Research
Plasmonic Metasurfaces
Plasmonic metasurfaces are thin surfaces incorporating many metallic nanostructures of subwavelength dimensions. Such metasurfaces offer the ability to control and manipulate the various properties of light-beams (profile, shape, polarization, etc.) almost arbitrarily. This can be achieved by properly designing the shapes of various nanostructures comprising the metasurface to obtain a desired amplitude, phase and polarization response. Consequently, much research has been focused on plasmonic metasurfaces as a new paradigm and building block for various applications such as flat optics, holography, nano-imaging, beam shaping, and many more.
​Our Group has demonstrated a novel approach for realizing wide angle, highly efficient and broadband holograms employing metallic scatterers. Currently we are working on new and exciting applications for metasurfaces such as dynamically reconfigurable metasurfaces for displays and LIDAR, on-chip particle accelerators, and sensors.
Plasmonic Nano-Antennas for Refractive index (RI) Sensing
Good sensors require the ability to detect small changes in the properties of the surrounding environment. We use refractive index (RI) detection technique based on an array of nanometer scale slot-antennas milled in a thin gold layer using a single lithographic step. The high sensitivity and resolution obtained for the slot antenna arrays indicate to the attractiveness of plasmonic devices for bio-medical sensing and points out the countless possibilities for further innovation. Future research in this direction involves adapting the slot array structure to different spectral domains and incorporating it into devices which are suitable for medical diagnostics or lab on chip frameworks.
An example of such an approach, which is currently being pursued, is slot antennas placed on the facet of an optical fiber. Such a system does not require free-space optical components and optical alignment, and could be operated as a black box by medical technicians without optical background. The scheme is simple to use and exhibits high sensitivity, rendering it ideal for applications such as precise point of care medical measurements of protein concentration or water contamination.
Dielectric Metasurfaces for beam forming and manipulation
During the last few years, much research efforts have been focused on designing flat metasurfaces facilitating subwavelength light control and flat optical components such as lenses, holograms and more. All-dielectric metamaterials exhibit low absorption losses in the infrared (IR) and visible spectral ranges. Such dielectric metasurfaces implement the Huygens surface principle, by utilizing an overlap of electric and magnetic Mie-type resonances of the constituent high refractive index elements to reduce significantly backscattering of the impinging light. Consequently, complete 2π phase shift coverage and very high transmission can be achieved over relatively broad wavelength range
Our Group is working on a new design approach for obtaining wide angle, highly efficient, all-dielectric metasurface beam deflectors based on a genetic optimization algorithm. Based on deflector optimization it is possible to design more complex phase masks which realize a variety of highly-efficient flat optical devices such as lenses, holograms, etc.
Perovskite Photonics
Organo-metal halide perovskites have emerged in recent years as one of the most interesting and promising materials in the area of photonics. It revolutionized the field of photovoltaics reaching high efficiencies in no time while showing great promise in the field of photonic sources. This family of materials, similar to organic dyes, can be processed without the need of high temperatures and low vacuum simply by spin casting in room temperature. In addition, their wavelength is broadly tunable throughout the visible spectrum. As opposed to organic dyes, these materials are conductive which paves the way towards the realization of electrically pumped laser based on perovskites.
In our group we combine our experience in photonic devices such as waveguides, resonators, gratings etc. with perovskite materials in order to explore exciting new applications.
3D Polymer Photonics
Polymeric materials have interesting optical and mechanical properties, making them an attractive choice for future photonic systems. In addition to low optical losses and material dispersion, polymers are simple to manipulate and to cast using a wide variety of fabrication methods including soft-lithography techniques. The ability to dope polymeric materials with molecules that exhibit a large electro-optic coefficient, nonlinear response or optical gain, paves the way to all-polymer integrated optical circuits that include on-chip sources, processors and detectors.
In our group, we use various techniques, mainly soft lithography, to pattern 3D active and passive polymeric photonic devices. The ability to fabricate high quality devices from various materials and integrate them on the same chip sets a good infrastructure for photonic integrated circuits and paves the way towards new applications in the future.
Radial Bragg Lasers
Radial Bragg lasers (RBLs) are a class oh Photonic Crystal cavities which utilizes Bragg confinement in circular geometry to realize disk or ring cavities. The Bragg confinement mechanism allows great flexibility in engineering the radial mode profile. For example, it is possible to design a resonator in which the light is confined within a defect composed of low refractive index material or even air. By contrast, this would be impossible in the case of conventional, total-internal-reflection-based, resonators. Such configuration is useful for high power surface emitting lasers, sensing applications, optical gyroscope and more. The Bragg reflection concept allows one to tailor the reflector structure to a desired radial field profile and decouple between the modal volume (or cavity dimensions) and the radiation losses. Another unique property of RBLs is that, in contrast to conventional laser, their tendency to lase in a single mode increases with their size.
Our Group has demonstrated single mode lasing from large RBLs (100mm in diameter) utilizing InGaAs quantum wells embedded in GaAs vertical waveguide structure. The design fabrication and characterization of the RBLs are done in house using the TAU nanotechnology center facilities. Currently we are working towards realizing new compact optical gyroscope and electrically pumped broad are surface emitting lasers.
Ultra-Long Fiber Lasers for Secure Key Distribution
Most modern encryption algorithms make use of a shared secret – the encryption key, known only to the legitimate parties. This encryption key is a number/sequence of bits of varying length that is used by the encryption algorithm to scramble the original content of the message in a way that only other bearers of a relevant key could unscramble. Once all parties have the relevant keys, they can start transferring secure messages back and forth, however, the longer the same key is being used, the higher the risks are that an ill purposed individual might manage to “guess” the right key and gain access to the content of the encrypted messages. To reduce the chances of that ever happening, it’s necessary to generate and exchange encryption keys securely, for long distances and at the fastest possible rates. One of the most studied solution to this is Quantum Key Distribution (QKD) which theoretically offers absolute security; however, its physical implementations require expensive equipment which is often not completely secure and its bit-rate drops exponentially with the distance due to attenuation making it impractical for distances in excess of 150km.
In recent years efforts have been made to find a practical solution to the key distribution problem using methods that involve classical physics. One such method involves stabilizing the lasing dynamics of ultra-long fiber lasers where matching Bragg mirrors on both ends of a fiber cavity alter its spectral transmission profile and measurements of lasing frequency indicate, to each of the legitimate parties, if the other party placed an identical mirror or not and thus allow them to synchronize a secure key bit while an outsider, lacking any knowledge of the mirror selection of both parties, can not gain any insight into the choices of bits.
White Light Cavities
White Light Cavities (WLCs) are characterized by a resonance which fulfills the phase condition over a band of frequencies. This is in contrast to conventional cavities in which this condition is satisfied at discreet frequencies (known as the resonance frequencies). This non-conventional property can be obtained by inserting a dispersive element into the cavity while tailoring the parameters of the cavity and the element in a manner where the frequency dependent phase accumulation of the empty cavity is cancelled out by the negative phase slope of the dispersive element. The utilization of this WLC phenomenon was suggested for various important applications in sensing and communication.
Our Group has demonstrated single mode lasing from large RBLs (100mm in diameter) utilizing InGaAs quantum wells embedded in GaAs vertical waveguide structure. The design fabrication and characterization of the RBLs are done in house using the TAU nanotechnology center facilities. Currently we are working towards realizing new compact optical gyroscope and electrically pumped broad are surface emitting lasers.
Thermal feedback in Brillouin Fiber Lasers
The main drawback of fiber lasers is their high sensitivity to fluctuation in the properties of their surroundings (temperature, pressure, vibration, etc.). Even a minuscule fluctuation in the ambient parameters can destabilize them. Consequently, complex techniques are often utilized to isolate and stabilize fiber lasers.
A new, passive, feedback mechanism inherent to Brillouin fiber lasers (BFLs) was discovered and studied in our group. This mechanism, stemming from the interplay between thermal optical-length variations and the gain-line induced frequency dependent lasing power, triggers unexpected and counter-intuitive phenomena such as self-frequency-stabilization, multi-stability and memory effects. The direct benefit of this feedback mechanism is that it allows for passive self-stabilization of fiber lasers with minimal means of isolation. This nonlinear process can be controlled and modified by engineering the gain lineshape, rendering BFLs highly attractive as a platform for studying nonlinear dynamics and providing novel tools for various possible applications.
Optical Nano Rectennas – The new solar cell
Nano rectennas are nanoscale devices that collect light and produce dc power. Building on the strong foundations of antenna theory, a rectenna is constructed from a carefully designed antenna and an integral diode rectifier. At submicrometer wavelengths, the antenna must have nanoscale features, and with frequencies over 200 THz the diode must be extremely fast, a tunnel junction. Combining the two into fully functional, scalable, efficient devices is a challenge that has been taken up by many and has yet to be accomplished.
In our group we have designed and fabricated several types of antennas, including the dual-Vivaldi antenna, an ultra-wideband optical range antenna with scattering efficiency over 85%. Additionally, we have fabricated a variety of metal-insulator-metal (MIM) tunneling junctions from various materials (Au, Al, Ag, Ti, Ni, Pt, Ca) and measured their characteristics. Putting them together is our next goal. The size constraints and impedance matching are essentially conflicting, and creative solutions will be required.