Among the various material growth techniques, physical vapor deposition of thin films encompasses many experimental forms. They can be categorized into equilibrium and none-quilibrium processes according to the ways in which the source materials are vaporized. Equilibrium process is typified by thermal evaporation including the conventional resistive heating, the more elaborate e-beam evaporation. as well as molecular beam epitaxy (MBE). Nonequilibrium processes include sputtering and many of its derivative forms such as ion beam deposition. In comparison with these techniques, thin-film deposition by laser evaporation is less well known and does not fall cleanly into either category. Depending on the laserconditions, the evaporation process can be either thermal, nonthermal, or a mixture of the two.

Growth of thin films by laser-induced evaporation

The principal aspects of rugate filter theory are reviewed and expanded to show how the Fouriertransform technique can be used to design rugate filters that fulfill many optical coating functions.

Rugate filter theory: an overview

We report an experimental study of porous silicon-based rugate filters. We performed filter apodization, following a half-apodization approach, which successfully attenuated the sidelobes at both sides of the photonic stop band. We achieved successful reduction of interference ripples through the insertion of index-matching layers on the first and last interfaces. An apodized dielectric mirror and a rugate filter are compared: Appreciable differences in the harmonic presence and stop-band performance were observed and are commented on. Bandwidth control when index contrast is modified is also demonstrated. Finally, the possibility of combining different rugate filter designs to attain more complex responses is demonstrated by the achievement of a multi-stop-band filter. Numerical calculations for design optimization and comparison with experimental data are reported too.

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Porous silicon-based rugate filters

We demonstrate the use of aqueous-based layer-by-layer (LbL) processing and in situ nanoreactor chemistry for creating polyelectrolyte multilayer (PEM) nanocomposites in which the concentration of a nanoparticle species is modulated throughout the depth of the film in a complex way. Using the in situ growth of silver nanoparticles to selectively increase the refractive index of precisely defined regions of the PEM film, we have produced a digitized rugate filter, a special type of dielectric mirror possessing many thin layers that approximate a continuous, periodic refractive index profile. The design of the digital rugate from an initial smooth index profile was aided by a computer, and construction of the device was accomplished entirely by automated means. Poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) bilayer regions within the film supported subsequent nanoreactor chemistry and created the high refractive index regions of the film, while PAH and poly(styrenesulfonate) (PSS) bilayer regions remained unmodified and served as the low index regions. Spectroscopic results obtained from the assembled device confirm the presence of a reflection band that grows in amplitude with increasing silver incorporation, reaching a peak reflectance of 75% after five silver loading and exchange cycles. The rugate structure suppresses reflection sidebands, features common to Bragg reflectors and simpler dielectric mirror designs. Cross-sectional TEM reveals nanoscale control of the size and location of silver-containing domains within the PEM film. The ability of this method to control the placement of nanoparticles within PEMs enables the inexpensive, easy production of conformable optical coatings with tunable properties. In addition, these nanocomposite films may have applications in a number of other fields where precise control of effective gradients in physical properties throughout thin films has been shown to affect the overall performance in a desirable way.

Creating effective refractive index gradients within polyelectrolyte multilayer films: molecularly assembled rugate filters.

Laser-assisted evaporation is an emerging novel thin film deposition technique. It has been successfully applied to a variety of dielectric and semiconductor materials. Characteristics of the evaporation process and of the evaporants, resulting from the interaction of high power radiation with matter, often result in better structural and chemical properties of the films than can be obtained by conventional evaporative techniques. Congruent evaporation, the presence of energetic vapor species, and precise rate control are important advantageous features of this technique. The physics of laser-induced evaporation and plasma formation, some engineering aspects, and properties of dielectric and semiconductor thin films deposited by this technique are discussed in this review.

Formation of Dielectric and Semiconductor Thin Films by Laser-Assisted Evaporation

Rugate filters are optical thin film interference structures with sinusoidal refractive index profiles. Two-wavelength reflection filters have been fabricated by codeposition of SiO2 and TiO2. 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.

Codeposition of continuous composition rugate filters.

A multicolor focal plane array, which detects and distinguishes between incoming electromagnetic radiation within first and second bands of wavelengths, includes a substrate and a two dimensional array of detectors disposed on the substrate and responsive to electromagnetic radiation within a predetermined range of wavelengths including the first and second wavelength bands. A first multilayer thin film filter is disposed on the substrate and interposed between the incoming radiation and a first subset of the detectors in the array, while a second multilayer thin film filter is disposed on the substrate and interposed between the incoming radiation and a second subset of the detectors in the array. The first and second filters include a dielectric/thin metal/dielectric layer combination, the filters differing only in that the thickness of the dielectric layers in the first filter combination are adjusted to select the first wavelength band, while the thicknesses of the dielectric layers in the second filter combination are adjusted to select the second wavelength band. In this manner, the detectors in the first subset are made sensitive to radiation within the first wavelength band, while the detectors in the second subset are sensitive to radiation within the second wavelength band.

Multicolor focal plane arrays

A gradient index filter can be fabricated with integral antireflection properties without degrading the inband performance of the filter. The filter is for placement between an incident medium and a substrate to reflect incident electromagnetic energy having wavelengths within a predetermined band of wavelengths and to transmit incident electromagnetic energy having wavelengths outside of the reflection band. The filter includes an antireflecting portion having a nominal refractive index profile selected to reflect a minimum amount of electromagnetic energy from a theoretical interface between a first medium having a first predetermined refractive index and a second medium having a second predetermined refractive index. A band rejecting portion has a periodic refractive index profile whose periodicity and amplitude are selected to reflect a maximum amount of electromagnetic energy within the relfection band. At least part of the nominal refractive index profile is modulated by the superposition thereon of at least part of the periodic refractive index profile. The entire periodic refractive index profile maybe superimposed on the monial refractive index profile, while the periodic refractive index profile may be a rugate profile. In addition, the nominal incident and substrate refractive index profiles may be quintic refractive index profiles.

Spectral filter with integral antireflection coating

Electric fields in grazing incidence laser mirror coatings.

The properties of metal fluoride films (MgF2 CeF3, LaF3 CaF2 and SrF2) deposited using cw laser and electron-beam evaporation were compared. The differences in film properties were subtle, but important. Films obtained by laser evaporation have lower losses in the UV (185–250 nm) and slightly better crystallinity than the films deposited by e-beam evaporation. Other optical and mechanical properties were independent of preparation method. Differences in the film properties were tentatively attributed to the higher proportion of dissociated species in the evaporant plume of the laser-evaporated materials.

Frank J. Woodberry

Papers

Codeposition of continuous composition rugate filters.

Multicolor focal plane arrays

Spectral filter with integral antireflection coating

Electric fields in grazing incidence laser mirror coatings.

Properties of Metal Fluoride Films Deposited by Laser and E-Beam Evaporation