Nanophotonics community has shown great interest in 2D materials because of their unique properti... more Nanophotonics community has shown great interest in 2D materials because of their unique properties of electromagnetic field manipulation. Many of these materials exhibit strong natural anisotropy, which further opens possibilities of polarization manipulation. Here, we show that α-MoO 3 , an emerging natural hyperbolic 2D material, can be combined with plasmonic nanostructures to provide strong extrinsic chirality in the visible range. A combination of biaxial anisotropy in α-MoO 3 and Fabry−Perot cavities with nanoscale features leads to different absorption of left and right circularly polarized photons, hence exhibiting circular dichroism (CD). Our simulation results predict that multilayer nanoscale-thick films including α-MoO 3 are potential candidates for achieving extrinsic chirality across the visible range. Furthermore, we show a significant CD increase when the α-MoO 3 layer is coupled with plasmonic nanohole arrays or plasmonic nanocubes. Such designs are achiral in geometry and therefore easier to fabricate. Moreover, we optimize the CD dissymmetry factor g CD for the nanocube-based design at 780 nm, obtaining 84%. We believe that utilizing biaxially anisotropic α-MoO 3 films to control and engineer chiro-optical properties in the visible frequency range will open research directions and enable enhanced functionalities in chiro-optical control at the nanoscale, further leading to applications in chiral sensing and CD.
ABSTRACT The design of aperture shape is a promising approach for enhanced transmission through a... more ABSTRACT The design of aperture shape is a promising approach for enhanced transmission through a subwavelength aperture. We designed split-ring-resonator (SRR)-shaped apertures in order to increase the transmission through subwavelength apertures by making use of the strong localization of the electromagnetic field in SRR-shaped apertures. We obtained a promising result of 104-fold enhancement by utilizing SRR-shaped apertures. It is possible to use these proposed structures at optical frequencies by making several modifications such as decreasing the sharpness of edges and increasing the gap width. Since SRRs are already being realized at optical frequencies, our proposed SRR-shaped aperture structures are promising candidates for novel applications. (C) 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3599873]
We experimentally investigate the semiconductor-to-metal transition (SMT) in vanadium dioxide thi... more We experimentally investigate the semiconductor-to-metal transition (SMT) in vanadium dioxide thin films using an infrared thermographic technique. During the semiconductor to metal phase change process, VO2 optical properties dynamically change and infrared emission undergoes a hysteresis loop due to differences between heating and cooling stages. The shape of the hysteresis loop was accurately monitored under different dynamic heating/cooling rates. In order to quantify and understand the effects of different rates, we used a numerical modelling approach in which a VO2 thin layer was modeled as metamaterial. The main experimental findings are interpreted assuming that both the rate of formation and shape of metallic inclusions are tuned with the heating/cooling rate. The structural transition from monoclinic to tetragonal phases is the main mechanism for controlling the global properties of the phase transition. However, our experimental results reveal that the dynamics of the heat...
Integrated optical sensors have garnered much interest for lab‐on‐chip applications such as chemi... more Integrated optical sensors have garnered much interest for lab‐on‐chip applications such as chemical and biological sensing and detection. Among various ways for detection schemes, spectral analysis performed by a spectrometer has shown great promise. Typically, such spectrometry is carried out in a relatively large device, owing to the fact that spectral resolution is often dictated by large optical path length. Here, a high‐resolution compact spectrometer utilizing random scattering events in a disordered medium is demonstrated. The spectrometer is inverse designed by objective‐first method and fabricated using two‐photon polymerization technique. The compact spectrometer consists of a disordered photonic structure and an inversed‐designed mode decomposer. A spectral resolution of 0.25 nm with a bandwidth of 30 nm in the near‐infrared regime is realized from a spectrometer occupying a relatively small footprint of 30 × 12.8 µm2. The proposed platform has a great potential to be used as a versatile lightweight and compact spectrometer for various applications including on‐chip spectrometer and sensing.
Plasmonic nanoparticles (NPs) [1] give rise to strong electric field (E-field) confinements, and ... more Plasmonic nanoparticles (NPs) [1] give rise to strong electric field (E-field) confinements, and therefore, when coupled with nearby emitters such as dye molecules or quantum dots, act as antennas to enhance their emission rates and intensities. [2,3] These so-called plasmonic nanoantennas can be used in lasing, [4]
that it typically uses a 780 nm wavelength, and only over a small volume at the focal point of a ... more that it typically uses a 780 nm wavelength, and only over a small volume at the focal point of a microscope objective is the intensity high enough for 2PP to occur (the polymer would normally polymerize in the UV at ≈390 nm). While the current 2PP resolution is still an order of magnitude larger than the best conventional lithographic methods (≈10 nm), 2PP direct laser writing lends itself to true 3D printing. It was demonstrated that such 2PP-based system can be widely used for fabricating high-quality micromachines for photonics and biomedicine. [6] However, in generally these structures does not have an urgent requirement for nanoresolution while this is particularly important for the types of highly complex geometries that emerge from inverse-designed free-form photonic devices, especially polarization handling devices such as polarization beamsplitters (PBSs), which play an important role in applications of nanophotonic chip design. There are many examples in the literatures focusing on waveguide-based PBS, of which the working principle is mainly based on modal evolution and modal coupling. [7] Generally, the latter including directional couplers (DCs), [8] multimode interference couplers, [9] and Mach-Zehnder interferometers, [10] are based on the mode beating behavior, providing a compact size especially for DCs. However, the fabrication could be challenging since the gap is very stringent although this can be solved by the other mechanism using a surface plasmon polariton. [11a] In addition, there are some plasmonic structures [8c,11] and photonic crystal-based PBSs, [12] etc. Recently, photonic metasurfaces have garnered increasing attention for their ability to replicate the functionality of classic optical devices such as lenses, axicons, holograms, or beamsplitters [13] but using only subwavelengththin structures. In particular, silicon-based dielectric metasurfaces have enabled the fabrication of devices working with high efficiency in transmission mode. [14] However, all the metasurfaces demonstrated so far have been planar, often fabricated with conventional photolithography, and act on light normally incident to the substrate. In this paper, we demonstrate for the first time that twophoton direct laser writing-based 3D-printing [2a] can be used for the fabrication of a free-form inverse-designed PBS in the near-infrared range to control light propagation parallel to the substrate. The PBS acts as a meta-grating that can split parallel and perpendicular polarizations into the left and right first This paper presents an inverse-designed 3D-printed polymer-based broadband free-form polarization beamsplitter in the near-infrared. A computational inverse-design method is used to design a thin free-form meta-grating to split normally incident light with different polarizations to different diffraction orders, one toward the left first order and one toward the right first order. The grating is 3D-printed using two-photon direct laser writing. Polarization splitting behavior is experimentally observed in the near-infrared region for wavelengths of 1.3 and 1.55 µm, and with performance metrics close to the simulation values. The proposed platform combining inverse-design and 3D-printing can be extended to the design, fabrication, and integration of multiple broadband photonic structures to build devices with complex functionalities.
Enhanced transmission is essential in many application cases. However, as an ordinary method to e... more Enhanced transmission is essential in many application cases. However, as an ordinary method to enhance wave transmission, traditional extraordinary transmission based on nanohole array structures has no capability to widen its operation bandwidth. Here, we use a continuous monolayer black phosphorus film to enhance transmission through gold nanostructure arrays at the mid-infrared region. By exciting surface plasmon polaritons at the BP/gold nanostructure arrays interface, enhanced transmission over a broad range of wavelengths was theoretically demonstrated. Using finite-difference time-domain simulations, we analyzed the effects of geometric parameters on the transmission spectra and demonstrated unique polarization-dependent transmission enhancement in BP/gold silt arrays and BP/gold patch arrays, which originates from the anisotropic properties of BP. Our work provides new guidance to the design of broadband, polarization-dependent extraordinary transmission enhancement.
In this paper we present an inverse-designed 3D-printed all-dielectric stretchable millimeter wav... more In this paper we present an inverse-designed 3D-printed all-dielectric stretchable millimeter wave metalens with a tunable focal distance. Computational inverse-design method is used to design a flat metalens made of disconnected polymer building blocks with complex shapes, as opposed to conventional monolithic lenses. Proposed metalens provides better performance than a conventional Fresnel lens, using lesser amount of material and enabling larger focal distance tunability. The metalens is fabricated using a commercial 3D-printer and attached to a stretchable platform. Measurements and simulations show that focal distance can be tuned by a factor of 4 with a stretching factor of only 75%, a nearly diffraction-limited focal spot, and with a 70% relative focusing efficiency, defined as the ratio between power focused in the focal spot and power going through the focal plane. The proposed platform can be extended for design and fabrication of multiple electromagnetic devices working from visible to microwave radiation depending on scaling of the devices.
Phase engineered 2D MoSe 2 exhibits exciting physics but challenges for electronics. • Li-induced... more Phase engineered 2D MoSe 2 exhibits exciting physics but challenges for electronics. • Li-induced MoSe2 1T′ transition leads to increased, tunable, optical transparency. • Li should diffuse between chip and monolayer making monolayer, patterning unstable. • Instability indicates Li-based TMD phase engineering is unreliable for electronics.
Multiplexed surface encoding is achieved by positioning two different sizes of gold nanocubes on ... more Multiplexed surface encoding is achieved by positioning two different sizes of gold nanocubes on gold surfaces with precisely defined locations for each particle via template-confined, DNA-mediated nanoparticle assembly. As a proof-of-concept demonstration, cubes with 86 and 63 nm edge lengths are assembled into arrangements that physically and spectrally encrypt two sets of patterns in the same location. These patterns can be decrypted by mapping the absorption intensity of the substrate at λ = 773 and 687 nm, respectively. This multiplexed encoding platform dramatically increases the sophistication and density of codes that can be written using colloidal nanoparticles, which may enable high-security, high-resolution encoding applications.
DNA programmable assembly has been combined with top-down lithography to construct superlattices ... more DNA programmable assembly has been combined with top-down lithography to construct superlattices of discrete, reconfigurable nanoparticle architectures on a gold surface over large areas. Specifically, the assembly of individual colloidal plasmonic nanoparticles with different shapes and sizes is controlled by oligonucleotides containing "locked" nucleic acids and confined environments provided by polymer pores to yield oriented architectures that feature tunable arrangements and independently controllable distances at both nanometer and micrometer length scales. These structures, which would be difficult to construct via other common assembly methods, provide a platform to systematically study and control light-matter interactions in nanoparticle-based optical materials. The generality and potential of this approach are explored by identifying a broadband absorber with a solvent polarity response that allows dynamic tuning of visible light absorption.
Metasurfaces offer tremendous opportunities in controlling wave propagation in unusual ways that ... more Metasurfaces offer tremendous opportunities in controlling wave propagation in unusual ways that cannot be achieved with conventional optical devices. Common approach in designing metasurfaces has been the use of spatially varying, subwavelength-thick metallic and/or dielectric nanostructures for obtaining required phase change locally that yields desired optical performance. Here, we theoretically demonstrate an alternative metasurface based on time-varying resonant elements rather than space-varying ones. Our metasurface design utilizes graphene microribbon arrays that exhibit resonant behavior at terahertz wavelengths. By controlling the Fermi level, and therefore the doping of graphene, one can induce time-varying changes in the complex refractive indices of graphene, resulting in active control of the reflection amplitude and phase. Time-varying phase changes that can be achieved by applying a 1 GHz alternative current signal has been shown to change the frequency of a reflected terahertz wave on the...
Graphene is a monolayer plasmonic material that has been widely studied in the area of plasmonics... more Graphene is a monolayer plasmonic material that has been widely studied in the area of plasmonics and nanophotonics. Combining graphene with traditional plasmonic structures provides new opportunities and challenges. One particular application for nanostructured metals is enhanced optical transmission. However, extraordinary transmission (EOT) is known to have a frequency-selective performance due to size and periodicity of the nanohole arrays. Here, we propose to use a continuous graphene layer to enhance transmission through gold nanoslit arrays at mid-infrared (mid-IR) wavelengths. Although graphene absorbs 2.3% of light, by exciting surface plasmon polaritons (SPPs) at the graphene/gold nanoslit arrays interface, we have theoretically demonstrated enhanced infrared transmission over broad range of wavelengths in the mid-IR region. Our analyses of the effects of various structure parameters on the transmittance spectra shows that surface plasmon polaritons excited at the graphene...
Metamaterials, Metadevices, and Metasystems 2016, 2016
The objective-first inverse-design algorithm is used to design an ultra-compact all-dielectric op... more The objective-first inverse-design algorithm is used to design an ultra-compact all-dielectric optical diode. Based on silicon and air only, this optical diode relies on asymmetric spatial mode conversion between the left and right ports. The first even mode incident from the left port is transmitted to the right port after being converted into an odd mode. On the other hand, same mode incident from the right port is reflected back by the optical diode dielectric structure. The convergence and performance of the algorithm are studied, along with a transform method that converts continuous permittivity medium into a binary material design. The optimal device is studied with full electromagnetic simulations to compare its behavior under right and left incidences, in 2D and 3D settings as well. A broadband optical diode is reported with a large ratio between the two transmission directions. This illustrates the potential of the objective-first inverse-design method to design ultra-compact broadband photonic devices.
We propose and analyze theoretically an approach for realizing a tunable optical phased-array ant... more We propose and analyze theoretically an approach for realizing a tunable optical phased-array antenna utilizing the properties of VO 2 for electronic beam steering applications in the near-IR spectral range. The device is based on a 1D array of slot nano-antennas engraved in a thin Au film grown over VO 2 layer. The tuning is obtained by inducing a temperature gradient over the device, which changes the refractive index of the VO 2 , and hence modifies the phase response of the elements comprising the array, by producing a thermal gradient within the underlying PCM layer. Using a 10-element array, we show that an incident beam can be steered up to 22 ±° with respect to the normal, by applying a gradient of less than 10°C.
Resonant light absorption in metallic nanostructures results from highly localized electric field... more Resonant light absorption in metallic nanostructures results from highly localized electric fields that are crucial for many processes such as Raman scattering, photoluminescence, hot-carrier creation and photovoltaics. In recent years, there have been substantial amount of studies related to the design and realization of resonant optical absorbers from microwave to optical frequencies. However, there has been little thought that went into investigation of directiondependent absorption, and reflection characteristics of optical materials. In this study, we introduce and realize an absorber that is capable of absorbing light asymmetrically depending on the illumination direction. By designing asymmetrical dielectric permittivity profile along the propagation direction, we have been able to control the optical resonance strength of the absorber resulting in asymmetric light absorption and reflection at resonance wavelengths. The proposed structure consists of a square hole lattice etched into a free standing Si 3 N 4 /Ag bilayer of total thickness 200 nm. A 9-fold front to back absorption contrast was measured with the fabricated structures, whereas 13-fold contrast was achieved in finite difference time domain simulations. Moreover, mode profiles of observed resonances were discussed in detail to understand the physical mechanism of the asymmetric light absorption. Our analyses indicate that the same
Nanophotonics community has shown great interest in 2D materials because of their unique properti... more Nanophotonics community has shown great interest in 2D materials because of their unique properties of electromagnetic field manipulation. Many of these materials exhibit strong natural anisotropy, which further opens possibilities of polarization manipulation. Here, we show that α-MoO 3 , an emerging natural hyperbolic 2D material, can be combined with plasmonic nanostructures to provide strong extrinsic chirality in the visible range. A combination of biaxial anisotropy in α-MoO 3 and Fabry−Perot cavities with nanoscale features leads to different absorption of left and right circularly polarized photons, hence exhibiting circular dichroism (CD). Our simulation results predict that multilayer nanoscale-thick films including α-MoO 3 are potential candidates for achieving extrinsic chirality across the visible range. Furthermore, we show a significant CD increase when the α-MoO 3 layer is coupled with plasmonic nanohole arrays or plasmonic nanocubes. Such designs are achiral in geometry and therefore easier to fabricate. Moreover, we optimize the CD dissymmetry factor g CD for the nanocube-based design at 780 nm, obtaining 84%. We believe that utilizing biaxially anisotropic α-MoO 3 films to control and engineer chiro-optical properties in the visible frequency range will open research directions and enable enhanced functionalities in chiro-optical control at the nanoscale, further leading to applications in chiral sensing and CD.
ABSTRACT The design of aperture shape is a promising approach for enhanced transmission through a... more ABSTRACT The design of aperture shape is a promising approach for enhanced transmission through a subwavelength aperture. We designed split-ring-resonator (SRR)-shaped apertures in order to increase the transmission through subwavelength apertures by making use of the strong localization of the electromagnetic field in SRR-shaped apertures. We obtained a promising result of 104-fold enhancement by utilizing SRR-shaped apertures. It is possible to use these proposed structures at optical frequencies by making several modifications such as decreasing the sharpness of edges and increasing the gap width. Since SRRs are already being realized at optical frequencies, our proposed SRR-shaped aperture structures are promising candidates for novel applications. (C) 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3599873]
We experimentally investigate the semiconductor-to-metal transition (SMT) in vanadium dioxide thi... more We experimentally investigate the semiconductor-to-metal transition (SMT) in vanadium dioxide thin films using an infrared thermographic technique. During the semiconductor to metal phase change process, VO2 optical properties dynamically change and infrared emission undergoes a hysteresis loop due to differences between heating and cooling stages. The shape of the hysteresis loop was accurately monitored under different dynamic heating/cooling rates. In order to quantify and understand the effects of different rates, we used a numerical modelling approach in which a VO2 thin layer was modeled as metamaterial. The main experimental findings are interpreted assuming that both the rate of formation and shape of metallic inclusions are tuned with the heating/cooling rate. The structural transition from monoclinic to tetragonal phases is the main mechanism for controlling the global properties of the phase transition. However, our experimental results reveal that the dynamics of the heat...
Integrated optical sensors have garnered much interest for lab‐on‐chip applications such as chemi... more Integrated optical sensors have garnered much interest for lab‐on‐chip applications such as chemical and biological sensing and detection. Among various ways for detection schemes, spectral analysis performed by a spectrometer has shown great promise. Typically, such spectrometry is carried out in a relatively large device, owing to the fact that spectral resolution is often dictated by large optical path length. Here, a high‐resolution compact spectrometer utilizing random scattering events in a disordered medium is demonstrated. The spectrometer is inverse designed by objective‐first method and fabricated using two‐photon polymerization technique. The compact spectrometer consists of a disordered photonic structure and an inversed‐designed mode decomposer. A spectral resolution of 0.25 nm with a bandwidth of 30 nm in the near‐infrared regime is realized from a spectrometer occupying a relatively small footprint of 30 × 12.8 µm2. The proposed platform has a great potential to be used as a versatile lightweight and compact spectrometer for various applications including on‐chip spectrometer and sensing.
Plasmonic nanoparticles (NPs) [1] give rise to strong electric field (E-field) confinements, and ... more Plasmonic nanoparticles (NPs) [1] give rise to strong electric field (E-field) confinements, and therefore, when coupled with nearby emitters such as dye molecules or quantum dots, act as antennas to enhance their emission rates and intensities. [2,3] These so-called plasmonic nanoantennas can be used in lasing, [4]
that it typically uses a 780 nm wavelength, and only over a small volume at the focal point of a ... more that it typically uses a 780 nm wavelength, and only over a small volume at the focal point of a microscope objective is the intensity high enough for 2PP to occur (the polymer would normally polymerize in the UV at ≈390 nm). While the current 2PP resolution is still an order of magnitude larger than the best conventional lithographic methods (≈10 nm), 2PP direct laser writing lends itself to true 3D printing. It was demonstrated that such 2PP-based system can be widely used for fabricating high-quality micromachines for photonics and biomedicine. [6] However, in generally these structures does not have an urgent requirement for nanoresolution while this is particularly important for the types of highly complex geometries that emerge from inverse-designed free-form photonic devices, especially polarization handling devices such as polarization beamsplitters (PBSs), which play an important role in applications of nanophotonic chip design. There are many examples in the literatures focusing on waveguide-based PBS, of which the working principle is mainly based on modal evolution and modal coupling. [7] Generally, the latter including directional couplers (DCs), [8] multimode interference couplers, [9] and Mach-Zehnder interferometers, [10] are based on the mode beating behavior, providing a compact size especially for DCs. However, the fabrication could be challenging since the gap is very stringent although this can be solved by the other mechanism using a surface plasmon polariton. [11a] In addition, there are some plasmonic structures [8c,11] and photonic crystal-based PBSs, [12] etc. Recently, photonic metasurfaces have garnered increasing attention for their ability to replicate the functionality of classic optical devices such as lenses, axicons, holograms, or beamsplitters [13] but using only subwavelengththin structures. In particular, silicon-based dielectric metasurfaces have enabled the fabrication of devices working with high efficiency in transmission mode. [14] However, all the metasurfaces demonstrated so far have been planar, often fabricated with conventional photolithography, and act on light normally incident to the substrate. In this paper, we demonstrate for the first time that twophoton direct laser writing-based 3D-printing [2a] can be used for the fabrication of a free-form inverse-designed PBS in the near-infrared range to control light propagation parallel to the substrate. The PBS acts as a meta-grating that can split parallel and perpendicular polarizations into the left and right first This paper presents an inverse-designed 3D-printed polymer-based broadband free-form polarization beamsplitter in the near-infrared. A computational inverse-design method is used to design a thin free-form meta-grating to split normally incident light with different polarizations to different diffraction orders, one toward the left first order and one toward the right first order. The grating is 3D-printed using two-photon direct laser writing. Polarization splitting behavior is experimentally observed in the near-infrared region for wavelengths of 1.3 and 1.55 µm, and with performance metrics close to the simulation values. The proposed platform combining inverse-design and 3D-printing can be extended to the design, fabrication, and integration of multiple broadband photonic structures to build devices with complex functionalities.
Enhanced transmission is essential in many application cases. However, as an ordinary method to e... more Enhanced transmission is essential in many application cases. However, as an ordinary method to enhance wave transmission, traditional extraordinary transmission based on nanohole array structures has no capability to widen its operation bandwidth. Here, we use a continuous monolayer black phosphorus film to enhance transmission through gold nanostructure arrays at the mid-infrared region. By exciting surface plasmon polaritons at the BP/gold nanostructure arrays interface, enhanced transmission over a broad range of wavelengths was theoretically demonstrated. Using finite-difference time-domain simulations, we analyzed the effects of geometric parameters on the transmission spectra and demonstrated unique polarization-dependent transmission enhancement in BP/gold silt arrays and BP/gold patch arrays, which originates from the anisotropic properties of BP. Our work provides new guidance to the design of broadband, polarization-dependent extraordinary transmission enhancement.
In this paper we present an inverse-designed 3D-printed all-dielectric stretchable millimeter wav... more In this paper we present an inverse-designed 3D-printed all-dielectric stretchable millimeter wave metalens with a tunable focal distance. Computational inverse-design method is used to design a flat metalens made of disconnected polymer building blocks with complex shapes, as opposed to conventional monolithic lenses. Proposed metalens provides better performance than a conventional Fresnel lens, using lesser amount of material and enabling larger focal distance tunability. The metalens is fabricated using a commercial 3D-printer and attached to a stretchable platform. Measurements and simulations show that focal distance can be tuned by a factor of 4 with a stretching factor of only 75%, a nearly diffraction-limited focal spot, and with a 70% relative focusing efficiency, defined as the ratio between power focused in the focal spot and power going through the focal plane. The proposed platform can be extended for design and fabrication of multiple electromagnetic devices working from visible to microwave radiation depending on scaling of the devices.
Phase engineered 2D MoSe 2 exhibits exciting physics but challenges for electronics. • Li-induced... more Phase engineered 2D MoSe 2 exhibits exciting physics but challenges for electronics. • Li-induced MoSe2 1T′ transition leads to increased, tunable, optical transparency. • Li should diffuse between chip and monolayer making monolayer, patterning unstable. • Instability indicates Li-based TMD phase engineering is unreliable for electronics.
Multiplexed surface encoding is achieved by positioning two different sizes of gold nanocubes on ... more Multiplexed surface encoding is achieved by positioning two different sizes of gold nanocubes on gold surfaces with precisely defined locations for each particle via template-confined, DNA-mediated nanoparticle assembly. As a proof-of-concept demonstration, cubes with 86 and 63 nm edge lengths are assembled into arrangements that physically and spectrally encrypt two sets of patterns in the same location. These patterns can be decrypted by mapping the absorption intensity of the substrate at λ = 773 and 687 nm, respectively. This multiplexed encoding platform dramatically increases the sophistication and density of codes that can be written using colloidal nanoparticles, which may enable high-security, high-resolution encoding applications.
DNA programmable assembly has been combined with top-down lithography to construct superlattices ... more DNA programmable assembly has been combined with top-down lithography to construct superlattices of discrete, reconfigurable nanoparticle architectures on a gold surface over large areas. Specifically, the assembly of individual colloidal plasmonic nanoparticles with different shapes and sizes is controlled by oligonucleotides containing "locked" nucleic acids and confined environments provided by polymer pores to yield oriented architectures that feature tunable arrangements and independently controllable distances at both nanometer and micrometer length scales. These structures, which would be difficult to construct via other common assembly methods, provide a platform to systematically study and control light-matter interactions in nanoparticle-based optical materials. The generality and potential of this approach are explored by identifying a broadband absorber with a solvent polarity response that allows dynamic tuning of visible light absorption.
Metasurfaces offer tremendous opportunities in controlling wave propagation in unusual ways that ... more Metasurfaces offer tremendous opportunities in controlling wave propagation in unusual ways that cannot be achieved with conventional optical devices. Common approach in designing metasurfaces has been the use of spatially varying, subwavelength-thick metallic and/or dielectric nanostructures for obtaining required phase change locally that yields desired optical performance. Here, we theoretically demonstrate an alternative metasurface based on time-varying resonant elements rather than space-varying ones. Our metasurface design utilizes graphene microribbon arrays that exhibit resonant behavior at terahertz wavelengths. By controlling the Fermi level, and therefore the doping of graphene, one can induce time-varying changes in the complex refractive indices of graphene, resulting in active control of the reflection amplitude and phase. Time-varying phase changes that can be achieved by applying a 1 GHz alternative current signal has been shown to change the frequency of a reflected terahertz wave on the...
Graphene is a monolayer plasmonic material that has been widely studied in the area of plasmonics... more Graphene is a monolayer plasmonic material that has been widely studied in the area of plasmonics and nanophotonics. Combining graphene with traditional plasmonic structures provides new opportunities and challenges. One particular application for nanostructured metals is enhanced optical transmission. However, extraordinary transmission (EOT) is known to have a frequency-selective performance due to size and periodicity of the nanohole arrays. Here, we propose to use a continuous graphene layer to enhance transmission through gold nanoslit arrays at mid-infrared (mid-IR) wavelengths. Although graphene absorbs 2.3% of light, by exciting surface plasmon polaritons (SPPs) at the graphene/gold nanoslit arrays interface, we have theoretically demonstrated enhanced infrared transmission over broad range of wavelengths in the mid-IR region. Our analyses of the effects of various structure parameters on the transmittance spectra shows that surface plasmon polaritons excited at the graphene...
Metamaterials, Metadevices, and Metasystems 2016, 2016
The objective-first inverse-design algorithm is used to design an ultra-compact all-dielectric op... more The objective-first inverse-design algorithm is used to design an ultra-compact all-dielectric optical diode. Based on silicon and air only, this optical diode relies on asymmetric spatial mode conversion between the left and right ports. The first even mode incident from the left port is transmitted to the right port after being converted into an odd mode. On the other hand, same mode incident from the right port is reflected back by the optical diode dielectric structure. The convergence and performance of the algorithm are studied, along with a transform method that converts continuous permittivity medium into a binary material design. The optimal device is studied with full electromagnetic simulations to compare its behavior under right and left incidences, in 2D and 3D settings as well. A broadband optical diode is reported with a large ratio between the two transmission directions. This illustrates the potential of the objective-first inverse-design method to design ultra-compact broadband photonic devices.
We propose and analyze theoretically an approach for realizing a tunable optical phased-array ant... more We propose and analyze theoretically an approach for realizing a tunable optical phased-array antenna utilizing the properties of VO 2 for electronic beam steering applications in the near-IR spectral range. The device is based on a 1D array of slot nano-antennas engraved in a thin Au film grown over VO 2 layer. The tuning is obtained by inducing a temperature gradient over the device, which changes the refractive index of the VO 2 , and hence modifies the phase response of the elements comprising the array, by producing a thermal gradient within the underlying PCM layer. Using a 10-element array, we show that an incident beam can be steered up to 22 ±° with respect to the normal, by applying a gradient of less than 10°C.
Resonant light absorption in metallic nanostructures results from highly localized electric field... more Resonant light absorption in metallic nanostructures results from highly localized electric fields that are crucial for many processes such as Raman scattering, photoluminescence, hot-carrier creation and photovoltaics. In recent years, there have been substantial amount of studies related to the design and realization of resonant optical absorbers from microwave to optical frequencies. However, there has been little thought that went into investigation of directiondependent absorption, and reflection characteristics of optical materials. In this study, we introduce and realize an absorber that is capable of absorbing light asymmetrically depending on the illumination direction. By designing asymmetrical dielectric permittivity profile along the propagation direction, we have been able to control the optical resonance strength of the absorber resulting in asymmetric light absorption and reflection at resonance wavelengths. The proposed structure consists of a square hole lattice etched into a free standing Si 3 N 4 /Ag bilayer of total thickness 200 nm. A 9-fold front to back absorption contrast was measured with the fabricated structures, whereas 13-fold contrast was achieved in finite difference time domain simulations. Moreover, mode profiles of observed resonances were discussed in detail to understand the physical mechanism of the asymmetric light absorption. Our analyses indicate that the same
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Papers by Koray Aydin