PTB >> Creating High Performance, Integrated Optical Circuits

Nano-optics is a valuably novel class of technology that takes advantage of light’s unique interaction with subwavelength, nano-scale patterned materials, and nanotechnology- enabled fabrication methods to create a broadly applicable optical device and manufacturing platform.

In optical circuits used in consumer electronics, communications, and security applications, nano-optic devices offer significant benefits by virtue of their novel behavior, small size, ease of integration, and lower overall costs. Through these virtues, nano-optics delivers compelling alternatives to conventional bulk optical solutions, and offers functionality and form factors that conventional optics cannot match. The following is a basic review of nano-optics theory and applications.

Function & Structure
The unique optical properties of nanooptic devices derive from appropriately patterning material on the nano-scale with structures whose critical dimensions are sized to be several times less than the wavelength of light at which they operate, thereby creating optical materials with highly useful modified optical functions.

Through a proper combination of materials and structures, nano-optic devices can perform any passive optical function including polarization filtering, phase retardation, spectral filtering, and propagation management (e.g., lenses and beam splitters). Various functions can be designed for both free-space- and waveguide- based applications. The former is far and away the most common in commercial applications.

Nano-optic devices also can be designed to operate over any wavelength range. The basic methodology is applicable to UV, visible, and IR wavelengths with appropriate variations in structural dimensions and materials. Additionally, they can form the core of an optical system. Practical nano-optic applications require a complete optical system comprised of the nano-patterned material and adjacent materials including the optical substrate and thin-film coatings.

The nano-optic structures are defined as a combination of material and physical attributes, as illustrated in Figure 1. Attributes can be divided in to three types:

• Structural: pattern (e.g., linear, mesh, and circular), dimensions (e.g., period, thickness, and duty cycle), and spatial variations (e.g., “chirping”, arrays, and multiple layers)
• Material: nano-structure material (e.g., dielectrics, metals, and polymers), interstructure fill material, adjacent thin film materials, and substrate material (e.g., glass, plastics, dielectrics, and crystalline material)
• System: relationship of the nano-optic structures with the optical beam path and any actuator materials and structures (e.g., liquid crystal, solid state materials, electro-optic polymers, and MEMS)

Subwavelength structure size is a key to nano-optics’ optical and physical advantages. Nano-optic elements employ microstructures one or more orders of magnitude smaller than the wavelengths of the incident light — with dimensions typically on the order of 10s to a few 100s of nanometers. At such dimensions, these structures may interact with light both according to the principles of classical optics and also in a realm of comparatively small-scale local interactions; rigorous application of the boundary conditions of Maxwell’s equation describes the interactions of light with nano-optic structures. For very small structures, single electron or quantum effects may be observed. As a consequence, many nano-optic devices exhibit unique optical behaviors — such as high polarization discrimination in a micron-thin space, achromatic phase retardance, and single layer combinations of optical functions like polarization and spectral filtering — either by coincidence or intent. These nano-optics provide useful alternatives for optical circuit design; others can simply be designed as drop-in replacements for classical bulk optics.

In addition to unique behaviors, the devices also achieve their optical effects over relatively short distances — nano-optic devices are generally very thin, usually one micron or less in thickness. This has strong positive repercussions for integrated optics since nano-optic structures can often be applied, similar to thin films, as a “coating layer” during a complex manufacturing process. Single electron and quantum effects also enable optical performance to be readily customized. Modifying material properties by adding nano-structures results in a continuous spectrum of optical functionality — essentially this combination creates hybrid, “artificial” optical materials — that allows performance specifications and optical wavelengths to be incrementally and accurately varied to meet specific application requirements.

Manufacturing & Integration
Nano-optic structures can be realized through a variety of manufacturing methods. For commercial usefulness, an optical device is not optimally practical unless it can be manufactured in high volumes, in multiple application specific variations, and readily integrated with other relevant technologies. The first criterion is necessary to achieve economies of scale; the second, to ensure that the relevant market opportunity is broad enough to be interesting; and the third, to facilitate easy integration of nano-optic elements into more complex systems. The available fabrication approaches (self-assembly and nano-lithography) achieve these objectives to varying degrees.

Some materials can be caused to form regular, nano-scale structures under appropriate, controlled conditions — a.k.a. self-assembly. The difficulty with this approach is lack of flexibility in achievable structures and usable materials, which limits the functions that can be realized.

Nano-lithography, which is akin to standard semi-conductor manufacturing, generally uses some method to create an image in a polymer resist layer; this image is then used as an etching mask to transfer the nano-scale pattern into the target material. Nano-lithography methods include interference lithography, e-beam (electron beam) lithography, and nanopattern replication (see sidebar at bottom for definitions). Overall, each of these methods is optimal for some set of nanostructure patterns, materials, and volume requirements.

As a manufacturing method, nanolithography supports a range of integration modes for optical circuits:

• Optimization of traditional multi-element, multi-technology optical circuit architectures is possible by taking advantage of the unique optical behavior of nano-optics.
• Monolithic “self-integration” can be achieved by layering one atop the other to create aggregate optical effects.
• Spatial integration can be accomplished by organizing varying optical
functions into an array structure via nano-pattern replication.
• Hybrid integration is achieved by adding a nano-optic layer(s) to functional optical materials.
• Integration of nano-optics with optically active layers, such as liquid crystal or MEMS structures, can create electronically controllable optical devices.

Integrated optical devices and subassemblies based on nano-optic elements realize a range of benefits. Compared to conventional bulk optics, their size and optical properties make alignment in assembly more forgiving, less labor-intensive, and less costly. When physically combined with other nano-optics or other materials in manufacturing to create monolithic integrated optics, this eliminates the complication and cost of multidevice lamination, while improving reliability. Also, arrayed nano-optic devices can be applied in multi-beam or multi-path optical circuits, eliminating the need to individually align discrete optics.

In general, the process by which nanooptic elements are fabricated is flexible and robust, allowing for cost-saving manufacturing process integration in creating hybrid and monolithic optics; again, the resultant optics are often smaller, more robust, more functional, and easier to assemble. Because nano-optic devices are manufactured utilizing wafer-based processes proven in semi-conductor fabrication, this enables flexible sharing of manufacturing capacity and supports high-volume manufacturing throughput. Figure 2 shows a processed wafer and nano-optic devices.

From Concept to Application
Nano-optics draws on a growing “design vocabulary” that is functionally equivalent to the design libraries used in semiconductor applications. Multiple design elements can be used with complex, multi-step, and multi-process methods to create particularly sophisticated nano-patterns and multi-layer devices. The broad range of different nano-structure patterns, materials, and modes of integration means that there are no inherent limits in the functionality and operational wavelength of nanooptic devices. These attributes are solely driven by the needs of the application and the optical circuit.

To date, the commercialization of nano-optics has demonstrated the portability of the manufacturing platform and its products across multiple markets including optical data storage, digital imaging, projection display, and telecommunications. As nano-optic technology continues to propagate, it promises to further expand both in applications and in benefits realized by users of nano-opticbased devices.

Sidebar 1
Nano-Lithography Methods
Interference Lithography: Readily creates interference patterns with useful dimensions using UV light sources. The benefit of this method is simplicity; the difficulty is in creating complex shapes and arrays.

E-Beam Lithography: Writes arbitrary, complex patterns using a focused beam of electrons. The benefit is that almost any pattern can be created; a drawback for commercial production is that individual wafer processing times can be fairly long (tens to thousands of hours).

Nano-Pattern Replication: Relies on other methods — e.g., the two above — to create a master plate capturing the inverse of the desired pattern. A printing-like transfer method then patterns the polymer resist. The benefit is high-fidelity reproduction allowing extremely small feature size, complex patterns, effective material independence during repetitive production, and for different structures in the same production line. It is not applicable for limited runs or when only a fixed interference pattern is required.

Sidebar 2
Sample Applications
Optical Disk Drives, Communications, & Projection Displays: A nano-optic
polarization beam splitter/combiner (PBS/C) provides 180 degrees of effective
separation — meaning that one polarization is transmitted and the other is reflected — via a submicron-thick nano-structural layer. This device’s beam separation and thin form allow adjacent components to be closer coupled compared with conventional optics, reducing existing optical path lengths by up to one-half.
In turn, shorter optical path lengths reduce beam divergence, improving coupling efficiency and reducing power loss. Many optical circuit designs also incorporate a wave plate adjacent to the PBS/C.

Digital Imaging, Communications, & Sensors: Physically, a nano-optic enabled
variable optical attenuator (VOA) consists of a crossed pair of polarizing nano-structures sandwiching a liquid crystal cell; applying an electrical field across the liquid crystal will rotate the liquid crystal molecules and control the fraction of light that is passed through the structure. This tunable optic addresses the frequent need to control and block the output power of polarized light sources, such as optical transceivers, in order to dynamically optimize their signals and to facilitate maintenance. Because the nano-optic polarizer is directly integrated into the liquid crystal cell construction, this allows the entire component to be less than 1 mm thick.

This article was written by Hubert Kostal, vice president of Marketing and Sales, for NanoOpto Corporation, 1600 Cottontail Lane, Somerset, NJ 08873. For more information, call (732) 627-0808 or email hkostal@NanoOpto.com. Visit NanoOpto online at www.nanoopto.com.