摘要:

For the modeling of infrared spectra it is a common approach to use a dielectric function that treats the vibrational modes as damped harmonic oscillators. This model was found to be rather crude for some applications to amorphous solids. A dielectric function model yielding a Gaussian shape of the absorption lines and satisfying Kramers–Kronig relations is suggested. The model function is constructed by a convolution of a Gaussian function with the dielectric function of the damped harmonic oscillator model. An analytical solution of this integral is given. It is demonstrated that this model describes the spectra of thermally grown ultrathin (1.3 nm) silicon oxide films, plasma‐deposited silicon films, plasma‐deposited silicon nitride films, and amorphous aluminum oxide films very well. The physical motivation of the dielectric function model suggested is the randomness of the vibrational frequencies in an amorphous structure.

ISSN:0021-8979    

DOI:10.1063/1.350737

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Photon scanning tunneling microscope (PSTM) images are made by scanning an optically conducting probe tip in an evanescent field above a sample surface. These images contain a mixture of topographical information, scattering effects, and variations in optical properties across the sample. Proper interpretation of PSTM images is therefore dependent upon knowledge of the field‐surface interactions giving rise to these features. The subject of this work is the calculation of PSTM image features arising from the topography of the sample surface. Image features of a model sinusoidal grating structure are calculated using small perturbation theory. Image amplitudes are shown to depend on the sample orientation and the shape and position of the probe tip. The calculated results are compared with actual PSTM images of a holographically produced sinusoidal grating surface.

ISSN:0021-8979    

DOI:10.1063/1.350650

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Elastic differential and total cross sections of electrons and positrons scattered by bound phosphorus, indium, and antimony atoms have been calculated using the method of partial waves. The interaction potential consists of an electrostatic potential, a Buckingham‐type polarization potential, and, in case of electron impact, the Mittleman–Watson exchange potential. The incident energies range from a few eV to 10 keV. A parameterization of the computed partial‐wave total cross sections in terms of the screened Rutherford total cross section is presented.

ISSN:0021-8979    

DOI:10.1063/1.350741

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have measured and report the total gas‐number‐density normalized effective ionization coefficient, &agr;¯/N, as a function of the density‐reduced electric field,E/Nfor Ar, CO2, CF4, the binary gas mixtures CF4:Ar (20:80), CO2:Ar(20:80), CO2:CH4(20:80) and for the ternary gas mixtures CO2:CF4:Ar (10:10:80) and H2O:CF4:Ar(2:18:80). We also report the limiting value, (E/N)limof the above gaseous systems. A comparison of our results on the unitary gases shows good agreement with previous measurements. The results on the ternary mixtures show that the addition of CO2or H2O to the binary gas mixture CF4:Ar lowers the electron energies and increases &agr;¯/N(E/N).

ISSN:0021-8979    

DOI:10.1063/1.350730

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A novel technique is proposed and experimentally demonstrated to achieve a quasiphase matching (QPM) condition for second‐harmonic generation (SHG) in a nonlinear optical waveguide with a periodically poled LiNbO3waveguide. The fundamental wave satisfying the QPM conditions is automatically generated when the laser diode (LD) and the optical waveguide are optically connected. Using the proton‐exchanged LiNbO3waveguide with a periodically poled grating, which was fabricated by Ti diffusion and the InP/InGaAsP LD with antireflection‐coated facets, it was confirmed that the LD oscillated at the wavelength satisfying the QPM condition, and SHG was observed simultaneously. Using a Ti:sapphire laser, it was also confirmed that the SHG was realized at the third order of QPM condition, as was expected.  

ISSN:0021-8979    

DOI:10.1063/1.350748

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The absorption and fluorescence spectra and fluorescence decay kinetics of Cd1−xMnxF2crystals withx=0.018, 0.063, 0.084, 0.26 were measured as a function of temperature under argon‐ion and nitrogen laser excitation. In addition, laser‐induced grating measurements were made as a function of write beam crossing angle for various temperatures between 15 and 50 K. It was found that a characteristic intrashell luminescence comes from two Mn subsystems, one composed of single Mn2+ions and the second of Mn ion aggregates. Based upon the four‐wave‐mixing experiment, a relatively strong energy migration was found in each of these subsystems, but with a weak, if any, communication between them. The diffusion coefficients for both subsystems increase as the temperature is raised, and above 50 K strong diffusion erases the light‐induced gratings beyond the possibility of detection. The long‐lived components of the four‐wave‐mixing transient signals are associated with the other point defects, either created or photoexcited by the strong laser beams.

ISSN:0021-8979    

DOI:10.1063/1.350703

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The optimum performance of a photorefractive two‐beam coupling fluence limiter is presented. A two‐level, single‐dopant‐species model is used to determine the minimum transmitted fluence for different crystal dopant densities and incident fluences. The effects on limiter performance for different beam parameters such as the modulation ratio and crossing angle and for different crystal parameters such as the mobility, electro‐optic coefficient, and dielectric constant are determined.

ISSN:0021-8979    

DOI:10.1063/1.351382

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Generation of the second Stokes order with a photon efficiency of 68% has been obtained from a single‐pass hydrogen Raman oscillator. The third‐order Stokes generated by four‐wave mixing is effectively suppressed by phase mismatching. The 1.6 times diffraction‐limited pump‐beam quality used in the present experiment was found to be essential to the high‐efficiency second‐Stokes‐order generation.

ISSN:0021-8979    

DOI:10.1063/1.350678

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Optical waveguides have been formed by helium‐ion implantation in KTaO3. The implantation forms a confinement barrier near the projected range of the ion by decreasing the refractive index as much as 16% for ion doses of 4×1016ions/cm2. This is the highest refractive‐index change yet reported for ion‐implanted crystalline planar waveguides. Guiding modes (with moderately low loss) are produced without the need for annealing out of color centers. During annealing studies, the index change reduces during an anneal stage near 400 °C, but waveguiding is maintained even after anneals to 900 °C. Loss measurements indicate a planar‐waveguide loss of <1 dB/cm after a 400 °C anneal.

ISSN:0021-8979    

DOI:10.1063/1.350684

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Photothermal deflection (PTD) techniques have been used to monitor various laser‐heating processes, including melt, vaporization, and ablation of solids. To interpret the complex signal response resulting from transient phase changes at a surface, the temporal profile of the PTD signal response must be considered. In doing so, the case of the linear heating of a target without phase change is first studied here. Numerical and experimental work is presented to show the effect on the shape, magnitude, and phase of a PTD signal due to changes in (1) the thermophysical properties of the target material and deflecting medium, (2) the dimensions and boundary conditions of the target, (3) the distance of the probe beam from the surface of the target, and (4) the modulation frequency of the heating source. Copper and lead target materials heated in air are used in the experimental work. The PTD signals show qualitative agreement with the temperature gradient normal to the surface calculated using a numerical finite‐difference two‐dimensional thermal‐diffusion model. The results also show that an unusual phenomenon occurs when heating with a laser or other finite‐sized heating source. When the thermal diffusivity of the target and deflecting medium are different and the probe beam is close to the surface, a local maximum is observed in the time‐response profile of the PTD signal during the heating cycle. The maximum occurs as a result of asymmetric changes in the temperature field over time. The shape of the PTD signal, therefore, can provide information about the laser‐heating process at a surface in real time.

ISSN:0021-8979    

DOI:10.1063/1.350645

出版商:AIP    

年代:1992

数据来源: AIP