Nano-electro-optics are devices that combine nano-optical devices — optical devices that pattern materials with nano-scale structures that create unique optical interactions — with a range of actuation technologies, including MEMS and liquid crystal, to create optical signal conditioning, selection and switching components. Among the examples that have been demonstrated are variable optical attenuators, polarization rotators, polarimeters, and tunable wavelength filters. In this note, we provide a short overview of the base technologies for nano-electro-optic devices, the general design platforms used, and examples of their application to specific optical components.

Nano-optic devices take advantage of the unique physical properties realized when light interacts with structures have critical physical dimensions smaller than the wavelength of the light that passes through it. For near IR, visible, and UV light, the required features are a few hundred or tens of nano-meters in size, sometimes with tolerances in the single nano-meter range — thus the term nano-optics.

While the optical properties of nano-scale grating structures have been studied for several decades, viable commercial manufacturing methods have only recently been introduced. Manufacturing methods include molecular self assembly, holographic lithography, e-beam lithography, and nano-imprint methods. Molecular self assembly has been used to produce large surface area polarizers, but can be limited by the usable materials available and the optical efficiency that can be achieved. Holographic lithography can be used to create fine scale grating structures, but requires a high degree of control over the laser output and vibration during exposure to ensure repeatability. Electron beam lithography allows the writing of complex patterns on a fine scale, but must deal with scattering issues that affect precision as well as the length of time required to write areas of commercially significant size, make this approach best for low volume, small area, and complex nano-scale patterns.

Nano-imprint methods are the most flexible manufacturing approach in terms of combined breadth of patterns support, pattern complexity, and integration with other manufacturing methods. These methods introduce an intermediate step in a direct lithography process, which is patterning a master plate or mold as a preliminary stage, and then using that master to imprint wafers — or more accurately, to imprint an etching mask in subsequent runs. Overall, this approach has the advantages of reducing run to run variability in creating the etching mask, increasing the speed with which the etching mask can be applied for complex patterns, and providing a ready archiving of nano-structure patterns for future reuse. The most significant value for manufacturing economics is that a range methods, such masking, step-and-repeat, and post-processing can be used to create complexly patterned masters, including pixilated optics, essentially for a one time set up cost. This cost is amortized across ensuing manufacturing runs, allowing costs to be minimized in high volume manufacturing. Even more intriguingly, because the same manufacturing line can be used to process resulting from applying different master plates, there is an economy of scale, for the first time, across multiple basic optical devices.

Nano-optic functionality and manufacturing methods readily support hybrid integration — the combination of nano-optic structures with dynamic optical materials within an end-to-end integrated manufacturing processes. Nano-optic structures can be patterned into a material prior to, as an intermediate step, or following other wafer-based manufacturing processes. For instance, a nano-optic structure can be applied to a glass or silicon wafer that will serve as the input for other manufacturing operations. Because nano-optic structures can be built from a broad range of materials, material properties can usually be chosen to be compatible with the handling and environmental conditions required in ensuing manufacturing steps.

Physically, the interaction of light with an electro-optical device will either modify a particular optical parameter (e.g., an electronic variable optical attenuator changes the output power of the optical signal) or selects a subset of the optical signal based on a particular optical parameter (e.g., a tunable filter selects a pass band of wavelengths from the input optical signal). There are two types of nano-electro-optical devices that address the modification and selection functions: Type 1, where a nano-optic device can be combined with an actuation technology to reduce the form factor, improve optical performance, or reduce manufacturing costs; or Type 2, where a nano-optic structure is optically coupled with the actuation technology so that applying an electrical field to the actuator changes the optical properties of the nano-optic device itself. The optical parameters that can be affected are polarization, phase, wavelength, direction and focus. These generic approaches are best understood through illustrative examples.

As a first example, a MEMS actuator can be combined with a nano-optic wavelength filter to create a tunable filter. In this Type 1/selective, nano-electro-optic device, a redirectable MEMS surface, similar to a MEMS mirror but optically transparent, is patterned with a nano-optic resonant grating filter. Because nano-structure regions can be masked in the manufacturing process, this can be done locally on the MEMS component of interest. By changing the angle of incidence of light on the filter surface, the center wavelength of the reflected pass band is continuously changed. The nano-optic filter allows the remaining optical signal to transmit through this signal can either be used or discarded.

In general, MEMS structures can be used to modify how a nano-optic device is positioned relative to the optical beam path, or the physical relationship between nano-optic structures to create a range of optical functions, including fine scale phase adjustment and polarization selection. In manufacturing, these nano-structures are integrated as a layer into the complete MEMS construct. Both Type 1 and Type 2 nano-electro-optic structures can be created with MEMS actuators.

As a second illustration, a liquid crystal actuator can be combined with a pair of nano-optic polarizers to create a variable optical attenuator (VOA) for use in transceivers. Such a device is constructed by patterning a nano-optic polarizer layer on the glass substrates that are used to sandwich the liquid crystal layer. Functionally, the first nano-optic polarizer selects the input polarization and is aligned with the output polarization of the light source; the liquid crystal layer is used to rotate the polarization of the light to a greater or lesser extent depending on the strength of electrical field applied; and the second nano-optic polarizer blocks light which does not match its polarization from being transmitted. Using the liquid crystal layer to rotate polarizations so that the two nano-optic polarizers are effectively parallel provides maximum output power; rotating polarization so that the polarizers are effectively crossed provides minimum output power. Variation of the electric field across the liquid crystal allows any power level in between to be realized. This compact VOA design allows for easy integration into transceiver packages, and the minimal width allows for improved optical efficiency.

The structure described for the VOA using one or more nano-optic layers in the construction of a liquid crystal cell or array along with the choice of whether to place the nano-optic layer on the exterior of the cell or immediately adjacent to (or even in) the liquid crystal layer provides a general architecture for this integration. While the VOA described above is a Type 1/modifying, nano-electro-optic device, other designs that place the nano-optic layer adjacent to liquid crystal layer can result in Type 2 devices. Additional functions that can be achieved with this architecture are optical switching, tunable filters, polarization selection, and phase adjustment. These functions can be used in optical transmissions systems, display systems, optical sensors, and optical feedback devices.

Hubert Kostal is vice president of marketing and sales at NanoOpto Corp. (Somerset, NJ).