Wide field of view optical phased array with a high-directionality antenna.

Light detection and ranging (LiDAR) is a widely utilized technology for extracting information from the outside world in fields such as automotive, robotics, and aerospace. Optical phased array (OPA) is a promising solution for LiDAR technology, although its application is limited by loss and alias-free steering range. In this paper, we propose a dual-layer antenna that achieves a peak directionality of over 92%, thereby mitigating antenna loss and enhancing power efficiency. Based on this antenna, we design and fabricate a 256-channel non-uniform OPA that achieves 150° alias-free steering.

Two-dimensional scanning of silicon-based focal plane array with field-of-view splicing technology.

All-solid-state beam scanning chip is ideal for next-generation LiDAR due to its reliability and small size. Here we propose a focal plane array chip for two-dimensional scanning using field-of-view splicing technology on silicon photonics platform. The chip has two rotationally symmetric structures, each including a 1 × 64 antenna array accompanied by a 1 × 64 micro-ring optical switch array. We demonstrate a two-dimensional scanning equivalent to an 8-line LiDAR with a field-of-view of 82° × 32°, a beam divergence angle of 0.07° × 0.07°, and a background suppression ratio of over 20 dB. Our chip works in such a simple way that only one optical switch needs to be turned on each time the beam is emitted. And the chip is scalable that a larger range of two-dimensional scanning can be achieved when using more antennas for field-of-view splicing and cascading more optical switches.

Adoption of large aperture chirped grating antennas in optical phase array for long distance ranging.

Optical phased array can be widely used in many fields benefiting from its superior performance. We designed and fabricated chirped grating antennas and uniform grating antennas for the optical phased array. The effective aperture of the chirped grating antennas is about twice that of the uniform grating antennas. The chirped grating optical phased array can receive the reflected signal of the object at a distance of 100 m, while the uniform grating optical phased array can only receive 50 m under the same conditions. Additionally, a ranging distance of 25 m is achieved when two chirped grating optical phased arrays are set as the transmitter and receiver.

Non-uniform optical phased array with a large steering angle and high side mode suppression ratio.

An optical phased array (OPA) is one of the most promising methods of light detection and ranging. A non-uniform array with different emitter distances is a method to realize OPA steering without grating lobes or a distance between two adjacent emitters larger than λ/2. However, the side mode suppression ratio (SMSR) will decrease as OPA turns into a large angle. In this paper, 64-, 128-, and 256-channel non-uniform OPAs are optimized by non-dominated sorting genetic algorithm-II (NSGA-II), which is a multi-objective optimization algorithm. Compared with arrays optimized by a genetic algorithm, the SMSR at 80° improves by 2.18, 2.61, and 2.56 dB, respectively.

InGaAs/InP multi-quantum-well nanowires with a lower optical leakage loss on v-groove-patterned SOI substrates.

InGaAs/InP multi-quantum-well nanowires were directly grown on the v-groove-patterned SOI substrate by metal organic chemical vapor deposition. The surface morphology of the nanowires, the thickness of the quantum wells, and the photoluminescence spectra were characterized by scanning electron microscope, transmission electron microscopy, and micro-photoluminescence, respectively. We found in the experiments that the work of removing part of top Si on both sides of the nanowire to further reduce the optical leakage loss could be completed perfectly without complicated processes, such as a lithography process. Numerical simulations showed that the III-V nanowire was able to support an extraordinarily stable optical guided mode with a lower optical leakage loss of 0.21 cm-1 when etching away part of top Si on both sides of the nanowire, and the optical confinement factor of the multi-quantum-well active region was about 8.8%. This approach opens up a way for monolithic photonic integration of III-V compound semiconductors on Si to occur.

A flexible control on electromagnetic behaviors of graphene oligomer by tuning chemical potential.

In this work, we demonstrate that the electromagnetic properties of graphene oligomer can be drastically modified by locally modifications of the chemical potentials. The chemical potential variations of different positions in graphene oligomer have different impacts on both extinction spectra and electromagnetic fields. The flexible tailoring of the localizations of the electromagnetic fields can be achieved by precisely adjusting the chemical potentials of the graphene nanodisks at corresponding positions. The proposed nanostructures in this work lead to the practical applications of graphene-based plasmonic devices such as nanosensing, light trapping and photodetection.

Dynamic tailoring of electromagnetic behaviors of graphene plasmonic oligomers by local chemical potential.

In the mid-infrared and terahertz (THz) regime, graphene supports tunable surface plasmon resonance (SPR) by controlling the chemical potential, which promotes light-matter interaction at the selected wavelength, showing exceptional promise for optoelectronic applications. In this article, we show that the electromagnetic (EM) response of graphene oligomers can be substantially modified by the modification of the local chemical potential, strengthening or reducing the intrinsic plasmonic modes. The effect mechanism is corroborated by a graphene nanocluster composed of 13 nanodisks with D6h symmetry; by transforming to D3h symmetry, the effect mechanism was retained and more available plasmonic resonance modes appeared. The intriguing properties open a new way to design nanodevices made of graphene oligomers with highly efficient photoresponse enhancement and tunable spectral selectivity for highly accurate photodetection.

Pseudospin Dependent One-Way Transmission in Graphene-Based Topological Plasmonic Crystals.

Originating from the investigation of condensed matter states, the concept of quantum Hall effect and quantum spin Hall effect (QSHE) has recently been expanded to other field of physics and engineering, e.g., photonics and phononics, giving rise to strikingly unconventional edge modes immune to scattering. Here, we present the plasmonic analog of QSHE in graphene plasmonic crystal (GPC) in mid-infrared frequencies. The band inversion occurs when deforming the honeycomb lattice GPCs, which further leads to the topological band gaps and pseudospin features of the edge states. By overlapping the band gaps with different topologies, we numerically simulated the pseudospin-dependent one-way propagation of edge states. The designed GPC may find potential applications in the fields of topological plasmonics and trigger the exploration of the technique of the pseudospin multiplexing in high-density nanophotonic integrated circuits.

Inclined emitting slotted single-mode laser with 1.7° vertical divergence angle for PIC applications.

In this Letter, a new type of single-mode slotted laser used for an on-chip light source in photonic integrated circuits is proposed. An inclined light beam with a low vertical divergence angle can be directly coupled into the surface grating of the silicon to form an integrated light source. Experimentally, a III-V laser with a 54.6° inclined angle and a vertical divergence angle of 1.7° is achieved by introducing a kind of specially distributed microstructure. The side mode suppression ratio is better than 45 dB, and the continuous wave output power reaches 6.5 mW at room temperature. We report the inclined emitting microstructured single-mode laser with a low divergence angle for the first time, to the best of our knowledge.

Topologically protected edge states in graphene plasmonic crystals.

A two-dimensional graphene plasmonic crystal composed of periodically arranged graphene nanodisks is proposed. We show that the band topology effect due to inversion symmetry broken in the proposed plasmonic crystals is obtained by tuning the chemical potential of graphene nanodisks. Utilizing this kind of plasmonic crystal, we constructed N-shaped channels and realized topologically edged transmission within the band gap. Furthermore, topologically protected exterior boundary propagation, which is immune to backscattering, was also achieved by modifying the chemical potential of graphene nanodisks. The proposed graphene plasmonic crystals with ultracompact size are subject only to intrinsic material loss, which may find potential applications in the fields of topological plasmonics and high density nanophotonic integrated systems.

Investigation of beam splitter in a zero-refractive-index photonic crystal at the frequency of Dirac-like point.

The Dirac-like cone dispersion of the photonic crystal induced by the three-fold accidental degeneracy at the Brillouin center is calculated in this paper. Such photonic crystals can be mapped to zero-refractive-index materials at the vicinity of the Dirac-like point frequency, and utilized to construct beam splitter of high transmission efficiency. The splitting ratio is studied as a function of the position of the input/output waveguides. Furthermore, variant beam splitters with asymmetric structures, bulk defects, and some certain bending angles are numerically simulated. Finally, we show that 1 × 2 to 1 × N beam splitting can be realized with high transmission efficiency in such a zero-refractive-index photonic crystal at the frequency of Dirac-like point. The proposed structure could be a fundamental component of the high density photonic integrated circuit technique.

Optimization of the Fano Resonance Lineshape Based on Graphene Plasmonic Hexamer in Mid-Infrared Frequencies.

In this article, the lineshape of Fano-like resonance of graphene plasmonic oligomers is investigated as a function of the parameters of the nanostructures, such as disk size, chemical potential and electron momentum relaxation time in mid-infrared frequencies. Also, the mechanism of the optimization is discussed. Furthermore, the environmental index sensing effect of the proposed structure is revealed, and a figure of merit of 25.58 is achieved with the optimized graphene oligomer. The proposed nanostructure could find applications in the fields of chemical or biochemical sensing.

Hybrid single-mode laser based on graphene Bragg gratings on silicon.

We exploit distributed optoelectronic properties enabled by graphene Bragg gratings (GBGs) to realize a hybrid single-mode laser on silicon. This hybrid laser achieves single-mode, continuous-wave operation at 1540 nm with a remarkable side-mode suppression ratio of 48 dB, benefitting from the coupling of the GBGs. These results suggest that graphene thin films can be used as an essential and cost-saving component for hybrid photonic integration on silicon.

Dynamically Tunable Plasmon-Induced Transparency in On-chip Graphene-Based Asymmetrical Nanocavity-Coupled Waveguide System.

A graphene-based on-chip plasmonic nanostructure composed of a plasmonic bus waveguide side-coupled with a U-shaped and a rectangular nanocavities has been proposed and modeled by using the finite element method in this paper. The dynamic tunability of the plasmon-induced transparency (PIT) windows has been investigated. The results reveal that the PIT effects can be tuned via modifying the chemical potential of the nanocavities and plasmonic bus waveguide or by varying the geometrical parameters including the location and width of the rectangular nanocavity. Further, the proposed plasmonic nanostructure can be used as a plasmonic refractive index sensor with a sensing sensibility of 333.3 nm/refractive index unit (RIU) at the the PIT transmission peak. Slow light effect is also realized in the PIT system. The proposed nanostructure may pave a new way towards the realization of graphene-based on-chip integrated nanophotonic devices.

Electromagnetic field coupling characteristics in graphene plasmonic oligomers: from isolated to collective modes.

In this paper, we propose a plasmonic tetramer composed of coupled graphene nanodisks. The transformation from the isolated to the collective modes of the proposed structure is investigated by analysing the whispering-gallery modes and extinction spectra with various inter-nanodisk gap distances. In addition, the effect of introducing a central nanodisk into the tetramer on the extinction spectra is explored, which leads to Fano resonance. Furthermore, the refractive index sensing properties of the proposed graphene plasmonic oligomer have been demonstrated. The proposed nanostructures might pave the road toward the application of graphene plasmonic oligomers in fields such as nanophotonics, and chemical or biochemical sensing.

[A Methane Detection System Using Distributed Feedback Laser at 1 654 nm].

A methane (CH4) detection system based on tunable diode laser absorption spectroscopy (TDLAS) technique was experimentally demonstrated. A distributed feedback (DFB) laser around 1 654 nm, an open reflective sensing probe and two InGaAs photodiodes were adopted in the system. The electrical part of the system mainly includes the laser temperature control & modulation module and the orthogonal lock-in amplifier module. Temperature and spectrum tests on the DFB laser indicate that, the laser temperature fluctuation can be limited to the range of -0.02-0.02 degrees C, the laser's emitting wavelength varies linearly with the temperature and injection current, and also good operation stability of the laser was observed through experiments. Under a constant working temperature, the center wavelength of the laser is varied linearly by adjusting the driving current. Meanwhile, a 5 kHz sine wave signal and a 10 Hz saw wave signal were provided by the driving circuit for the harmonic extraction purpose. The developed orthogonal lock-in amplifier can extract the If and 2f harmonic signals with the extraction error of 3.55% and 5% respectively. By using the open optical probe, the effective optical pass length was doubled to 40 cm. Gas detection experiment was performed to derive the relation between the harmonic amplitude and the gas concentration. As the concentration increases from 1% to 5%, the amplitudes of the 1f harmonic and the 2f harmonic signal were obtained, and good linear ration between the concentration and the amplitude ratio was observed, which proves the normal function of the developed detection system. This system is capable to detect other trace gases by using relevant DFB lasers.

Investigation of the Band Structure of Graphene-Based Plasmonic Photonic Crystals.

In this paper, one-dimensional (1D) and two-dimensional (2D) graphene-based plasmonic photonic crystals (PhCs) are proposed. The band structures and density of states (DOS) have been numerically investigated. Photonic band gaps (PBGs) are found in both 1D and 2D PhCs. Meanwhile, graphene-based plasmonic PhC nanocavity with resonant frequency around 175 THz, is realized by introducing point defect, where the chemical potential is from 0.085 to 0.25 eV, in a 2D PhC. Also, the bending wvaguide and the beam splitter are realized by introducing the line defect into the 2D PhC.

[Online Detection System on Acetylene with Tunable Diode Laser Absorption Spectroscopy Method].

Based on tunable diode laser absorption spectroscopy (TDLAS) technique, an acetylene (C2H2) online detection system was developed by using the absorption band at the wavelength of 1.534 µm of C2H2 molecule. The sensing system consists of four modules including a distributed feedback (DFB) laser, a DFB laser driver, a gas cell with single optical path and a data processing module. With the prepared standard C2H2 gas sample, detailed measurements were carried out to study the detection performance of the system. Experimental results reveal that, the limit of the system (LOD) is about 0.02%; a good linear relationship is observed between C2H2 gas concentration and the amplitude of the 2f signal is within the range of 0.02%~1%. A long-term measurement lasting for 20 h on a 0.5% C2H2 gas sample was carried out to test the stability of the system. Compared with the C2H2 detection systems utilizing quantum cascaded lasers (QCLs) and wideband incandescence, this system has great advantage due to the capability of using long-distance and low-loss optical fiber for remote monitoring. With self-developed DFB laser driver and lock-in amplifier, the system has good prospects in industrial field because of its simple structure, low price and capability of easy to be integrated.

Investigation of plasmonic whispering-gallery mode characteristics for graphene monolayer coated dielectric nanodisks.

In this Letter, we theoretically studied high-quality (Q) factor plasmonic whispering-gallery modes (WGMs) with ultrasmall mode volumes in graphene monolayer coated semiconductor nanodisks in the mid-infrared range. The influence of the chemical potential, the relaxation time of graphene, and the radius of the nanodisk on the cavity Q factor and the mode volume was numerically investigated. The numerical simulations showed that the plasmonic WGMs excited in this cavity had a deep subwavelength mode volume of 1.4×10(-5)(λ(0)/2n)(3), a cavity Q factor as high as 266 at a temperature lower than 250 K, and, consequently, a large Purcell factor of ∼1.2×10(7) when the chemical potential and relaxation time were assumed to be 0.9 eV and 1.4 ps, respectively. The results provide a possible application of plasmonic WGMs in the integration of nano-optoelectronic devices based on graphene.

Ultrashort pulse generation by semiconductor mode-locked lasers at 760 nm.

We demonstrate the first semiconductor mode-locked lasers for ultrashort pulse generation at the 760 nm waveband. Multi-section laser diodes based on an AlGaAs multi-quantum-well structure were passively mode-locked, resulting in the generation of pulses at around 766 nm, with pulse durations down to ~4 ps, at pulse repetition rates of 19.4 GHz or 23.2 GHz (with different laser cavity lengths of 1.8 mm and 1.5 mm, respectively). The influence of the bias conditions on the mode-locking characteristics was investigated for these new lasers, revealing trends which can be ascribed to the interplay of dynamical processes in the saturable absorber and gain sections. It was also found that the front facet reflectivity played a key role in the stability of mode-locking and the occurrence of self-pulsations. These lasers hold significant promise as light sources for multi-photon biomedical imaging, as well as in other applications such as frequency conversion into the ultraviolet and radio-over-fibre communications.