Efficient sum-frequency generation of a yellow laser in a thin-film lithium niobate waveguide.

Yellow lasers with high efficiency and tunability play an essential role in many applications. Here, we demonstrate the sum-frequency generation (SFG) of yellow light on a periodically poled thin-film lithium niobate (PP-TFLN) waveguide. Taking advantage of large χ(2) nonlinearity, a high normalized conversion efficiency of 10,097% (W·cm2) is obtained with pump wavelengths of 1317.7 and 1064 nm. An absolute conversion efficiency of 24.17% is recorded with on-chip pump powers of 10.4 dBm (O-band) and 13.5 dBm (1064 nm).

Virtual-State Model for Analyzing Electro-Optical Modulation in Ring Resonators.

Ring resonators play a crucial role in optical communication and quantum technology applications. However, these devices lack a simple and intuitive theoretical model to describe their electro-optical modulation. When the resonance frequency is rapidly modulated, the filtering and modulation within a ring resonator become physically intertwined, making it difficult to analyze the complex physical processes involved. We address this by proposing an analytical solution for electro-optic ring modulators based on the concept of a "virtual state." This approach equates a lightwave passing through a dynamic ring modulator to one excited to a virtual state by a cumulative phase and then returning to the real state after exiting the static ring. Our model simplifies the independent analysis of the intertwined physical processes, enhancing its versatility in analyzing various incident signals and modulation formats. Experimental results, including resonant and detuning modulation, align with the numerical simulation of our model. Notably, our findings indicate that the dynamic modulation of the ring resonator under detuning driving approximates phase modulation.

Sustainable technologies for the recycling and upcycling of precious metals from e-waste.

E-waste is the fastest growing solid waste in the world. Each year, over 50 million tonnes of e-waste are produced, with its rate increasing by 3-5 % annually. Currently, only 17 % of e-waste is properly recycled, leaving the majority managed unsustainably, thereby causing detrimental environmental and economic effects. Cleaner e-waste management technologies are essential to address this urgent and rapidly expanding issue. Precious metals within e-waste significantly contribute to recycling revenues. In this paper, we review state-of-the-art technologies for sustainable recycling and upcycling of these metals from e-waste, including cleaner extractive metallurgy, solution purification technologies, and direct synthesis of green nanomaterials. We also discuss the potential impacts and constraints of these technologies and provide recommendations for improving and implementing both existing and prospective technologies.

Ultra-broadband 1 × 2 3 dB power splitter using a thin-film lithium niobate from 1.2 to 2 µm wave band.

The 3 dB power splitters are fundamental building blocks for integrated photonic devices. As data capacity requirements continue to rise, there is a growing interest in integrated devices that can accommodate multiple spectral bands, including the conventional O-, C-, and L-bands, and the emerging 2 µm band. Here we propose and experimentally demonstrate a 3 dB power splitter based on adiabatic mode evolution using a thin-film lithium niobate, with ultra-broadband operation bandwidth from 1200 to 2100 nm. The fabricated power splitter exhibits low insertion losses of 0.2, 0.16, and 0.53 dB for wavelengths at 1310, 1550, and 2000 nm, respectively. The measured 1 dB bandwidth covers 1260-1360, 1480-1640, and 1930-2030 nm, which we believe that the proposed device is capable of operating in both O-, C-, L-, and 2 µm bands.

Thermo-optic tunable optical filters with GHz-bandwidth and flat-top passband on thin film lithium niobate platform.

Lithium niobate on insulator (LNOI) is a new photonic integrated platform that provides high optical confinement and retains the inherent excellent properties of lithium niobate (LN). Tunable filters are one of the indispensable devices for integrated optics. Here we design and fabricate a thermo-optic (TO) tunable optical filter using two cascaded racetrack microring resonators (MRRs) based on LNOI. The filter shows a narrow and flat top passband with intra band ripple less than 0.3 dB, 3 dB bandwidth of 4.8 GHz and out-of-band rejection of about 35 dB. The insertion loss of the filter is about -14 dB, including grating coupling loss about -6.5 dB and on-chip loss less than -1 dB. The heating power for center wavelength shift of the filter is about 89.4 mW per free spectral range (FSR). Relevant applications of such filters include optical information processing and microwave photonics.

All-optical wavelength conversion of a 92-Gb/s 16-QAM signal within the C-band in a single thin-film PPLN waveguide.

Tunable all-optical wavelength conversion (AOWC) within 151 nm bandwidth is demonstrated in a thin-film periodically poled lithium niobate (PPLN) waveguide, which utilizes the cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) process. Also, in the same waveguide, AOWC of a 92-Gb/s 16-ary quadrature amplitude modulated (16-QAM) signal within the C-band is successfully achieved. For Bit-error ratio (BER) measurements, we obtain a negligible optical signal-to-noise ratio (OSNR) penalty (<0.2 dB) for the converted idler wave at a BER of 1e-3.

Enhanced bound states in the continuum in a DBR-assisted photonic crystal slab.

Bound states in the continuum (BICs) are perfectly confined resonances within the radiation continuum. The novel characteristics of single BICs have been studied in great detail in various wave systems, including electromagnetic waves, acoustic waves, water waves, and elastic waves in solids. In practice, the performance of BICs is limited by the finite size of the structure, while the combination of multiple BICs can further improve the localization of resonances. In this study, we experimentally demonstrate the combination of Fabry-Perot and symmetry-protected BICs at near infrared wavelengths by employing a compound photonic crystal system composed of a photonic crystal slab and a distributed Bragg reflector, resulting in an enhanced high quality factor.

Photonic sampling analog-to-digital conversion based on time and wavelength interleaved ultra-short optical pulse train generated by using monolithic integrated LNOI intensity and phase modulator.

High-speed analog-to-digital conversion (ADC) is experimentally demonstrated by employing a time and wavelength interleaved ultra-short optical pulse train to achieve photonic sampling and using wavelength division demultiplexing to realize speed matching between the fast optical front-end and the slow electronic back-end. The sampling optical pulse train is generated from a cavity-less ultra-short optical pulse source involving a packaged device that monolithically integrates an intensity modulator and a phase modulator into a chip based on lithium niobate on insulator (LNOI). In the experiment, the fiber-to-fiber insertion loss of the packaged modulation device is measured to be 6.9 dB. In addition, the half-wave voltages of the Mach-Zehnder modulator and the phase modulator in the LNOI-based modulation device are measured to be 3.6 V and 3.4 V at 5 GHz, respectively. These parameters and the device size are superior to those based on cascaded commercial devices. Through using the packaged modulation device, two ultra-short optical pulse trains centered at 1541.40 nm and 1555.64 nm are generated with time jitters of 19.2 fs and 18.9 fs in the integral offset frequency range of 1 kHz to 10 MHz, respectively, and are perfectly time interleaved into a single pulse train with a repetition rate of 10 GHz and a time jitter of 19.8 fs. Based on the time and wavelength interleaved ultra-short optical pulse train, direct digitization of microwave signals within the frequency range of 1 GHz to 40 GHz is demonstrated by using a two-channel wavelength demultiplexing photonic ADC architecture, where the effective number of bits are 5.85 bits and 3.75 bits for the input signal at 1.1 GHz and 36.3 GHz, respectively.

Efficient polarization splitter-rotator on thin-film lithium niobate.

Recently, thin-film lithium niobate coherent modulators have emerged as a promising candidate for the next generation coherent communication system. High performance polarization splitter-rotators (PSRs) are essential to further achieve dual polarization coherent modulators. Here we present a PSR on the lithium niobate on insulator (LNOI) platform with the measured insertion loss less than 1 dB, extinction ratio exceeding 26.6 dB and 19.6 dB for TE0 and TM0 modes, working bandwidth of 1520-1580 nm and total length of 440 µm. In addition, a relatively large fabrication tolerance for waveguide width is also proved. This demonstrated PSR can find its potential application in polarization-division multiplexing (PDM) optical transmitter based on LNOI.

Efficient Gold Recovery from Cyanide Solution Using Magnetic Activated Carbon.

Activated carbon has been used for gold recovery in the gold mining industry commercially for decades. The high specific surface area and porosity, good affinity to aurocyanide ions, and abundant resources make activated carbon an efficient and economical material for the adsorption of aurocyanide. However, the separation of activated carbon from the slurry is usually a challenge, and the adsorption rate of activated carbon is limited by the coarse particle size. Herein, a simple and sustainable way to recover gold from cyanide solution using magnetic activated carbon synthesized via a solvothermal method has been developed. The synthesized magnetic activated carbon possesses good magnetism (44.57 emu/g) and specific surface area equal to 249.7 m2/g. The magnetic activated carbon showed 99.1% recovery efficiency of gold from 10 mg/L solution within 5 h, which is much faster compared to the commercial granular activated carbon, and the magnetic activated carbon can be easily separated from the solution with an external magnet. The adsorption ability of this magnetic activated carbon has been tested under different conditions in the cyanide solution, the adsorption isotherm and kinetics are also investigated. The magnetic activated carbon was also recycled in the adsorption-desorption tests and showed good reusability.

Strongly Enhanced Second Harmonic Generation in a Thin Film Lithium Niobate Heterostructure Cavity.

Boosting second-order optical nonlinear frequency conversion over subwavelength thickness has long been pursued through optical resonance in micro- and nanophotonics. However, the availability of thin film materials with high second-order nonlinearity is limited to III-V semiconductors, which have low transparency in the visible. Here, we experimentally demonstrated strongly enhanced second harmonic generation in one-dimensional heterostructure cavities on thin film lithium niobate. A guided-mode resonance resonator and distributed Bragg reflectors are combined for both efficient coupling and electromagnetic field localization. Over 1200 times second harmonic generation enhancement is experimentally realized compared with flat thin film lithium niobate through optimizing the trade-off between quality factor and mode volume, leading to a record high normalized conversion efficiency of 2.03×10^{-5} cm^{2}/GW under 1.92 MW/cm^{2} pump intensity. Our approach could inspire the miniaturization and integration of compact resonant nonlinear photonic devices on thin film lithium niobate.

Direct Imaging of Integrated Circuits in CPU with 60 nm Super-Resolution Optical Microscope.

Far-field super-resolution optical microscopies have achieved incredible success in life science for visualization of vital nanostructures organized in single cells. However, such resolution power has been much less extended to material science for inspection of human-made ultrafine nanostructures, simply because the current super-resolution optical microscopies modalities are rarely applicable to nonfluorescent samples or unlabeled systems. Here, we report an antiphase demodulation pump-probe (DPP) super-resolution microscope for direct optical inspection of integrated circuits (ICs) with a lateral resolution down to 60 nm. Because of the strong pump-probe (PP) signal from copper, we performed label-free super-resolution imaging of multilayered copper interconnects on a small central processing unit (CPU) chip. The label-free super-resolution DPP optical microscopy opens possibilities for easy, fast, and large-scale electronic inspection in the whole pipeline chain for designing and manufacturing ICs.

High-efficient coupler for thin-film lithium niobate waveguide devices.

Lithium niobate (LN) devices have been widely used in optical communication and nonlinear optics due to its attractive optical properties. The emergence of the thin-film lithium niobate on insulator (LNOI) improves performances of LN-based devices greatly. However, a high-efficient fiber-chip optical coupler is still necessary for the LNOI-based devices for practical applications. In this paper, we demonstrate a highly efficient and polarization-independent edge coupler based on LNOI. The coupler, fabricated by a standard semiconductor process, shows a low fiber-chip coupling loss of 0.54 dB/0.59 dB per facet at 1550 nm for TE/TM light, respectively, when coupled with an ultra-high numerical aperture fiber (UHNAF) of which the mode field diameter is about 3.2 µm. The coupling loss is lower than 1dB/facet for both TE and TM light in the wavelength range of 1527 nm to 1630 nm. A relatively large tolerance for optical misalignment is also proved, due to the coupler's large mode spot size up to 3.2 µm. The coupler shows a promising stability in high optical power and temperature variation.

Photonic crystal enabled manipulation of optical and electric field in germanium avalanche photodetectors.

We demonstrate the use of a photonic crystal (PhC) structure to improve the performance of a germanium avalanche photodetector (APD) by simultaneously manipulating the distribution of the optical and electric fields. The PhC is fabricated at the top center of the vertical germanium APD. For a 14 µm diameter device, the 1550 nm responsivity increases from 0.2 to 0.6 A W-1 at unity gain, owing to the resonance-enhanced absorption. Moreover, the structure separates the absorption and multiplication regions of the device, resulting in an increase of the avalanche gain and the gain-bandwidth product. Under -10 dBm input optical power, a 3 dB bandwidth of 34 GHz before avalanche and a clear 40 Gbps eye diagram under avalanche demonstrates good high-speed performance of the device.

Dynamic 3D meta-holography in visible range with large frame number and high frame rate.

The hologram is an ideal method for displaying three-dimensional images visible to the naked eye. Metasurfaces consisting of subwavelength structures show great potential in light field manipulation, which is useful for overcoming the drawbacks of common computer-generated holography. However, there are long-existing challenges to achieving dynamic meta-holography in the visible range, such as low frame rate and low frame number. In this work, we demonstrate a design of meta-holography that can achieve 228 different holographic frames and an extremely high frame rate (9523 frames per second) in the visible range. The design is based on a space channel metasurface and a high-speed dynamic structured laser beam modulation module. The space channel consists of silicon nitride nanopillars with a high modulation efficiency. This method can satisfy the needs of a holographic display and be useful in other applications, such as laser fabrication, optical storage, optics communications, and information processing.

Precisely ordered Ge quantum dots on a patterned Si microring for enhanced light-emission.

Semiconductor microcavities can greatly enhance the light-emission of embedded quantum dots (QDs). Here, a new route toward the microcavity-QD system by fabricating microcavities followed by growing ordered QDs on a patterned microresonator is proposed, which keeps QDs from being etched. Self-assembled Ge QDs prefer to form at the rims of Si microrings or microdisks. The Ge QDs on the pit- or groove-patterned microring resonator (MRR) show better size uniformity and position accuracy. These features are explained by the evolutions of surface morphology and surface chemical potential distribution. Sharp photoluminescence peaks in the telecommunication band with the quality factors in the range of 450-850 from groove-patterned MRR are observed at 295 K due to efficient overlap between Ge QDs and resonant modes. Our schemes shed light on the exactly site-controlled growth of QDs on micro- and nano-structures, which further facilitates the investigation of light-matter interactions.

Fundamental mode hybridization in a thin film lithium niobate ridge waveguide.

The high performance of thin film lithium niobate on insulator (LNOI) platform shows potential for electro-optical signal processing and nonlinear optics systems. To realize precise polarization management for sub-wavelength devices, we theoretically and experimentally investigate fundamental transverse electric (TE) and transverse magnetic (TM) mode hybridization in an x-cut LNOI ridge waveguide. Sudden jumps in the free-spectrum-range (FSR) of these modes in a fabricated microring resonator demonstrate the mode hybridization. The measured Q-factor of the lithium niobate (LN) microring is 1.78 million near the critical coupling condition. The hybridization wavelength was designed at 1562 nm and observed at 1537 nm. Potential applications include fundamental mode conversion, polarization rotation, polarization splitter, and polarization insensitive waveguides in optical receiver module.

Light emission driven by magnetic and electric toroidal dipole resonances in a silicon metasurface.

Dielectric nanoparticles supporting pronounced toroidal and anapole resonances have enabled a new class of optical antennas with unprecedented functionalities. In this work, we propose a light-emitting silicon metasurface which simultaneously supports both magnetic toroidal dipole and electric toroidal dipole resonances in the near-infrared region. The metasurface consists of a square array of split nanodisks with embedded germanium quantum dots. By varying the width of the split air-gap, the spectral positions and quality factors of the two toroidal dipoles are flexibly tuned. Large photoluminescence enhancement is experimentally demonstrated at the toroidal resonances, which is attributed to the unique near- and far-field characteristics of the resonant modes. Moreover, the light emissions driven by the two toroidal dipoles are of different polarization, which further suggests versatile polarization-engineered radiation properties. Our work shows enormous potential in light emission manipulation and provides a route for high-efficiency, ultra-compact LEDs and potentially functional dielectric metasurface lasers.

Generalized Hartmann-Shack array of dielectric metalens sub-arrays for polarimetric beam profiling.

To define and characterize optical systems, obtaining the amplitude, phase, and polarization profile of optical beams is of utmost importance. Traditional polarimetry is well established to characterize the polarization state. Recently, metasurfaces have successfully been introduced as compact optical components. Here, we take the metasurface concept to the system level by realizing arrays of metalenses, allowing the determination of the polarization profile of an optical beam. We use silicon-based metalenses with a numerical aperture of 0.32 and a mean measured focusing efficiency in transmission mode of 28% at a wavelength of 1550 nm. Our system is extremely compact and allows for real-time beam diagnostics by inspecting the foci amplitudes. By further analyzing the foci displacements in the spirit of a Hartmann-Shack wavefront sensor, we can simultaneously detect phase-gradient profiles. As application examples, we diagnose the profiles of a radially polarized beam, an azimuthally polarized beam, and of a vortex beam.

Integrated four-channel directly modulated O-band optical transceiver for radio over fiber application.

We have fabricated a compact and integrated 4-channel analog optical transceiver for radio over fiber application. In the fabricated module, the transmitter optical sub-assembly is composed of four directly modulated DFB laser chips integrated with an optical multiplexer based on an arrayed waveguide grating (AWG) using silica-based planar lightwave circuit (PLC) technology. The receiver optical sub-assembly consists of a PIN photodiode array integrated with an AWG-PLC-type optical de-multiplexer. For all the lanes, the 3 dB bandwidth exceeds 19.1 GHz and the measured spurious-free dynamic range (SFDR) is up to 90.5 dB⋅Hz2/3 when the input RF frequency is from 2 GHz to 14 GHz. Meanwhile, the electrical inter-channel crosstalk of the transceiver is less than -20 dB when the carry frequency is below 18.5 GHz. This module shows a great transmission performance in radio over fiber system. Under simultaneous 4-channel different 600 Mb/s 5-band 64QAM-OFDM RF signal operation, the measured error vector magnitude (EVM) performance below 8% is achieved after 15.5 km single-mode fiber propagation for all lanes. This scheme has potential applications in guiding high-dense, cost-effective and high-linearity analog optical transceiver design to realize high-capacity radio over fiber transmission systems.