Metasurfaces have become a key focus in research and are applied in numerous fields because of their exceptional capability to control electromagnetic waves across microwave to optical frequencies. These artificial sheet materials have the advantages of lightweight and ability to control wave propagation both on the surface and in the surrounding free space. The complexity of fabricating metasurfaces via two-photon lithography (TPL) is addressed through sophisticated modeling. Critical to the success of TPL is the ability to predict the effects of the fabrication process on the final product. This paper introduces three distinct modeling approaches that vary in complexity and predictive capabilities. We evaluate the performance and limitations of a simple threshold model, a compact model and a full model of polymerization. Through application examples, we demonstrate how these models can guide the fabrication of metasurfaces.
Electromagnetic metasurfaces have shown immense potential for wave control in diverse frequency regimes ranging from radio frequencies to the visible. In visible frequencies, materials with low loss and the ability to tightly confine light (high-index dielectrics, plasmonic metals) become limited. Optical metasurfaces are additionally challenging from a practical point of view, since they require nanofabrication techniques with high resolution and precision. Here, we consider polymer-based optical metasurfaces to be fabricated with two-photon lithography, which is a fast, scalable and cost-effective nanofabrication technique. We focus on a beam steering scenario, which is the archetypical example of wavefront manipulation functionalities. The proposed design operates in transmission and the principle of operation is based on phase accumulation within short vertical waveguide segments. We start from idealized, perfectly-cylindrical meta-atom shapes, which are typically studied in the literature, and proceed to conical shapes which exhibit increased mechanical stability and smooth half-ellipsoid-like structures that are compatible with the voxels of the laser writing process. We show that the adopted realistic meta-atom shapes lead to only a small deterioration of the steering performance and by employing numerical optimization we are able to recover the performance obtained with the idealized meta-atom shapes. Our work aspires to enable high-performance, practical optical metasurfaces taking fabrication limitations and particularities thoroughly into account.
A multimode framework that can be utilized in the analysis and design of general, non-Hermitian periodic systems that comprise contemporary 2D materials is developed. The theoretical framework is based on the concept of Quasinormal Modes (QNMs) that can be used to efficiently retrieve the spectral response of periodic systems. Both sub- and super-wavelength lattice constants are examined with extra care to include propagating higher diffraction orders. The framework is employed in two indicative configurations, a graphene-based periodic metasurface that supports tightly confined graphene surface plasmons, and a periodic dielectric metasurface enhanced with a transition metal dichalcogenide layer that supports quasi bound states in the continuum. Both configurations are of fundamental and practical interest since they exhibit controllable resonant response and, in addition, the electromagnetic properties of the involved 2D materials can be electrically tuned. The Finite Element Method (FEM) is used to retrieve the QNMs, which are then fed to the framework to specify the spectral response for the zeroth and higher diffraction orders. Full-wave FEM simulations are used to verify the validity of obtained results.
A novel 3D electromagnetic metamaterial design for Electromagnetically Induced Transparency in THz frequencies is reported. Simulations were done using finite elements method in order to optimize the geometry of the metamaterial. The structure was fabricated using Multiphoton Lithography on high resistivity Silicon substrate and further processed with Electroless Silver Plating to get the highly conductive metallic metamaterial.
The optical fiber sensing field is in continuous seek of new processing methods, light localization structures and transduction mechanisms for developing devices with novel functionalities and/or improved performance, while targeting existing or emerging application fields. Herein, we are reviewing work performed on the imprinting of optical resonators, onto optical fibers using multi-photon, three-dimensional lithography. Two major resonating cavity designs are presented: hollow Fabry-Perot resonators imprinted onto the endface of optical fibers, and micro-ring resonators attached onto micronic diameter optical fiber tapers; both types of devices operate at the 1.5μm spectral band. Results are presented on the design, spectral characterization and simulation of those hybrid type of photonic devices, while their sensing capabilities are exemplified in the tracing of organic solvent vapors, which upon case can reach sub-ppm detectivity levels.
We present a thorough numerical investigation of end-fire coupling between dielectric-loaded surface plasmon
polariton (DLSPP) and compact rib/wire silicon-on-insulator (SOI) waveguides. Simulations are based on the
three-dimensional vector finite element method. The interface geometrical parameters leading to optimum performance,
i.e., maximum coupling efficiency or, equivalently, minimum insertion loss (IL), are identified. We
show that coupling efficiencies as high as 85 % are possible. In addition, we quantify the fabrication tolerances
about the optimum parameter values. In the same context, we assess the effect of a metallic stripe gap and
that of a horizontal offset between waveguides on insertion loss. Finally, we demonstrate that by benefiting
form the low-loss coupling between the two waveguides, hybrid silicon-plasmonic 2 x 2 thermo-optic switching
elements can outperform their all-plasmonic counterparts in terms of IL. Specifically, we examine two hybrid
SOI-DLSPP switching elements, namely, a Mach-Zehnder Interferometer (MZI) and a Multi-Mode-Interference
(MMI) switch. In particular, in the MZI case the IL improvement compared to the all-plasmonic counterpart
is 4.5 dB. Moreover, the proposed hybrid components maintain the high extinction ratio, small footprint, and
efficient tuning traits of plasmonic technology.
We rst report on design, fabrication and characterizations of thermally-controlled plasmonic routers relying on
the interference of a plasmonic and a photonic mode supported by wide enough dielectric loaded waveguides. We
show that, by
owing a current through the gold lm on which the dielectric waveguides are deposited, the length
of the beating created by the interference of the two modes can be controlled accurately. By operating such a
plasmonic dual-mode interferometer switch, symmetric extinction ratio of 7dB are obtained at the output ports
of a 2x2 router. Next, we demonstrate ber-to-ber characterizations of stand-alone dielectric loaded surface
plasmon waveguide (DLSPPW) devices by using grating couplers. The couplers are comprised of dielectric loaded
gratings with carefully chosen periods and duty-cycles close to 0.5. We show that insertion loss below 10dB per
coupler can be achieved with optimized gratings. This coupling scheme is used to operate Bit-Error-Rate (BER)
measurements for the transmission of a 10Gbits/s signal along a stand-alone straight DLSPPW. We show in
particular that these waveguides introduce a rather small BER power penalty (below 1dB) demonstrating the
suitability of this plasmonic waveguiding platform for high-bit rate transmission.
Surface plasmons polaritons are electromagnetic waves propagating along the surface of a conductor. Surface plasmons photonics is a promising candidate to satisfy the constraints of miniaturization of optical interconnects. This contribution reviews an experimental parametric study of dielectric loaded surface plasmon waveguides ring resonators and add-drop filters within the perspective of the recently suggested hybrid technology merging plasmonic and silicon photonics on a single board (European FP7 project PLATON "Merging Plasmonic and Silicon Photonics Technology towards Tb/s routing in optical interconnects"). Conclusions relevant for dielectric loaded surface plasmon switches to be integrated in silicon photonic circuitry will be drawn. They rely on the opportunity offered by plasmonic circuitry to carry optical signals and electric currents through the same thin metal circuitry. The heating of the dielectric loading by the electric current enables to design low foot-print thermo-optical switches driving the optical signal flow.
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