Fabrication of high-quality VO2 thin films by ion-assisted dual ac magnetron sputtering.

The technique of cathodic ac dual magnetron sputtering along with the high energy ionic bombardment is known to yield high-quality thin films in terms of their uniformity and high density. This technique has been applied for the first time to achieve thermochromic VO2 thin films that show a high optical and electrical contrast between normal and switched states. In this two-step process Vanadium metal films were deposited and subsequently oxidized in optimum conditions to achieve stoichiometric VO2 films. Typical films switched between more than 40 % to less than 5 % transmission in the infrared region while undergoing an electrical sheet resistance change between 1 × 10(5) and 1 × 10(2) Ω/cm(2). The application potential of such VO2 films in integrated optics is deemed high.

Mid-IR laser beam quality measurement through vanadium dioxide optical switching.

We present a beam characterization system for infrared lasers which can measure both wavefront and beam profile with visible detectors. While previous studies demonstrated the conversion from the visible to the near infrared, this device exploits the wavelength conversion from the infrared to the visible, which is based on the refractive index change because of the optical switching of a vanadium dioxide layer. This technique can be applied over a broad spectral range from the visible to the infrared and potentially to the terahertz.

Mid-IR to near-IR image conversion by thermally induced optical switching in vanadium dioxide.

We demonstrate the image conversion from mid-IR to near-IR (NIR) exploiting high-contrast optical switching in vanadium oxide thin-film layers. The intensity distribution of a mid-IR beam is converted to NIR wavelengths exploiting the strong reflectivity changes induced by optical pumping in the mid-IR. We show an experimental setup in which the radiation of a Tm-doped fiber laser at 1940 microm and a carbon dioxide at 10.6 microm has been converted to both 850 nm and 1064 nm. The resolution was 35 microm and was reached by using an inexpensive CCD camera. The sensitivity of the device increases linearly with sample temperature. We measured a threshold of 144 mW/cm(2), with a sample temperature of 62 degrees C.

Design of a physical vapor transport cell for time controlled deposition of nucleation phase organic thin films.

A portable high vacuum chamber has been designed to implement a solenoid operated shutter used as a substrate cache during short duration deposition of organic thin films via the physical vapor transport (PVT) method. This PVT cell was designed for the study of gravity effects on nucleation phase organic thin films obtained in laboratory unit g conditions and especially low g conditions found onboard parabolic flights. The design challenges met were, notably, the timely control of deposition on the substrate during parabolas and maintenance of the experimental cell pressure during operation of the shutter. Nucleation phase thin films of the organic hole transporting semiconductor N,N' -bis(3-methylphenyl)-N,N' -bis(phenyl)benzidine (TPD), obtained with the use of the PVT cells, show that the moving shutter has an effect on the convective PVT gas flow; however, as convection is reduced, this effect is observed to be equally reduced.

Transmission and reflection ellipsometry of thin films and multilayer systems.

Ellipsometric studies are generally carried out in the reflection mode rather than in the transmission mode, requiring invariably opaque substrates or substrates in which the backreflection is minimized or suppressed by different methods. In the present research we used a transmission and reflection photoellipsometry method to study electrochromic materials and their multilayer systems deposited on thick substrates. The role of the substrate is examined carefully, and the contributions from multiple reflections in the substrate are taken into account in the theoretical treatment. This procedure not only allows the study of thin films deposited on quasi-transparent substrates, but when carried out in conjunction with reflection measurements it greatly improves the accuracy in the determination of the optical constants. Optical measurements are carried out on an automatic reflection transmission spectroscopic ellipsometer. Solid-state ionic materials used in electrochromic systems such as indium tin oxide, tungsten oxide, and their multilayer structures deposited on glass substrates are used as examples. A software based on the above theory, optikan, was developed to model and analyze such systems. It is demonstrated that the photoellipsometry method proposed is especially suited to analyzing electrochromic materials and transmitting devices in a nondestructive way.

Reflection-transmission photoellipsometry: theory and experiments.

We propose a method that uses reflection and transmission photoellipsometry to analyze samples consisting of thin films combined with semitransparent thick layers or substrates in the form of multilayer structures. Athick film or substrate is defined as a layer for which no interference effects can be observed for a given wavelength resolution, and contributions from multiple reflections in the substrate are taken into account in the theoretical treatment. An automatic reflection-transmission spectroscopic ellipsometer was built to test the theory, and satisfactory results have been obtained. Examples corresponding to a strongly absorbing film deposited on a glass substrate and a highly transmitting film also deposited on glass are shown. In both cases a good fit between theory and experiment is found. The photoellipsometric method presented is particularly suited to the analysis of actual samples of energy-efficient coatings for windows.