**4.1 Methods for fabrications of PhC's**

In Section 3.1 it was mentioned that the widely accepted approach for production of 1D PhC is the alternating deposition of high and low refractive index materials with quarterwave thicknesses. Depending on the materials chosen, different deposition methods as vacuum condensation, spin and dip coating, pulsed laser deposition, sol-gel etc. are used. The experimental conditions for deposition thin films of 1D PhC is presented below in section 4.2.

The most widely recognized methods for preparation of 2D and 3D photonic crystals are colloidal self-assembly, direct laser writing and lithography with deep etching, that can be E-beam, X-ray or holographic lithography (Lopez, 2003).

Typical colloidal PhC are face centred 3D arrays of self assembled monodispersed silica or polymer microspheres with diameter from 200 – 2000 nm. The sedimentation is often used for fabrication of thick samples, which are called bulk opals. Besides, deposition on vertical substrates with controlled moving meniscus (Egen et al., 2004) is used for thinner samples. Because the refractive index of silica and polymers spheres is around 1.5, the optical contrast of 0.5 is not sufficient for opening of a complete PBG. A post fabrication infiltration of colloidal crystals with high refractive index material is necessary. Following this strategy a colloidal crystals infiltrated with Si and Ge exhibiting complete PBG at wavelength of 1.5 µm are fabricated (A. Blanco et al., 2000 ).

The comparison between the real and ideal 1D-PhC from As2S3 / PMMA is presented in Fig. 7. It is seen that there is an insignificant shift between the reflectance bands of both structures. The difference of 1.1 % between the measured and calculated reflectance can be due to slight absorption and scattering of the layers as well as to measurements errors. The most significant difference between the fabricated and simulated structures is in the side peaks. Note for example the first minima that are very high for the real structure and zero for the simulated one. Most probably this difference is due to random deviations of phase thicknesses of the constituent layers leading to violation of the conditions for destructive

100 Ideal

Real

1000 1500 2000 2500

Wavelength [nm]

In Section 3.1 it was mentioned that the widely accepted approach for production of 1D PhC is the alternating deposition of high and low refractive index materials with quarterwave thicknesses. Depending on the materials chosen, different deposition methods as vacuum condensation, spin and dip coating, pulsed laser deposition, sol-gel etc. are used. The experimental conditions for deposition thin films of 1D PhC is presented below in

The most widely recognized methods for preparation of 2D and 3D photonic crystals are colloidal self-assembly, direct laser writing and lithography with deep etching, that can be

Typical colloidal PhC are face centred 3D arrays of self assembled monodispersed silica or polymer microspheres with diameter from 200 – 2000 nm. The sedimentation is often used for fabrication of thick samples, which are called bulk opals. Besides, deposition on vertical substrates with controlled moving meniscus (Egen et al., 2004) is used for thinner samples. Because the refractive index of silica and polymers spheres is around 1.5, the optical contrast of 0.5 is not sufficient for opening of a complete PBG. A post fabrication infiltration of colloidal crystals with high refractive index material is necessary. Following this strategy a colloidal crystals infiltrated with Si and Ge exhibiting complete PBG at wavelength of 1.5

0

E-beam, X-ray or holographic lithography (Lopez, 2003).

Fig. 7. Measured and simulated reflectance band of As2S3/PMMA stack

25

50

Reflectance [%]

75

interference.

**4. Device fabrication** 

section 4.2.

**4.1 Methods for fabrications of PhC's** 

µm are fabricated (A. Blanco et al., 2000 ).

Direct laser writing method through multi-photon absorption is another method for producing 3D PhC. The light from the laser is focused on a small spot of the dye-doped polymer used as a recording medium. The energy of the laser excites the dye molecules that initiate a local polymerization in the spot thus changing the refractive index of the polymer in the illuminated spot. A spatial resolution of 120 nm is reported in the literature (Kawata et al., 2001). The problem is that the optical contrast is very small and infiltration of the structure is needed. Another possibility is using the laser writing in high refractive index material. Promising candidates are chalcogenide glasses, particularly As2S3 that undergo changes in solubility upon exposure to light.

Conventional lithography and selective etching were used mainly for fabrication of 2D PhC. For producing 3D PhC a concept of layer-by-layer deposition has to be implemented that comprises repeated cycles of photolithography, wet and dry etching, planarization, and growth of layers (Blanco et. al., 2004).

The holographic lithography is very promising method enabling large-area defect-free 2D and 3D periodic structure to be produced in a single-step. In holographic lithography the sensitive medium (a photoresist) is exposed to a multiple-beam interference pattern (for Ndimensional structures at least N+1 beams are required) and subsequently developed, producing a porous structure. The interbeam angles and polarizations and the number of beams determine the type of symmetry of the recorded structures. It has been theoretically shown that all 14 Bravais lattices could be produced (Cai et. al., 2002). Usually SU-8 photoresist with refractive index of around 1.67 is used. A recognized drawback is the need for infiltration of the produced structure with high refractive index material. Otherwise the optical contrast is not sufficiently high for opening a complete photonic band. Difficulties such as optical alignment, vibrational instability, and reflection losses on the interface air/photoresist further complicate the recording processes. One possibility for overcoming the problem is rotating the sample between two consecutive exposures (Lai et. al., 2005). The second approach is implementation of specially design diffraction mask that provides the required number of beams with correct directions and polarizations reducing the alignment complexity and vibration instabilities in the optical setup (Divlianski et. al., 2002).

Two-dimensional structures have already been fabricated in chalcogenide glasses using holographic lithography (Feigel et. al., 2005; Su et. al., 2009). To the best of our knowledge holographic lithography has not been used yet for fabrication of 3D PhC from chalcogenide glasses. The three dimensional wood-pile photonic crystals made in chalcogenide glasses are fabricated by direct laser writing (Nicoletti et. al., 2008) or through layer-by-layer deposition (Feigel et. al., 2003).

### **4.2 Experimental procedure for deposition of 1D photonic crystals**

In the present work we used the concept of layer-by-layer deposition of quarter-wave stacks of alternating suitably chosen films with low and high refractive indices for producing of one-dimensional photonic crystals.

The bulk chalcogenide glass was synthesized in a quartz ampoule by the method of melt quenching from elements of purity 99.999 %. The chalcogenide layers were deposited by thermal evaporation at deposition rate of 0.5 – 0.7 nm/s. The X-ray microanalysis showed that the film composition is close to that of the bulk samples (Todorov et al., 2010b).

Thin Chalcogenide Films for Photonic Applications 157

In this part the optical properties are presented of real multilayered structures consisting of alternating layers chalcogenide-chalcogenide glass and chalcogenide glass-organic polymer. The possibility of tuning their properties under external factors such as annealing or strong

As it is mentioned in section 3.2, the knowledge of the optical properties of the single layers is important for the successful engineering of a multilayer structure. Firstly we have investigated the optical parameters of the single layers. The results of the investigation of the thickness dependence of the refractive indices of thin films from some basic

10 100 1000

Fig. 8. Refractive index of GeS2 , As2S3 and As2Se3 thin films at λ = 1550 nm as a function of

The Swanepoel's method (Swanepoel, 1983) was used for optical constants determination of the thin films with d > 300 nm and a combination of double (T, R) and triple (T, R, R) methods was applied in calculations for thinner layers (d < 300 nm) (Konstantinov et al., 1998; Babeva et al. 2001). The calculated values for optical parameters of thin films show that their refractive index is independent on the layer's thickness for d > 50 – 70 nm (Fig. 8). The calculated values for the refractive indices of thin films with composition As2Se3 and GeS2 were 2.83 and 2.07 (λ = 1550 nm), respectively. Through the addition of metal in As2Se3 such as Ag and Cu thin films with a higher refractive index can be produced (Ogusu et al., 2004). The Ag-As2Se3 layer cannot be obtained by direct evaporation of the ternary Ag-As-Se glass since phase decomposition occurs. On the account of this, photodoping technique is used to produce As-As-Se film: evaporating Ag layer with a certain thickness on the As2Se3 layer, and exposing the stack to induce migration of the silver (Tasseva et al., 2010, Suzuki et al., 2005). We found a refractive index of 3.06 at λ = 1550 nm for thin As2Se3 film photodoped with 10 at % Ag (Tasseva et al., 2010). The photosensitivity of the chalcogenides arises from structural rearrangements induced by the absorption of photons at energies near the optical band gap of the material (Shimakawa et al., 1995). These structural rearrangements lead to changes in the optical properties. The magnitude and sign of these photoinduced changes can be dependant on the chemical composition of the glasses (Tanaka et al., 1979) and on the

Thickness of the film [nm]

chalcogenide glass formers As2S3, As2Se3 and GeS2 are given in Fig. 8.

**5. Results and discussion** 

electric field is examined.

**5.1 Optical properties of thin chalcogenide films** 

2.0 2.2 2.4 2.6 2.8 3.0

Refractive index

GeS2

thickness before (solid symbols) and after irradiation (open symbols).

As2 S3

As2 Se3

The multilayer structures formed only from chalcogenide sublayers were produced in one cycle of thermal evaporation using two sources - As2Se3 and GeS2. After deposition the samples were exposed in air to a mercury lamp (20 mW.cm-2).

Multilayered stacks from chalcogenide glass and organic polymer comprising 19 layers are prepared by alternating vacuum evaporation of As2S3 with target thickness of 170 nm and spin coating of PMMA with target thickness of 260 nm. The stock solution of the polymer was prepared by dissolution at ambient temperature of one gram of PMMA (Poly (methyl methacrylate)) in 10 ml of Dichloroethane (Aldrich) using magnetic stirrer for accelerating the process. The polymer films with different thicknesses are obtained by the method of spin coating using the stock solution further diluted by adding dichloroethane. Polymer layers with thicknesses of 260 nm are obtained by dripping a drop of 0.5 ml of 2.2 wt % polymer solution on the preliminarily cleaned substrate. The speed and duration of spinning were 2000 rpm and 30 s. To remove the extra solvent the samples are annealed for 30 minutes at temperature of 60 C.

#### **4.3 Optical methods for control and characterization of thin films for photonic crystals fabrication**

It is known that thin film's thickness can significantly affect their optical constants. In literature there is no unanimity about the dependence of the refractive index of thin chalcogenide films on their thickness (Abdel-Aziz et al. 2001). In photonic crystals it is necessary that the thin films are deposited with an exact optical thickness, (*nd*). For example, when a thin chalcogenide film is a part of a quarterwave stack and its refractive index has a value between 2.00 and 3.60 (see Fig.1), the thickness of the sublayers must be 110 – 193 nm for the working wavelength λ = 1550 nm.

The optical constants of thin films are usually determined by optical methods as spectrophotometry, prism-coupling technique and ellipsometry. We have demonstrated that the spectrophotometric and ellipsomeric methods offer a good accuracy for determination of the optical parameters of thin chalcogenide films from λ/30 to 2λ (λ is working wavelength) (Konstantinov et al. 1998, Babeva et al. 2001, Todorov et al. 2010a). Results on the reflectance response of photonic crystals from chalcogenide glass/polymer (DeCorby et al., 2005; Kohoutek et al., 2007a) or chalcogenide glass/chalcogenide glass, e.g. GeS2/Sb2Se3 (Kohoutek et al., 2009b) by variable angle spectroscopic ellipsometry have been reported.

In the present work optical transmittance and reflectance measurements at normal incidence of light beam were carried out in the spectral range from 350 to 2500 nm using an UV–VIS–NIR spectrophotometer (Cary 05E, Australia). Reflectance measurements at oblique incidence of linearly polarized light were performed with VASRA (Variable Angle Specular Reflectance Accessory). For polarizing the incident radiation, a high quality Glan–Taylor polarizer is used that provides an extremely pure linear polarization with a ratio 100 000:1. The computer controlled stepper motor of the VASRA accessory ensures reproducible adjustment of the incident angle with an accuracy Δθ = ±0.25º (according to the Cary Operation Manual). A self-made reference Al-mirror, whose preparation and characterization are described in details elsewhere (Babeva et al., 2002), is used as a standard mirror.

#### **5. Results and discussion**

156 Photonic Crystals – Innovative Systems, Lasers and Waveguides

The multilayer structures formed only from chalcogenide sublayers were produced in one cycle of thermal evaporation using two sources - As2Se3 and GeS2. After deposition the

Multilayered stacks from chalcogenide glass and organic polymer comprising 19 layers are prepared by alternating vacuum evaporation of As2S3 with target thickness of 170 nm and spin coating of PMMA with target thickness of 260 nm. The stock solution of the polymer was prepared by dissolution at ambient temperature of one gram of PMMA (Poly (methyl methacrylate)) in 10 ml of Dichloroethane (Aldrich) using magnetic stirrer for accelerating the process. The polymer films with different thicknesses are obtained by the method of spin coating using the stock solution further diluted by adding dichloroethane. Polymer layers with thicknesses of 260 nm are obtained by dripping a drop of 0.5 ml of 2.2 wt % polymer solution on the preliminarily cleaned substrate. The speed and duration of spinning were 2000 rpm and 30 s. To remove the extra solvent the samples are annealed for 30 minutes at

**4.3 Optical methods for control and characterization of thin films for photonic** 

It is known that thin film's thickness can significantly affect their optical constants. In literature there is no unanimity about the dependence of the refractive index of thin chalcogenide films on their thickness (Abdel-Aziz et al. 2001). In photonic crystals it is necessary that the thin films are deposited with an exact optical thickness, (*nd*). For example, when a thin chalcogenide film is a part of a quarterwave stack and its refractive index has a value between 2.00 and 3.60 (see Fig.1), the thickness of the sublayers must be 110 – 193 nm

The optical constants of thin films are usually determined by optical methods as spectrophotometry, prism-coupling technique and ellipsometry. We have demonstrated that the spectrophotometric and ellipsomeric methods offer a good accuracy for determination of the optical parameters of thin chalcogenide films from λ/30 to 2λ (λ is working wavelength) (Konstantinov et al. 1998, Babeva et al. 2001, Todorov et al. 2010a). Results on the reflectance response of photonic crystals from chalcogenide glass/polymer (DeCorby et al., 2005; Kohoutek et al., 2007a) or chalcogenide glass/chalcogenide glass, e.g. GeS2/Sb2Se3 (Kohoutek et al., 2009b) by variable angle spectroscopic ellipsometry

In the present work optical transmittance and reflectance measurements at normal incidence of light beam were carried out in the spectral range from 350 to 2500 nm using an UV–VIS–NIR spectrophotometer (Cary 05E, Australia). Reflectance measurements at oblique incidence of linearly polarized light were performed with VASRA (Variable Angle Specular Reflectance Accessory). For polarizing the incident radiation, a high quality Glan–Taylor polarizer is used that provides an extremely pure linear polarization with a ratio 100 000:1. The computer controlled stepper motor of the VASRA accessory ensures reproducible adjustment of the incident angle with an accuracy Δθ = ±0.25º (according to the Cary Operation Manual). A self-made reference Al-mirror, whose preparation and characterization are described in details elsewhere (Babeva et al., 2002), is used as a

samples were exposed in air to a mercury lamp (20 mW.cm-2).

temperature of 60 C.

**crystals fabrication** 

have been reported.

standard mirror.

for the working wavelength λ = 1550 nm.

#### **5.1 Optical properties of thin chalcogenide films**

In this part the optical properties are presented of real multilayered structures consisting of alternating layers chalcogenide-chalcogenide glass and chalcogenide glass-organic polymer. The possibility of tuning their properties under external factors such as annealing or strong electric field is examined.

As it is mentioned in section 3.2, the knowledge of the optical properties of the single layers is important for the successful engineering of a multilayer structure. Firstly we have investigated the optical parameters of the single layers. The results of the investigation of the thickness dependence of the refractive indices of thin films from some basic chalcogenide glass formers As2S3, As2Se3 and GeS2 are given in Fig. 8.

Fig. 8. Refractive index of GeS2 , As2S3 and As2Se3 thin films at λ = 1550 nm as a function of thickness before (solid symbols) and after irradiation (open symbols).

The Swanepoel's method (Swanepoel, 1983) was used for optical constants determination of the thin films with d > 300 nm and a combination of double (T, R) and triple (T, R, R) methods was applied in calculations for thinner layers (d < 300 nm) (Konstantinov et al., 1998; Babeva et al. 2001). The calculated values for optical parameters of thin films show that their refractive index is independent on the layer's thickness for d > 50 – 70 nm (Fig. 8). The calculated values for the refractive indices of thin films with composition As2Se3 and GeS2 were 2.83 and 2.07 (λ = 1550 nm), respectively. Through the addition of metal in As2Se3 such as Ag and Cu thin films with a higher refractive index can be produced (Ogusu et al., 2004). The Ag-As2Se3 layer cannot be obtained by direct evaporation of the ternary Ag-As-Se glass since phase decomposition occurs. On the account of this, photodoping technique is used to produce As-As-Se film: evaporating Ag layer with a certain thickness on the As2Se3 layer, and exposing the stack to induce migration of the silver (Tasseva et al., 2010, Suzuki et al., 2005). We found a refractive index of 3.06 at λ = 1550 nm for thin As2Se3 film photodoped with 10 at % Ag (Tasseva et al., 2010). The photosensitivity of the chalcogenides arises from structural rearrangements induced by the absorption of photons at energies near the optical band gap of the material (Shimakawa et al., 1995). These structural rearrangements lead to changes in the optical properties. The magnitude and sign of these photoinduced changes can be dependant on the chemical composition of the glasses (Tanaka et al., 1979) and on the

Thin Chalcogenide Films for Photonic Applications 159

= 0 <sup>o</sup>

= 70 <sup>o</sup>

0 50 100

Fig. 10. Spectra of transmittance, *T* and reflectance, *R* of a multilayer coating on a glass substrate at normal incidence; and reflectance spectra at angle of light incidence 70 for p –

**5.3 Thermo-induced changes in chalcogenide glasses and tuning of the photonic** 

The ability to tune or modulate the optical properties of photonic crystals would increase their functionality and open up new possibilities for a variety of applications for integrated optics (Lee et al., 2007). Photosensitivity has been successfully utilized for post-tuning in 2D photonic crystals (Lee et al., 2007). In (Todorov et al., 2010b) we demonstrated a possibility of shifting the fundamental reflectance band of as-deposited multilayer As2Se3/GeS2 quarter-wave slab exposing it to light and thus inducing changes in both materials building the coating. It is seen from Fig. 1 that thin films from Ge - S and As - Se systems ensure high optical contrast Δ*n* ~ 0.8. The exposure to light leads to opposite effects in the thin films – photodarkening in As2Se3 and photobleaching in GeS2 layers and increase of Δ*n* up to 1.0. The good knowledge of the photoinduced changes enabled a designed 50 nm expansion of the high reflectance band width. The presence of reversible changes in chalcogenide glasses

0

20

2 3 1

771 763

754

40

Transmission [%]

Fig. 11. Spectra of transmittance at normal light incidence of multilayer coating consisting of 19 alternating As2Se3 and GeS2 layers in different consequences of treatment: as-deposited -

60

80

100

0 50 100

Rs

<sup>70</sup> <sup>o</sup> 0 50 100 Rp 70 <sup>o</sup>

700 800 900 1000 1100

Wavelength [nm]

b)

1. as-deposited 2. annealed 3. exposed after annealing

= 70 <sup>o</sup>

R

0 50 100

T

and s- light polarization. The shaded area represents the ODR band.

allows production of photonic structures with dirigible optical properties.

700 800 900 1000 1100

Wavelength [nm]

1. as-depsoited 2. exposed

3. annealed after exposure 4. second exposure after annnealing

exposed - annealed - exposed (a); and as-deposited - annealed – exposed (b).

1 2

4, 3

= 0 <sup>o</sup>

700

800

900

Wavelength [nm]

**band gap** 

0

20

40

Transmission [%]

60

754 764 760

80

100 a) 1000

1100

processing history of the sample (Okuda et al., 1979). It is seen from Fig.8 that the arsenic containing thin chalcogenide films with compositions As2S3 and As2Se3 demonstrate increasing of the refractive index up to 2.39 and 2.93 after exposure to light, respectively while for thin GeS2 layer n decreases to 2.03.

Furthermore, these changes may be either reversible or irreversible. The irreversible changes result from light exposure or thermal annealing of as-deposited thin films in a non-equilibrium state (Biegelsen et al., 1980) while the reversible changes result from structural rearrangement leading from one quasi-stable state to another (Street, 1977, Biegelsen et al., 1980). In Fig. 9 the changes of the refractive index are shown of thin As2Se3 film after exposure and annealing. The as-deposited layer demonstrates an increase of the refractive index (Δ*n* = +0.14 at λ = 1550 nm) that is an expression of the photo-darkening effect. A reversible change of Δ*n* = -0.02 after annealing at 160C in vacuum was observed.

Fig. 9. Dispersion of the refractive index, *n* of as-deposited, exposed and subsequently annealed at 160C in vacuum thin As2Se3 film.

#### **5.2 Optical properties of a double layered structure and multilayered coatings**

Further, we use the data plotted in Fig. 8 for modelling of double- and multilayered structures. The reflectance spectrum of a double layered coating from As2Se3 and GeS2 (BK-7/As2Se3/GeS2) was discussed in (Todorov et al. 2010b). Having determined the optical parameters and thickness of the single layers we calculated the theoretical reflectance spectrum of the complex structure using the equations from section 3.1. A good coincidence between the theoretical and measured spectrum of the double layered structure was obtained.

In Fig. 10 the measured transmittance and reflectance spectra at normal light incidence (*θ* = 0) and angle *θ* = 70 of 19 layers quarter-wave structure from alternating As2Se3 and GeS2 layers deposited on a glass and on an absorbing Si wafer substrate are presented. The shift of the photonic band gap to shorter wavelengths with the increase of the angle of light incidence, discussed in section 3, is clearly seen.

processing history of the sample (Okuda et al., 1979). It is seen from Fig.8 that the arsenic containing thin chalcogenide films with compositions As2S3 and As2Se3 demonstrate increasing of the refractive index up to 2.39 and 2.93 after exposure to light, respectively

Furthermore, these changes may be either reversible or irreversible. The irreversible changes result from light exposure or thermal annealing of as-deposited thin films in a non-equilibrium state (Biegelsen et al., 1980) while the reversible changes result from structural rearrangement leading from one quasi-stable state to another (Street, 1977, Biegelsen et al., 1980). In Fig. 9 the changes of the refractive index are shown of thin As2Se3 film after exposure and annealing. The as-deposited layer demonstrates an increase of the refractive index (Δ*n* = +0.14 at λ = 1550 nm) that is an expression of the photo-darkening effect. A reversible change of Δ*n* = -0.02 after annealing at 160C in

500 1000 1500 2000 2500

Wavelength [nm]

Fig. 9. Dispersion of the refractive index, *n* of as-deposited, exposed and subsequently

**5.2 Optical properties of a double layered structure and multilayered coatings** 

Further, we use the data plotted in Fig. 8 for modelling of double- and multilayered structures. The reflectance spectrum of a double layered coating from As2Se3 and GeS2 (BK-7/As2Se3/GeS2) was discussed in (Todorov et al. 2010b). Having determined the optical parameters and thickness of the single layers we calculated the theoretical reflectance spectrum of the complex structure using the equations from section 3.1. A good coincidence between the theoretical and measured spectrum of the double layered structure was

In Fig. 10 the measured transmittance and reflectance spectra at normal light incidence (*θ* = 0) and angle *θ* = 70 of 19 layers quarter-wave structure from alternating As2Se3 and GeS2 layers deposited on a glass and on an absorbing Si wafer substrate are presented. The shift of the photonic band gap to shorter wavelengths with the increase of the angle of light

As40Se60

 virgin exposed annealed

while for thin GeS2 layer n decreases to 2.03.

2.6 2.8 3.0 3.2 3.4 3.6 3.8

Refractive Index

annealed at 160C in vacuum thin As2Se3 film.

incidence, discussed in section 3, is clearly seen.

vacuum was observed.

obtained.

Fig. 10. Spectra of transmittance, *T* and reflectance, *R* of a multilayer coating on a glass substrate at normal incidence; and reflectance spectra at angle of light incidence 70 for p – and s- light polarization. The shaded area represents the ODR band.

#### **5.3 Thermo-induced changes in chalcogenide glasses and tuning of the photonic band gap**

The ability to tune or modulate the optical properties of photonic crystals would increase their functionality and open up new possibilities for a variety of applications for integrated optics (Lee et al., 2007). Photosensitivity has been successfully utilized for post-tuning in 2D photonic crystals (Lee et al., 2007). In (Todorov et al., 2010b) we demonstrated a possibility of shifting the fundamental reflectance band of as-deposited multilayer As2Se3/GeS2 quarter-wave slab exposing it to light and thus inducing changes in both materials building the coating. It is seen from Fig. 1 that thin films from Ge - S and As - Se systems ensure high optical contrast Δ*n* ~ 0.8. The exposure to light leads to opposite effects in the thin films – photodarkening in As2Se3 and photobleaching in GeS2 layers and increase of Δ*n* up to 1.0. The good knowledge of the photoinduced changes enabled a designed 50 nm expansion of the high reflectance band width. The presence of reversible changes in chalcogenide glasses allows production of photonic structures with dirigible optical properties.

Fig. 11. Spectra of transmittance at normal light incidence of multilayer coating consisting of 19 alternating As2Se3 and GeS2 layers in different consequences of treatment: as-deposited exposed - annealed - exposed (a); and as-deposited - annealed – exposed (b).

Thin Chalcogenide Films for Photonic Applications 161

According to (Kolomiets et al., 1970) the change in the transmission, Δ*T*, induced by electric field, depends on the photon energy (*E* = *hν*). He observed that the maximal change Δ*T* occured at *hν* = 2.30 eV applying sinusoidal electric voltage of 100 V. In Fig. 12a the change in transmission spectrum, Δ*T* = *T*voltage – *T*as-dep. is presented of thin film from As2S3 with thickness 2 μm due to the application of dc electric voltage U = 100 V. The maximal value for Δ*T* was observed at 500 nm, i.e. for photon energy 2.48 eV. The changes of the spectrum at longer wavelengths are smaller and are possibly "shadowed" by the interference fringes. In Fig. 12b the variation is shown of the transmittance at *λ* = 500 nm depending on the applied electric voltage. Mathematical extrapolations showed that when the electric voltage is applied the transmittance decreases following an exponential low of the type – *T* = *T*0 + A1.*exp*(-*t*/*B*1), where *T*0 is the initial value of the transmittance, *T*0 = 19.9%; *A*1 = 1.85 and *B*1 = 2.58 min are parameters. The switching off of the electric field restores the initial value of *T*. In this case the increase can be described through the exponential low of the type

*T* = *T*0 + A2.[1 - *exp*(-*t*/*B*2)] where *A*2 = 136.45 and *B*2 = 3.63 min are the parameters.

As2

18

b)

(U = 0 V) El. field OFF

19

Transmission [%]

Fig. 12. Spectral dependence of the change of transmission spectrum, Δ*T* =Tvoltage– T as dep. of thin As2S3 film, under dc electric voltage 100 V (a); Time evolution of the transmittance at λ = 500 nm depending on the applied voltage (open symbols) and extrapolations with

The changes in the transmission at wavelength λ = 1915 nm are given in Fig. 13b. We observed cyclic reduction or enhancing of the transmission when switch *on* or *off* the electric field. It was observed that the changes of the transmission coefficient followed the same exponential laws as the thin film from As2S3 with parameters *A*1 = 3.59 and *B*1 = 2.12 min, when decreasing under the influence of the electric field, and *A*2 = 11120.68 and *B*2 = 2.22

The observations imply that the electric field induces reversible changes of the refractive index of the sublayers in the photonic structure. The applied electric voltage of 100 V is

20

21

(U = 100 V) El. field ON

0 20 40 60

(U = 100 V) El. field ON

 experimental measurement exponential extrapolation

Time [min]

400 450 500 550 600

exponential functions (dashed line) (b).

Wavelength [nm]

min for the relaxation process after switching off the voltage.

S a) <sup>3</sup>




0.0

T%

Considering the relationship between optical band gap and refractive index (see Penn, 1962; Wemple & DiDomenico, 1971) it is expected the value of *n* to increase when *E*g decreases due to electroabsorption. The changes in the refractive indices of the sublayers in 1Dphotonic crystals would shift the position of their stop band. An example of the influence of the electric field on the stop band of 1D quaterwave structure from As2S3/PMMA is presented on Fig. 13. The shift of the transmission spectrum of such a multilayer system sandwiched between two thin transparent chromium electrodes is clearly seen (Fig. 13a).

In Fig. 11 the changes in the transmittance spectra are presented of the multilayer coatings referred to in Fig. 10 after exposure to light from a mercury lamp and after annealing at 160C in vacuum. The annealing temperature was selected to be with 20C lower then the glass forming temperature for As2Se3 glass (Li et al., 2002). The exposure to light leads to 17 nm or 30 nm red shift of the resonant band for as-deposited or annealed samples, respectively, at T = 10 %. It is seen that light illumination results in expansion of the resonant band to longer wavelengths and parallel shift of the pass band in both as-deposited and thermally treated samples. The annealing of the samples causes the stop band to move to shorter wavelengths - with 14 and 28 nm (T = 10 %) for exposed and as-deposited samples, respectively. Due to the effect of thermobleching, observed in thin As2Se3 and GeS2 films (De Neufville et al. 1974, Tichy et al. 1993) the optical contrast between sublayers is changed after annealing. It is known that the reversible changes are increased with the increasing of the arsenic in the AsxSe100-x system for x > 40 at % and allow making of many cycles between both states applying the light 'recording' and 'erasing' by annealing of the films (Lyubin, 1984). Unfortunately, the increase of the arsenic content for 40 ≤ *x* ≤ 60 leads to a reduction of the refractive index (Petkov et al., 2009). Further investigations would involve the selection of suitable chalcogenide glasses with optimal reversible changes for creating of multilayered structure with variable width of the stop band.

#### **5.4 Electrostatic tuning of the photonic band gap**

The phenomena of electroabsorption due to the effect of Franz-Keldysh in crystals and glasses from As2S3 were observed by (Kolomiets et al., 1970). The absorption edge is shifted to longer wavelengths due to decrease of the optical gap, *E*g following the formula:

$$
\Delta E\_{\chi} = e^2 h^2 S^2 F^2 / 24m^\* \tag{16}
$$

where *e* and *m\** are electric charge and mass of the electron, respectively, h is Plank's constant, *S* is slope of the absorption edge in the absence of the field and *F* is the intensity of the electric field. The phenomena of the electroabsorption in chalcogenide glasses are explained through the barrier-cluster model (Banik, 2010). It assumes that an amorphous semiconductor consists of microscopic regions - perhaps closed clusters - separated from each other by potential barriers. The strong electric field increases the probability of tunnelling and optical absorption, α as well. The increase of α in non-crystalline semiconductors is proportional to the squire of the intensity of the electric field – *F*2:

$$\frac{\Delta a}{a} = \text{const.} F^2 \tag{17}$$

Samples from "sandwich" type wеre prepared for investigation of the effect of electroabsorption on the optical properties of the thin films or multilayered structure. Firstly, thin transparent electrode from chromium with 10 nm thickness was deposited on a glass substrate by electron beam evaporation. The transmission coefficient of the electrode in the spectral range 1200-2500 nm is between 70 and 80 % and the electrical resistivity, *ρ* = 2 mΩ.cm. Subsiquently the investigated samples - thin layer or multilayered stack, were deposited on the chromium electrode. The structure was terminated by deposition of the second chromium electrode on the top.

In Fig. 11 the changes in the transmittance spectra are presented of the multilayer coatings referred to in Fig. 10 after exposure to light from a mercury lamp and after annealing at 160C in vacuum. The annealing temperature was selected to be with 20C lower then the glass forming temperature for As2Se3 glass (Li et al., 2002). The exposure to light leads to 17 nm or 30 nm red shift of the resonant band for as-deposited or annealed samples, respectively, at T = 10 %. It is seen that light illumination results in expansion of the resonant band to longer wavelengths and parallel shift of the pass band in both as-deposited and thermally treated samples. The annealing of the samples causes the stop band to move to shorter wavelengths - with 14 and 28 nm (T = 10 %) for exposed and as-deposited samples, respectively. Due to the effect of thermobleching, observed in thin As2Se3 and GeS2 films (De Neufville et al. 1974, Tichy et al. 1993) the optical contrast between sublayers is changed after annealing. It is known that the reversible changes are increased with the increasing of the arsenic in the AsxSe100-x system for x > 40 at % and allow making of many cycles between both states applying the light 'recording' and 'erasing' by annealing of the films (Lyubin, 1984). Unfortunately, the increase of the arsenic content for 40 ≤ *x* ≤ 60 leads to a reduction of the refractive index (Petkov et al., 2009). Further investigations would involve the selection of suitable chalcogenide glasses with optimal reversible changes for

The phenomena of electroabsorption due to the effect of Franz-Keldysh in crystals and glasses from As2S3 were observed by (Kolomiets et al., 1970). The absorption edge is shifted

where *e* and *m\** are electric charge and mass of the electron, respectively, h is Plank's constant, *S* is slope of the absorption edge in the absence of the field and *F* is the intensity of the electric field. The phenomena of the electroabsorption in chalcogenide glasses are explained through the barrier-cluster model (Banik, 2010). It assumes that an amorphous semiconductor consists of microscopic regions - perhaps closed clusters - separated from each other by potential barriers. The strong electric field increases the probability of tunnelling and optical absorption, α as well. The increase of α in non-crystalline

*2 2 2 2 \**

*<sup>g</sup> ΔE e h S F /24m* (16)

(17)

to longer wavelengths due to decrease of the optical gap, *E*g following the formula:

semiconductors is proportional to the squire of the intensity of the electric field – *F*2:

**<sup>2</sup>** *const*.*<sup>F</sup>*

Samples from "sandwich" type wеre prepared for investigation of the effect of electroabsorption on the optical properties of the thin films or multilayered structure. Firstly, thin transparent electrode from chromium with 10 nm thickness was deposited on a glass substrate by electron beam evaporation. The transmission coefficient of the electrode in the spectral range 1200-2500 nm is between 70 and 80 % and the electrical resistivity, *ρ* = 2 mΩ.cm. Subsiquently the investigated samples - thin layer or multilayered stack, were deposited on the chromium electrode. The structure was terminated by deposition of the

creating of multilayered structure with variable width of the stop band.

**5.4 Electrostatic tuning of the photonic band gap** 

second chromium electrode on the top.

According to (Kolomiets et al., 1970) the change in the transmission, Δ*T*, induced by electric field, depends on the photon energy (*E* = *hν*). He observed that the maximal change Δ*T* occured at *hν* = 2.30 eV applying sinusoidal electric voltage of 100 V. In Fig. 12a the change in transmission spectrum, Δ*T* = *T*voltage – *T*as-dep. is presented of thin film from As2S3 with thickness 2 μm due to the application of dc electric voltage U = 100 V. The maximal value for Δ*T* was observed at 500 nm, i.e. for photon energy 2.48 eV. The changes of the spectrum at longer wavelengths are smaller and are possibly "shadowed" by the interference fringes. In Fig. 12b the variation is shown of the transmittance at *λ* = 500 nm depending on the applied electric voltage. Mathematical extrapolations showed that when the electric voltage is applied the transmittance decreases following an exponential low of the type – *T* = *T*0 + A1.*exp*(-*t*/*B*1), where *T*0 is the initial value of the transmittance, *T*0 = 19.9%; *A*1 = 1.85 and *B*1 = 2.58 min are parameters. The switching off of the electric field restores the initial value of *T*. In this case the increase can be described through the exponential low of the type *T* = *T*0 + A2.[1 - *exp*(-*t*/*B*2)] where *A*2 = 136.45 and *B*2 = 3.63 min are the parameters.

Considering the relationship between optical band gap and refractive index (see Penn, 1962; Wemple & DiDomenico, 1971) it is expected the value of *n* to increase when *E*g decreases due to electroabsorption. The changes in the refractive indices of the sublayers in 1Dphotonic crystals would shift the position of their stop band. An example of the influence of the electric field on the stop band of 1D quaterwave structure from As2S3/PMMA is presented on Fig. 13. The shift of the transmission spectrum of such a multilayer system sandwiched between two thin transparent chromium electrodes is clearly seen (Fig. 13a).

Fig. 12. Spectral dependence of the change of transmission spectrum, Δ*T* =Tvoltage– T as dep. of thin As2S3 film, under dc electric voltage 100 V (a); Time evolution of the transmittance at λ = 500 nm depending on the applied voltage (open symbols) and extrapolations with exponential functions (dashed line) (b).

The changes in the transmission at wavelength λ = 1915 nm are given in Fig. 13b. We observed cyclic reduction or enhancing of the transmission when switch *on* or *off* the electric field. It was observed that the changes of the transmission coefficient followed the same exponential laws as the thin film from As2S3 with parameters *A*1 = 3.59 and *B*1 = 2.12 min, when decreasing under the influence of the electric field, and *A*2 = 11120.68 and *B*2 = 2.22 min for the relaxation process after switching off the voltage.

The observations imply that the electric field induces reversible changes of the refractive index of the sublayers in the photonic structure. The applied electric voltage of 100 V is

Thin Chalcogenide Films for Photonic Applications 163

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considerably high for applications in modern optoelectronic devices such as electrooptical modulators for high-speed time-domain-multiplexing (TDM) and wavelength-divisionmultiplexing systems (WDM). Further investigations would involve determination of the dependence of the elеctroabsorption effect on the composition of the chalcogenide glasses striving to reduce the voltage of the applied electric field.

Fig. 13. Transmission spectrum of As2S3/PMMA multilayer slab between thin transparent chromium electrodes. In the inset the transmission spectrum of the same sample in wide spectral range - 800-2500 nm - is presented (a); Evolution of the transmission coefficient at λ = 500 nm in cyclic switching on and off of dc electric voltage U = 100 V. The exponential extrapolations are given with a dashed line (b).
