Photodoping phenomenon is observed when a double-layer consisting of an amorphous chalcogenide film (As2S3, GeS2, GeSe2 etc.) and a metal (Ag, Cu etc.) film is illuminated by light. The metal diffuses abnormally into the amorphous chalcogenide layer. Amorphous chalcogenide films of GeS2 with an Ag over layer exhibited large increase of refractive index through the abnormal doping of Ag by irradiating the light around the absorption edge of the GeS2 chalcogenide. In this study, we aimed the application of this effect for the fabrication of optical devices and fabricated various micro doped patterns by using a laser beam. Mask less pattering with refractive index modified films are possible by manipulating the scanning of the laser beam. Micro gratings were fabricated using a confocal laser microscope to work as both fabrication and observation system. Waveguides were also fabricated by scanning the laser beam for photodoping. Holographic gratings were fabricated by utilizing the photodoping of the two beam interference pattern, which showed the possibility to produce large scale optical devices or mass production.
When metal layers like Ag or Cu deposited on the amorphous chalcogenide films such as As2S3, As2Se3, GeS2, and GeSe2, are illuminated by light, the metal elements diffuse abnormally into the amorphous chalcogenide layer. This abnormal diffusion of metal was found by Kostyshin in Russia in 1965 and called photodoping. The large modulation of refractive index is realized by the doping of large amount of Ag atoms into the amorphous network of chalcogenide film which gives the possibility for various photonic device applications. In this study, the quantitative characterization of photodoping phenomena was carried out for amorphous GeS2 films using Ag as a doping metal element to obtain the basic information to the photonic device fabrication. Quantum efficiency to reach the saturation of the doping was derived using the laser diodes with different wavelengths which cause a photodoping phenomenon. As for the wavelength dependence of the photodoping, it was suggested that the quantum efficiency was enhanced for the irradiated photon energy near the optical gap energy (ca. 3.3eV) of a-GeS2 and showed the tendency similar to the absorption spectrum of the amorphous GeS2 film. Photodoping rate was studied for various irradiated power densities of a He-Cd laser (441.5 nm) and the photodoping rate was proportional to the number of incident photons at low intensity. For the intensity over 10mW/cm2, the enhancement of photodoping rate was observed due to some extra effects like a thermal effect.
The photodoping phenomenon was found by Kostyshin in Russia in 1965. Many researches have carried out studying on
the photodoping phenomenon of Ag /a-As2S3 system, and various interesting behaviors have been reported. However,
there are few works on the research of the photodoping phenomenon of Ag /a-GeS2 double layer system. It was reported
that the lateral diffusion was small at the photodoping phenomenon of Ag /a-GeS2 system compared with the Ag /a-
As2S3 system. Therefore, the feasibility of application for fine patterning is expected for the Ag /a-GeS2 system. In
general, a light beam with the photon energy near the optical gap energy (ca. 3.3eV) of a-GeS2 is irradiated to cause
photodoping. In this study, the photodoping characteristics of multilayer films, GeS2/Ag/GeS2 and Ag/GeS2/Ag, were
fabricated and characterized. The photodoping characteristics are compared with the conventional two layer films. In
the GeS2/Ag/GeS2 three layered system, strong selectivity of photodoping property relating with the incident light
wavelength was found. The feasibility to apply three layered films to optical memories and waveguides were suggested.
The photodoping phenomenon of Ag is one of the light induced phenomena in GeS2 amorphous chalcogenide films.
Photonic structures like wave guides and micro-optics may be possible to fabricate utilizing the refractive modulation
caused by the doping process of Ag, but the practical fabrication method has not been established yet. The method to
monitor the doping also has to be paid attention for precise processing. In this report, a dual functional laser scanning
system integrating micro-machining and micro-scope systems is proposed. An UV laser, effective for photodoping
phenomenon, is used to process the doping patterns, and a VIS laser, not influential upon the material, is used to monitor
the fabricated patterns. In-situ nano scale manipulation and observation processes were possible under same setup in one
system. As a typical photonic pattern, diffraction gratings of Ag/GeS2 were fabricated and optical performances were
evaluated. An optical model of the light propagation in the doped layers is proposed and simulated. The mechanism for
the enhanced broadening of the doped region was suggested well through the simulation. This system will open new
device fabrication and help to advance the research of nanostructures and photonic crystals.
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