The X band ir mP.,rnm~.,t PI- nLn~l,inn ,.>.,En~.-fr\rmJ .rri,b tk~ &?4-~ Q nm
Fig.
295
P. Riihola et al. /Optical Materials 5 (1996) 293-299 4
,,.I”‘#*““‘.”
.
2.01
-FBANDATRT - - - ANOMALOUS SPECTRUM AT RT
t 1 .5
’
4
r-
**
__----_
‘0.
‘\
d
1 .o
0.5’ 400
500
600 WAVELENGTH
700 (nm)
800
Fig. 2. Anomalous bleaching behaviour. Sample has been exposed with the 632.8 nm line. The initial F band at RT and the anomalous band at RT and LNT are also shown.
nm Kr+ laser line is the closest one at the recording temperature (A, = 645 nm>. Our measurements showed that some samples did not have typical absorption spectra of the X centres after the bleaching (Fig. 2). These spectra appeared only when the 632.8 nm line was used for the bleaching. Hereafter we will call this type of absorption as “anomalous” although for 632.8 nm bleaching it was observed almost in every case. In some cases we have observed both normal and anomalous spectra for the same sample but at different reusing cycles. The integrated absorption (sum of the F and X bands) should stay constant in the F-X conversion [ 131 but according to the Fig. 2 the integrated absorption after the HeNe line bleaching is smaller than the integrated absorption of the initial F band. The situation is displayed in Fig. 3 where relative integrated absorption RA is presented as a function of the bleaching temperature. Here, RA is defined as the ratio of integrated absorptions before and after the bleaching. The exposures were done with a single Gaussian beam.
3.2. A model for anomalous
\
\
‘\ 0
-
180
1
1
0 GAUSSIAN, NORMAL 0 GAUSSIAN, ANOMALOUS
‘.
---@-_ *
190 TEMPERATURE
200 (OC)
--.?
i '
210
Fig. 3. Relative integrated absorption RA (the ratio of integrated absorptions before and after bleaching) as a function of bleaching temperature.
sured at RT in the far infrared region up to wavelength 50 pm, but no other absorption bands were found. This means that there are no large colloids (diameter > 10 nm) in the crystal [ 14,151, which could account for the low number of the F and X centres. Furthermore, when the sample was quenched after the bleaching process, the F centres were recovered as expected. We suggest that the absorption in the anomalous case (peak around 600 nm at LNT) is not solely caused by F centres or perturbed F centres. Rather it must be caused partly by some traps which capture the F electrons. The traps are most likely created by thermal erasure. The traps are located near the F centres spatially and spectrally. We propose an energy band diagram as seen in Fig. 4 at the recording
CB
absorption
According to Fig. 2 the anomalous spectrum at LNT is resolved into two separate bands whose maxima are situated around 600 and 760 nm. The 760 nm band is the X band. The spectra of these anomalously behaving KBr crystals were also mea-
VB Fig. 4. Energy level diagram for the trap model before bleaching at the recording temperature. CB is the conduction band and VB is the valence band. The F centre ground state 1s and the first excited state 2p are also labelled.
I’. Riihola et al. /Optical Materials 5 (1996) 293-299
296
w L
trap
level 2p
IS
3.3. Bleaching eficiency
CB
We have defined relative
~Zl?V
_..
*4l?V 42eV
i
1
/ Fig. 5. Energy level diagram for the trap model LNT. CB is the conduction band and VB is the F centre ground state Is and the first excited labelled. L, is one of the higher excited states
after bleaching at valence band. The state 2p are also of the F centre.
temperature before the bleaching and in Fig. 5 at LNT after the bleaching. When the samples are bleached with HeNe laser the F electrons are captured by the traps (Fig. 4.), which are in resonance with the 632.8 nm line (1.96 eV). The energy for the F transition from 1s to 2p state corresponds the wavelength A, = 645 nm (1.92 eV>. The proposed model is supported by the fact that other laser lines (Ar+ laser; 514.5 nm, Kr+ laser; 647.1 and 676.4 nm) did not give rise to the anomalous effect. It is therefore suggested that the trap level lies approximately 2 eV above the 1s state. In Fig. 2 the number of F electrons at the 2p state is low compared to the number of electrons captured by the traps. In Fig. 5 it is seen that at LNT the energy to lift an F electron from 1s state to the conduction band (CB) is about 4 eV [l], while the transition energy from the 1s state to the trap level is about 2 eV. So 2 eV in energy is required to shift the trapped electron to the conduction band. We suggest that it is this trap to conduction band transition together with F centre absorption that is responsible for the broad 600 nm band at LNT. The oscillator strength of the transition from the trap state to the conduction band is less than the oscillator strength of the transition from state 1s to 2p. This explains the reduced area of the anomalous absorption band (the area is directly proportional to the oscillator strength).
bleaching
parameter
mF = 1 - OD/OD,,
as (4)
where OD, is the initial optical density before the bleaching at the wavelength involved and OD is the optical density after the bleaching at the same wavelength. The parameter mF has an advantage that it does not depend on the thickness of the sample. Fig. 6 presents this factor for Ar+ and Kr+ laser wavelengths. Also HeNe laser wavelength is included when the normal bleaching process is involved. According to the figure there is no difference in bleaching whether the samples have been exposed with one single beam (Gaussian intensity distribution) or with two interfering beams (sinusoidal intensity distribution). To compare the Gaussian and sinusoidally periodical exposures we have defined the total average bleaching intensity I,” on the surface of the sample as I,, = 4P/rrd,2.
(5)
Here P is the total average incident light power and d, is the l/e’ diameter of the laser beam. Neglecting small elipticity of the beams (the angle of incidence N 10’) we can write the general expression describing intensity distributions in both cases: Z=Z,exp[-a2(..?+~2)](1
+McosKx),
where M is the fringe visibility,
0.7
- 0
is the
_o_____@-----Q
GAUSSIAN 0 SINUSOIDAL
0.6
(6)
K = 2r/A
___Q_---
,Q-=
0.5 -
, a’
i
o.ou. 0
40 60 EXPOSURE ENERGY (J/cm*)
20
60
’
100
Fig. 6. Relative bleaching as a function of exposing energy for Gaussian and sinusoidal light intensity distributions. The sample has been exposed with the 632.8 nm HeNe laser line and the type of the absorption is anomalous.
P. Riihola et al. /Optical
modulus of the grating vector and a = 2fi/d,,. For a single beam M = 0 while for the periodic distribution created by two interfering beams with total maximum intensity I,, 0 < A4 I 1. Let us show that the average bleaching intensities for the Gaussian and sinusoidal intensity distributions are equal. The total average power over the intensity distribution is
Materials
297
5 (I 9%) 293-299
not influence the saturation state of the bleaching. Therefore the most important parameter must be VI 2 al/ax. The derivation results to the equation aZ/ax=I,exp[-a’(x*+Y*)] X[-2x(w2(l
+McosKx)
-MKsinfi]. (8)
P=Io/_“,/”exp[-cz’(x*+$)] -r
x(1 =
Because d, s A, 1XI I d,/2 and K z=- LYwe have For the Gaussian beam M = 0 and for the periodical distribution we suppose M = 1. Now it can be easily be easily seen from Eq. (8) that (CU/&x),M,‘,‘/(aZ/axl,M,“,’ = IOK/(I,cxfi) = s-d,&/(2A> 2 1900 Accordingly the intensity gradient for the grating is more than 1900 times larger than for Gaussian beam. To show theoretically that VI is responsible for the differences of the bleaching parameters in Fig. 6 and 7 we apply the F-X bleaching model of 0~01s [6] to calculate m,(t). For the initial bleaching stage according to Eqs. (l)-(3) and (6) we obtain the relative bleaching parameter mF MK sin Kx z=- 2 X(Y2( 1 + M cos Kx).
+McosKx)dxdv
(al,/a*)[
I + Mexp(
-K’/4a’)].
Typically K’/(4a’> s 1 (in our case > 105), Mexp[ - K/(4a2>] = 0 and the equation simplifies I,, = (Y*P/T = to P = aI,/a *. Rearranging 8 P/(s-di) = 2 I,“. i.e. we have for both the Gaussian and periodical intensity distributions I,” = 4P/(rdi) if d, B- A. Fig. 6 shows the bleaching efficiency for the 632.8 nm line. This is the case of the anomalous absorption. According to the figure the sinusoidal exposure bleaches the samples better than the exposure with two beams. The behaviour can be explained if the bleaching process is shown to be dependent on the intensity gradient and/or local intensity maxima. The value of the local intensity maximum is I, for a Gaussian beam and (1 + M)I, I 2 I,, for the sinusoidal intensity distribution. However, our measurements for normal F-X reaction (Fig. 7) show that differing values of the local intensity maxima do
0.7 0 SINUSOIDAL
AOD mF =
OD
=
[
WTFt + 0.2171nlO
czoFlot]
x (1 - ax/a,>.
(9)
We see that mF does not depend on spatial intensity distribution given by the parameter M. This is the case (within experimental error) for the bleaching by 514.5, 632.8 (normal), 647.1 and 676.4 nm light. The reason for the anomalous bleaching differences of the two intensity distributions at 632.8 nm (Fig. 6) is most probably in AI affecting anomalous bleaching. This is not taken into account in the derivation of Eqs. (l)-(3).
0.6 0.5
3.4. Diffraction t$iciency
IL 0.4 E 0.3 0.2 0.1 Al-l
U.”
0
10
20
30
40
EXPOSURE Fig. 7. Relative
50
60
70
a0
90
100
ENERGY (J/cmz)
bleaching as a function of exposing energy different wavelengths and spatial light intensity distributions normal F-X reactions.
for for
Fig. 8 displays diffraction efficiencies for five different holographic gratings. The grating recorded with the green Ar’ laser line (514.5 nm) represents mainly phase grating [ 161. Therefore the highest diffraction efficiency of 5% have been obtained at this wavelength. Its behaviour is analysed in detailed in one of our studies [8]. According to the calculations [17] the diffraction efficiency can be much higher upto 70%. In additively coloured KBr crystals
298
P. Riihola et al. / Optical Materials
- -.
“‘.
- -
514.5nm 632.6 nm. ~nomolous recording 632.6 nm, normal recording 647.1 nm 676.4 nm ,----_-
EXPOSURE
ENERGY
_*
**
5 (1996) 293-299
dently the recording nonlinearity arises from the “anomalous” recording mechanism which we have found in our experiments. The reason why anomalous recording takes only in electrolytically coloured KBr crystals remains unclear. It may be connected with larger non-homogeneity in the F centre distribution compared to the additively coloured crystals. The diffraction efficiency can be substantially improved if homogenous plane waves instead of Gaussian shaped beams were used [8,17].
(J/m-12)
Fig. 8. Diffraction efficiency of volume holographic gratings for five different recording wavelengths as a function of exposure energy. The right v-axis is for the 632.8 nm normal HeNe laser recording.
diffraction efficiency of 12.1% is achieved [ 161. However, in these kind of samples the light scattering from the crystal imperfections, the inhomogeneity of the spatial F centre and temperature distributions as well as the Gaussian shaped intensity profile decrease the efficiency. Gaussian shaped beam profiles induce F centres to migrate towards the centre of the light intensity distribution thus increasing the average absorption and decreasing the grating contrast [S]. But the change of absorption spectra without any doubt show that the information storage at 514.5 nm is based on the F X colour centre conversion. The F + X colour centre conversion takes place also when Kr+ laser 647.1 and 676.4 nm lines are used for the recording (Fig. 8) and in some cases (as explained above) also when HeNe laser 632.8 nm is used, The diffraction efficiency of these gratings is low because they are predominantly absorption gratings [16,17]. Instead, the gratings recorded by 632.8 nm line behave differently when anomalous bleaching takes place. The anomalous bleaching is the most common case for 632.8 nm recording in electrolytically coloured KBr crystals but phenomenon has never been observed in additively coloured KBr crystals [6,11]. In anomalous bleaching process the diffraction efficiency is high and exceeds the maximum value of 3.7% calculated for the linear absorption grating [18]. High diffraction efficiencies are possible only for nonlinear absorption gratings [ 111. Evi-
4. Conclusions Spectral dependence of the holographic recording in electrolytically coloured KBr crystals has been experimentally studied. It is found that the holographic recording at 514.5, 647.1, 676.4 nm and in some cases at 632.8 nm laser lines is based on the F -+ X colour centre conversion. In these cases there is no difference in the bleaching parameters when exposures with the Gaussian and sinusoidal light intensity distributions are compared. In most cases when bleaching was performed by the 632.8 nm HeNe line a different (anomalous) type of absorption spectra changes take place. For this type of photoreaction bleaching by the sinusoidal intensity distribution is much more efficient than the Gaussian one. We believe that this difference is caused by the intensity gradient dependence of anomalous photoreaction. A model is proposed to explain this anomalous behaviour. Accordingly the photoexcited F electrons are captured by traps which are in resonance with 632.8 nm line and which are created during the thermal treatment of the crystals. The 632.8 nm holographic recording is much efficient for the anomalous photoreaction than for the normal F -+ X colour centre conversion. The maximum diffraction efficiency of the normal recording (514.5 nm line) did not exceed 5%. The reason for the low diffraction efficiency values is mainly due to the Gaussian beam profile which induces undesirable F centre diffusion. This obstacle can be removed by using plane waves for the recording instead of Gaussian beams. We have demonstrated that electrolytically coloured KBr crystals can be used as moderately recyclable holographic recording material in the wide
P. Riihola et al. /Optical Materials 5 (19%) 293-299
spectral range. They do not need any postprocessing and the readout is nondestructive. The possible applications include high density optical information storage and cheap holographic optical elements. These crystals can be applied as a model of the holographic photochromic materials and used also for demonstration purposes.
References [I] J.H. Schulman and W.D. Compton, Colour Centres in Solids (Pergamon, Oxford, 1963). [2] B. Stadnik and Z. Tronner, Nouvelle Rev. Opt. Appl. 3 (1972) 347. [3] G.E. Scrivener and M.R. Tubbs, Optics Commun. 6 (1972) 242. [4] B. Stadnik and Z. Tronner, Optics Commun. 6 (1972) 122. [5] B. Stadnik and Z. Tronner, Opt. Spectrosc. 41 (1976) 66.
299
[6] A. 0~01s. Latvijas PSR ZinBtnu Akadgmijas VEstis N 4 (1978) 28. [7] A. Hughes and S.C. Jain, Adv. Phys. 28 (1979) 717. [8] P. Riihola, 0. Salminen, A. 0~01s and T. Viitala, Spatially Recorded F ++ X Colour Centre Conversion Study in Electrolytically Coloured KBr Crystals, to be published. [9] A. Ozols, 0. Salminen, P. Raerinne and P. Ketolainen, J. Mod. Optics 40 (1993) 707. [lo] 0. Salminen, A. Ozols and P. Riihola, J. Mod. Optics 41 (1994) 1507. [l II E. Raita, A. 0~01s and 0. Salminen, Appl. Optics 34 (1995) 838. 1121 P. Raerinne, Meas. Sci. Technol. 3 (1992) 75. [I31 W.T. Doyle, Phys. Rev. 111 (1958) 1067. [14] M.Z. Savostianova, Physik 64 (1930) 262. [I51 C.J. Penley and R.S. Witte, J. Appl. Phys. 33 (1962) 2875. [I61 A. Ozols, Latvijas PSR Zinatnu AkadEmijas VFstis N 3 (1979) 138. 1171 A. Ozols, Latvija PSR Zinatnu Akademijas VEstis. Fiz. Tehn. Zin. SErija N 5 (1978) 16. [I81 H. Kogelnik. Bell Syst. Tech. J. 48 (1969) 2909.