Angus Macleod
The ability of design software to precisely design and calculate the performance of complex assemblies has led to a burgeoning of designs and applications for thin-film optical coatings. The term "optical coating" is unfortunate, however, because it suggests a largely supporting role for thin films in optics. Whatever the perception, the reality is that optical coatings are currently playing all kinds of roles, from completely dominant to purely supporting. Frequently, optical systems are built around the coatings rather than the opposite.
Current state-of-the-art advances in optical coatings would be impossible without the ability to design and calulate the performance of the assemblies of thin films involved. The most successful calculation techniques for optical coatings are still those published by Abelès more than 40 years ago. However, in spite of the beautiful symmetrical simplicity of the equations, the manual evaluation of the performance of a multilayer of only modest complexity is impossibly daunting. The computer is the solution invariably used.
In dense wavelength division multiplexing (DWDM), for example, the channels are separated in terms of carrier wavelength at intervals just wide enough to avoid overlapping sidebands and to permit adding and extraction, or dropping, of individual channels from the mainstream. Standard separations are 50, 100, or 200 GHz corresponding to 0.4, 0.8, or 1.6 nm, respectively.
Deposition rate fluctuations and any residual fractional rotation of the substrates during deposition of each layer in DWDM beamsplitters are sources of random thickness errors. Errors are minimized by very high substrate rotation speeds, of the order of 1000 rpm with no perceptible wobble or speed variation. Deposition is only the beginning, however. The filters must finally be diced into squares, usually 1.4 mm in size and individually tested.
Thin-film narrowband filters that transmit one designated channel and reflect all others are commonly used as beamsplitters for dropping or adding. Insertion loss of -0.5 dB translates into reflectance or transmittance of 89% while crosstalk of -20 dB implies residual reflectance or transmittance of 1%. Such requirements imply multiple-cavity designs of three or more cavities and perhaps 100 or more layers. Coherent cross-talk, or interference between the added and dropped channels, is a serious problem. A value of less than -20 dB implies a residual reflection loss of less than 1% in the pass band of the filter. Since this is still difficult to achieve consistently in production, current filters are not normally used to drop and add channels simultaneously.
Multiple-cavity filter design is a mature subject and problems are almost entirely related to manufacture. It is easy to show that random thickness errors should not be greater than 0.002%, an impossible figure. Fortunately, optical monitoring of quarterwave layers directly on the filter at its center wavelength contains a natural error compensation that makes production possible (see photo).
Display applications
A new reflective display device, Digital Paper (Iridigm Display Corporation; San Francisco, CA), uses a combination of MEMS technology and interferometric modulators, or IMods. Most MEMS devices simply redirect the light, a problem for visual displays. The IMod tunes an interference condition by varying the thickness of a gap in a coating so that only the light with the desired characteristic is reflected and the rest absorbed. Part of the coating is deposited on a transparent support and part on a movable membrane that defines a gap. Both parts are sufficiently conducting so that an applied voltage can make the membrane collapse by electrostatic forces.
Each IMod thus has two stable states. Switching from black to white or to a color (usually an additive primary) are all possible. Viewing angles of ±45° with contrast ratios of 4:1 and switching speeds of around 100 KHz have already been demonstrated on working devices, but tests promise contrasts of better than 12:1 with viewing angles greater than ±60° and speeds greater than 1 MHz. Inherent electromechanical hysteresis permits the enormous advantage of a passive matrix addressing scheme. This and the use of ambient reflected light makes it ideal for displays in portable electronic devices of all kinds.
Antireflection was the earliest application of optical interference coatings and yet there are still exciting things happening in the topic. Glare from the surface of a display device such as a cathode ray tube is reduced if an absorber is placed over it. The signal light travels through the absorber only once and its brightness can easily be returned to its original level. But the glare must travel twice through the absorber and is therefore reduced. Added antireflection coatings avoid extra glare from the absorber itself. It is simpler if the glass of the display is absorbing because then only one broad-band antireflection coating is required, or if the antireflection coating itself is absorbing.
Recently T. Oyama and Y. Katayama of Asahi Glass Company (Tokyo, Japan) introduced a very simple two-layer antireflection coating that presents very low reflectance over the whole of the visible region, absorbs light sufficiently to reduce glare and conducts sufficiently to afford an additional degree of electromagnetic screening. A quite thin layer next to the glass is absorbing with an extinction coefficient increasing suitably with wavelength. As with any thin film, the absorbing layer transforms the optical admittance of the substrate surface but the changing extinction coefficient ensures that over the visible region the transformed optical admittance remains close to a value that can be antireflected by a layer of silica rather thinner than the normal quarterwave. One of the enormous advantages of this coating is the very low total thickness implying a short and therefore more economical production cycle.
Polarization applications
A feature of optical interference coatings is their dependence on polarization. And a new concept, giant birefringent optics (GBO; 3M Company; St. Paul, MN), has begun to address the well-known polarization splitting where p-properties become progressively weaker with increasing incidence. A GBO stack is a thin film interference assembly where individual layers are polymeric and some exhibit massive birefringence, typical indexes being as far apart as 1.50 and 1.63. A particularly useful arrangement has alternate films uniaxial with index for electric field normal to the interface larger than for electric field parallel to the interface. This boosts p-polarized reflectance at the interfaces and the p-characteristic of the coating is no longer necessarily the weak one. These thin films are interference devices just as conventional optical coatings and should not be confused with decorative multilayer foils with essentially random thicknesses that have been available for some time.
ANGUS MACLEOD, is president and CEO of Thin Film Center Inc., 2745 E Via Rotonda, Tucson, AZ 85716-5227; e-mail: [email protected].