Bessel beams are of great interest due to their unique non-diffractive properties.Using a conical prism or an objective paired with an annular aperture are two typical approaches for generating zeroth-order Bessel bea...Bessel beams are of great interest due to their unique non-diffractive properties.Using a conical prism or an objective paired with an annular aperture are two typical approaches for generating zeroth-order Bessel beams.However,the former approach has a limited numerical aperture(NA),and the latter suffers from low efficiency,as most of the incident light is blocked by the aperture.Furthermore,an additional phase-modulating element is needed to generate higher-order Bessel beams,which in turn adds complexity and bulkiness to the system.We overcome these problems using dielectric metasurfaces to realize meta-axicons with additional functionalities not achievable with conventional means.We demonstrate meta-axicons with high NA up to 0.9 capable of generating Bessel beams with full width at half maximum about as small as~λ/3(λ=405 nm).Importantly,these Bessel beams have transverse intensity profiles independent of wavelength across the visible spectrum.These meta-axicons can enable advanced research and applications related to Bessel beams,such as laser fabrication,imaging and optical manipulation.展开更多
We directly measure optical bound states in the continuum(BICs)by embedding a photodetector into a photonic crystal slab.The BICs observed in our experiment are the result of accidental phase matching between incident...We directly measure optical bound states in the continuum(BICs)by embedding a photodetector into a photonic crystal slab.The BICs observed in our experiment are the result of accidental phase matching between incident,reflected and in-plane waves at seemingly random wave vectors in the photonic band structure.Our measurements were confirmed through a rigorously coupled-wave analysis simulation in conjunction with temporal coupled mode theory.Polarization mixing between photonic crystal slab modes was observed and described using a plane wave expansion simulation.The ability to probe the field intensity inside the photonic crystal and thereby to directly measure BICs represents a milestone in the development of integrated opto-electronic devices based on BICs.展开更多
The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics,stereochemistry and spintronics.However,these intrinsically chiral metamateria...The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics,stereochemistry and spintronics.However,these intrinsically chiral metamaterials are restricted to a complicated three-dimensional(3D)geometry,which leads to significant fabrication challenges,particularly at visible wavelengths.Their planar two-dimensional(2D)counterparts are limited by symmetry considerations to operation at oblique angles(extrinsic chirality)and possess significantly weaker chiro-optical responses close to normal incidence.Here,we address the challenge of realizing strong intrinsic chirality from thin,planar dielectric nanostructures.Most notably,we experimentally achieve near-unity circular dichroism with~90%of the light with the chosen helicity being transmitted at a wavelength of 540 nm.This is the highest value demonstrated to date for any geometry in the visible spectrum.We interpret this result within the charge-current multipole expansion framework and show that the excitation of higher-order multipoles is responsible for the giant circular dichroism.These experimental results enable the realization of high-performance miniaturized chiro-optical components in a scalable manner at optical frequencies.展开更多
We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron nitride thin flakes on a gold substrate.Such modes show substantially improved confinement and a group v...We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron nitride thin flakes on a gold substrate.Such modes show substantially improved confinement and a group velocity that is hundreds of times slower than the speed of light,thereby providing a new way to create slow light in the midinfrared range with a simple structure that does not require nano-patterning.One mode is the fundamental mode in the first Restrahlen band of hexagonal boron nitride thin crystals on a gold substrate;the other mode is equivalent to the second mode of the second Restrahlen band of hexagonal boron nitride flakes that are suspended in vacuum.The new modes also couple efficiently with incident light at the hexagonal boron nitride edges,as we demonstrate experimentally using photo-induced force microscopy and scanning near-field optical microscopy.The high confinement of these modes allows for Purcell factors that are on the order of tens of thousands directly above boron nitride and a wide band,with new perspectives for enhanced light-matter interaction.Our findings demonstrate a new approach to engineering the dispersion of polaritons in 2D materials to improve confinement and light-matter interaction,thereby paving the way for new applications in mid-infrared nano-optics.展开更多
Complex non-local behavior makes designing high efficiency and multifunctional metasurfaces a significant challenge.While using libraries of meta-atoms provide a simple and fast implementation methodology,pillar to pi...Complex non-local behavior makes designing high efficiency and multifunctional metasurfaces a significant challenge.While using libraries of meta-atoms provide a simple and fast implementation methodology,pillar to pillar interaction often imposes performance limitations.On the other extreme,inverse design based on topology optimization leverages non-local coupling to achieve high efficiency,but leads to complex and difficult to fabricate structures.In this paper,we demonstrate numerically and experimentally a shape optimization method that enables high efficiency metasurfaces while providing direct control of the structure complexity through a Fourier decomposition of the surface gradient.The proposed method provides a path towards manufacturability of inverse-designed high efficiency metasurfaces.展开更多
Structural colors have drawn wide attention for their potential as a future printing technology for various applications,ranging from biomimetic tissues to adaptive camouflage materials.However,an efficient approach t...Structural colors have drawn wide attention for their potential as a future printing technology for various applications,ranging from biomimetic tissues to adaptive camouflage materials.However,an efficient approach to realize robust colors with a scalable fabrication technique is still lacking,hampering the realization of practical applications with this platform.Here,we develop a new approach based on large-scale network metamaterials that combine dealloyed subwavelength structures at the nanoscale with lossless,ultra-thin dielectric coatings.By using theory and experiments,we show how subwavelength dielectric coatings control a mechanism of resonant light coupling with epsilon-near-zero regions generated in the metallic network,generating the formation of saturated structural colors that cover a wide portion of the spectrum.Ellipsometry measurements support the efficient observation of these colors,even at angles of 70°.The network-like architecture of these nanomaterials allows for high mechanical resistance,which is quantified in a series of nano-scratch tests.With such remarkable properties,these metastructures represent a robust design technology for real-world,large-scale commercial applications.展开更多
We report on electrical and optical properties of p＋-i-n＋ photodetectors/solar cells based on square millimeter arrays of InP nanowires （NWs） grown on InP substrates. The study includes a sample series where the p...We report on electrical and optical properties of p＋-i-n＋ photodetectors/solar cells based on square millimeter arrays of InP nanowires （NWs） grown on InP substrates. The study includes a sample series where the p＋-segment length was varied between 0 and 250 nm, as well as solar cells with 9.3% efficiency with similar design. The electrical data for all devices display clear rectifying behavior with an ideality factor between 1.8 and 2.5 at 300 K. From spectrally resolved photocurrent measurements, we conclude that the photocurrent generation process depends strongly on the p^-segment length. Without a p＋-segment, photogenerated carriers funneled from the substrate into the NWs contribute strongly to the photocurrent. Adding a p＋-segment decouples the substrate and shifts the depletion region, and collection of photogenerated carriers, to the NWs, in agreement with theoretical modeling. In optimized solar cells, clear spectral signatures of interband transitions in the zinc blende and wurtzite InP layers of the mixed-phase i-segments are observed. Complementary electroluminescence, transmission electron microscopy （TEM）, as well as measurements of the dependence of the photocurrent on angle of incidence and polarization, support our interpretations.展开更多
基金supported in part by the Air Force Office of Scientific Research(MURI,grant#FA9550-14-1-0389)Charles Stark Draper Laboratory,Inc.(SC001-0000000959)+4 种基金Thorlabs Incsupport from the Ministry of Science and Technology,Taiwan(104-2917-I-564-058)supported by a Charles Stark Draper Fellowshiparvard SEAS and A*STAR Singapore under the National Science Scholarship schemesupported by the National Science Foundation under NSF award no.1541959.CNS is a part of Harvard University.
文摘Bessel beams are of great interest due to their unique non-diffractive properties.Using a conical prism or an objective paired with an annular aperture are two typical approaches for generating zeroth-order Bessel beams.However,the former approach has a limited numerical aperture(NA),and the latter suffers from low efficiency,as most of the incident light is blocked by the aperture.Furthermore,an additional phase-modulating element is needed to generate higher-order Bessel beams,which in turn adds complexity and bulkiness to the system.We overcome these problems using dielectric metasurfaces to realize meta-axicons with additional functionalities not achievable with conventional means.We demonstrate meta-axicons with high NA up to 0.9 capable of generating Bessel beams with full width at half maximum about as small as~λ/3(λ=405 nm).Importantly,these Bessel beams have transverse intensity profiles independent of wavelength across the visible spectrum.These meta-axicons can enable advanced research and applications related to Bessel beams,such as laser fabrication,imaging and optical manipulation.
基金supported by the Austrian Science Funds(FWF):F 4909-N23(Next Lite)the PLATON project 35N within the Austrian NANO Initiative+1 种基金the‘Gesellschaft für Mikro-und Nanoelektronik’GMesupport from the European Research Council under the European Union’s Horizon 2020 research and innovation program(Grant Agreement No.639109).
文摘We directly measure optical bound states in the continuum(BICs)by embedding a photodetector into a photonic crystal slab.The BICs observed in our experiment are the result of accidental phase matching between incident,reflected and in-plane waves at seemingly random wave vectors in the photonic band structure.Our measurements were confirmed through a rigorously coupled-wave analysis simulation in conjunction with temporal coupled mode theory.Polarization mixing between photonic crystal slab modes was observed and described using a plane wave expansion simulation.The ability to probe the field intensity inside the photonic crystal and thereby to directly measure BICs represents a milestone in the development of integrated opto-electronic devices based on BICs.
基金supported in part by the Air Force Office of Scientific Research(MURI,Grant Nos FA9550-14-1-0389 and FA9550-16-1-0156)support from King Abdullah University of Science and Technology under Award OSR-2016-CRG5-2995+3 种基金Harvard SEAS and A*STAR Singapore under the National Science Scholarship schemesupport from the Ministry of Science and Technology,Taiwan(Grant No.104-2917-I-564-058)supported by the National Research Foundation,Prime Minister’s Office,Singapore under its Competitive Research Program(CRP Award No.NRF-CRP15-2015-03)supported by the National Science Foundation under NSF Award No.1541959.CNS is a part of Harvard University.
文摘The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics,stereochemistry and spintronics.However,these intrinsically chiral metamaterials are restricted to a complicated three-dimensional(3D)geometry,which leads to significant fabrication challenges,particularly at visible wavelengths.Their planar two-dimensional(2D)counterparts are limited by symmetry considerations to operation at oblique angles(extrinsic chirality)and possess significantly weaker chiro-optical responses close to normal incidence.Here,we address the challenge of realizing strong intrinsic chirality from thin,planar dielectric nanostructures.Most notably,we experimentally achieve near-unity circular dichroism with~90%of the light with the chosen helicity being transmitted at a wavelength of 540 nm.This is the highest value demonstrated to date for any geometry in the visible spectrum.We interpret this result within the charge-current multipole expansion framework and show that the excitation of higher-order multipoles is responsible for the giant circular dichroism.These experimental results enable the realization of high-performance miniaturized chiro-optical components in a scalable manner at optical frequencies.
基金supported by the National Science Foundation under NSF Award No.1541959supported by the National Science Foundation EFRI 2-DARE program through grant No.1542807the support of the Swiss National Science Foundation(SNSF)grant No.168545.
文摘We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron nitride thin flakes on a gold substrate.Such modes show substantially improved confinement and a group velocity that is hundreds of times slower than the speed of light,thereby providing a new way to create slow light in the midinfrared range with a simple structure that does not require nano-patterning.One mode is the fundamental mode in the first Restrahlen band of hexagonal boron nitride thin crystals on a gold substrate;the other mode is equivalent to the second mode of the second Restrahlen band of hexagonal boron nitride flakes that are suspended in vacuum.The new modes also couple efficiently with incident light at the hexagonal boron nitride edges,as we demonstrate experimentally using photo-induced force microscopy and scanning near-field optical microscopy.The high confinement of these modes allows for Purcell factors that are on the order of tens of thousands directly above boron nitride and a wide band,with new perspectives for enhanced light-matter interaction.Our findings demonstrate a new approach to engineering the dispersion of polaritons in 2D materials to improve confinement and light-matter interaction,thereby paving the way for new applications in mid-infrared nano-optics.
文摘Complex non-local behavior makes designing high efficiency and multifunctional metasurfaces a significant challenge.While using libraries of meta-atoms provide a simple and fast implementation methodology,pillar to pillar interaction often imposes performance limitations.On the other extreme,inverse design based on topology optimization leverages non-local coupling to achieve high efficiency,but leads to complex and difficult to fabricate structures.In this paper,we demonstrate numerically and experimentally a shape optimization method that enables high efficiency metasurfaces while providing direct control of the structure complexity through a Fourier decomposition of the surface gradient.The proposed method provides a path towards manufacturability of inverse-designed high efficiency metasurfaces.
基金the Air Force Office of Scientific Research(MURI:FA9550-14-1-0389)for financial supportthe Center for Nanoscale Systems(CNS),a member of the National Nanotechnology Coordinated Infrastructure(NNCI)+3 种基金supported by the National Science Foundation under NSF award no.1541959.CNS is part of Harvard Universitysupport from KAUST(Award CRG-1-2012-FRA-005)the financial support of the‘Size matters’project(TDA Capital Ltd,London,UK)the financial support by the Master Thesis Grant of the Zeno Karl Schindler Foundation(Switzerland).
文摘Structural colors have drawn wide attention for their potential as a future printing technology for various applications,ranging from biomimetic tissues to adaptive camouflage materials.However,an efficient approach to realize robust colors with a scalable fabrication technique is still lacking,hampering the realization of practical applications with this platform.Here,we develop a new approach based on large-scale network metamaterials that combine dealloyed subwavelength structures at the nanoscale with lossless,ultra-thin dielectric coatings.By using theory and experiments,we show how subwavelength dielectric coatings control a mechanism of resonant light coupling with epsilon-near-zero regions generated in the metallic network,generating the formation of saturated structural colors that cover a wide portion of the spectrum.Ellipsometry measurements support the efficient observation of these colors,even at angles of 70°.The network-like architecture of these nanomaterials allows for high mechanical resistance,which is quantified in a series of nano-scratch tests.With such remarkable properties,these metastructures represent a robust design technology for real-world,large-scale commercial applications.
文摘We report on electrical and optical properties of p＋-i-n＋ photodetectors/solar cells based on square millimeter arrays of InP nanowires （NWs） grown on InP substrates. The study includes a sample series where the p＋-segment length was varied between 0 and 250 nm, as well as solar cells with 9.3% efficiency with similar design. The electrical data for all devices display clear rectifying behavior with an ideality factor between 1.8 and 2.5 at 300 K. From spectrally resolved photocurrent measurements, we conclude that the photocurrent generation process depends strongly on the p^-segment length. Without a p＋-segment, photogenerated carriers funneled from the substrate into the NWs contribute strongly to the photocurrent. Adding a p＋-segment decouples the substrate and shifts the depletion region, and collection of photogenerated carriers, to the NWs, in agreement with theoretical modeling. In optimized solar cells, clear spectral signatures of interband transitions in the zinc blende and wurtzite InP layers of the mixed-phase i-segments are observed. Complementary electroluminescence, transmission electron microscopy （TEM）, as well as measurements of the dependence of the photocurrent on angle of incidence and polarization, support our interpretations.
