Generalized Model for Photoinduced Surface Structure in Amorphous Thin Films

We present a generalized model to explain the spatial and temporal evolution of photoinduced surface structure in photosensitive amorphous thin films. The model describes these films as an incompressible viscous fluid driven by a photoinduced pressure originating from dipole rearrangement. This derivation requires only the polarizability, viscosity and surface tension of the system. Using values of these physical parameters, we check the validity of the model by fitting to experimental data of ${\mathrm{As}}_2{\mathrm{S}}_3$ and demonstrating good agreement.

Figure 1

Schematic of the typical experimental setup used to generate surface structure. Two beams, $W1$ and $W2$, interfere on the surface of a thin film of amorphous, light-sensitive material, leading to the pictured definitions of the coordinate system and polarization directions.

Figure 2

Schematic of photoinduced dipoles (on the scale of about 3 coordination spheres [19]) and their rearrangement leading to mass transport [22]. Dipoles can lower their energy by changing configurations in response to the optical electric field. In the case of linear polarization the dipoles will align with the optical electric field.

Figure 3

Plots of maximum grating amplitude evolution versus irradiation fluence (time). The curve is produced by taking snapshots of the surface profile at different fluence and recording the maximum grating amplitude for each. Each curve corresponds to a set of material parameters: {$p_1$, $\sigma$, $\eta$}, where $p_1$ is directly related to the polarizability ${ϵ}_{ij}$ (see Table ), $\sigma$ is surface tension, while $\eta$ is the dynamic viscosity. The upper curve, generated by {$p_1=1.8\times {10}^{-2}\mathrm{J}/{\mathrm{m}}^3$, $\sigma =0.060\mathrm{J}/{\mathrm{m}}^2$, $\eta =0.208\times {10}^{11}\mathrm{Pa}·\mathrm{s}$}, simulates the response of ${\mathrm{As}}_2{\mathrm{S}}_3$.

Figure 4

The model’s fit to a section of a surface relief profile data from 3. Results by the model agree well with experimental measurements, both in amplitude and frequency.

Figure 5

Fluence dependence of maximum surface relief amplitude, with measurements data taken from 3.