Volume 86, Issue 1, 01 July 1999
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
Microstructural and nonlinear optical properties of silica–titania sol-gel film doped with PbS quantum dots86(1999); http://dx.doi.org/10.1063/1.370702View Description Hide Description
Thin silica–titania filmsdoped with different concentration of PbSquantum dots (PbS/oxide molar ratios ranging from 5% to 25%) were fabricated via a sol-gel route. The structuralproperties were studied by x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering spectrometry. The PbS crystals were found to have a mean diameter in the range 2.3–3.5 nm with narrow size distribution. The resonant nonlinear optical properties were studied by the nonlinear m-line technique and degenerate four-wave mixing. High negative nonlinear refractive indices were measured at 1.064 μm. Different values were obtained for nanosecond excitation and for picosecond excitation The differences can be explained by saturation effects. Measurements at 532 nm showed values ten times higher than at 1.064 μm. The response time of the nonlinearity for both wavelengths was below 35 ps.
86(1999); http://dx.doi.org/10.1063/1.370703View Description Hide Description
Thin films of tris-(8, hydroxyquinoline) aluminum and (TPD) were measured using synchrotron radiation-based core and valence level photoemission and core level photoabsorption to elucidate the element-specific electronic structure of organic electroluminescent materials. The energy level alignment of an /TPD interface is given for both occupied and unoccupied states. A comparison of freshly evaporated films of and TPD with films that have been exposed to intense radiation or oxidative conditions sheds light on possible damage mechanisms of the molecular solid.
86(1999); http://dx.doi.org/10.1063/1.370704View Description Hide Description
The fundamental optical properties of a nonlinear optical semiconductor are reported. These properties include birefringence, indices of refraction, infrared transmission, and the temperature dependence of the band gap. The average index for wavelengths greater than several microns was found to be 3.0. The birefringence was found to be rather large and to range from a near band edge value of 0.038 at 1.3 μm to a value of 0.017 at 15 μm. Additionally, native defect related sub-bandgap absorption, photoluminescence, and electrical transport have been studied in these nominally undoped p-type crystals. An activation energy associated with these defects was determined to be 0.3 eV and the corresponding photoluminescence and absorption data showed, respectively, a broad asymmetric emission band centered at 0.8 eV and two bands at 0.95 and 1.01 eV, the absorption band at 0.95 eV being the most intense. The measured properties were utilized to assess the potential of for the wavelength conversion processes of second-harmonic generation and degenerate optical parametric oscillation. It was found that will not phase match at room temperature. However, it is estimated that mixed crystals of the form for a Te addition of 19% can exceed the conversion efficiency of by more than 100%.
86(1999); http://dx.doi.org/10.1063/1.370705View Description Hide Description
The distance between sample and probe in a scanning near-field optical microscope is regulated via tracing the shear-force on the tip which is glued to a tuning fork piezo. A lock-in technique is used. We demonstrate that the bandwidth of the control loop is increased if not only amplitude or phase, but a favorable combination of both is used as feedback signal. The enhancement of bandwidth is connected with a reduction of signal-to-noise ratio. The optimum combination of both, bandwidth and signal-to-noise ratio, can be adjusted purely electronically to the specific needs of an experiment. A theoretical model is developed that discloses the relation between the mechanical and electrical properties of the combination of tuning fork and fiber tip. The frequency response of the shear-force detection system is calculated with a numerical simulation based on this model. Experimental frequency response curves are well fitted by these simulations. Our results are especially important for low-temperature scanning microscopy, where the bandwidth enhancement is essential for obtaining a reasonable scanning speed.
86(1999); http://dx.doi.org/10.1063/1.370706View Description Hide Description
We report on measurements of two-dimensional potential distribution with nanometer spatial resolution of operating light emitting diodes. By measuring the contact potential difference between an atomic force microscope tip and the cleaved surface of the light emitting diode, we were able to measure the device surface potential distribution. These measurements enable us to accurately locate the metallurgical junction of the light emitting device, and to measure the dependence of the built-in voltage on applied external bias. As the device is forward biased, the junction built-in voltage decreases up to flat band conditions, and then inverted. It is shown that the potential distribution across the pn junction is governed by self-absorption of the sub-bandgap diode emission.