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Research Articles|122 Article(s)
Research Articles
On-chip digitally tunable positive/negative dispersion controller using bidirectional chirped multimode waveguide gratings
Shujun Liu, Ruitao Ma, Zejie Yu, Yaocheng Shi, and Daoxin Dai
A silicon-based digitally tunable positive/negative dispersion controller (DC) is proposed and realized for the first time using the cascaded bidirectional chirped multimode waveguide gratings (CMWGs), achieving positive and negative dispersion by switching the light propagation direction. A 1 × 2 Mach–Zehnder switch (MZS) and a 2 × 1 MZS are placed before and after to route the light path for realizing positive/negative switching. The device has Q stages of identical bidirectional CMWGs with a binary sequence. Thus the digital tuning is convenient and scalable, and the total dispersion accumulated by all the stages can be tuned digitally from - ( 2Q - 1 ) D0 to ( 2Q - 1 ) D0 with a step of D0 by controlling the switching states of all 2 × 2 MZSs, where D0 is the dispersion provided by a single bidirectional CMWG unit. Finally, a digitally tunable positive/negative DC with Q = 4 is designed and fabricated. These CMWGs are designed with a 4-mm-long grating section, enabling the dispersion D0 of about 4.16 ps / nm in a 20-nm-wide bandwidth. The dispersion is tuned from -61.53 to 63.77 ps / nm by switching all MZSs appropriately, and the corresponding group delay is varied from -1021 to 1037 ps. A silicon-based digitally tunable positive/negative dispersion controller (DC) is proposed and realized for the first time using the cascaded bidirectional chirped multimode waveguide gratings (CMWGs), achieving positive and negative dispersion by switching the light propagation direction. A 1 × 2 Mach–Zehnder switch (MZS) and a 2 × 1 MZS are placed before and after to route the light path for realizing positive/negative switching. The device has Q stages of identical bidirectional CMWGs with a binary sequence. Thus the digital tuning is convenient and scalable, and the total dispersion accumulated by all the stages can be tuned digitally from - ( 2Q - 1 ) D0 to ( 2Q - 1 ) D0 with a step of D0 by controlling the switching states of all 2 × 2 MZSs, where D0 is the dispersion provided by a single bidirectional CMWG unit. Finally, a digitally tunable positive/negative DC with Q = 4 is designed and fabricated. These CMWGs are designed with a 4-mm-long grating section, enabling the dispersion D0 of about 4.16 ps / nm in a 20-nm-wide bandwidth. The dispersion is tuned from -61.53 to 63.77 ps / nm by switching all MZSs appropriately, and the corresponding group delay is varied from -1021 to 1037 ps.
Advanced Photonics
- Publication Date: Nov. 22, 2023
- Vol. 5, Issue 6, 066005 (2023)
Resolution enhancement with deblurring by pixel reassignment
Bingying Zhao, and Jerome Mertz
Improving the spatial resolution of a fluorescence microscope has been an ongoing challenge in the imaging community. To address this challenge, a variety of approaches have been taken, ranging from instrumentation development to image postprocessing. An example of the latter is deconvolution, where images are numerically deblurred based on a knowledge of the microscope point spread function. However, deconvolution can easily lead to noise-amplification artifacts. Deblurring by postprocessing can also lead to negativities or fail to conserve local linearity between sample and image. We describe here a simple image deblurring algorithm based on pixel reassignment that inherently avoids such artifacts and can be applied to general microscope modalities and fluorophore types. Our algorithm helps distinguish nearby fluorophores, even when these are separated by distances smaller than the conventional resolution limit, helping facilitate, for example, the application of single-molecule localization microscopy in dense samples. We demonstrate the versatility and performance of our algorithm under a variety of imaging conditions. Improving the spatial resolution of a fluorescence microscope has been an ongoing challenge in the imaging community. To address this challenge, a variety of approaches have been taken, ranging from instrumentation development to image postprocessing. An example of the latter is deconvolution, where images are numerically deblurred based on a knowledge of the microscope point spread function. However, deconvolution can easily lead to noise-amplification artifacts. Deblurring by postprocessing can also lead to negativities or fail to conserve local linearity between sample and image. We describe here a simple image deblurring algorithm based on pixel reassignment that inherently avoids such artifacts and can be applied to general microscope modalities and fluorophore types. Our algorithm helps distinguish nearby fluorophores, even when these are separated by distances smaller than the conventional resolution limit, helping facilitate, for example, the application of single-molecule localization microscopy in dense samples. We demonstrate the versatility and performance of our algorithm under a variety of imaging conditions.
Advanced Photonics
- Publication Date: Oct. 27, 2023
- Vol. 5, Issue 6, 066004 (2023)
Integrated terahertz vortex beam emitter for rotating target detection
Jingya Xie, Jun Qian, Tengjiao Wang, Linjie Zhou, Xiaofei Zang, Lin Chen, Yiming Zhu, and Songlin Zhuang
We propose a terahertz (THz) vortex emitter that utilizes a high-resistance silicon resonator to generate vortex beams with various topological charges. Addressing the challenge of double circular polarization superposition resulting from the high refractive index contrast, we regulate the transverse spin state through a newly designed second-order grating partially etched on the waveguide’s top side. The reflected wave can be received directly by a linearly polarized antenna, simplifying the process. Benefiting from the tuning feature, a joint detection method involving positive and negative topological charges identifies and detects rotational Doppler effects amid robust micro-Doppler interference signals. This emitter can be used for the rotational velocity measurement of an on-axis spinning object, achieving an impressive maximum speed error rate of ∼2 % . This approach holds promise for the future development of THz vortex beam applications in radar target detection and countermeasure systems, given its low cost and potential for mass production. We propose a terahertz (THz) vortex emitter that utilizes a high-resistance silicon resonator to generate vortex beams with various topological charges. Addressing the challenge of double circular polarization superposition resulting from the high refractive index contrast, we regulate the transverse spin state through a newly designed second-order grating partially etched on the waveguide’s top side. The reflected wave can be received directly by a linearly polarized antenna, simplifying the process. Benefiting from the tuning feature, a joint detection method involving positive and negative topological charges identifies and detects rotational Doppler effects amid robust micro-Doppler interference signals. This emitter can be used for the rotational velocity measurement of an on-axis spinning object, achieving an impressive maximum speed error rate of ∼2 % . This approach holds promise for the future development of THz vortex beam applications in radar target detection and countermeasure systems, given its low cost and potential for mass production.
Advanced Photonics
- Publication Date: Oct. 19, 2023
- Vol. 5, Issue 6, 066002 (2023)
Photoswitchable vibrational nanoscopy with sub-100-nm optical resolution
Jianpeng Ao, Xiaofeng Fang, Liyang Ma, Zhijie Liu, Simin Wu, Changfeng Wu, and Minbiao Ji
Stimulated Raman scattering (SRS) microscopy has shown superior chemical resolution due to the much narrower vibrational spectral bandwidth than its fluorescence counterpart. However, breaking the diffraction-limited spatial resolution of SRS imaging is much more challenging because of the intrinsically weak scattering cross section and inert/stable nature of molecular bond vibrations. We report superresolution SRS (SR-SRS) nanoscopy based on reversible-switchable vibrational photochromic probes integrated with point spread function engineering strategy. By introducing a Gaussian-shaped ultraviolet excitation beam and a donut-shaped visible depletion beam in addition to the pump and Stokes beams, SR-SRS could reach sub-100 nm resolution on photoswitchable nanoparticles (NPs). Furthermore, NP-treated live cell imaging was demonstrated with resolution improvement by a factor of ∼4. Our proof-of-principle work provides the potential for SR vibrational imaging to assist research on complex biological systems. Stimulated Raman scattering (SRS) microscopy has shown superior chemical resolution due to the much narrower vibrational spectral bandwidth than its fluorescence counterpart. However, breaking the diffraction-limited spatial resolution of SRS imaging is much more challenging because of the intrinsically weak scattering cross section and inert/stable nature of molecular bond vibrations. We report superresolution SRS (SR-SRS) nanoscopy based on reversible-switchable vibrational photochromic probes integrated with point spread function engineering strategy. By introducing a Gaussian-shaped ultraviolet excitation beam and a donut-shaped visible depletion beam in addition to the pump and Stokes beams, SR-SRS could reach sub-100 nm resolution on photoswitchable nanoparticles (NPs). Furthermore, NP-treated live cell imaging was demonstrated with resolution improvement by a factor of ∼4. Our proof-of-principle work provides the potential for SR vibrational imaging to assist research on complex biological systems.
Advanced Photonics
- Publication Date: Sep. 30, 2023
- Vol. 5, Issue 6, 066001 (2023)
On-chip metamaterial-enabled high-order mode-division multiplexing
Yu He, Xingfeng Li, Yong Zhang, Shaohua An, Hongwei Wang, Zhen Wang, Haoshuo Chen, Yetian Huang, Hanzi Huang, Nicolas K. Fontaine, Roland Ryf, Yuhan Du, Lu Sun, Xingchen Ji, Xuhan Guo, Yingxiong Song, Qianwu Zhang, and Yikai Su
Mode-division multiplexing (MDM) technology enables high-bandwidth data transmission using orthogonal waveguide modes to construct parallel data streams. However, few demonstrations have been realized for generating and supporting high-order modes, mainly due to the intrinsic large material group-velocity dispersion (GVD), which make it challenging to selectively couple different-order spatial modes. We show the feasibility of on-chip GVD engineering by introducing a gradient-index metamaterial structure, which enables a robust and fully scalable MDM process. We demonstrate a record-high-order MDM device that supports TE0–TE15 modes simultaneously. 40-GBaud 16-ary quadrature amplitude modulation signals encoded on 16 mode channels contribute to a 2.162 Tbit / s net data rate, which is the highest data rate ever reported for an on-chip single-wavelength transmission. Our method can effectively expand the number of channels provided by MDM technology and promote the emerging research fields with great demand for parallelism, such as high-capacity optical interconnects, high-dimensional quantum communications, and large-scale neural networks. Mode-division multiplexing (MDM) technology enables high-bandwidth data transmission using orthogonal waveguide modes to construct parallel data streams. However, few demonstrations have been realized for generating and supporting high-order modes, mainly due to the intrinsic large material group-velocity dispersion (GVD), which make it challenging to selectively couple different-order spatial modes. We show the feasibility of on-chip GVD engineering by introducing a gradient-index metamaterial structure, which enables a robust and fully scalable MDM process. We demonstrate a record-high-order MDM device that supports TE0–TE15 modes simultaneously. 40-GBaud 16-ary quadrature amplitude modulation signals encoded on 16 mode channels contribute to a 2.162 Tbit / s net data rate, which is the highest data rate ever reported for an on-chip single-wavelength transmission. Our method can effectively expand the number of channels provided by MDM technology and promote the emerging research fields with great demand for parallelism, such as high-capacity optical interconnects, high-dimensional quantum communications, and large-scale neural networks.
Advanced Photonics
- Publication Date: Sep. 13, 2023
- Vol. 5, Issue 5, 056008 (2023)
Fluorescence interference structured illumination microscopy for 3D morphology imaging with high axial resolution|On the Cover
Yile Sun, Hongfei Zhu, Lu Yin, Hanmeng Wu, Mingxuan Cai, Weiyun Sun, Yueshu Xu, Xinxun Yang, Jiaxiao Han, Wenjie Liu, Yubing Han, Xiang Hao, Renjie Zhou, Cuifang Kuang, and Xu Liu
Imaging three-dimensional, subcellular structures with high axial resolution has always been the core purpose of fluorescence microscopy. However, trade-offs exist between axial resolution and other important technical indicators, such as temporal resolution, optical power density, and imaging process complexity. We report a new imaging modality, fluorescence interference structured illumination microscopy (FI-SIM), which is based on three-dimensional structured illumination microscopy for wide-field lateral imaging and fluorescence interference for axial reconstruction. FI-SIM can acquire images quickly within the order of hundreds of milliseconds and exhibit even 30 nm axial resolution in half the wavelength depth range without z-axis scanning. Moreover, the relatively low laser power density relaxes the requirements for dyes and enables a wide range of applications for observing fixed and live subcellular structures. Imaging three-dimensional, subcellular structures with high axial resolution has always been the core purpose of fluorescence microscopy. However, trade-offs exist between axial resolution and other important technical indicators, such as temporal resolution, optical power density, and imaging process complexity. We report a new imaging modality, fluorescence interference structured illumination microscopy (FI-SIM), which is based on three-dimensional structured illumination microscopy for wide-field lateral imaging and fluorescence interference for axial reconstruction. FI-SIM can acquire images quickly within the order of hundreds of milliseconds and exhibit even 30 nm axial resolution in half the wavelength depth range without z-axis scanning. Moreover, the relatively low laser power density relaxes the requirements for dyes and enables a wide range of applications for observing fixed and live subcellular structures.
Advanced Photonics
- Publication Date: Sep. 19, 2023
- Vol. 5, Issue 5, 056007 (2023)
Nonrelativistic and nonmagnetic terahertz-wave generation via ultrafast current control in anisotropic conductive heterostructures
Sheng Zhang, Yongwei Cui, Shunjia Wang, Haoran Chen, Yaxin Liu, Wentao Qin, Tongyang Guan, Chuanshan Tian, Zhe Yuan, Lei Zhou, Yizheng Wu, and Zhensheng Tao
Precise and ultrafast control over photo-induced charge currents across nanoscale interfaces could lead to important applications in energy harvesting, ultrafast electronics, and coherent terahertz sources. Recent studies have shown that several relativistic mechanisms, including inverse spin-Hall effect, inverse Rashba–Edelstein effect, and inverse spin-orbit-torque effect, can convert longitudinally injected spin-polarized currents from magnetic materials to transverse charge currents, thereby harnessing these currents for terahertz generation. However, these mechanisms typically require external magnetic fields and exhibit limitations in terms of spin-polarization rates and efficiencies of relativistic spin-to-charge conversion. We present a nonrelativistic and nonmagnetic mechanism that directly utilizes the photoexcited high-density charge currents across the interface. We demonstrate that the electrical anisotropy of conductive oxides RuO2 and IrO2 can effectively deflect injected charge currents to the transverse direction, resulting in efficient and broadband terahertz radiation. Importantly, this mechanism has the potential to offer much higher conversion efficiency compared to previous methods, as conductive materials with large electrical anisotropy are readily available, whereas further increasing the spin-Hall angle of heavy-metal materials would be challenging. Our findings offer exciting possibilities for directly utilizing these photoexcited high-density currents across metallic interfaces for ultrafast electronics and terahertz spectroscopy. Precise and ultrafast control over photo-induced charge currents across nanoscale interfaces could lead to important applications in energy harvesting, ultrafast electronics, and coherent terahertz sources. Recent studies have shown that several relativistic mechanisms, including inverse spin-Hall effect, inverse Rashba–Edelstein effect, and inverse spin-orbit-torque effect, can convert longitudinally injected spin-polarized currents from magnetic materials to transverse charge currents, thereby harnessing these currents for terahertz generation. However, these mechanisms typically require external magnetic fields and exhibit limitations in terms of spin-polarization rates and efficiencies of relativistic spin-to-charge conversion. We present a nonrelativistic and nonmagnetic mechanism that directly utilizes the photoexcited high-density charge currents across the interface. We demonstrate that the electrical anisotropy of conductive oxides RuO2 and IrO2 can effectively deflect injected charge currents to the transverse direction, resulting in efficient and broadband terahertz radiation. Importantly, this mechanism has the potential to offer much higher conversion efficiency compared to previous methods, as conductive materials with large electrical anisotropy are readily available, whereas further increasing the spin-Hall angle of heavy-metal materials would be challenging. Our findings offer exciting possibilities for directly utilizing these photoexcited high-density currents across metallic interfaces for ultrafast electronics and terahertz spectroscopy.
Advanced Photonics
- Publication Date: Sep. 12, 2023
- Vol. 5, Issue 5, 056006 (2023)
Unconventional bound states in the continuum from metamaterial-induced electron acoustic waves
Wenhui Wang, Antonio Günzler, Bodo D. Wilts, Ullrich Steiner, and Matthias Saba
Photonic bound states in the continuum (BICs) are spatially localized modes with infinitely long lifetimes, which exist within a radiation continuum at discrete energy levels. These states have been explored in various systems, including photonic and phononic crystal slabs, metasurfaces, waveguides, and integrated circuits. Robustness and availability of the BICs are important aspects for fully taming the BICs toward practical applications. Here, we propose a generic mechanism to realize BICs that exist by first principles free of fine parameter tuning based on non-Maxwellian double-net metamaterials (DNMs). An ideal warm hydrodynamic double plasma (HDP) fluid model provides a homogenized description of DNMs and explains the robustness of the BICs found herein. In the HDP model, these are standing wave formations made of electron acoustic waves (EAWs), which are pure charge oscillations with vanishing electromagnetic fields. EAW BICs have various advantages, such as (i) frequency-comb-like collection of BICs free from normal resonances; (ii) robustness to symmetry-breaking perturbations and formation of quasi-BICs with an ultrahigh Q-factor even if subject to disorder; and (iii) giving rise to subwavelength microcavity resonators hosting quasi-BIC modes with an ultrahigh Q-factor. Photonic bound states in the continuum (BICs) are spatially localized modes with infinitely long lifetimes, which exist within a radiation continuum at discrete energy levels. These states have been explored in various systems, including photonic and phononic crystal slabs, metasurfaces, waveguides, and integrated circuits. Robustness and availability of the BICs are important aspects for fully taming the BICs toward practical applications. Here, we propose a generic mechanism to realize BICs that exist by first principles free of fine parameter tuning based on non-Maxwellian double-net metamaterials (DNMs). An ideal warm hydrodynamic double plasma (HDP) fluid model provides a homogenized description of DNMs and explains the robustness of the BICs found herein. In the HDP model, these are standing wave formations made of electron acoustic waves (EAWs), which are pure charge oscillations with vanishing electromagnetic fields. EAW BICs have various advantages, such as (i) frequency-comb-like collection of BICs free from normal resonances; (ii) robustness to symmetry-breaking perturbations and formation of quasi-BICs with an ultrahigh Q-factor even if subject to disorder; and (iii) giving rise to subwavelength microcavity resonators hosting quasi-BIC modes with an ultrahigh Q-factor.
Advanced Photonics
- Publication Date: Sep. 04, 2023
- Vol. 5, Issue 5, 056005 (2023)
Precise photoelectrochemical tuning of semiconductor microdisk lasers
Debarghya Sarkar, Paul H. Dannenberg, Nicola Martino, Kwon-Hyeon Kim, Yue Wu, and Seok-Hyun Yun
Micro- and nanodisk lasers have emerged as promising optical sources and probes for on-chip and free-space applications. However, the randomness in disk diameter introduced by standard nanofabrication makes it challenging to obtain deterministic wavelengths. To address this, we developed a photoelectrochemical (PEC) etching-based technique that enables us to precisely tune the lasing wavelength with subnanometer accuracy. We examined the PEC mechanism and compound semiconductor etching rate in diluted sulfuric acid solution. Using this technique, we produced microlasers on a chip and isolated particles with distinct lasing wavelengths. These precisely tuned disk lasers were then used to tag cells in culture. Our results demonstrate that this scalable technique can be used to produce groups of lasers with precise emission wavelengths for various nanophotonic and biomedical applications. Micro- and nanodisk lasers have emerged as promising optical sources and probes for on-chip and free-space applications. However, the randomness in disk diameter introduced by standard nanofabrication makes it challenging to obtain deterministic wavelengths. To address this, we developed a photoelectrochemical (PEC) etching-based technique that enables us to precisely tune the lasing wavelength with subnanometer accuracy. We examined the PEC mechanism and compound semiconductor etching rate in diluted sulfuric acid solution. Using this technique, we produced microlasers on a chip and isolated particles with distinct lasing wavelengths. These precisely tuned disk lasers were then used to tag cells in culture. Our results demonstrate that this scalable technique can be used to produce groups of lasers with precise emission wavelengths for various nanophotonic and biomedical applications.
Advanced Photonics
- Publication Date: Sep. 01, 2023
- Vol. 5, Issue 5, 056004 (2023)
Finding the superior mode basis for mode-division multiplexing: a comparison of spatial modes in air-core fiber
Hongya Wang, Jianzhou Ai, Zelin Ma, Siddharth Ramachandran, and Jian Wang
Diverse spatial mode bases can be exploited in mode-division multiplexing (MDM) to sustain the capacity growth in fiber-optic communications, such as linearly polarized (LP) modes, vector modes, LP orbital angular momentum (LP-OAM) modes, and circularly polarized OAM (CP-OAM) modes. Nevertheless, which kind of mode bases is more appropriate to be utilized in fiber still remains unclear. Here, we aim to find the superior mode basis in MDM fiber-optic communications via a system-level comparison in air-core fiber (ACF). We first investigate the walk-off effect of four spatial mode bases over 1-km ACF, where LP and LP-OAM modes show intrinsic mode walk-off, while it is negligible for vector and CP-OAM modes. We then study the mode coupling effect of degenerate vector and CP-OAM modes over 1-km ACF under fiber perturbations, where degenerate even and odd vector modes suffer severe mode cross talk, while negligible for high-order degenerate CP-OAM modes based on the laws of angular momentum conservation. Moreover, we comprehensively evaluate the system-level performance for data-carrying single-channel and two-channel MDM transmission with different spatial mode bases under various kinds of fiber perturbations (bending, twisting, pressing, winding, and out-of-plane moving). The obtained results indicate that the CP-OAM mode basis shows superiority compared to other mode bases in MDM fiber-optic communications without using multiple-input multiple-output digital signal processing. Our findings may pave the way for robust short-reach MDM optical interconnects for data centers and high-performance computing. Diverse spatial mode bases can be exploited in mode-division multiplexing (MDM) to sustain the capacity growth in fiber-optic communications, such as linearly polarized (LP) modes, vector modes, LP orbital angular momentum (LP-OAM) modes, and circularly polarized OAM (CP-OAM) modes. Nevertheless, which kind of mode bases is more appropriate to be utilized in fiber still remains unclear. Here, we aim to find the superior mode basis in MDM fiber-optic communications via a system-level comparison in air-core fiber (ACF). We first investigate the walk-off effect of four spatial mode bases over 1-km ACF, where LP and LP-OAM modes show intrinsic mode walk-off, while it is negligible for vector and CP-OAM modes. We then study the mode coupling effect of degenerate vector and CP-OAM modes over 1-km ACF under fiber perturbations, where degenerate even and odd vector modes suffer severe mode cross talk, while negligible for high-order degenerate CP-OAM modes based on the laws of angular momentum conservation. Moreover, we comprehensively evaluate the system-level performance for data-carrying single-channel and two-channel MDM transmission with different spatial mode bases under various kinds of fiber perturbations (bending, twisting, pressing, winding, and out-of-plane moving). The obtained results indicate that the CP-OAM mode basis shows superiority compared to other mode bases in MDM fiber-optic communications without using multiple-input multiple-output digital signal processing. Our findings may pave the way for robust short-reach MDM optical interconnects for data centers and high-performance computing.
Advanced Photonics
- Publication Date: Sep. 01, 2023
- Vol. 5, Issue 5, 056003 (2023)
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