**3. Theory**

148 Photonic Crystals – Innovative Systems, Lasers and Waveguides

still keeping low values of the two photon absorption. The values of the non-linear refractive index predicted for thin As-S-Se films are commensurable with those measured for bulk samples (Cardinal et al., 1999). The ultrafast response, i.e. the induction of nonlinear refractive index in chalcogenides exposed to the influence of strong electromagnetic field could be explained in the terms of the electronic structure of the glasses, considering

0

4x10-11

[m/W]

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -3x10<sup>2</sup>

Fig. 3. Dispersion of the a) nonlinear refractive index, b) two photon absorption and c) FOM

As40S60 1.96 71 0 As40Se60 6.32 231 0 As40S30Se30 3.58 131 0 As39.2Se55.1Ag5.7 19.1 697 1 As41Se28S26Ge5 4.46 163 0 Ge33S67 1.47 54 0 Table 1. Nonlinear refractive index values, and FOM for low-loss thin chalcogenide films

In the process of selection of chalcogenide compositions for optimized photonic applications, as discussed in the lines above, one should be careful in tailoring the properties of the materials. It should be taken account for transparency of the films in the designation spectral region, high nonlinearities and low nonlinear absorption, high photosensitivity.

Photon energy [eV]

x10-18 / SiO2 FOM

1,55 m 1,06 m

 As2 S3 As2 Se3 GeS2

8x10-11

1x10-10

2x10-10

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

c)

Photon energy [eV]

 As b) <sup>2</sup> S3 As3 Se3 GeS2

ionization of the atom and distortion of the electron orbits (Liu et al., 2005).

 As2 S3 As2 Se3 GeS2

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -5x10-18

Photon energy [eV]


of thin chalcogenide films, calculated from eqs (2), (5) and (7)

[m2/W]

FOM

0

Eg /2

Composition of the thin film

of various compositions at λ = 1.55 µm

5x10-18

[m2/W]

1x10-17

a)
