Ultra-high-speed, ultra-large-capacity and ultra-long-haul (3U) are the forever pursuit of optical communication. As a new mode of optical communication, 3U transmission can greatly promote next generation optical internet and broadband mobile communication network development and technological progress, therefore it has become the focus of international high-tech intellectual property competition ground. This paper introduces the scientific problems, key technologies and important achievements in 3U transmission research.

Subwavelength electromagnetics is a discipline that deals with light-matter interaction at subwavelength scale and innovative technologies that control electromagnetic waves with subwavelength structures. Although the history can be dated back to almost one hundred years ago, the flourish of these researching areas have been no more than 30 years. In this paper, we gave a brief review of the history, current status and future trends of subwavelength electromagnetics. In particular, the milestones related with metamaterials, plasmonics, metasurfaces and photonic crystals are highlighted.

This paper summarizes our research work on optoelectronic devices with nanostructures. It was indicated that by manipulating so called “general energy-bands” of fundamental particles or quasi-particles, such as photon, phonon, and surface plasmon polariton (SPP), novel optoelectronic characteristics can be obtained, which results in a series of new functional devices. A silicon based optical switch with an extremely broadband of 24 nm and an ultra-compact (8 mm × 17.6 mm) footprint was demonstrated with a photonic crystal slow light waveguides. By proposing a nanobeam based hetero optomechanical crystal, a high phonon frequency of 5.66 GHz was realized experimentally. Also, we observed and verified a novel effect of two-surface-plasmon-absorption (TSPA), and realized diffraction-limit-overcoming photolithography with resolution of ~1/11 of the exposure wavelength.

Microwave photonics (MWP) is an interdisciplinary field that combines two different areas of microwave engineering and photonics. It has several key features by transferring signals between the optical domain and microwave domain, which leads to the advantages of broad operation bandwidth for generation, processing and distribution of microwave signals and high resolution for optical spectrum measurement. In this paper, we comprehensively review past and current status of MWP in China by introducing the representative works from most of the active MWP research groups. Future prospective is also discussed from the national strategy to key enabling technology that we have developed.

This paper presents an introduction to the researches in microwave photonics based packages and its application, a 973 project (No. 2012CB315600), which focuses on addressing new requirements for millimeter wave (MMW) system to work with higher frequency, wider bandwidth, larger dynamic range and longer distance of signal distribution. Its key scientific problems, main research contents and objectives are briefed, and some latest achievements by the project team, including generation of linear frequency modulation wave (LFMW), tunable optoelectronic oscillator (OEO) with lower phase noise, reconfigurable filter with higher Q value, time delay line with wider frequency range, down conversion with gain, and local oscillator (LO) transmission with stable phase, are introduced briefly.

Optical vortices (OVs) refer to a class of cylindrical optical modes with azimuthally varying phase terms arising either from polarization rotation or from the angular projection of the wave vector that at the quantum level corresponds to photon spin or orbital angular momenta. OVs have attracted the attention of researchers in many areas of optics and photonics, as their potential applications range from optical communications, optical manipulation, imaging, sensing, to quantum information. In recent years, integrated photonics has becomes an effective method of manipulating OVs. In this paper, the theoretical framework and experimental progress of integrated photonics for the manipulation of OVs were reviewed.

In this paper, we reviewed the design principles of two-dimensional (2D) silicon photonic crystal microcavity (PCM) biosensors coupled to photonic crystal waveguides (PCWs). Microcavity radiation loss is controlled by engineered the cavity mode volume. Coupling loss into the waveguide is controlled by adjusting the position of the microcavity from the waveguide. We also investigated the dependence of analyte overlap integral (also called fill fraction) of the resonant mode as well as the effect of group index of the coupling waveguide at the resonant wavelength of the microcavity. In addition to the cavity properties, absorbance of the sensing medium or analyte together with the affinity constant of the probe and target biomarkers involved in the biochemical reaction also limits the minimum detection limits. We summarized our results in applications in cancer biomarker detection, heavy metal sensing and therapeutic drug monitoring.

A novel type of high power edge-emitting semiconductor laser (SL) with extended vertical photonic band crystal (PBC) waveguide was reviewed. Simulations predict narrow beam divergence, resulting from the thick PBC waveguide, to be independent of realistic variations of the growth parameters. Narrow ridge lasers fabricated along the simulations indeed demonstrate superior output power, narrow beam divergence, circular beam profile, excellent beam quality and very low astigmatism. Efficient fiber coupling decisive for most applications was thus eased. Stability of the laser under a wide range of operating temperature was demonstrated. Ultrashort pulses with few ps of duration at GHz repetition rates were generated by passively mode locking the lasers.

Microresonator frequency combs (microcombs) are very promising as ultra-compact broadband sources for microwave photonic applications. Conversely, microwave photonic techniques are also employed intensely in the study of microcombs to reveal and control the comb formation dynamics. In this paper, we reviewed the microwave photonic techniques and applications that are connected with microcombs. The future research directions of microcomb-based microwave photonics were also discussed.

Optical interconnects (OIs) are the only solution to fulfil both the requirements on large bandwidth and minimum power consumption of data centers and high-performance computers (HPCs). Vertical-cavity surface-emitting lasers (VCSELs) are the ideal light sources for OIs and have been widely deployed. This paper will summarize the progress made on modulation speed, energy efficiency, and temperature stability of VCSELs. Especially VCSELs with surface nanostructures will be reviewed in depth. Such lasers will provide new opportunities to further boost the performance of VCSELs and open a new door for energy-efficient OIs.

Mid-infrared (mid-IR) (2-20 μm) photonics has numerous chemical and biologic “fingerprint” sensing applications due to characteristic vibrational transitions of molecules in the mid-IR spectral region. Unfortunately, compared to visible light and telecommunication band wavelengths, photonic devices and applications have been difficult to develop at mid-IR wavelengths because of the intrinsic limitation of conventional materials. Breaking a new ground in the mid-IR science and technology calls for revolutionary materials. Graphene, a single atom layer of carbon arranged in a honey-comb lattice, has various promising optical and electrical properties because of its linear dispersion band structure and zero band gap features. In this review article, we discuss recent research developments on mid-IR graphene photonics, in particular ultrafast lasers and photodetectors. Graphene-photonics-based biochemical applications, such as plasmonic sensing, photodynamic therapy, and florescence imaging are also reviewed.

Space-division multiplexing (SDM) using multi-core fibers (MCFs) and few-mode fibers (FMFs) was proposed as a solution to increase capacity and/or reduce the cost per bit of fiber-optic transmission. Advances in passive and active SDM devices as well as digital signal processing have led to impressive SDM transmission demonstrations in the laboratory. Although the perceived advantages in terms of capacity and cost per bit that SDM offers over parallel SMF bundles are not universally accepted, SDM is beginning to emerge as an indispensable solution in major network segments. The introduction of the spatial degree of freedom allows optical networks to overcome fundamental limitations such as fiber nonlinearity as well practical limitations such as power delivery. We describe these application scenarios that the optical communications industry has already began to explore. From a fundamental science point of view, concepts such as the principal modes, generalized Stokes space, and multi-component solitons discovered in SDM research will likely have a broad impact in other areas of science and engineering.

Light trapping is of critical importance for constructing high efficiency solar cells. In this paper, we first reviewed the progress we made on the plasmonic light trapping on Si wafer solar cells, including Al nanoparticle (NP)/SiN_x hybrid plasmonic antireflection and the Ag NP light trapping for the long-wavelength light in ultrathin Si wafer solar cells. Then we numerically explored the maximum light absorption enhancement by a square array of Ag NPs located at the rear side of ultrathin solar cells with wavelength-scale Si thickness. Huge absorption enhancement is achieved at particular long wavelengths due to the excitation of the plasmon-coupled guided resonances. The photocurrent generated in 100 nm thick Si layers is 6.8 mA/cm², representing an enhancement up to 92% when compared with that (3.55 mA/cm²) of the solar cells without the Ag NPs. This study provides the insights of plasmonic light trapping for ultrathin solar cells with wavelength-scale Si thickness.

In this paper, we demonstrate a color tunable white organic light-emitting devices (WOLEDs) based on the two complementary color strategies by introducing two-dimensional (2-D) dual periodic gratings. It is possible to tune the color in a range between cold-white and warm-white by simply operating the polarization of polarizer in front of the microstructured WOLEDs. Experimental and numerical results demonstrate that color tunability of the WOLEDs comes from the effect of the 2-D dual periodic gratings by exciting the surface plasmon-polariton (SPP) resonance associated with the cathode/organic interface. The electroluminescence (EL) performance of the WOLEDs have also been improved due to the effective light extraction by excitation and out-coupling of the SPP modes, and a 39.65% enhancement of current efficiency has been obtained compared to the conventional planar devices.

In this paper, we investigated coherent beam combining of angled-grating broad-area lasers in a completely integrated approach. We obtained the simultaneous coherent beam combining and single transverse mode operation on a single chip through the integrated coupling regions and the transverse Bragg resonance (TBR) gratings, respectively. The proposed combining method can be easily extended to a zigzag-like laser array. We analyzed the scalability of the zigzag-like combining structure and compared it with other coherent combining methods. Two and six angled-grating broad-area lasers are fabricated and coherently combined by use of the proposed method. The high contrast interference fringes within an overall single lobe envelope in the measured far field prove that the emitters in the array are indeed coherently combined. By p-side-down bonding, we obtained over 1 W output power with over 90% combining efficiency in the two coherently combined lasers.

Pumping combiner is a kernel component of high power fiber laser (HPFL). We demonstrate two types of tapered fiber bundle (TFB) end-pumping combiner able to combining multi-kilowatts of pumping laser. After the experimental test of coupling performance, the 3×1 coupler is proved to have a power handling capacity of 2.11 kW with a coupling efficiency of 95.1%, and the 7×1 coupler is capable of handling pumping power of 4.72 kW with a coupling efficiency of 99.4%. These two coupler have obtained the ability to be used in laser diodes (LDs) direct beam combining and the pumping coupling of multi-kilowatts level fiber lasers.

In this paper, influence of temperature and reverse bias on photocurrent spectrum and spectral response of a monolithic GaInP/GaAs double-junction solar cell was investigated in detail. Two sharp spectral response offsets, corresponding to the bandedge photo absorption of the bottom GaAs and the top GaInP subcells, respectively, show the starting response points of individual subcells. More interestingly, the cell photocurrent was found to enhance significantly with increasing the temperature. In addition, the cell photocurrent also increases obviously as the reverse bias voltage increases. The integrated photocurrent intensity of the top GaInP subcell was particularly addressed. A theoretical model was proposed to simulate the reverse bias dependence of the integrated photocurrent of the GaInP subcell at different temperatures.

Integrated photonics requires high gain optical materials in the telecom wavelength range for optical amplifiers and coherent light sources. Erbium (Er) containing materials are ideal candidates due to the 1.5 μm emission from Er³⁺ ions. However, the Er density in typical Er-doped materials is less than 10²⁰ cm^-3, thus limiting the maximum optical gain to a few dB/cm, too small to be useful for integrated photonic applications. Er compounds could potentially solve this problem since they contain much higher Er³⁺ density. So far the existing Er compounds suffer from short lifetime and strong upconversion effects, mainly due to poor crystal qualities. Recently, we explore a new Er compound: erbium chloride silicate (ECS, Er₃(SiO₄)₂Cl) in the form of nanowire, which facilitates the growth of high quality single crystal with relatively large Er³⁺ density (1.62 × 10²² cm^–3). Previous optical results show that the high crystal quality of ECS material leads to a long lifetime up to 1 ms. The Er lifetime-density product was found to be the largest among all the Er containing materials. Pump-probe experiments demonstrated a 644 dB/cm signal enhancement and 30 dB/cm net gain per unit length from a single ECS wire. As a result, such high-gain ECS nanowires can be potentially fabricated into ultra-compact lasers. Even though a single ECS nanowire naturally serves as good waveguide, additional feedback mechanism is needed to form an ultra-compact laser. In this work, we demonstrate the direct fabrication of 1D photonic crystal (PhC) air hole array structure on a single ECS nanowire using focused ion beam (FIB). Transmission measurement shows polarization-dependent stop-band behavior. For transverse electric (TE) polarization, we observed stop-band suppression as much as 12 dB with a 9 μm long airholed structure. Through numerical simulation, we showed that Q-factor as high as 11000 can be achieved at 1.53 μm for a 1D PhC micro-cavity on an ECS nanowire. Such a high Q cavity combined with the high material gain of ECS nanowires provides an attractive solution for ultra-compact lasers, an important goal of this research.

Quantum dots in nanowires (DINWs) are considered as important building blocks for novel nanoscale semiconductor optoelectronic devices. In this paper, pure axial heterojunction InGaN/GaN DINWs are grown by using plasma-assisted molecular beam epitaxy (PA-MBE) system. The InGaN quantum dots (QDs) are disk-like observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The diameter of QDs can be controlled by the growth conditions of nanowires (NWs), while the thickness of QDs can be controlled by the growth time of InGaN. Temperature-dependent photoluminescence (TDPL) measurements demonstrate that the PL peak of DINWs with small and uniform sizes shows a general red shift with increasing temperature. However, the PL peak of DINWs with non-uniform sizes shows an abnormal blue shift with increasing temperature, which is due to different internal quantum efficiencies of the DINWs with different sizes.

Laser annealing of silicon dioxide (SiO₂) film formed by inductively coupled plasma enhanced chemical vapor deposition (ICPECVD) is studied for the fabrication of low loss silicon based waveguide. The influence of laser annealing on ICPECVD-deposited SiO₂ film is investigated. The surface roughness, refractive index, and etch rate of annealed samples are compared with those of SiO₂ film obtained by thermal oxidation. It is demonstrated that the performance of ICPECVD-deposited SiO₂ film can be significantly improved by laser annealing. Al₂O₃/SiO₂ waveguide has been fabricated on silicon substrate with the SiO₂ lower cladding formed by ICPECVD and laser annealing process, and its propagation loss is found to be comparable with that of the waveguide with thermally oxidized lower cladding.

A simple method is proposed to design freeform surface for Lambertian extended source. In this method, it can take advantage of the designing method for point source via substituting each incident ray with a dynamically calculated equivalent ray. For each facet on the freeform surface, the equivalent ray emits from the energy weighted average-emitting-position for the corresponding incident beam, and redirects into the direction which is determined by a source-to-target mapping. The results of the designing examples show that the light distributions’ uniformities can be improved by this method, e.g., even the improvement of 59% can be achieved.