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The thermodynamics of laser interacting with AgOx nano-thin-film on
glass is numerically simulated. In order to be applicable for realistic experimental
conditions, methods of dynamical both spatial and temporal sizes
are developed. The simulation results may provide an explanation for the
nonlinear optical transmissions observed experimentally.
The silver oxide nano-thin-film has been used as a mask layer in the super
resolution near-field structure (super-RENS) disk. The experimental results
of Liu et al.[6] show that this structure induces strong near-field intensity and
unusual nonlinear optical effect. Another experiment[7] exhibited an increase
of optical transmittance after the furnance heating of a glass substract with
a thin AgOx layer to a temperature higher than the decomposition temperature.
Our theoretical calculation based on the experimental parameters indicates
the possibility of the optical transmittance increase after the AgOx film
is annealed. Therefore, we employ a numerical model to study the thermodynamical
interaction between laser and AgOx nano-thin-film on glass.
The thermodynamical model employs numerical methods to solve heat
flow equation. First, the system is set up with hundreds finite cells in one
dimension. Then, the absorption of laser energy, the temperature, the enthalpy
and the thermal conductivity of each cell are dynamically updated
in each time step. Thus, the dynamical temperature profiles including the
AgOx film temperature and other thermodynamical properties for various
laser intensity are obtained.
The nano-thin-film is in nanometer scale so as the finite cell Δx, but
the thickness of glass is in millimeter. Thus, a method is developed for dynamically
simulating the physical region depending the instant temperature
profile. In addition, the key problem is in time domain because the time
step is limited by Δt = 0.5Δx2 (in cgs unit) and the interaction time can
be in a few seconds. In order to finish the simulation with present computer
power, we take advantage of our understanding of the physical processes by
dynamically doubling the size of each cell for increasing both the physical
region simulated and time step size.
The simulation results seem to be consistent with the experimental results[6].
Under the laser power density of 30 W/cm2, the surface temperature of Ag2O
nano-thin-film on glass can be raise to it’s decomposition temperature within
a couple seconds. The dynamics and the relationship between required laser
power and irradiation time for raising the surface to decomposition temperature
are studied in detail. Also, the dynamics for the decomposition to occur
within microsecond time scale is also investigated for the purpose of data
storage.

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