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Optical nonlinearities in amorphous silicon-carbon alloys
Journal of Non-CrystallineSolids 115 (1989) 99-101 North-Holland
99
OPTICAL NONLINEARITIES IN AMORPHOUS SILICON-CARBON ALLOYS Ursula EICKER, Ayad K. DARZI, Brian S. WHERRETF, John I.B. WILSON Department of Physics, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, U.K.
Picosecond excite/probe and degenerate four-wave mixing experiments in hydrogenated amorphous silicon-carbon alloys have been used to determine absorptive and refractive cross-sections of the photoinduced carriers. A maximum steady-state nonlinear refractive index nz of ~ 10Scm2/W was derived from the results for excitation energy (2.33 eV) close to the optical bandgap (Eo~= 2.25 eV). Amorphous all-dielectric interference filters have been designed for 514 nm Ar and 633 nm HeNe l~er emissions, using silicon-carbon alloys as the spacer material and multilayers of a-SiC:H and a-SiN:H for the reflecting stacks. We report the first demonstration of optical bistability in these devices.
1. INTRODUCTION The wide range of bandgap tuning (1.4-5 eV) of thin-film amorphous alloys (a-Si,Cr:H, a-Si,Gey:H and a-Si,NF:H) which are easily prepared by plasma enhanced chemical vapour deposition (PECVD), evokes considerable interest for applications in nonlinear optics. Semiconductor optical nonlinearities in combination with feedback result in the phenomenon of optical bistability. Switching between two stable transmission states is characterised by hysteretic behaviour which can be used for optical memory devices. Changes in both nonlinear refractive index and absorption coefficient can generate bistable responses which are "optoelectronic", if the changes are due to carder generation or "optothermal", if the nonlinearities are caused by sample temperature changes. Picosecond excite/probe and degenerate four-wave mixing experiments are widely used to investigate fast electronic carrier redistribution and relaxation of photoexcited electron-hole pairs 1~'3. The derivation of nonlinear absorptive and refractive cross-sections o,h and o~, allows the extrapolation to steady state nonlinearities, if the recombination time of the material is known4. To obtain dispersive optical bistability, refractive index changes (An = o~AN) of 10.3 are typically required. In this paper, we report electronic nonlinearities in bandgap tuned a-SixCy:H alloys and optical bistable switching in a-Si,Ny/a-Si~Cy:H nonlinear interference filters.
0022 3093/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland)
2. EXPERIMENTAL a-SixCy:Hand a-Si,Ny:H thin films were deposited on glass substrates at 230-260°C by capacitively-coupled rf plasma enhanced chemical vapour deposition from 0.1-0.2 tort mixtures of silane, propane and ammonia, using 10 W rf power applied to 100 mm diameter electrodes. The multilayer nonlinear interference filters were designed for operation at laser wavelengths of 514 nm and 633 nm (Fig. 1).
11111 [[[[[
1]1111 I [11
HLHLHLHL
I
LHLHLHLH
[11
4PAIRS OF HIGH & LOW NDEX
a-Si:C:H SPACER (n=28 for 633nm)
4 PAIRS OF HIGH & LOW INDEX
/ 7 0 5 9 GLASS SUBSTRATE
Fig. 1 Amorphous nonlinear interference filter structure. The carbon content of the spacer layer (D ~ 1 pan) was adjusted to give an absorption coefficient (a) fulfilling the cavity optimisation condition: (zD ~- (1 -RI) + (1 -R~) where Rr (P~) is the front (back) filter reflectance.
(1) Each
reflecting stack consisted of four pairs of high/low index quarterwave layers, using a-SixNy:H with n - 1.8 for the low index and a-Si, Cy:H with n ~ 2.3 for the high index material. Despite the absence of in situ optical monitoring, broadband reflecting stacks were deposited with maximum peak reflectivities of 95% (Fig. 2).
U. Eicker et M. / Optical nonlinearities in amorphous silicon-carbon alloys
100
All picosecond experiments were performed at room • t"'--'"~8.57
/ /
temperature using a passively mode-locked Nd:YAG oscillator/amplifier system (20ps pulses) frequency doubled
\
/
\ _ 867 '\
to 532 rim (4-16 I.tJ per pulse). The beams were focussed
z 8o
/i
7" /
O400
'\'.
/
_./',,'i
25
\
/
i 500
down to 220 lanafor the excite beam and 60 lain for the probe beam.
.
\\
. . . .
3. RESULTS AND DISCUSSION
i
i
i
600
700
800
900
WAVELENGTH (nm)
Fig. 2 Reflectivity spectrum of 4 pairs of high-low index quarterwave layers. Linear transmission spectra for two amorphous interference filters designed for ~. = 514 nm (filter A) and 2~ = 633 nm (filter B) are shown in Fig. 3 and Fig. 4. FILTER A/ STACK 867
At zero delay time between excite and probe pulses all a-Si~C~:H samples showed photoinduced absorption (A(z). The absorption cross-section (a,h), derived from the relation Acx= a, hN (where N is the number of excited carrier pairs), decreased strongly with increase in optical bandgap and doping ratio as the excitation wavelength was kept constant at 532 nm~. We attribute the low c,h values to the partial blocking of interband transitions as carriers in high bandgap materials were excited into bandtail states rather than into the conduction band with a higher density of states.
100
Refractive cross-sections (~,) can be extracted from the 75
measurement of the effective cross-section (c,H) of the two-beam transient grating in degenerate four-wave mixing
~ so
experiments 4'~ because this grating consists of both absorptive and refractive contributions:
25
0
i
400
500
i
600 700 WAVELENGTH (nm)
i
800
900
Fig. 3 Linear transmission spectrum for filter A designed for an operation wavelength ~ = 514 nm (cavity finesse = 4.9).
Figure 5 summarises the results:
~.
increases for excitation
energies close to the optical bandgap, whereas excitation high into the conduction band results in high absorptive cross-sections o,h and low o,. 102;
FILTER B/ STACK 887
/-
100 10
75
~6
i00
10 22 ~ -
D~
co z
~7
/
10
I--25
10 .8 0 400
500
600 700 WAVELENGTH (nm)
800
Fig. 4 Linear transmission spectrum for filter B designed for ~ = 633 nm (cavity finesse = 5.3).
102a 1.7
1.8
1.9
2.0
2.1
212
2.3
BANDGAP (eV)
Fig. 5 The variation of the cross-section o,h(*) and o,(a) with optical bandgap.
101
U. Eicker et al. / Optical nonlinearities in amorphous silicon-carbon alloys
Kramers-Kronig analysis of semiconductor absorption spectra 7 have shown resonances of the nonlinear refractive
In cw steady state optical bistability
experiments,
index (nz) at bandgap energies, which is consistent with the
refractive index changes are mostly attributed to sample temperature rises. The sign of the observed index change
results obtained for our samples. It has to be noted that the
was positive, indicating the dominance of optothermal
excitation energy of 2.33 eV is always higher than the optical bandgap (Eop0 of these samples (< 2.25 eV) so that further increases of n2 are to be expected for excitation energies
refractive index changes. A short switching time of around 10-20 IXSwas observed for small (10 lain) radiation spot sizes.
closer to Eopt. The highest steady-state n2 observed here was - 10s cm2/W (E~t = 2.25 eV) assuming a recombination time of 1 ~ts. The critical irradiance required to generate the 10.3
4. CONCLUSION
refractive index change typically required for optical switching is thus estimated to be only 100 W/cm2. Nonlinear refraction in a Fabry-Perot cavity changes the optical pathlength and switches the filter onto resonance from an initial detuning. Figure 6 shows nonlinear transmission
These first results are encouraging, as low power optical bistability has been achieved without elaborate optimization of the Fabry-Perot interference filter structure. The cw electronic nonlinear refractive index in a-Si~Cy:H (E.~ = 2.25 eV) has been determined from picosecond excite/probe and degenerate four-wave mixing experiments to be ~ 1(7s cm2/'~/y.
characteristics for filter A at 514 nm. ACKNOWLEDGEMENTS Ursula Eicker was supported by a Heriot-Watt Studentship I
I
Z L
i
I
l
i
i
0
10
20
0
10
20
iNCIDENT POWER (row)
INCIDENT POWER (mWl
Fig. 6 Dispersive and "butterfly" bistability in filter A. As the bandgap energy is only 88 meV above the excitation energy, absorption is high and this results in dispersive and combined dispersive/absorptive, or "butterfly", bistability at different spot positions of the wedge shaped sample s. In filter B, E~t exceeded the excitation energy of 1.96 eV (633 nm) by 440 meV and stable, dispersive bistability has been observed for critical irradiances of 15 mW in a spotsize of 10 tarn (Fig. 7).
10 C3
Z I
I
I
I
0
5
10
15
INCJDENT POWER (mW)
0
15
30
INCIDENT POWER (mW)
Fig. 7 Dispersive bistability in transmission and reflection for filter B.
and Ayad K. Darzi by the Government of Iraq. REFERENCES 1. D.A.B. Miller, C.T. Seaton, M.E. Prise, S.D. Smith, Phys. Rev. Lett. 47, (1981) 197. 2. J.L. Oudar, I. Abram, C. Minot, Appl. Phys. Lett. 44, (1984) 689. 3. R.K. Jain, R.C. Lind, J. Opt. Soc. Am. 73, (1983) 647. 4. M.N. Islam, E.P. Ippen, J. Appl. Phys. 59, (1986) 261. 5. U. Eicker, A.K. Darzi, B.S. Wherrett, J.I.B. Wilson, Phys. Rev. B 39, (1989) 3664. 6. M.N. Islam, E.P. Ippen, Appl. Phys. lett. 47, (1985) 1042. 7. B.S. Wherrett, A.C. Walker, F.A.P. Tooley, "Optical Nonlinearities and Instabilities in Semiconductors", Academic Press, (1988) 239. 8. D.C. Hutchings, A.D. Lloyd, I. Janossy, B.S. Wherrett, Opt. Commun. 61, (1987) 354.
99
OPTICAL NONLINEARITIES IN AMORPHOUS SILICON-CARBON ALLOYS Ursula EICKER, Ayad K. DARZI, Brian S. WHERRETF, John I.B. WILSON Department of Physics, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, U.K.
Picosecond excite/probe and degenerate four-wave mixing experiments in hydrogenated amorphous silicon-carbon alloys have been used to determine absorptive and refractive cross-sections of the photoinduced carriers. A maximum steady-state nonlinear refractive index nz of ~ 10Scm2/W was derived from the results for excitation energy (2.33 eV) close to the optical bandgap (Eo~= 2.25 eV). Amorphous all-dielectric interference filters have been designed for 514 nm Ar and 633 nm HeNe l~er emissions, using silicon-carbon alloys as the spacer material and multilayers of a-SiC:H and a-SiN:H for the reflecting stacks. We report the first demonstration of optical bistability in these devices.
1. INTRODUCTION The wide range of bandgap tuning (1.4-5 eV) of thin-film amorphous alloys (a-Si,Cr:H, a-Si,Gey:H and a-Si,NF:H) which are easily prepared by plasma enhanced chemical vapour deposition (PECVD), evokes considerable interest for applications in nonlinear optics. Semiconductor optical nonlinearities in combination with feedback result in the phenomenon of optical bistability. Switching between two stable transmission states is characterised by hysteretic behaviour which can be used for optical memory devices. Changes in both nonlinear refractive index and absorption coefficient can generate bistable responses which are "optoelectronic", if the changes are due to carder generation or "optothermal", if the nonlinearities are caused by sample temperature changes. Picosecond excite/probe and degenerate four-wave mixing experiments are widely used to investigate fast electronic carrier redistribution and relaxation of photoexcited electron-hole pairs 1~'3. The derivation of nonlinear absorptive and refractive cross-sections o,h and o~, allows the extrapolation to steady state nonlinearities, if the recombination time of the material is known4. To obtain dispersive optical bistability, refractive index changes (An = o~AN) of 10.3 are typically required. In this paper, we report electronic nonlinearities in bandgap tuned a-SixCy:H alloys and optical bistable switching in a-Si,Ny/a-Si~Cy:H nonlinear interference filters.
0022 3093/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland)
2. EXPERIMENTAL a-SixCy:Hand a-Si,Ny:H thin films were deposited on glass substrates at 230-260°C by capacitively-coupled rf plasma enhanced chemical vapour deposition from 0.1-0.2 tort mixtures of silane, propane and ammonia, using 10 W rf power applied to 100 mm diameter electrodes. The multilayer nonlinear interference filters were designed for operation at laser wavelengths of 514 nm and 633 nm (Fig. 1).
11111 [[[[[
1]1111 I [11
HLHLHLHL
I
LHLHLHLH
[11
4PAIRS OF HIGH & LOW NDEX
a-Si:C:H SPACER (n=28 for 633nm)
4 PAIRS OF HIGH & LOW INDEX
/ 7 0 5 9 GLASS SUBSTRATE
Fig. 1 Amorphous nonlinear interference filter structure. The carbon content of the spacer layer (D ~ 1 pan) was adjusted to give an absorption coefficient (a) fulfilling the cavity optimisation condition: (zD ~- (1 -RI) + (1 -R~) where Rr (P~) is the front (back) filter reflectance.
(1) Each
reflecting stack consisted of four pairs of high/low index quarterwave layers, using a-SixNy:H with n - 1.8 for the low index and a-Si, Cy:H with n ~ 2.3 for the high index material. Despite the absence of in situ optical monitoring, broadband reflecting stacks were deposited with maximum peak reflectivities of 95% (Fig. 2).
U. Eicker et M. / Optical nonlinearities in amorphous silicon-carbon alloys
100
All picosecond experiments were performed at room • t"'--'"~8.57
/ /
temperature using a passively mode-locked Nd:YAG oscillator/amplifier system (20ps pulses) frequency doubled
\
/
\ _ 867 '\
to 532 rim (4-16 I.tJ per pulse). The beams were focussed
z 8o
/i
7" /
O400
'\'.
/
_./',,'i
25
\
/
i 500
down to 220 lanafor the excite beam and 60 lain for the probe beam.
.
\\
. . . .
3. RESULTS AND DISCUSSION
i
i
i
600
700
800
900
WAVELENGTH (nm)
Fig. 2 Reflectivity spectrum of 4 pairs of high-low index quarterwave layers. Linear transmission spectra for two amorphous interference filters designed for ~. = 514 nm (filter A) and 2~ = 633 nm (filter B) are shown in Fig. 3 and Fig. 4. FILTER A/ STACK 867
At zero delay time between excite and probe pulses all a-Si~C~:H samples showed photoinduced absorption (A(z). The absorption cross-section (a,h), derived from the relation Acx= a, hN (where N is the number of excited carrier pairs), decreased strongly with increase in optical bandgap and doping ratio as the excitation wavelength was kept constant at 532 nm~. We attribute the low c,h values to the partial blocking of interband transitions as carriers in high bandgap materials were excited into bandtail states rather than into the conduction band with a higher density of states.
100
Refractive cross-sections (~,) can be extracted from the 75
measurement of the effective cross-section (c,H) of the two-beam transient grating in degenerate four-wave mixing
~ so
experiments 4'~ because this grating consists of both absorptive and refractive contributions:
25
0
i
400
500
i
600 700 WAVELENGTH (nm)
i
800
900
Fig. 3 Linear transmission spectrum for filter A designed for an operation wavelength ~ = 514 nm (cavity finesse = 4.9).
Figure 5 summarises the results:
~.
increases for excitation
energies close to the optical bandgap, whereas excitation high into the conduction band results in high absorptive cross-sections o,h and low o,. 102;
FILTER B/ STACK 887
/-
100 10
75
~6
i00
10 22 ~ -
D~
co z
~7
/
10
I--25
10 .8 0 400
500
600 700 WAVELENGTH (nm)
800
Fig. 4 Linear transmission spectrum for filter B designed for ~ = 633 nm (cavity finesse = 5.3).
102a 1.7
1.8
1.9
2.0
2.1
212
2.3
BANDGAP (eV)
Fig. 5 The variation of the cross-section o,h(*) and o,(a) with optical bandgap.
101
U. Eicker et al. / Optical nonlinearities in amorphous silicon-carbon alloys
Kramers-Kronig analysis of semiconductor absorption spectra 7 have shown resonances of the nonlinear refractive
In cw steady state optical bistability
experiments,
index (nz) at bandgap energies, which is consistent with the
refractive index changes are mostly attributed to sample temperature rises. The sign of the observed index change
results obtained for our samples. It has to be noted that the
was positive, indicating the dominance of optothermal
excitation energy of 2.33 eV is always higher than the optical bandgap (Eop0 of these samples (< 2.25 eV) so that further increases of n2 are to be expected for excitation energies
refractive index changes. A short switching time of around 10-20 IXSwas observed for small (10 lain) radiation spot sizes.
closer to Eopt. The highest steady-state n2 observed here was - 10s cm2/W (E~t = 2.25 eV) assuming a recombination time of 1 ~ts. The critical irradiance required to generate the 10.3
4. CONCLUSION
refractive index change typically required for optical switching is thus estimated to be only 100 W/cm2. Nonlinear refraction in a Fabry-Perot cavity changes the optical pathlength and switches the filter onto resonance from an initial detuning. Figure 6 shows nonlinear transmission
These first results are encouraging, as low power optical bistability has been achieved without elaborate optimization of the Fabry-Perot interference filter structure. The cw electronic nonlinear refractive index in a-Si~Cy:H (E.~ = 2.25 eV) has been determined from picosecond excite/probe and degenerate four-wave mixing experiments to be ~ 1(7s cm2/'~/y.
characteristics for filter A at 514 nm. ACKNOWLEDGEMENTS Ursula Eicker was supported by a Heriot-Watt Studentship I
I
Z L
i
I
l
i
i
0
10
20
0
10
20
iNCIDENT POWER (row)
INCIDENT POWER (mWl
Fig. 6 Dispersive and "butterfly" bistability in filter A. As the bandgap energy is only 88 meV above the excitation energy, absorption is high and this results in dispersive and combined dispersive/absorptive, or "butterfly", bistability at different spot positions of the wedge shaped sample s. In filter B, E~t exceeded the excitation energy of 1.96 eV (633 nm) by 440 meV and stable, dispersive bistability has been observed for critical irradiances of 15 mW in a spotsize of 10 tarn (Fig. 7).
10 C3
Z I
I
I
I
0
5
10
15
INCJDENT POWER (mW)
0
15
30
INCIDENT POWER (mW)
Fig. 7 Dispersive bistability in transmission and reflection for filter B.
and Ayad K. Darzi by the Government of Iraq. REFERENCES 1. D.A.B. Miller, C.T. Seaton, M.E. Prise, S.D. Smith, Phys. Rev. Lett. 47, (1981) 197. 2. J.L. Oudar, I. Abram, C. Minot, Appl. Phys. Lett. 44, (1984) 689. 3. R.K. Jain, R.C. Lind, J. Opt. Soc. Am. 73, (1983) 647. 4. M.N. Islam, E.P. Ippen, J. Appl. Phys. 59, (1986) 261. 5. U. Eicker, A.K. Darzi, B.S. Wherrett, J.I.B. Wilson, Phys. Rev. B 39, (1989) 3664. 6. M.N. Islam, E.P. Ippen, Appl. Phys. lett. 47, (1985) 1042. 7. B.S. Wherrett, A.C. Walker, F.A.P. Tooley, "Optical Nonlinearities and Instabilities in Semiconductors", Academic Press, (1988) 239. 8. D.C. Hutchings, A.D. Lloyd, I. Janossy, B.S. Wherrett, Opt. Commun. 61, (1987) 354.