Andrew Skumanich

Optical absorption spectra of carbon 60 thin films from 0.4 to 6.2 eV

Abstract The optical absorption spectrum for thin films of sublimed C 60 is presented. Both transmission spectroscopy and photothermal deflection spectroscopy are used to obtain the spectrum over a wide energy range (0.4 to 6.2 eV) for the same film. The optical gap is ≈ 1.6 eV and the corresponding transition appears to be optically forbidden given the weakness of the absorption. Structure is observed on the “gap edge” which can be attributed to vibronic transitions. The gap region can be described in terms used for amorphous semiconductors, having features such as an Urbach edge, and sub-gap defect (“dangling bond”) absorption. The similarity suggests that the nature of C 60 thin films may be analogous to amorphous silicon and carbon films.

Absorption and scattering in nonlinear optical polymeric systems

Fundamental absorption measurements were made on organic films which are candidates for nonlinear optical materials applications, in order to gain insight into the loss mechanisms. Photothermal deflection spectroscopy was used to obtain the full absorption spectrum and to explain observed variations in waveguide attenuation. The absorption data provide a complete characterization of the optical spectrum and allow us to select optimal polymer‐chromophore systems. The variations in the attenuation at 832 nm are due to the chromophore absorption, while for wavelengths longer than 1 μm, the C—H vibrational overtones and combination bands of the polymer dominate the spectrum and are likely to be the main source of waveguide attenuation.

A contactless method for investigating the thermal properties of thin films

The photothermal deflection technique has been extended as a contactless method to investigate thermal transport in thin films. A theoretical model is developed which quantitatively describes the transport behavior, and is shown to be in excellent agreement with experimental results. This approach yields the thermal diffusivity directly and in a spatially-resolved manner.

Light‐induced defects in hydrogenated amorphous silicon alloys

The effect of light illumination on gap state absorption of hydrogenated amorphous silicon (a‐Si:H) alloys has been investigated using photothermal deflection spectroscopy. The alloys studied were the large gap materials a‐SiC:H and a‐SiO:H and the narrow gap a‐SiGe:H and a‐Ge:H. The results indicate a direct relationship between the gap energy and defect formation. As the gap opens, the number of metastable defects increases; whereas for the narrow gap materials, significantly fewer defects are observed. This behavior is consistent with the interpretation of defect formation by electron‐hole recombination.

Photothermal deflection spectroscopy : a sensitive absorption technique for organic thin films

Abstract An important property of an organic film is its absorption spectrum. However, because of the short optical path length in a thin film, only very strong electronic transitions can be measured by standard spectrometric techniques. With the sensitive photothermal deflection approach it is possible to measure the absorption from transitions which can be as much as four orders of magnitude weaker than the main electronic transitions. We have applied photothermal deflection spectroscopy to a variety of organic thin films with the thickness ranging from sub-micron to hundreds of microns. Several small molecule and polymer binding matrix systems have been studied. Some results are presented and compared with standard UV-Vis spectra.

The vibrational overtone spectrum of a thin polymer film

Abstract Using photothermal deflection spectroscopy (PDS), vibrational overtone spectra have been obtained in organic thin films for the first time. Overtone and combination resonances for several orders above the fundamental are observed. The frequencies, relative intensities, and linewidths can be precisely determined. Assignments to specific CH bond resonances are made using the theory of anharmonic vibrations based on a Morse potential for local modes. Details of the molecular potential can be studied, as can be transition from normal to local made behavior.

Substrate birefringence in coated optical storage disks.

The in-plane and vertical birefringences of polycarbonate optical disk substrates are important parameters that can adversely affect the system performance. However, these parameters are difficult to measure in coated disks with current techniques. An approach based on a variable-angle spectroscopic ellipsometer allows measurement of the birefringence for coated disks. In particular, it is possible to obtain the vertical birefringence, which is of increasing importance, in a relatively simple manner. Comparison between coated and uncoated substrates shows that the birefringence can decrease after coating with magneto-optical layers.

Nonlinear optical polymeric waveguide electro-optic phase modulator

Three nonlinear optical polymeric systems based on nonlinear optical chromophores attached as side chains to poly(methyl methacrylate) were synthesized and their nonlinear optical properties measured in thin film form by second harmonic generation and by the electro-optic effect. Also, attenuation of light propagation in optical waveguides and photothermal deflection infrared spectra were measured on these films to aid in selecting the best system for device development. Based on these results, a phase modulator was constructed and with sub- carrier multiplexing the signals from five TV channels were placed on an infrared laser beam as sidebands. After a short transmission through space, these signals were then heterodyne detected and displayed on a TV monitor with good signal to noise and with no crosstalk.

Observation of light‐induced defect formation in hydrogenated amorphous silicon by subgap illumination

The effect of subgap illumination on the formation of light‐induced defects in hydrogenated amorphous silicon was investigated using photothermal deflection spectroscopy. As with broadband illumination, an enhancement in the subgap absorption is observed and is related to the silicon dangling bonds. The magnitude of the enhancement varies with the photon energy and with the amount of doping. The results are consistent with the mechanism of recombination of photoexcited electrons with trapped holes producing metastable defects.

Highly sensitive absorption measurements in organic thin films and optical media

An important signature of material properties is the absorption spectrum. However, for weak absorption in thin films or for subtle spectral features, standard measurement techniques are inadequate. Ideally suited for this regime is photothermal deflection spectroscopy (PDS) which can give several orders of magnitude greater sensitivity than transmission techniques. With the sensitivity of PDS, it is possible to detect features that cannot be observed by other means, such as singlet-triplet transitions, weak charge-transfer bands, and weak photo-chemical changes at the surface. The films studied were those commonly used as charge transport materials in organic photoconductors, and are comprised of a small organic molecule and a polymer binder. The PDS spectra indicate the formation of charge transfer complexes between the two components, in the solid state. The strength depends on the various combinations and can be correlated to relative energy level offsets between the HOMO of the small molecule and the LUMO of the polymer binder. Reversible photochemistry resulting from UV exposure can also be detected at the surface. In addition, using PDS, it has been possible to obtain, for the first time, the vibrational overtone spectra in the range from the near IR to the visible (0.4 - 2 eV) for polymeric thin films. The absorption spectra of polycarbonate, for instance, exhibits C-H stretch modes for (Delta) n > 1 where n is the vibrational quantum level. Previous measurements were constrained to either long fibers, which pose the problem of scattering, or to a restricted wavelength region in the visible accessible by dye laser sources. The unparalleled sensitivity of PDS allows precise determination of frequency, lineshape, and intensity of the various modes, even for films just a few microns thick over a very broad energy range. The overtone spectra can be used as a probe of various basic molecular properties such as bond energy, anharmonicity, and vibrational energy localization. In a manner similar to NMR spectroscopy, it is possible to study specific atomic bonds.