Natalya Tokranova

Fluorescent polymer-porous silicon microcavity devices for explosive detection

Conjugated polymers entrapped in porous silicon microcavity have been studied as optical sensors for low volatility explosives such as trinitrotoluene. The fluorescence spectra of entrapped polymers were modulated by the microcavity via a spectral “hole” that matches the resonance peak of the microcavity reflectance. Exposure of the porous silicon microcavity containing entrapped polymer to explosives vapor results in a red shift of the resonance peak and the spectral hole, accompanied by the quenching of the fluorescence. This multiplexed response provides multiple monitoring parameters, enabling the development of an optical sensor array for the detection of target explosive vapor.

Hybrid solar cells based on porous Si and copper phthalocyanine derivatives

We demonstrate a solar cell based on n-type nanoporous Si (PSi) filled with copper phthalocyanine (CuPC) and its derivatives (including a discotic liquid crystal form). The CuPC device shows conversion efficiency up to 2% under white light illumination (20–30mW∕cm2), distinct from cells filled with CuPC derivatives with alkyl chains attached to the core. It is concluded that a critical issue for efficient photocarrier generation is the distance between the CuPC core and the PSi surface. Both organic and inorganic components contribute to photoinduced charge transfer and transport processes. The influence of the PSi structure and pore filling on the solar cell performance is discussed.

SU8-based Static Diffractive Optical Elements : Wafer-level Integration with VCSEL arrays

With the continued miniaturization and sophistication of current generations of semiconductor devices, it is the limitations of data transfer rates that are beginning to impact system performance. Although conventional pathways continue progressing, researchers are moving toward optical interconnects as a potential solution. Optical interconnection is a promising way to replace existing global or chip-to-chip interconnects in future integrated circuits. In contrast to existing metallic wiring, optical interconnects exhibit smaller distance-related loss or distortion of the signal, no deleterious fringing effects and no heat dissipation in the interconnect itself. Pioneering interconnect schemes are currently being developed using both planar waveguides and fibers to distribute optical signals around printed circuit boards. However, researchers are now attempting to incorporate novel, freespace optical interconnects, which will boost data transfer rates by a factor of a thousand. These systems consist of a number of components including vertical cavity surface emitting lasers (VCSELs), lenses, diffractive optical elements and detectors. Integration of single components into sub-systems will help to minimize the optical system footprint for both on-chip and chip-to-chip interconnects. This paper will present the development of both independent and integrated with VCSELs,static diffractive optical element (DOEs) made of SU8 and prove the feasibility of such an approach. SU8 is a negative tone photoresist, conventionally used for high aspect ratio MEMS-based structures. Recent developments in thin film SU8 along with its low absorption at long wavelengths makes it a suitable material for optical applications. By developing a low cost lithography based process, SU-8 DOEs can be efficiently integrated directly on laser sources with minimal effect to VCSEL performance. This approach could have a significant impact on the creation of next generation optical I/O fabrics.

New approaches for chip-to-chip interconnects: integrating porous silicon with MOEMS

We describe two types of active optical devices developed for use as free-space optical interconnects FSOIs for chip-to-chip communications. The design of both types of devices—membrane and freestanding structures—includes both optical and mechanical components. The optical component contains porous silicon PSi with customized optical properties fabricated by electrochemical etching of silicon. The mechanical part of the devices is composed of metal/nitride bimorph thermal actuators. The membrane devices form concave mirrors when actuated, and can be used to focus the incoming optical signals and correct any optical misalignment within the input/output I/O fabric. The freestanding devices have out-of-plane optical components, whose tilting angle is controlled by the current applied to the actuator. These devices can function as either reflectors or tunable optical filters. By incorporating the developed PSi diffractive optical element DOE into the freestanding structure, another type of freestanding device is realized for beamsplitting applications. Details of the fabrication, testing, and integration of these PSi-based devices are presented.

Hybrid solar cells based on organic material embedded into porous silicon

Solar cells based on organic and inorganic materials are an emerging technology for a new generation of photovoltaics (PV). Hybrid solar cells, which use both organic and inorganic components, have advantages such as cost-effective processing and the ability to fabricate devices on flexible substrates. The combination of organic materials with semiconductor nanostructures allows enhancement of the conversion efficiency due to the fast electron transport in semiconductors and a high interface area between organic and inorganic components. In our work, anodized porous Si (PSi) was chosen as a host matrix filled with Copper Phthalocyanine (CuPC) molecules. The resulting nanocomposite can yield high performance novel materials for solar cells. The fabrication of PSi was completed using electrochemical etching of Si in diluted hydrofluoric acid (HF). Also, this process, with some modifications, can be applied to produce free-standing PSi films of desired thickness. PSi layer was filled with CuPC dissolved in concentrated sulfuric acid. The top contact was made by sputtering of Au or ITO. A power conversion efficiency (PCE) of 3% (33 mW/cm2) was obtained for 12 um thick n-type pSi layer with pore sizes of approximately 15 nm filled with CuPC. The electrochemical etching of Si under different conditions was carried out to optimize the photovoltaic parameters. A detailed investigation of the solar cell performance depending on porous layer thicknesses and pore sizes is presented. The use of free-standing films of PSi can lead to the fabrication of novel PV solar cells on flexible substrates with high conversion efficiency.

Active silicon components for chip to chip interconnects

Among the major challenges confronting the current initiatives to incorporate optical interconnect capabilities for chip to chip I/O is to define, develop and implement the necessary components required for a complete pipeline from source to receiver. For next generation integrated circuits, the need for multifunctionality and multidimensional integration has resulted in new demands on interface technology to yield massively parallel data and clock lines. At this point, such methods are primarily limited to static reflectors, filters and gratings for interface and optical routing. One of the crucial elements is to develop a high performance and flexible optical network to transform an incoming optical pulse train into a widely distributed set of optical signals whose direction, alignment and power can be independently controlled. This coupling can be achieved using several methods including active (primarily, MEMS-based) beam steering arrays. For chip to chip applications, the overwhelming majority of the recent research and development effort has been focused on source and detector technologies, but less attention has been devoted to flexible, reconfigurable beam steering modalities. A variety of approaches for such beam steering and distribution of both timing and data lines has been examined. This paper will present an overview of active, silicon components under development at the College of Nanoscale Science and Engineering for arraybased I/O management with an emphasis on reconfigurable diffractive devices and adjustable, porous silicon- based components which combine optical beam steering, filtering and focusing capabilities. Design details along with initial performance data from prototype components will be presented.

Microfabricated devices for bio-applications

This paper focuses on the development of two MEMS-based devices for lab-on-a-chip bio-applications. The first device is designed to facilitate cell secretion studies by enabling parallel electrochemical detection with millisecond resolution. Initial prototypes of micro-arrays have been fabricated with Cr/Au microelectrodes on various substrates such as polyimide, SU-8 and SiO2. An FT cell-line (bullfrog fibroblast, American Tissue Culture Collection) has been successfully established and cultured directly on these prototype micro-arrays. It is well known that the FT cells can uptake hormones or other macromolecules from the culture media through a non-specific uptake mechanism which is still under investigation. After culturing on micro-arrays, FT cells were loaded with norepinephrine of various concentrations by incubation in the culture media supplied with norepinephrines. Rapid elevation of intracellular Ca2+ levels triggers the exocytosis of norepinephrine which then can be detected by the Cr/Au electrodes. Microfabrication of these prototype micro-arrays as well as cell culture and electrochemical detection results will be presented in this paper. The second device is designed for 3-dimensional transportation of living cells on chips. Initial prototypes of micro-arrays were fabricated with SU-8 buried channels on a silicon substrate. Both single-layered and double-layered SU-8 buried channels have been realized to enable 2D and 3D cell transportation. Stained solutions were used to visualize fluid transport through the channel networks. Following this, living FT cells in solution were successfully transported through single-layered SU-8 channels. Testing of 3D transportation of living FT cells is underway. Microfabrication of these prototype micro-arrays and living cell transportation on chips will also be presented in this paper.

A novel optical component for the development of an integrated interferometric system

Optical interferometry is a well established technique for high resolution displacement measurements. It is commonly used in the semiconductor industry as a sub-system of manufacturing and metrology tools. As the industry progresses, the tools continue to evolve, requiring the concomitant reduction of size and cost in sensors. Existing interferometric systems are bulky and therefore difficult to incorporate in equipment. Efforts are ongoing to miniaturize these systems but with optical components (beam splitters, detectors and lasers) still in the millimeter range, it is difficult to realize ultra compact systems. Thus, it is imperative to focus on development of micron scale components that would provide the necessary high spatial resolution in a compact format. The focus of this paper is on the development of a micron size optical component that combines multiple optical elements and can be integrated with VCSELs at the wafer level to yield a compact, low cost interferometric system. The design and development of this component containing the beam splitter and reference mirror will be presented including the investigation of suitable polymeric materials with desirable optical properties and appropriate fabrication techniques. Preliminary optical measurements of the integrated system will also be demonstrated. This approach has the potential to impact the next generation of micron scale interferometers as precise position/proximity sensors.

Porous silicon 2D photonic crystals

Porous silicon (PSi) is an attractive material for fabrication of multilayer optical devices such as Bragg reflectors, Fabry-Perot resonators and other novel (optical) components. Such devices are characterized by a periodic modulation of the refractive indices in alternating layers and can be classified as 1D photonic crystals. 2D photonic bandgap structures can be also obtained using a variation of applied potential on the back side of the sample during electrochemical formation of the multilayers. This technique allows a fabrication of spatially distributed filters on the millimeter size scale. In this paper, a new method is presented which uses a front side protective mask for the creation of 2D photonic bandgap structures on the micron scale. The devices obtained by this technique can be used for the creation of spatially distributed filters. The front side protective mask controls lateral undercut in multiple ways depending on the mask material. By varying the design and material of the protective mask, PSi interference filters with desired optical parameters across a field of view can be realized. In this paper, a novel, simple method to produce 2D periodic multilayer structures is described. In particular, the focus is on the changes in the photonic crystal cavities when various mask materials are used. In addition, a new type of active optical components for a chip-to chip interconnection based on the combination of our method and MEMS technology is presented.

Novel spatially distributed porous Si optical bandpass filters

To assist the growth of the telecommunication sector, new types of optical components such as those based on optical interference filter technology are critical. Existing technologies based on thin-film processing for production of optical communications filters have rapidly advanced. Although the Fabry-Perot bandpass filters made by deposition of alternate layers with high- and low- refractive index have a broad rejection band and a narrow passband, this technique does not allow for the control of filter parameters such as specification and adjustment of the transmitted wavelength at any place across the surface of the filter. The new approach discussed in the paper is directed toward the anodization of silicon to fabricate not only multilayer optical filters with a uniform passband across the field of view but also specially designed passbands at any single point in the field of view of the optical system. In particular, the realization and characterization of spatially distributed filters made of porous silicon are presented. These filters are able to select various passbands in the visible and IR regions. The filters were fabricated on p+ and p - type doped substrates. By varying the electrode configuration on the backside of wafer and the applied potential during electrochemical etching, the desired spatially distributed filter can be formed. The impact of wafer resistivity on filter parameters is discussed.