Yuan Meng | Massachusetts Institute of Technology (MIT) (original) (raw)

Papers by Yuan Meng

Nanoscale Horizons, 2023

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered... more The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Nature Reviews Materials, 2023

The integration of functional nanomaterials and heterostructures with photonic architectures has ... more The integration of functional nanomaterials and heterostructures with photonic architectures has laid the foundation for important photonic and optoelectronic applications. The advent of epitaxy and layer lift-off techniques has enabled a wide spectrum of two-dimensional materials and three-dimensional single-crystalline freestanding thin films with diverse optical functionalities, featuring van der Waals (vdW) interfaces suitable for photonic vdW integration. Physical assembly leveraging vdW interactions eliminates the constraints of epitaxial lattice-matching, introducing unprecedented freedom to combine dissimilar materials with appealing optoelectronic properties but radically distinct crystal structures. Various prefabricated vdW building blocks can be combined in novel hetero-integrated photonic architectures and hybrid vdW heterostructures to prototype new devices and explore exotic nanophotonic phenomena at mixed-dimensional vdW interfaces. The ultrathin nature of these freestanding nanomembranes also enables flexible and lightweight photonic devices for low-cost wearable and multifunctional health-care applications. In this Review, we survey the recent progress in photonic nanomembranes with vdW interfaces, discussing a broad range of delaminated freestanding nanomembranes from film preparation to device implementation. We also analyse the remaining challenges and highlight emerging opportunities for advanced vdW hetero-integration.

Scientific reports, Jan 6, 2018

Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic ... more Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic devices with high flexibility. We propose a set of ultracompact tunable on-chip waveguide couplers with mode-selectivity and polarization sensitivity around the telecom wavelength of 1.55 μm, under the configuration of graphene-laminated silicon waveguides patterned with gold nanoantennas. Versatile couplings can be achieved in a widely tunable fashion within a deep-subwavelength area (210 × 210 nm), by marrying the advantages of tight field confinement in plasmonic antennas and the largely tunable carrier density of graphene. Incident light signals can be selectively coupled into different fundamental modes with good mode quality and high directionality exceeding 25 dB. Design scenarios for asymmetric couplings are presented, where the operation wavelength can be tuned across a 107-nm range around 1.55 mm by altering the chemical potential of graphene from 0 to 1.8 eV. Furthermore, the ...

Light: Science & Applications, 2021

The growing maturity of nanofabrication has ushered massive sophisticated optical structures avai... more The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous metawaveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-ba...

Communications Physics, 2021

Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for e... more Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for enhancing the spontaneous emission process towards both classical and quantum optical applications. Conventionally, light cannot be directly emitted from the plasmonic metal itself. Here, we explore the large field confinement and slow-light effect near the epsilon-near-zero (ENZ) frequency of the light-emitting material itself, to greatly enhance the "forbidden" two-plasmon spontaneous emission (2PSE) process. Using degeneratelydoped InSb as the plasmonic material and emitter simultaneously, we theoretically show that the 2PSE lifetime can be reduced from tens of milliseconds to several nanoseconds, comparable to the one-photon emission rate. Furthermore, we show that the optical nonlocality may largely govern the optical response of the ultrathin ENZ film. Efficient 2PSE from a doped semiconductor film may provide a pathway towards on-chip entangled light sources, with an emis...

Light: Science & Applications, 2021

The growing maturity of nanofabrication has ushered massive sophisticated optical structures avai... more The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous metawaveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

Optics Express, 2021

Metasurface has achieved fruitful results in tailoring optical fields in free space. However, a s... more Metasurface has achieved fruitful results in tailoring optical fields in free space. However, a systematic investigation on applying meta-optics to completely control waveguide modes is still elusive. Here we present a comprehensive catalog to selectively and exclusively couple free space light into arbitrary high-order waveguide modes of interest, leveraging silicon metasurface-patterned silicon nitride waveguides. By simultaneously engineering the matched phase gradient of the nanoantennas and the vectorial spatial modal overlap between the antenna near-field and target waveguide mode profile, either single or multiple high-order modes are successfully launched with high purity reaching 98%. Moreover, on-chip twisted light generators are theoretically proposed with configurable OAM topological charge ℓ from −3 to +2. This work may serve as a comprehensive framework for guided mode meta-optics and motivates further applications such as versatile integrated couplers, multiplexers, and mode-division multiplexing-based communication systems.

Communications Physics, 2021

Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for e... more Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for enhancing the spontaneous emission process towards both classical and quantum optical applications. Conventionally, light cannot be directly emitted from the plasmonic metal itself. Here, we explore the large field confinement and slow-light effect near the epsilon-near-zero (ENZ) frequency of the light-emitting material itself, to greatly enhance the "forbidden" two-plasmon spontaneous emission (2PSE) process. Using degeneratelydoped InSb as the plasmonic material and emitter simultaneously, we theoretically show that the 2PSE lifetime can be reduced from tens of milliseconds to several nanoseconds, comparable to the one-photon emission rate. Furthermore, we show that the optical nonlocality may largely govern the optical response of the ultrathin ENZ film. Efficient 2PSE from a doped semiconductor film may provide a pathway towards on-chip entangled light sources, with an emission wavelength and bandwidth widely tunable in the mid-infrared.

Optics Express, 2021

Energy transfer is an essential light-matter interaction. The transfer efficiency is critical for... more Energy transfer is an essential light-matter interaction. The transfer efficiency is critical for various applications such as light-emitting, optical modulation, and the photoelectric effect. Two primary forms of light-matter energy transfer, including absorption and emission, can be enhanced in optical cavities. Both forms can reach an extremum inside the cavity according to the coupled-mode theory. Graphene conductivity at the terahertz frequency can be tuned from positive to negative, providing a suitable material to study switchable extremums of these two forms. We integrate graphene with a nested cavity where an infrared cavity is inserted in a terahertz cavity, thereby achieving terahertz perfect absorption at the static state and optimal gain under photoexcitation. Leveraging an inserted infrared cavity, we can elevate the working efficiency by strongly absorbing the infrared pump. We also numerically show the feasibility of electrically tunable extreme energy transfer. Our concept of the nested cavity can be extended to different materials and even to guided modes. A switchable synergy of loss and gain potentially enables high-contrast dynamic modulation and photonic devices with multiplexing functions.

Photonics Research, 2020

Metasurface has found broad applicability in free-space optics, while its potential to tailor gui... more Metasurface has found broad applicability in free-space optics, while its potential to tailor guided waves remains barely explored. By synergizing the Jones matrix model with generalized Snell's law under the phase-matching condition, we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity, multiple working wavelengths, and high efficiency concurrently. The coupling direction, operation frequency, and excited mode type can be designed at will for arbitrary incident polarizations, outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability. Here, using silicon-nanoantennas-patterned silicon-nitride photonic waveguides, we numerically demonstrate a set of chip-scale optical couplers around 1.55 μm, including mode-selective directional couplers with high coupling efficiency over 57% and directivity about 23 dB. Polarization and wavelength demultiplexer scenarios are also proposed with 67% maximum efficiency and an extinction ratio of 20 dB. Moreover, a chip-integrated twisted light generator, coupling free-space linear polarization into an optical vortex carrying 1ℏ orbital angular momentum (OAM), is also reported to validate the mode-control flexibility. This comprehensive method may motivate compact wavelength/polarization (de)multiplexers, multifunctional mode converters, on-chip OAM generators for photonic integrated circuits, and high-speed optical telecommunications.

Optics Express, 2019

Epsilon-near-zero (ENZ) materials have recently been suggested as excellent candidates for constr... more Epsilon-near-zero (ENZ) materials have recently been suggested as excellent candidates for constructing all-optical and electro-optical switches in the infrared. The performance of previously reported ENZ material-based optical switches, however, has been greatly hampered by the low quality-(Q-) factor of the ENZ cavity, resulting in a large required optical pump fluence or applied voltage, a large insertion loss, or a small modulation depth. Here, we propose a solution by integrating the ENZ material into a Bragg microcavity, such that the Q-factor of the coupled cavity can be dramatically enhanced. Using high-mobility Dysprosium-doped cadmium oxide (CdO) as the prototype ENZ material, we numerically show an infrared all-optical switch with its reflectance modulated from near-zero to 94% under a pump fluence of only 7 μJ cm −2 , about a 59-time-reduction compared with a state-of-the-art Berreman-type cavity. Moreover, the high-Q coupled cavity can also be adopted to realize a reflective electro-optical switch. Its reflectance can be switched from near-zero to 89%, with a bias electric field well below the breakdown field of conventional gate dielectrics. The switching operation can further be extended to the transmission mode with a slightly modified cavity geometry, with its absolute transmittance modulated by 40%.

IEEE Photonics Journal, 2019

Dynamic manipulation of wavefront is vital for massive free-space optical applications. Here we p... more Dynamic manipulation of wavefront is vital for massive free-space optical applications. Here we propose a set of largely tunable circular polarization splitters leveraging graphene nanoantennas with high efficiency reaching 83% and wide frequency tunability range of 2 to 5 THz. By synergizing the electrically tunable surface plasmons of graphene with phase gradient metasurface, we numerically demonstrate two kinds of polarization split-ters with complimentary graphene patterns to realize electrical tuning of operation frequency and efficient circular polarization demultiplexing. Using antennas of different geometric sizes, the device performances are investigated in several different terahertz bands. Our proposed structures can facilitate dynamically tunable broadband and high-speed applications such as polarization demultiplexing and optical switches in terahertz regime.

Optics Express, 2019

While metasurfaces are now widely considered in free-space optics, their potential for coupling a... more While metasurfaces are now widely considered in free-space optics, their potential for coupling and tailoring guided waves is not fully explored. Here we transfer the Jones matrix method to target versatile on-chip coupling using metasurface-patterned photonic waveguides around the telecommunication wavelength of 1.55 µm, which can accommodate both propagation and Pancharatnam-Berry phase metasurfaces for guided waves. One can either encode two arbitrary and independent phase profiles to any pair of orthogonal polarizations or deploy complete control over both the phase and polarization of coupled modes. A set of design scenarios synergizing silicon nanoantennas and low-loss silicon-nitride waveguides are proposed, including directional couplers with mode-selectivity and polarization splitters with directionality ranging from 10 to 20 dB. Furthermore, our optimization method can be further extended to cover multiple working wavelengths. Exemplary on-chip color routers are also numerically demonstrated. This chip-integrated metasurface platform further translates the concept of a metasurface into photonic integrated circuits, serving as a positive paradigm for versatile and complete control over waveguide optical signals and motivating chip-scale applications such as polarization/wavelength demultiplexers, optical switches, and multifunctional mode converters.

Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic ... more Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic devices with high flexibility. We propose a set of ultracompact tunable on-chip waveguide couplers with mode-selectivity and polarization sensitivity around the telecom wavelength of 1.55 μm, under the configuration of graphene-laminated silicon waveguides patterned with gold nanoantennas. Versatile couplings can be achieved in a widely tunable fashion within a deep-subwavelength area (210 × 210 nm 2), by marrying the advantages of tight field confinement in plasmonic antennas and the largely tunable carrier density of graphene. Incident light signals can be selectively coupled into different fundamental modes with good mode quality and high directionality exceeding 25 dB. Design scenarios for asymmetric couplings are presented, where the operation wavelength can be tuned across a 107-nm range around 1.55 mm by altering the chemical potential of graphene from 0 to 1.8 eV. Furthermore, the proposed schemes can be leveraged as mode-sensitive on-chip directional waveguide signal detectors with an extinction ratio over 10 dB. Our results provide a new paradigm upon graphene-assisted tunable integrated photonic applications. Plasmonic nanoantennas attract tremendous research interest for their excellent capabilities to confine light in subwavelength volume 1. Judicious design and arrangement of these scatterers provide unprecedented degree of freedom in the manipulation of electromagnetic field, giving rise to a plethora of emerging applications such as enhanced light emission and detection 2,3 , sensing and optical metasurfaces 4,5. The investigation focus has been so far primarily centered around tailoring the propagation of light waves in free space 6 , while comparatively fewer attentions are devoted to their eminent potentials regarding guided waves in photonic waveguides 7,8. Meanwhile, photonic integrated circuits have been hailed as an appealing platform for optical information processing 9 , lab-on-a-chip systems and ultrafast chip-scale optical interconnects with low power dissipation 10,11 , holding the promise for revolutionizing conventional electronics and technologies. Practically, one may frequently require selectively in-coupling certain optical signals into certain guided mode with high directionality or out-coupling some specific wavelength channels out of the waveguide for processing or detection. Consequently, as an indispensable component that bridges connection between free-space light waves and guided signals in waveguides, compact directional couplers and waveguide detectors are of vital significance. However, conventional optical coupling components such as prisms and grating couplers are generally bulky 12,13. The introduction of plasmonic antennas can largely miniaturize device footprint to facilitate on-chip coupling applications. Recently demonstrated nanoantennas-based couplers are mainly concentrated on the directional launching of surface plasmon polaritons or their operation wavelengths are generally fixed once the devices are fabricated 14–19. Graphene was successfully leveraged in tunable metasurfaces working for free-space optics from near-to mid-infrared bands 20–24 , but the combination of graphene plasmonic antennas and photonic waveguides for the sake of tunable on-chip coupling applications still remains elusive. Here we numerically demonstrate a set of ultracompact tunable on-chip waveguide couplers under the syn-ergy of plasmonic nanoantennas and graphene-laminated silicon waveguides, possessing the capability of highly Published: xx xx xxxx OPEN

The concept of incorporating graphene into nanophotonic waveguides has pul-lulated into massive b... more The concept of incorporating graphene into nanophotonic waveguides has pul-lulated into massive broadband optoelectronic applications with compact footprint. We theoretically demonstrate that by solely altering the dimension design of graphene-laminated silicon waveguides, the phase, amplitude, and polarization of the fundamental propagating modes can all be effectively tailored under different bias voltages. Different device func-tionalities, including optical amplitude/phase modulators and polarizers, are ascribed into the devising of the effective mode index. A comprehensive analysis and unified design scenarios upon waveguide geometries are summarized, with fabrication robustness and moderate process complexity. Moreover, design examples are manifested. We report a TM-mode-based phase modulator, achieving a π phase shift within an active length of 49.2 μm with dual graphene layers. A feasible polarization-independent amplitude modulator is also demonstrated, where the discrepancy of the imaginary parts of the effective mode index between the two fundamental modes is kept at an order of 10 −5 over a broad wavelength range from 1.35 to 1.65 μm.

Nanoscale Horizons, 2023

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered... more The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Nature Reviews Materials, 2023

The integration of functional nanomaterials and heterostructures with photonic architectures has ... more The integration of functional nanomaterials and heterostructures with photonic architectures has laid the foundation for important photonic and optoelectronic applications. The advent of epitaxy and layer lift-off techniques has enabled a wide spectrum of two-dimensional materials and three-dimensional single-crystalline freestanding thin films with diverse optical functionalities, featuring van der Waals (vdW) interfaces suitable for photonic vdW integration. Physical assembly leveraging vdW interactions eliminates the constraints of epitaxial lattice-matching, introducing unprecedented freedom to combine dissimilar materials with appealing optoelectronic properties but radically distinct crystal structures. Various prefabricated vdW building blocks can be combined in novel hetero-integrated photonic architectures and hybrid vdW heterostructures to prototype new devices and explore exotic nanophotonic phenomena at mixed-dimensional vdW interfaces. The ultrathin nature of these freestanding nanomembranes also enables flexible and lightweight photonic devices for low-cost wearable and multifunctional health-care applications. In this Review, we survey the recent progress in photonic nanomembranes with vdW interfaces, discussing a broad range of delaminated freestanding nanomembranes from film preparation to device implementation. We also analyse the remaining challenges and highlight emerging opportunities for advanced vdW hetero-integration.

Scientific reports, Jan 6, 2018

Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic ... more Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic devices with high flexibility. We propose a set of ultracompact tunable on-chip waveguide couplers with mode-selectivity and polarization sensitivity around the telecom wavelength of 1.55 μm, under the configuration of graphene-laminated silicon waveguides patterned with gold nanoantennas. Versatile couplings can be achieved in a widely tunable fashion within a deep-subwavelength area (210 × 210 nm), by marrying the advantages of tight field confinement in plasmonic antennas and the largely tunable carrier density of graphene. Incident light signals can be selectively coupled into different fundamental modes with good mode quality and high directionality exceeding 25 dB. Design scenarios for asymmetric couplings are presented, where the operation wavelength can be tuned across a 107-nm range around 1.55 mm by altering the chemical potential of graphene from 0 to 1.8 eV. Furthermore, the ...

Light: Science & Applications, 2021

The growing maturity of nanofabrication has ushered massive sophisticated optical structures avai... more The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous metawaveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-ba...

Communications Physics, 2021

Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for e... more Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for enhancing the spontaneous emission process towards both classical and quantum optical applications. Conventionally, light cannot be directly emitted from the plasmonic metal itself. Here, we explore the large field confinement and slow-light effect near the epsilon-near-zero (ENZ) frequency of the light-emitting material itself, to greatly enhance the "forbidden" two-plasmon spontaneous emission (2PSE) process. Using degeneratelydoped InSb as the plasmonic material and emitter simultaneously, we theoretically show that the 2PSE lifetime can be reduced from tens of milliseconds to several nanoseconds, comparable to the one-photon emission rate. Furthermore, we show that the optical nonlocality may largely govern the optical response of the ultrathin ENZ film. Efficient 2PSE from a doped semiconductor film may provide a pathway towards on-chip entangled light sources, with an emis...

Light: Science & Applications, 2021

The growing maturity of nanofabrication has ushered massive sophisticated optical structures avai... more The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous metawaveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

Optics Express, 2021

Metasurface has achieved fruitful results in tailoring optical fields in free space. However, a s... more Metasurface has achieved fruitful results in tailoring optical fields in free space. However, a systematic investigation on applying meta-optics to completely control waveguide modes is still elusive. Here we present a comprehensive catalog to selectively and exclusively couple free space light into arbitrary high-order waveguide modes of interest, leveraging silicon metasurface-patterned silicon nitride waveguides. By simultaneously engineering the matched phase gradient of the nanoantennas and the vectorial spatial modal overlap between the antenna near-field and target waveguide mode profile, either single or multiple high-order modes are successfully launched with high purity reaching 98%. Moreover, on-chip twisted light generators are theoretically proposed with configurable OAM topological charge ℓ from −3 to +2. This work may serve as a comprehensive framework for guided mode meta-optics and motivates further applications such as versatile integrated couplers, multiplexers, and mode-division multiplexing-based communication systems.

Communications Physics, 2021

Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for e... more Plasmonic cavities can provide deep subwavelength light confinement, opening up new avenues for enhancing the spontaneous emission process towards both classical and quantum optical applications. Conventionally, light cannot be directly emitted from the plasmonic metal itself. Here, we explore the large field confinement and slow-light effect near the epsilon-near-zero (ENZ) frequency of the light-emitting material itself, to greatly enhance the "forbidden" two-plasmon spontaneous emission (2PSE) process. Using degeneratelydoped InSb as the plasmonic material and emitter simultaneously, we theoretically show that the 2PSE lifetime can be reduced from tens of milliseconds to several nanoseconds, comparable to the one-photon emission rate. Furthermore, we show that the optical nonlocality may largely govern the optical response of the ultrathin ENZ film. Efficient 2PSE from a doped semiconductor film may provide a pathway towards on-chip entangled light sources, with an emission wavelength and bandwidth widely tunable in the mid-infrared.

Optics Express, 2021

Energy transfer is an essential light-matter interaction. The transfer efficiency is critical for... more Energy transfer is an essential light-matter interaction. The transfer efficiency is critical for various applications such as light-emitting, optical modulation, and the photoelectric effect. Two primary forms of light-matter energy transfer, including absorption and emission, can be enhanced in optical cavities. Both forms can reach an extremum inside the cavity according to the coupled-mode theory. Graphene conductivity at the terahertz frequency can be tuned from positive to negative, providing a suitable material to study switchable extremums of these two forms. We integrate graphene with a nested cavity where an infrared cavity is inserted in a terahertz cavity, thereby achieving terahertz perfect absorption at the static state and optimal gain under photoexcitation. Leveraging an inserted infrared cavity, we can elevate the working efficiency by strongly absorbing the infrared pump. We also numerically show the feasibility of electrically tunable extreme energy transfer. Our concept of the nested cavity can be extended to different materials and even to guided modes. A switchable synergy of loss and gain potentially enables high-contrast dynamic modulation and photonic devices with multiplexing functions.

Photonics Research, 2020

Metasurface has found broad applicability in free-space optics, while its potential to tailor gui... more Metasurface has found broad applicability in free-space optics, while its potential to tailor guided waves remains barely explored. By synergizing the Jones matrix model with generalized Snell's law under the phase-matching condition, we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity, multiple working wavelengths, and high efficiency concurrently. The coupling direction, operation frequency, and excited mode type can be designed at will for arbitrary incident polarizations, outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability. Here, using silicon-nanoantennas-patterned silicon-nitride photonic waveguides, we numerically demonstrate a set of chip-scale optical couplers around 1.55 μm, including mode-selective directional couplers with high coupling efficiency over 57% and directivity about 23 dB. Polarization and wavelength demultiplexer scenarios are also proposed with 67% maximum efficiency and an extinction ratio of 20 dB. Moreover, a chip-integrated twisted light generator, coupling free-space linear polarization into an optical vortex carrying 1ℏ orbital angular momentum (OAM), is also reported to validate the mode-control flexibility. This comprehensive method may motivate compact wavelength/polarization (de)multiplexers, multifunctional mode converters, on-chip OAM generators for photonic integrated circuits, and high-speed optical telecommunications.

Optics Express, 2019

Epsilon-near-zero (ENZ) materials have recently been suggested as excellent candidates for constr... more Epsilon-near-zero (ENZ) materials have recently been suggested as excellent candidates for constructing all-optical and electro-optical switches in the infrared. The performance of previously reported ENZ material-based optical switches, however, has been greatly hampered by the low quality-(Q-) factor of the ENZ cavity, resulting in a large required optical pump fluence or applied voltage, a large insertion loss, or a small modulation depth. Here, we propose a solution by integrating the ENZ material into a Bragg microcavity, such that the Q-factor of the coupled cavity can be dramatically enhanced. Using high-mobility Dysprosium-doped cadmium oxide (CdO) as the prototype ENZ material, we numerically show an infrared all-optical switch with its reflectance modulated from near-zero to 94% under a pump fluence of only 7 μJ cm −2 , about a 59-time-reduction compared with a state-of-the-art Berreman-type cavity. Moreover, the high-Q coupled cavity can also be adopted to realize a reflective electro-optical switch. Its reflectance can be switched from near-zero to 89%, with a bias electric field well below the breakdown field of conventional gate dielectrics. The switching operation can further be extended to the transmission mode with a slightly modified cavity geometry, with its absolute transmittance modulated by 40%.

IEEE Photonics Journal, 2019

Dynamic manipulation of wavefront is vital for massive free-space optical applications. Here we p... more Dynamic manipulation of wavefront is vital for massive free-space optical applications. Here we propose a set of largely tunable circular polarization splitters leveraging graphene nanoantennas with high efficiency reaching 83% and wide frequency tunability range of 2 to 5 THz. By synergizing the electrically tunable surface plasmons of graphene with phase gradient metasurface, we numerically demonstrate two kinds of polarization split-ters with complimentary graphene patterns to realize electrical tuning of operation frequency and efficient circular polarization demultiplexing. Using antennas of different geometric sizes, the device performances are investigated in several different terahertz bands. Our proposed structures can facilitate dynamically tunable broadband and high-speed applications such as polarization demultiplexing and optical switches in terahertz regime.

Optics Express, 2019

While metasurfaces are now widely considered in free-space optics, their potential for coupling a... more While metasurfaces are now widely considered in free-space optics, their potential for coupling and tailoring guided waves is not fully explored. Here we transfer the Jones matrix method to target versatile on-chip coupling using metasurface-patterned photonic waveguides around the telecommunication wavelength of 1.55 µm, which can accommodate both propagation and Pancharatnam-Berry phase metasurfaces for guided waves. One can either encode two arbitrary and independent phase profiles to any pair of orthogonal polarizations or deploy complete control over both the phase and polarization of coupled modes. A set of design scenarios synergizing silicon nanoantennas and low-loss silicon-nitride waveguides are proposed, including directional couplers with mode-selectivity and polarization splitters with directionality ranging from 10 to 20 dB. Furthermore, our optimization method can be further extended to cover multiple working wavelengths. Exemplary on-chip color routers are also numerically demonstrated. This chip-integrated metasurface platform further translates the concept of a metasurface into photonic integrated circuits, serving as a positive paradigm for versatile and complete control over waveguide optical signals and motivating chip-scale applications such as polarization/wavelength demultiplexers, optical switches, and multifunctional mode converters.

Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic ... more Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic devices with high flexibility. We propose a set of ultracompact tunable on-chip waveguide couplers with mode-selectivity and polarization sensitivity around the telecom wavelength of 1.55 μm, under the configuration of graphene-laminated silicon waveguides patterned with gold nanoantennas. Versatile couplings can be achieved in a widely tunable fashion within a deep-subwavelength area (210 × 210 nm 2), by marrying the advantages of tight field confinement in plasmonic antennas and the largely tunable carrier density of graphene. Incident light signals can be selectively coupled into different fundamental modes with good mode quality and high directionality exceeding 25 dB. Design scenarios for asymmetric couplings are presented, where the operation wavelength can be tuned across a 107-nm range around 1.55 mm by altering the chemical potential of graphene from 0 to 1.8 eV. Furthermore, the proposed schemes can be leveraged as mode-sensitive on-chip directional waveguide signal detectors with an extinction ratio over 10 dB. Our results provide a new paradigm upon graphene-assisted tunable integrated photonic applications. Plasmonic nanoantennas attract tremendous research interest for their excellent capabilities to confine light in subwavelength volume 1. Judicious design and arrangement of these scatterers provide unprecedented degree of freedom in the manipulation of electromagnetic field, giving rise to a plethora of emerging applications such as enhanced light emission and detection 2,3 , sensing and optical metasurfaces 4,5. The investigation focus has been so far primarily centered around tailoring the propagation of light waves in free space 6 , while comparatively fewer attentions are devoted to their eminent potentials regarding guided waves in photonic waveguides 7,8. Meanwhile, photonic integrated circuits have been hailed as an appealing platform for optical information processing 9 , lab-on-a-chip systems and ultrafast chip-scale optical interconnects with low power dissipation 10,11 , holding the promise for revolutionizing conventional electronics and technologies. Practically, one may frequently require selectively in-coupling certain optical signals into certain guided mode with high directionality or out-coupling some specific wavelength channels out of the waveguide for processing or detection. Consequently, as an indispensable component that bridges connection between free-space light waves and guided signals in waveguides, compact directional couplers and waveguide detectors are of vital significance. However, conventional optical coupling components such as prisms and grating couplers are generally bulky 12,13. The introduction of plasmonic antennas can largely miniaturize device footprint to facilitate on-chip coupling applications. Recently demonstrated nanoantennas-based couplers are mainly concentrated on the directional launching of surface plasmon polaritons or their operation wavelengths are generally fixed once the devices are fabricated 14–19. Graphene was successfully leveraged in tunable metasurfaces working for free-space optics from near-to mid-infrared bands 20–24 , but the combination of graphene plasmonic antennas and photonic waveguides for the sake of tunable on-chip coupling applications still remains elusive. Here we numerically demonstrate a set of ultracompact tunable on-chip waveguide couplers under the syn-ergy of plasmonic nanoantennas and graphene-laminated silicon waveguides, possessing the capability of highly Published: xx xx xxxx OPEN

The concept of incorporating graphene into nanophotonic waveguides has pul-lulated into massive b... more The concept of incorporating graphene into nanophotonic waveguides has pul-lulated into massive broadband optoelectronic applications with compact footprint. We theoretically demonstrate that by solely altering the dimension design of graphene-laminated silicon waveguides, the phase, amplitude, and polarization of the fundamental propagating modes can all be effectively tailored under different bias voltages. Different device func-tionalities, including optical amplitude/phase modulators and polarizers, are ascribed into the devising of the effective mode index. A comprehensive analysis and unified design scenarios upon waveguide geometries are summarized, with fabrication robustness and moderate process complexity. Moreover, design examples are manifested. We report a TM-mode-based phase modulator, achieving a π phase shift within an active length of 49.2 μm with dual graphene layers. A feasible polarization-independent amplitude modulator is also demonstrated, where the discrepancy of the imaginary parts of the effective mode index between the two fundamental modes is kept at an order of 10 −5 over a broad wavelength range from 1.35 to 1.65 μm.