Randolph L. Hall

Codeposition of continuous composition rugate filters.

Rugate filters are optical thin film interference structures with sinusoidal refractive index profiles. Two-wavelength reflection filters have been fabricated by codeposition of SiO(2) and TiO(2). Composition modulation was monitored and controlled using quartz crystal rate controllers. The resulting filters exhibited two well-defined stopbands. Microscopic examination revealed that the structure is glasslike without pronounced thin film microstructure.

Rugate filter sidelobe suppression using quintic and rugated quintic matching layers

Sine wave rugate index profiles may be superimposed on a slowly varying average index in such a way as to reduce sidelobes over broad spectral regions and at the same time maintain the strength of the stopband reflectance.

Fabrication of refractive microlens arrays

Fabrication issues of microlens arrays, made by first forming photoresist microlenses, by patterning and reflowing photoresist islands under temperature, and then transferring this into the substrate by a dry etch process, were studied. Photoresist microlenses were reliably fabricated within a range of aspect ratios. The desired sag of the microlenses in the substrate was controllably achieved by adjusting the etch selectivity. Etching behavior of fused silica in mixtures of fluoroform with oxygen or sulfur hexafluoride was studied in detail. High quality microlens arrays were fabricated in fused silica, silicon and germanium, and selected lenses were characterized.

Thin-film deposition by laser-assisted evaporation

Laser-assisted evaporation of materials has several unique features and is being used increasingly as a thin-film deposition process. Characteristics of this technique and design parameters of a laser-assisted evaporation system are discussed. Types of source material, scanning and focusing of the laser beam, optical elements to deliver laser power to the sources, and emplacement of these elements to deposit thick films are explained.

In situ optical monitoring of OMVPE deposition of AlGaAs by laser reflectance

The feasibility of laser reflectometry for in situ monitoring and control of OMVPE AlGaAs was demonstrated. The optical constants, refractive index and extinction coefficient, of the AlGaAs alloys at the growth temperature were obtained for 633 nm wavelength. These data were used in closed loop control of thickness and composition during deposition of heterostructure layers. Laser reflectometry was also instrumental in the observation of unintentional composition gradients at the substrate-epilayer interfaces in atmospheric pressure vertical flow reactors.

Using wavelets to design gradient-index interference coatings

This paper proposes that wavelet construction of the gradient-index refractive index be used to design optical interference coatings. The basic wavelet used is a localized apodized rugate. By incorporating scaled and shifted copies of the basic wavelet, which are other elements of a basis set according to wavelet theory, a variety of interference coatings can be designed.

Properties of mixed composition Si/ZnSe and ZnSe/LaF 3 infrared optical thin films

The behavior of codeposited thin films of ZnSe/LaF(3) and Si/ZnSe has been investigated. Mixed ZnSe/LaF(3) films are polycrystalline with intermediate compositions with moderately high compressive stress (1500-2000 kg/cm(2)). Mixed Si/ZnSe films are amorphous with intermediate compositions with low compressive stress (<1000kg/cm(2)). Optical properties of the ZnSe/LaFcm(3) system vary monotonically with composition, but those of the Si/ZnSe system do not, which is explained by the possible chemical reaction of the two species.

Binary optics thin-film microlens array

Binary optics can produce microlenses and lens arrays with theoretical diffraction efficiency as high as 95% for eight-phase level devices. Due to shadowing, mask misalignment, and etching errors that accumulate during fabrication, the actual diffraction efficiency can be reduced to less than 70%. Advances in mask design and e-beam writing have reduced mask misalignment errors to less than 0.2 micrometers but the major issue is the accuracy of the RIE process that is used to transfer a lithographic pattern into the substrate. RIE has two limitations for binary optic applications. First, it cannot be readily employed for the wide range of possible optical substrates of interest (Al2O3 for example), and second, since the pattern is etched directly into the substrate, there is no simple means to calibrate the etch depth during the process. Thin film deposition of the binary structure addresses both of these limitations. It is applicable to a wide range of materials, and accurate in process monitoring of the deposit permits precise control of the feature height. In this paper, we report on eight-phase level binary optic microlenses processed by deposition of SiO2 on fused silica and Al2O3 on sapphire using a projection lithography system. Photoresist processing was achieved by image reversal and lift-off technique. The microlens arrays (in a square format) were designed for (lambda) equals 0.632 micrometers with two microlens sizes of 120 micrometers X 120 micrometers and 240 micrometers X 240 micrometers having speeds of F/12 and F/6 (at the corners), respectively. Optical characterization has demonstrated that the microlens arrays are near diffraction limited and diffraction efficiency is in excess of 80%.

Narrow-Bandpass Filter Using Partitioned Cavities

Numerical optimization studies have led to a new multilayer design for a narrow bandpass filter. Its unique feature is the absence of traditional spacer layers. Electric field intensity profiles indicate that it is similar to a double cavity but the spacer layers are defined by pairs of low- and high-index layers forming partitioned cavities. Other partitionings have been studied. It is shown that the partitioning configuration can control somewhat the electric field intensity profile, which should lead to higher transmission. Another advantage of partitioned cavity filters is that they are less sensitive to random thickness errors in fabrication and are amenable to an error compensation scheme. Some filters were fabricated using ellipsometric monitoring with error compensation including one which achieved 83% peak transmission and about 1% bandwidth.

Ellipsometry As A Real-Time Deposition Monitor

The use of an ellipsometer as an in situ thin film deposition monitor is usually complicated by the calculations required to arrive at an estimate of the index and thickness values. In general, the ellipsometer parameters Φ and Δ are derived from the measured readings. The calculation for index and thickness from the (Φ and Δ values is by successive approximation and can require considerable time. Because of this, real-time control is not possible even for modest evaporation rates. The direct use of the Fourier coefficients of the signal from the ellipsometer analyzer can significantly increase the data acquisition rate. One quick way to analyze the data from the ellipsometer is to compare the numerical Fourier analysis of the ellipsometer signal with the expected values calculated prior to the deposition. Several commercial ellipsometers can provide such a signal for accurate layer thickness measurements, in near real-time, during the thin film coating process, thus, enabling computer control of multiple evaporation sources. Variations of the measured ellipsometer Fourier parameters can provide a great deal of information on the index, thickness, and inhomogeneity of the film during growth. A method for realizing the above is presented, along with examples of films grown using this technique.