Research Statement
1. Overview
My research interests include linear and nonlinear optics in metamaterials (MMs), photonic devices, and optical fiber communications. Two current research directions of our Nanophotonics and Nonlinear Optics group are as follows:
i. Theoretical, numerical, and experimental investigations of fundamental properties and potential applications of photonic MMs
Photonic MMs are artificial nanostructures that emerge as a source of nearly unlimited opportunities for the realization of material properties that were not previously accessible, including positive, negative, and even zero indices of refraction. Unique applications of MMs include super-lenses that beat the fundamental diffraction limit and enable high-resolution optical imaging, and cloaking devices that render macroscale objects invisible
ii. Theoretical, numerical, and experimental studies of photonic crystal fiber (PCF) and waveguide based devices
Photonic crystal fibers are a new class of optical fibers containing air-holes ranging in diameter from 25 nanometers to 50 micrometers. The air-holes run along the fiber length and are distributed in the cladding in either periodic or random fashion. These fibers offer exceptional opportunities for the manipulation of light propagation and the realization of novel photonic devices. Potential device applications include sensor arrays, tunable filters, and biosensors that can readily be combined with microfluidic functionality
2. Photonic Metamaterials
Metamaterials constitute a new, 21st century area of research that is expanding fundamental knowledge of the behavior of electromagnetic wave propagation and present potential novel solutions to the realization of entirely new photonic functionalities such as sub-wavelength imaging, invisibility cloaking, and all-optical signal processing. Our research directions in this field include
2.1 Field enhancement effects in near-zero refractive index MMs
Significant progress in the understanding of MM fundamentals and recent developments in fabrication technologies have given rise to the field of transformation optics. Transformation optics is based on MMs with a tailored spatial distribution of the refractive index, which can vary from positive to negative values
The optical properties and potential applications of uniform MMs with constant refractive indices have been studied in detail and are quite well understood. However, graded-index MMs - artificial materials with refractive indices gradually varying in space in a wide range from positive to zero to negative values - have received significantly less attention so far. The enormous potential of graded-index MM structures was recently exemplified by the first experimental demonstration of an invisibility (cloaking) device. However, cloaking is just one of numerous prospective applications of these structures. Nevertheless, no fundamental physical models, design and optimization numerical tools, or experimental platforms exist to date to fully explore their unique properties. Our research is focused on the development of theoretical and numerical models for understanding, modeling and controlling linear and nonlinear interactions of light with graded-index photonic MMs and their device applications.
In particular, we predicted and investigated the resonant enhancement of electromagnetic waves propagating at oblique incidence in MMs near a point where the real part of the refractive index is zero. This effect occurs for both TE and TM polarizations near the point where the refractive index changes its sign at it transitions through zero. Our model elucidates the unique features of the resonant enhancement in "positive-to-negative transition" MMs for a broad frequency range from microwaves to optics. We performed a detailed study of the effect of resonant absorption in transition MMs with various refractive index profiles. The results of this parametric study may be of considerable interest for a variety of MMs-based applications. Depending on the application, the amount of resonant absorption may be minimized or maximized by changing the parameters of the transition layer and more generally the spatial profiles of material parameters. For example, resonant absorption may affect the performance of a superlens with diffused boundaries or other structures based, for example, on doped semiconductors, and therefore, needs to be minimized. On the other hand, applications such as "perfect" absorbers or nanodetectors would benefit from the resonant absorption effect that in this case should be maximized. Moreover, our results could be of considerable importance in the context of transformation optics as the inherent near zero index transition phenomena in graded-index structures could significant change spatial field distributions and lead to undesired absorption. Also, we demonstrated that more complex refractive index distributions with, for example, two zero-index crossings, may result in a formation of tunable resonant cavities. Currently, we are studying several applications of these local field enhancement effects for low-intensity nonlinear optical devices, including switching and antenna applications.
- Micro- to- nano-scale optical tapers: in this project, we design a micro- to- nano-scale optical taper: a device that guides and concentrates light to a sub-wavelength spot for applications including optoelectronic integrated circuits, sensors, and photovoltaics. This device would offer a viable solution to one of the major problems of modern optoelectronics - bridging the gap between micro-scale photonic waveguides (e.g., optical fibers) and nano-scale optical and electronic devices. In this project, we utilize the strong enhancement of the field near the zeroindex point and an anisotropic refractive index profile designed via the transformation optics technique.
- Novel nonlinear wavelength converters: this application relies on the predicted resonant field enhancement near the zero refractive index point in combination with the nonlinear response of the MMs host medium. Our research concentrates on applications based on both secondand third-order nonlinearities. An important advantage of this approach over the existing solutions is the potentially low input power requirements owing to the strong field localization effect that enables nonlinear effects at moderate intensities. New regimes of nonlinear conversion are expected owing to unusual phase-matching conditions that may be enabled at the PIM-NIM interface.
This project is funded by the US Army Research Office Single investigator Award (PI: Litchinitser) and by the US Air Force Office of Scientific Research Award for "Energy and Sensor Informatics" (PI: Cartwright, co-PI: Litchinitser).
Currently, two Ph.D. students, Ethan Gibson and Zhaxylyk Kudyshev (visiting from al - Farabi Kazakh National University) and one M.S. student, Mehdi Pakmehr, are working on this project.
Previously (2009-2010), Postdoctoral Researcher, Dr. Irene Mozjerin, investigated the effects of realistic MM parameters that include the fact that MMs have significant losses on the predicted field enhancement effects. It was found that considerable field enhancement can be achieved in optical transition MMs that have electromagnetic material properties obtained from experimental data despite the presence of significant losses. The field enhancement factor was found to be polarization-dependent and largely determined by the material parameters and the width of the transition layer, opening new opportunities for designing polarization sensitive optical devices.
2.2 Nonlinear Optics in Negative Index MMs
The emergence of negative index materials (NIMs) has given rise to numerous unusual phenomena that cannot be realized in conventional materials. These materials have revolutionized modern optics by providing unparalleled potential opportunities for designing novel applications, including nano-imaging and sensing devices, solar cells and light-emitting devices. Owing to the structural complexity of these composite materials, theoretical predictions and numerical analysis are the essential components of NIMs research.
Linear properties of MMs can be engineered by properly adjusting the dimension, periodicity and other properties of their constituent components. Nonlinear properties of NIMs are also expected to be significantly modified by their design and by their enhanced interactions with the magnetic field component. Most of the theoretical studies of nonlinear effects in NIMs to date either avoided specifying the origin of nonlinear response or utilized a model developed for a particular design of NIMs operating in microwave region without any justification. We develop models for nonlinear properties of optical NIMs taking into account the effects of nanostructures (such as strong local field enhancements) in studying the dynamics of ultrashort pulse and wave interactions in nonlinear NIMs.
In parallel to these fundamental theoretical studies, we perform detailed design and optimization of metal-dielectric and all semiconductor-based nonlinear NIMs. To date, nonlinear NIM have not been demonstrated at optical frequencies where they are expected to lead to a number of new phenomena and applications, including backward-phase matching, new regimes of second harmonic generation, and parametric amplification. In collaboration with two other groups in the EE department (Cartwright and Durbin), we progress toward nonlinear NIM fabrication (using focused ion beam lithography and molecular beam epitaxy) and optical characterization (using zscan, transmission and reflection measurements, and spectroscopic ellipsometry). The end goal of this project is to demonstrate the first nonlinear NIMs at optical frequencies and use them for sensing, photovoltaics, and other applications.
These efforts are funded through the US Army Research Office Single Investigator Award for "Nonlinear Optics in Negative Index Materials" (PI Litchinitser) and the US Air Force Office of Scientific Research Award for "Energy and Sensor Informatics" (PI: Cartwright, co-PI: Litchinitser).
Currently two Ph.D. students, Gayatri Venugopal, Apra Pandey, and one M.S. student (Mehdi Pakmehr) are working on this project.
2.3 Metamaterial components for Photonic Integrated Circuits
By leveraging the capabilities of photonics (speed) and of electronics (compactness) it will soon be possible to realize high performance integrated opto-plasmonic systems with applications from high bandwidth communications to sensing, and beyond. Such integration requires the availability of ultra-compact, ultra-fast, low loss components that can be efficiently coupled to the rest of a network, integrated such that all interconnected optical and electrical components are arranged on a single substrate, and be compatible with a complementary metal-oxidesemiconductor fabrication process. While some of the basic functionalities are presently available in the form of individual components, their integration is still challenging for a number of reasons, including their size, speed and power consumption.
One of the major challenges of existing integrated circuits is the so-called "interconnect bottleneck," which is an RC circuit delay occurring due to the small cross-section and close spacing of conducting metal lines above the integrated circuit chip. This becomes increasingly important as the characteristic feature size of interconnects is reduced. As a result, it is the number of communication channels, and not even computation speed itself, that is likely to be the main limiting factor in deep submicron technologies. To overcome this limitation, novel compact and scalable interconnect technologies capable of supporting ultra-high Gbit/s data rates are needed. Another essential step toward the realization of efficient performance of optoelectronics chips is buffering of optical signals to allow avoiding congestion of information traffic. An optical buffer is a device that slows down or even stops light to store it for a certain (controllable) period of time. Although several approaches to the realization of such "slow light" structures have been proposed and demonstrated, a majority of slow light schemes based on various waveguide geometries are not easily scalable to a chip-size footprint. Therefore, it is the goal of our on-going project to develop ultra-fast, reconfigurable, ultra-compact opto-plasmonic devices that are ready for on-chip integration and bridge existing gaps in electronic-photonic integration.
In particular, we investigate the effects of bistability and modulational instabilities in positivenegative index based nonlinear optical couplers. Optical bistability is a phenomenon in which two different values of output power are possible for the same input power. This phenomenon finds numerous applications in optical memory and storage devices. It should be mentioned that nonlinear optical couplers made of conventional positive index materials are not bistable (unless some additional components such as Bragg gratings or mirrors are introduced). However, in MMs-based couplers, bistability results from the effective feedback mechanism enabled by opposing directionality of the wave vector and the Poynting vector in NIMs. Moreover, such a coupler supports gap solitons-a feature commonly associated with periodic structures. These effects have no analogies in conventional couplers. These unusual properties of MM directional couplers form a basis for the development all-optical processing applications, including wavelength converters, flip-flops, and mirrorless lasers. Moreover, MMs allow for ultra-compact (subwavelength) design of such couplers.
This project is supported by the US Army Research Office Single Investigator Award for "Nonlinear Optics in Negative Index Materials" (PI Litchinitser).
Currently, two Ph.D. students, Gayatri Venugopal and Zhaxylyk Kudyshev (visiting from al - Farabi Kazakh National University) are working on this project.
2.4 Nonlinearly Tunable MMs
Since the first experimental demonstration of optical MMs it became clear that the possibility of controlling of their parameters at the post-fabrication stage - tunability - is one of the key steps toward their practical applications. Indeed, to take full advantage of the designer properties of MMs, a method to tune the electromagnetic response of the MM, preferably over a broad frequency range in as short of time as possible, is required. To date, nearly all demonstrations of actively tuned MMs have been achieved by varying the capacitance of a split ring resonator. This inherently limits the tuning to the magnetic response and to frequencies in the terahertz or lower. The very first designs of tunable MMs in the optical frequency range were based on using nematic liquid crystals. In both works, tunability was achieved by changing the dielectric function of the nematic liquid crystals using linear effects, either using DC voltage or thermal sources.
We investigated a radically different approach that uses strong Kerr nonlinearity of the nematic liquid crystals to control the location of the magnetic and electric resonances in the couplednanostrip and fishnet-based MM structures. We studied the interaction of intense fields with such structures which are covered by a nematic liquid crystal, a design that is most realistic and efficient in terms of fabrication and tunability. For example, our numerical simulations performed based on COMSOL Multiphysics platform showed that a 0.3 W/cm2 CW plane-wave can induce a shift of ~15 nm in the location of the transmission minimum at the magnetic resonance. This change is comparable to the relative change reported in MMs in the microwave range. Moreover, since the new approach employs the significant near-field enhancement of the electric field, and thus, the localized nonlinear response, the resulting optically-tunable shift is larger than the shift induced with a uniform linear bias-field of a similar magnitude.
Fulbright Postdoctoral Researcher Dr. Yonatan Sivan worked on this project in 2008-2009.
2.5 Transformation Optics based Nonlinear Converters for Photovoltaic Applications
We explore nonlinear optical applications of a broadband, omnidirectional light trapping device based on graded-index MM "black hole" recently proposed by Narimanov and Kildishev. A cylindrical version of such a "black hole" was already proposed and experimentally demonstrated in the microwave spectral range using non-resonant and resonant MM structures.
As light collectors can substantially boost the field intensity within a prescribed region, it is therefore reasonable to consider combining them with nonlinear optical materials for spectral conversion. The main mechanisms by which incident photons can be transformed to photons with higher energies are second harmonic generation (SHG), simultaneous two-photon absorption, and frequency up-conversion. Relatively low efficiency of SHG in conventional nonlinear materials typically precludes from using this nonlinear process for frequency upconversion in photovoltaic applications. However, most of nonlinear effects can be greatly enhanced if light is tightly confined in a waveguide or cavity. Therefore, we proposed to combine an omnidirectional "black-hole" absorber with nonlinear material in its "core" and explore the possibility of up-conversion of tightly confined radiation inside the nonlinear waveguide.
These efforts are funded through the US Air Force Office of Scientific Research Award for "Energy and Sensor Informatics" (PI: Cartwright, co-PI: Litchinitser).
One Ph.D. student, Apra Pandey, and EE undergraduate student, Yinnar Chen, are working on this project.
2.6 Beam Steering in Anisotropic MMs
Reconfigurability and low-loss performance are the key properties necessary for the realization of numerous practical applications of MMs. We proposed a new type of tunable anisotropic MM structure combined with microfluidic systems. There are numerous applications in the field of communication including the realization of ultra-compact reconfigurable optical components on a chip or novel microfluidic biosensors.
The anisotropic route to MMs is based on designing a structure that exhibits different electrical permittivity (e? and e?) along different axes. The motivations to move beyond conventional metal-dielectric resonant structures to anisotropic hyperbolic MMs are potentially lower losses, larger bandwidth, tunability and reconfigurability. We utilize a structure consisting of silver nanorods filled with liquids/gels that are used to control the effective permittivity of the composite columnar structure. Tunability and beam steering is achieved by changing the fill fraction of the rods in the structure and therefore, changing the effective index of refraction (and the angle of refraction) from positive to negative values.
Negative refractive properties of this structure were confirmed in numerical simulations using a finite elements based method implemented COMSOL® Multiphysics. We designed and numerically characterized the tunable, anisotropic MM composite. Experimental realization of this structure is in progress. Active tuning and beam steering may find applications in optical communication and sensing applications of MMs.
This project is supported in part by the US Army Research Office Single Investigator Award for "Nonlinear Optics in Negative Index Materials" (PI Litchinitser).
Currently, one Ph.D. student from my group, Gayatri Venugopal, and a Ph.D. student from Prof. Oh's group, Rajagopal Panchapaksan, are working on this project.
2.7 Novel approach to NIM diagnostics
To date, optical NIMs have been demonstrated only in the form of thin films of width ~150 nm. Unambiguous methods of detection of the negative refractive index are essential in laboratory experiments. Existing characterization methods are based on the measurement of the phase advance and are quite complicated. We explore a potentially simpler method of NIM diagnostics based on the nonlinear phenomenon of optical bistability in NIMs combined with nonlinear nematic liquid crystal layer.
This project is supported in part by the US Army Research Office Single Investigator Award for "Nonlinear Optics in Negative Index Materials" (PI Litchinitser).
One Ph.D. student, Katherine Byrnes, works on this project.
2.8 Future research
The U.S. Department of Energy Report on the Basic Research Needs for Solar Energy Utilization states: "Reaching the goals of ultra-high efficiencies and low cost will require basic research to achieve the revolutionary advances". To address this need, we proposed to use unparalleled opportunities offered by MM technology and transformation optics (TO) to develop novel solutions for ultra-high efficiency solar cells. Our two main future research thrusts include the development of:
- Nonlinear Transformation Optics: A generalized theory of TO for nonlinear media and fundamental understanding of light-matter interaction in nonlinear MMs,
- Transformation Optics based MM designs for advanced light manipulation in photovoltaic devices.
The TO approach will be used to design i) Efficient, omnidirectional, and broadband light collectors and spectral converters; ii) Light trapping structures for high-efficiency photovoltaic absorbers; iii) Flat, broadband, and omnidirectional "cloaking" coatings minimizing the front surface reflectivity of a solar cell and scattering from surface defects. Our ultimate goal is to develop theoretical tools for designing MMs-enhanced photovoltaic devices, and for modeling of complex phenomena in the solar conversion process. These include harvesting, absorption, and interface effects. Multi-scale modeling tools spanning all spatial scales from quantum mechanical (0.1-10nm) to the mesoscale (0.1-100μm) and device scale (1mm-10m) that are essential in the context of MMs and their applications will be developed. Accurate description of all these processes requires advanced multi-scale and multi-physics theoretical capabilities that currently do not exist. Development of these tools would enable us (in collaboration with nanofabrication groups) to build optimally performing structures without having to discover them in the laboratory by trial and error.
New Ph.D. student, Fatema Alali, started working on these ideas in Fall 2010.
3. Photonic Crystal Fibers and Waveguides
During my first year at UB I built a laboratory for optical characterization of fiber optic devices. The laboratory is equipped with the state-of-the-art fiber optic equipment, including a high quality fusion splicer, capable of splicing various types of fibers including those with complex transverse structures. Other equipment in the laboratory are optical sources, a high-resolution, broadband optical spectral analyzer, nanopositioners, and a variety of fibers and other optical components.
Our current experimental studies are focused on photonic crystal fibers (PCFs). Photonic crystal fibers are a new type of two-dimensional photonic crystal extended in a longitudinal direction over kilometers of length. PCFs are typically made using established fiber drawing techniques, and are emerging as a platform for building novel devices for applications ranging from optical communications and spectroscopy, to biomedicine and defense.
Most of the theoretical studies of light propagation in PCFs have been performed numerically. Powerful, direct numerical simulations of these structures are quite complex, time consuming, and do not always provide a sufficient understanding of the PCF's guiding properties. An analytical model would be highly desirable but is currently not available. Previously, we developed a simple model that accurately predicted the transmission properties of a novel class of PCFs that we call antiresonant PCFs. Antiresonant PCFs have air-holes filled with a material with a refractive index higher than that of the matrix material. They are emerging as an excellent platform for fundamental nonlinear optics studies and for the building of novel nonlinear photonic devices because they uniquely combine large nonlinear coefficients with a group velocity dispersion that can be tailored and tuned to almost any value.
Our research directions include:
- Development of analytical (or semi-analytical) models for the dispersive and loss properties of antiresonant PCFs, numerical simulations and optimization of antiresonant PCFs' dispersion profile for nonlinear applications, including pulse compression, wavelength conversion and supercontinuum generation.
- Design and experimental demonstration of PCF-based Sensors. Antiresonant PCFs with high index inclusions have several advantages for designing sensors owing to periodic spectral response characteristics and high sensitivity to the refractive index changes of the material placed in the fiber air-holes. We perform experimental and numerical studies of a novel refractometric sensor utilizing these unique properties of antiresonant PCFs. The sensor operation is based on the wavelength shift of the transmission spectrum in response to the refractive index change of a sample loaded in the air-holes as a function of the temperature change.
3.1 Antiresonant guiding PCFs for distributed gradient measurements
Photonic crystal fibers offer unprecedented opportunities for manipulation of light propagation and realization of novel photonic devices. In particular, PCFs provide a new platform for the development of compact, multifunctional optical sensors.
To date, a majority of PCF-based sensors have been designed for measuring uniform changes in the refractive index due to temperature, analyte concentration, or other factors. However, another important area of their potential applications is the development of advanced distributed fiberbased monitoring systems. While standard optical fibers are widely used in a variety of distributed systems applications ranging from environmental and commercial sensing to security and defense, PCF-based devices are expected to bring new degrees of freedom as their properties can be modified after the fiber has been made, e.g. , the air-holes can be filled with temperature sensitive fluids, liquid crystals, or other materials that change their optical properties in response to electric or magnetic fields, strain, or light intensity. Depending on the PCF structure and materials inside the air-holes, these fibers confine light through different physical mechanisms, including modified total internal reflection and bandgap guidance.
While initial theoretical studies and design of PCFs was almost exclusively done numerically, later, it was shown that some of their properties could be understood using a so-called antiresonant optical waveguide (ARROW) based model. The ARROW model, in particular, applies to the PCFs with air-holes filled with a material with a refractive index higher than that of the matrix material. According to this model, light is confined in the core by antiresonant reflections from the individual high-index rods. In contrast to other photonic bandgap structures, the positions of transmission minima in the spectra of antiresonant PCFs are largely determined by the individual properties of high-index cladding inclusions rather than their periodicity, spacing, and number. The accuracy of the antiresonant PCF model has been validated by numerical simulations and laboratory experiments. In addition, the model has facilitated the development of a number of novel fiber designs and device applications based on the transmission properties of these fibers, including sensors, fixed and continuously tunable filters. Recently, we proposed, designed and experimentally demonstrated a refractometric measurement technique utilizing antiresonant-guiding PCFs for measuring refractive index gradients resulting from spatially non-uniform temperature distributions along the fiber.
Two M.S. students, Roshni Biswas and Gaurav Mehta (both graduated with M.S. degree in May- June 2010), worked on this project. Currently, Ph.D. student, Jinwei Zeng, is continuing and expanding on this work. This project is conducted in collaboration with two other groups in EE Department (Fam and Cartwright).
3.2 Optofluidic biosensors
The interest in compact, portable and inexpensive biosensors has grown dramatically in recent years due in part to advances in microfluidics, especially lab-on-a-chip technology. A relatively new and promising approach to biosensing involves optofluidics where optic and fluidic functionality are integrated into a microsystem to leverage their combined advantages. Microfluidic functionality enables compact and rapid processing of small biofluid samples, and optical functionality enables high detection sensitivity of target biomaterials within these samples. To date, various optofluidic sensing devices have been developed. Many of these utilize a detection scheme based on some form of resonant optical behavior with input/output signals carried by optical fibers or integrated waveguides. Examples of these include waveguide coupled photonic crystal resonators and Whispering Gallery Mode based sensors. Such sensors are capable of high detection sensitivity, but they have potential drawbacks both in terms of input/output coupling efficiency and ease of multiplexing, i.e. addressing multiple independent sensing stations on a single platform.
We proposed, designed, and numerically investigated a novel optofluidic biosensor that operates in a transmission mode and can function using free-space illumination without the need for fiber or waveguide coupled input/output signals. The sensor consists of multiple parallel microchannels embedded in a substrate and oriented perpendicular to its surface. The central microchannel is illuminated by a focused beam of light. The periodic spacing of the channels and contrast in refractive index between the carrier fluid and the substrate act to confine and guide the incident light through the central channel similar to that of an antiresonant reflecting optical waveguide. The transmission spectrum of the sensor is obtained using a detector positioned beneath the central microchannel. The transmission is a function of the wavelength of the incident light, the dimensions, periodicity and refractive index of the substrate, and the refractive index of the carrier fluid. Sensing is achieved via accumulation of a thin (nanoscale) layer of target biomaterial on the surface of the microchannels, which are functionalized (e.g. using immobilized capture antibodies) to bind with the biomaterial as it flows through the system. Detection is based on a shift in the transmission spectrum caused by the contrast in refractive index between the carrier fluid and the target biomaterial.
One M.S. student, Roshni Biswas, worked on this project. This project was performed in collaboration with Dr. Furlani (Kodak).
3.3 Future work and other projects
Transversely illuminated PCF based devices
In contrast to most of the literature on PCFs where the fiber was used as a waveguide for light propagating along its longitudinal axis, we propose several applications of a PCF illuminated in transverse direction, i.e. at an angle to its longitudinal axis. Only few reports have been found on the characterization and potential applications of transversely illuminated PCFs. Numerous novel device applications are being developed based on this new geometry.
Plasmonic sensor array for label-free binding high-throughput screening based on the phenomenon of enhanced transmission in sub-wavelength engineered structures first discovered by Ebbesen. This concept will be realized based on a combination of PCFs, and plasmonic MMs.
In this project, we attempt to design, optimize, and experimentally demonstrate a resonant MMbased structures implemented on a cleaved end of a PCF to enhance the power transmission through sub-wavelength apertures using surface plasmons . These will be excited at a wavelength that will depend on the dielectric surrounding the plasmonic MM structure (analyte, biomolecules, etc.). Label-free surface plasmon detection techniques open a number of exciting possibilities for simultaneous monitoring for the presence/absence of a particular biomolecule as well as its absolute concentration. Also, an array allows the absolute concentrations of a number of molecules to be detected simultaneously to produce a pattern of molecular expression such as disease biomarkers. Finally, other parameters such as pH or temperature can be monitored on the same platform opening new possibilities for multi-modality sensing.
PCF-based MMs: The mature PCF technology can be merged with rapidly developing field of photonic MMs. In order to alter the refractive index of fiber material, or more precisely, the effective dielectric permittivity of the fiber material, we proposed to fill the air-holes with metals (gold or silver) or metallic nanoparticles.
PCF and MMs technologies may be highly complimentary in the sense that the PCF technology may provide a platform for developing bulk optical MMs, including negative index MMs and graded-index meta-coatings for cloaking applications, by filling air-holes with metals. No largescale bulk optical MMs have been reported to date. Fabrication of optical MMs using existing nanofabrication technologies is still considered to be a difficult task. Therefore, it may be even more challenging to produce graded-index structures with a particular refractive index distribution required for specific applications including those discussed above.
On a general level, one of the serious obstacles that the MMs field needs to overcome is establishing repeatable manufacturing capabilities on a large scale. On the other hand, nowadays, PCFs is a mature technology offering unprecedented control over the material and dimensional properties of the fiber. PCFs are unique structures owing to the fact that there is several orders of magnitude difference in their transverse and longitudinal dimensions: They possess subwavelength structures, air-holes, in their transverse cross-section and extend over tens of meters in longitudinal direction.
One of the significant advantages of using the PCF technology for MMs realization is that the required air-hole pattern (transverse fiber profile) is designed at a step when fiber preform is made, and therefore can be done on a millimeter instead of nanometer scale. The transverse structure of a preform is preserved when the preform is drawn into a fiber. Another advantage is that light (potentially at different wavelengths) can be launched in both transverse and longitudinal directions expanding the range of potential applications of these structures.
Currently, one Ph.D. student, Jinwei Zeng, is working in this direction.
Plasmonics for cancer treatment
Recently, the electromagnetic properties of metal nanoparticles have attracted an enormous attention owing to their unique tunable plasmonic properties enabling ultrasensitive diagnostic, imaging, and therapeutic technologies. In particular, they were proposed as promising candidates for injectable nanoantennas that target tumors and locally convert light energy to thermal energy for ablation. Currently, tumor ablation approaches used in clinical practice, including radio frequency, laser, and focused ultrasound waves lack the selectivity, i.e. are unable to selectively heat tumor tissues without affecting surrounding tissues. Therefore, the development of a tumortargeted heat source is of paramount importance. We collaborate with the researchers at Roswell Park Cancer Institute on the project aiming at the development of a novel cancer treatment technique that uses plasmon resonance of gold nanorods to selectively heat cancer cells with minimized heating of the surrounding tissue.
One Ph.D. student, Apra Pandey, is working on this project.
