Olli Nordman

Electron beam induced changes in the refractive index and film thickness of amorphous AsxS100-x and AsxSe100-x films

In this article, electron beam induced changes in the refractive index and film thickness of time relaxed amorphous AsxS100−x (with x=30–45) and AsxSe100−x (with x=40–70) are studied. The largest index change (∼0.08) in AsxS100−x films is found when x=40. The corresponding value (∼0.06) for AsxSe100−x films is met when x=55. The difference in the best compositions is attributed to the different relaxation processes of As–S and As–Se films. Electron beam irradiation causes surface shrinkage of the films. In AsxSe100−x films contractions are deepest (∼75 nm) when x=55.

Design and fabrication of circular grating coupled distributed Bragg reflector lasers

The design and fabrication of circular grating surface emitting lasers are presented. The influence of device parameters on the mode selection is described. The fabrication of an InGaAs/GaAs circular grating distributed Bragg reflector (DBR) laser is presented. Significant improvement is obtained by using a quantum-well structure with a multistack and an etch-stop layer. A threshold current as low as 15 mA and an output power in excess of 170 mW without saturation are obtained. The potential of circular grating DBR laser arrays for multiwavelength operation is demonstrated.

Two-color holographic-grating formation in amorphous As 2 S 3 films

A detailed experimental study of the holographic gratings recorded in nonannealed amorphous As2S3 films by 514.5-nm light in the presence of 632.8-nm readout light is carried out. A strong influence of a continuous 632.8-nm readout is found. The dependences of the maximal first-order diffraction efficiency and the corresponding specific recording energy on the holographic grating period were studied in a wide range of periods from 0.40 to 70.0 μm for 2-yr-old films. The obtained results are discussed in terms of photoinduced structural changes, relaxational structural changes, photoinduced anisotropy, and photoinduced recharging of the localized states in the bandgap. The photoinduced sulphur related D-- and D+-center orientation mechanism is proposed to explain the stimulating action of 632.8-nm light.

Relaxation process of holographic gratings in amorphous As 2 S 3 films

A detailed study of the relaxation self-enhancement (RSE) of the holographic gratings in amorphous As2S3 films is presented. The changes of the diffraction efficiency have been measured as functions of the aging time and the recording light intensity. The role of the internal mechanical stress of the films in the self-enhancement phenomenon has been discussed. It is shown that the RSE has a vectorial character owing to the uniaxial periodically distributed stress relaxation. A model based on the photoinduced stress relaxation and viscous flow of amorphous film is proposed to explain the experiments qualitatively. It is further shown that the stress-induced optical anisotropy of the films is responsible for the RSE.

Characterization of refractive index change induced by electron irradiation in amorphous thin As2S3 films

Binary diffraction gratings in As2S3 films were prepared with the aid of an electron beam. The dose of the electrons was varied. The gratings were read with a HeNe laser, and the zero-order and first-order diffraction efficiencies were noted. Rigorous diffraction theory was used to give a homogeneous approximation for the refractive index change. Reactive ion etching was applied to reduce the thickness of the film, and a new value for the refractive index change was evaluated. The refractive index change versus film thickness dependence was found to be linear at low electron doses and Gaussian shaped at higher doses. Through a simple mathematical analysis, the absolute value of the refractive index was determined as a function of the position inside the film. At higher doses, the refractive index change was found to have a maximum value of 3%, approximately 1.2 μm from the film surface.

Optical properties of two photoresists designed for charge-coupled device microlenses

Two different commercially available photoresists, specifically designed for the fabrication of charge-coupled device (CCD) microlens arrays, are compared for their effectiveness in industrial-scale array production. A method is used to characterize resist films, prepared under the same processing conditions as those for the actual lens arrays. Great differences in the transmission spectra are observed between the two resists. Similarly, the quality of the film surface, based on the interference patterns in transmission spectra, vary greatly as well. Finally, plots of absorption versus wavelength offer practical proof that power losses due to absorption vary significantly between the types of photoresist.

On thermal influence of laser beam irradiation on optical absorption of amorphous as-evaporated As2S3 films

Abstract Photoinduced changes of the optical absorption in amorphous as-evaporated As 2 S 3 thin films are studied. Before the measurements films were kept at dark for more than two years. The long storage time ensured that the relaxational structural changes caused by the deposition process were absent. As a light source a CW unfocused 488.0 nm Ar laser line was used. The intensity of the laser beam was varied from rather low intensity values up to intensities which raised the temperature of the films above the glass transition temperature. Based on the behavior of the saturation values of the absorption we were able to state that the exposed As 2 S 3 films started to polymerize at 63–69°C which corresponds to the light intensity level of ∼4 W cm −2 .

Refractive index change caused by electron irradiation in amorphous As–S and As–Se thin films coated with different metals

The refractive index change caused by electron irradiation was measured in amorphous As–S and As–Se thin films coated with different metals. Metal atoms/ions diffused into the films during irradiation. The diffusion was dependent on the metal and influenced the refractive index. The influence was smallest in As40S60 films although these films possessed the highest overall refractive index changes. Au atoms/ions were almost immobile in all films while Ag atoms/ions had the highest mobility. Their high mobility allowed them to diffuse laterally within the film.

Diffraction-efficiency oscillations in amorphous As 2 S 3 films

An experimental study of the holographic gratings recorded in nonannealed, thermally with time relaxed amorphous As2S3 films by 514.5-nm light in the presence of 632.8-nm readout light is carried out. The dependences of the maximal first-order diffraction efficiency on the holographic grating period was studied in a wide range of periods, from 0.40 to 70.0 µm. A peculiar oscillatory diffraction-efficiency temporal behavior occurring under certain conditions is reported. The obtained results are discussed in terms of photoinduced structural changes, relaxational structural changes, photoinduced anisotropy, and photoinduced recharging of the localized states in the bandgap. The diffraction-efficiency oscillations are explained by the model of annihilating defects and by multiwave mixing in a thin dynamic hologram.

Influence of the age of amorphous nonannealed As2S3 thin films on holographic properties

The dependences of the maximal first order diffraction efficiency and the corresponding specific recording energy on the holographic grating period were studied. Grating period was varied from 0.40 to 70.0 μm. Both fresh and aged films were used. A large holographic recording efficiency decrease in the course of aging is found to take place. These changes are due to the effective film grain size increase caused by the relaxational structural changes and atmospheric oxygen exposure. Results are explained with the aid of stress fields induced by the evaporation and holographic recording. The obtained results can be used to optimize the hologram recording in amorphous chalcogenide films.