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Quantum droplets (QDs) are self-trapped modes stabilized by the Lee-Huang-Yang correction to the mean-field Hamiltonian of binary atomic Bose-Einstein condensates. The existence and stability of quiescent and rotating dipole-shaped and vortex QDs with vorticity S=1 (DQDs and VQDs, respectively) are numerically studied in the framework of the accordingly modified two-component system. The rotating DQDs trapped in an annular potential are built of two crescentlike components, stretching along the azimuthal direction with the increase of the rotation frequency. Rotating quadrupole QDs (QQDs) bifurcate from the VQDs with S=2. Above a certain rotation frequency, they transform back into VQDs with a flat-top shape. Rotating DQDs and QQDs are stable in a broad interval of values of the chemical potential. The results provide the first example of stable modes which are intermediate states between the rotating DQDs and QQDs on the one hand and VQDs on the other.

We investigate the dynamic evolution of resonant radiation (RR) emitted from modulated Airy pulses in an optical fiber with emphasis on the third-order dispersion. We show that the process of RRs emission strongly depends on the parameters of the predesigned spectral phase imposed on the Airy pulses, which is attributed to its linear focusing behaviors. A localized pulse formed at the distance-controlled focusing can effectively generate a large amount of RR after a prescribed propagation distance. At variance with the case of fundamental soliton, the minimum value of third-order dispersion required for the onset of RRs emission becomes much smaller for the modulated Airy pulse. The conversion efficiency of RRs increases with an increasing linear focusing point, but decreases with an increasing truncated coefficient. There is an optimal value of the truncated coefficient for the RRs having highest peak intensity and largest frequency shift. The impact of Raman effects on the RRs is also revealed. Our results not only provide a simply route to actively manipulate the efficient emission of RRs in conventional optical fiber, also could have significant implications of pulse shaping technology in novel highly efficient light sources based on the RRs emission for a variety of application.

Abstract The emergence of cylindrical vector beam (CVB) multiplexing has opened new avenues for high-capacity optical communication. Although several configurations have been developed to couple/separate CVBs, the CVB multiplexer/demultiplexer remains elusive due to lack of effective off-axis polarization control technologies. Here we report a straightforward approach to realize off-axis polarization control for CVB multiplexing/demultiplexing based on a metal–dielectric–metal metasurface. We show that the left- and right-handed circularly polarized (LHCP/RHCP) components of CVBs are independently modulated via spin-to-orbit interactions by the properly designed metasurface, and then simultaneously multiplexed and demultiplexed due to the reversibility of light path and the conservation of vector mode. We also show that the proposed multiplexers/demultiplexers are broadband (from 1310 to 1625 nm) and compatible with wavelength-division-multiplexing. As a proof of concept, we successfully demonstrate a four-channel CVB multiplexing communication, combining wavelength-division-multiplexing and polarization-division-multiplexing with a transmission rate of 1.56 Tbit/s and a bit-error-rate of 10−6 at the receive power of −21.6 dBm. This study paves the way for CVB multiplexing/demultiplexing and may benefit high-capacity CVB communication.

The rapid and accurate identification of large-scale orbital angular momentum (OAM) modes is crucial for expanding the application of vortex beams (VBs). In this paper, an OAM mode recognition method based on convolutional neural networks (CNNs) is proposed and investigated. We construct an 8-layer CNN possesses complex feature extraction capability and train it to own powerful anti-turbulence competence by feeding the intensity patterns of VBs interfered by Gaussian beam. After supervised training of a large sample set, the CNN model takes on excellent network generalization ability and can well detect VBs with the mode range of [-50,50]. The simulation results indicate that under the influence of weak and medium turbulences, the average recognition accuracy exceeds 99%. Even under strong turbulence, the accuracy also reaches 98.54%. Meanwhile, the identification time is only 1.55ms per OAM mode with Intel(R) Xeon(R) Gold 6148 CPU. Moreover, the influence of different Gaussian beam waists, VB orders, input training sets, and CNN structures on OAM mode recognition performance, is fully studied. These results demonstrate that our proposed method can achieve higher accuracy and higher order OAM mode detection at a fast speed, which contributes a more effective method for the recognition of VBs.

The 1.7 μm band eye-safe laser sources have recently received lots of attention thanks to the development of various applications. Although a variety of lasing configurations operating in this band have been demonstrated, one still needs to seek a good candidate for particular applications with a reasonable compromise between the relative performance targets (e.g., stability, output power, and spectral purity) and the construction cost. Here, we demonstrate a high-power 1694 nm random fiber laser (RFL) in a cost-effective structure pumped by a high-powered 1565 nm RFL. The maximum output power reached the 10 W level, and the output showed extremely low-intensity fluctuations for both the short-time and long-time regimes. Meanwhile, an excellent spectral purity as high as 26.9 dB was also realized. This work provides one of the most attractive approaches for constructing high-performance 1.7 μm band laser sources for practical applications.

Fractional vortex beam (FVB) possessing helical phase can be applied in the shift-keying communication due to its fractional orbital angular momentum (FOAM) mode, which theoretically allows an infinite increase of the transmitted capacity. However, the discontinuity of spiral phase makes FVB more likely to be disturbed in turbulence environment, and the precise measurement of distorted FOAM modes is crucial for practical FOAM-based communication application. Here, we proposed a FOAM mode recognition method with feedforward neural network (FNN). Employing the diffraction preprocessing of a two-dimensional fork grating, the original optical features of FVBs can be extended along the far-field diffraction order, endowing FNN more feature information and saving calculation time, and enlarging the detection range to conjugate FOAM modes. The simulation results show that the 9-layer FNN can identify FOAM mode with interval of 0.1 with an accuracy of 99.1% under the turbulences of Cn2=1×10-14m-2/3 and Δz=10m. Furthermore, we experimentally constructed a 102-ary FOAM shift-keying communication link to transmit gray image, and the signals are successfully demodulated by the FNN model with the pixel-error-rate of 0.07160. It is anticipated that the proposed FNN-based FOAM recognition method will break the limitation of precision measurement under turbulence environment in practical FOAM applications.

Abstract Great success in 2D van der Waals (vdW) heterostructures based photodetectors is obtained owing to the unique electronic and optoelectronic properties of 2D materials. Performance of photodetectors based 2D vdW heterojunctions at atomic scale is more sensitive to the nanointerface of the heterojunction than conventional bulk heterojunction. Here, a nanoengineered heterostructure for the first‐time demonstration of a nanointerface using an inserted graphene layer between black phosphorus (BP) and InSe which inhibits interlayer recombination and greatly improves photodetection performances is presented. In addition, a transition of the transport characteristics of the device is induced by graphene, from diffusion motion of minority carriers to drift motion of majority carriers. These two reasons together with an internal photoemission effect make the BP/G/InSe‐based photodetector have ultrahigh specific detectivity at room temperature. The results demonstrate that high‐performance vdW heterostructure photodetectors can be achieved through simple structural manipulation of the heterojunction interface on nanoscale.

Abstract Highly stretchable and robust superhydrophobic surfaces have attracted tremendous interest due to their broad application prospects. In this work, silicone elastomers were chosen to fabricate superhydrophobic surfaces with femtosecond laser texturing method, and high stretchability and tunable adhesion of the superhydrophobic surfaces were demonstrated successfully. To our best knowledge, it is the first time flexible superhydrophobic surfaces with a bearable strain up to 400% are fabricated by simple laser ablation. The test also shows that the strain brings no decline of water repellency but an enhancement to the superhydrophobic surfaces. In addition, a stretching-induced transition from “petal” state to “lotus” state of the laser-textured surface was also demonstrated by non-loss transportation of liquid droplets. Our results manifest that femtosecond laser ablating silicone elastomer could be a promising way for fabricating superhydrophobic surface with distinct merits of high stretchability, tunable adhesion, robustness, and non-fluorination, which is potentially useful for microfluidics, biomedicine, and liquid repellent skin.

Structural anisotropy of surfaces determines properties relevant for applications. Here the authors observe a relationship between the shape of water droplets forming on graphene, MoS2and black phosphorous and the surface structure, proposing a method to determine lattice orientation by optical microscopy.

Singular beams with spatially variant field distributions have various fantastic applications. However, one of the significant challenges that hinder the wide application of singular beams is how to effectively identify the topological charge and the polarization order of arbitrary singular beams. We found that when a light beam with arbitrary polarization state illuminates a polarization-sensitive blazed-grating, the horizontal and vertical component can be separated from the incident beam. Based on this phenomenon, an effective method is proposed to probe the integral topological charge and polarization order of arbitrary singular beams. With this detection method, the vortex beam, cylindrical vector beam and cylindrical vector vortex beam with different topological charges and polarization orders have been experimentally identified. This effective detection method can be widely used to measure the topological charge and polarization order of arbitrary singular beams.

Orbital angular momentum (OAM) shift-keying (SK), which manifests as the rapid switching of OAM modes, is crucial but severely hampered for lacking effective demodulation techniques in OAM communication. In this paper, we propose and investigate a novel direct-demodulation method for the OAM-SK communication. After passing through a well-designed beam slit, the optical vortex beam will possess varying output energies associated with the OAM modes. Hence, splitting the optical vortex beam into two sub-beams and one of them passing through the slit, we can directly extract the OAM-SK signals from the energy ratios of the two sub-beams. The results show that the bit-error-rate (BER) of demodulated OAM-SK signals can reach 3.7 ×10-3 even under the influence of strong atmospheric turbulence with Cn2 = 3 ×10-12 and misalignment of 1.8 mm. Encoding digital signals to OAM modes and amplitude of optical vortex beams simultaneously, we construct a novel multi-modulation format composed of 16 QAM-OFDM and OAM-SK signals, which is also demodulated with the BER of 10-5 at the optical signal-to-noise ratio (OSNR) of 18 dB. These demonstrate that the direct-demodulation method provides a feasible way to demodulate OAM-SK signals and may show potential in complex OAM modulation communications.

The identification of orbital angular momentum (OAM) modes with high-accuracy and -speed is always a difficult issue in practically applying optical vortex beams (OVs). In this work, we propose and experimentally investigate a convolutional neural network (CNN) method for optical OAM mode identification and shift-keying (SK) communications. The CNN model, including convolution and pooling layers, was designed to extract mode information from the diffraction patterns produced by diffracting the OVs with a cylindrical lens. After trained with loads of studying samples, the CNN model has a good generation ability in recognizing the OAM modes of OVs ranging from -15 to 15. The recognition accuracy reaches 99% with the turbulence intensity of Cn2 = 1 × 10-13m-2/3, Δz = 50 m. Even under the turbulence of Cn2 = 1 × 10-12 m-2/3, Δz = 50 m, the accuracy still exceeds 89%. By mapping and encoding a Lena gray image with the size of 100 × 100 pixels to two OAM channels, the OAM-SK signals with 900 modulation orders were successfully demodulated by the CNN model, and the image was well recovered after transmission. With an I5-8500 Central Processing Unit, this recognition process only takes 1 ×10-3 s per mode. It is anticipated that the CNN methods might provide an effective way for identifying OAM modes with high-accuracy and -speed, which may have great potentials in OAM communication, quantum information processing, and astronomical application, etc.

The development of high-speed optical communication calls for compatible multiplexing dimensions to enlarge transmission capacity and process optical information. Although vortex beam multiplexing has been previously demonstrated by exploiting the orthogonality of orbital angular momentum (OAM) modes, the multidimensional demultiplexer remains elusive due to the limited phase and polarization manipulation capability. We introduce dielectric propagation phase metasurface based demultiplexers and investigate their application in simultaneously demultiplexing OAM and orthogonal linear polarization (OLP) channels. Utilizing the size-dependent anisotropy of dielectric rectangular nano-pillars, we reveal that the multimode fork grating phase masks with different off-axis phase items can be integrated on the same substrate with polarization selectivity, which allows simultaneously demultiplexing OAM and OLP channels. The demultiplexed position and the number of channels depend on its off-axis phase items, and a 16-channel (8-OAM modes and 2-polarization states) demultiplexer is demonstrated. The vortex beams with different OAM modes and OLP states are successfully demultiplexed with the maximum crosstalk is below −9.2 dB. Changing the number of superimposed complex amplitudes and the off-axis phase items in multimode fork grating phase masks, we further present two 12-channel demultiplexers with different channel distributions. Our results provide an approach that breaks the polarization-insensitive in manipulating OAM modes, introducing demultiplexers that apply to OAM mode and OLP states simultaneously.

Cylindrical vector vortex beams (CVVBs), possessing both spatially nonuniform phase and polarization distribution, have attracted research interests in many applications due to their unique optical properties. Although various methods have been previously demonstrated, the generation of CVVBs with arbitrary topological charge and polarization order remains elusive. We introduce cross-polarized modulation method to construct CVVBs via changing the phases of the orthogonal polarization components of light beams. Two cross-polarized modulation approaches are demonstrated for conjugate and consistent phase orthogonal circular polarization beams. The phases of the orthogonal circular polarization components of light beams can be flexibly manipulated by exploiting the polarization-sensitivity of spatial light modulators, which leads to changed topological charges and polarization orders, and a group of CVVBs with different topological charges and polarization orders are successfully generated. Because of the adjustable orders and the single light path, the CVVBs generation method possess not only flexibility but also high stability.

Dispersive waves (DWs) radiation in optical fibers allow to excite frequencies in spectral regions that are otherwise difficult to access. However, the resonant emission is weak and phase matching inhibits its shaping, making it ineffective in many applications. Here, we numerically investigated that DWs generated from high-energy pulses propagating in nonlinear optical fibers can be controlled through spectral phase modulation. Using tunable asymmetric and oscillatory temporal pulse shapes obtained by cubic and quadratic spectral phase modulation, we excite a multiple emission process that strongly enhances the energy and peaks of DWs. Successive collisions between the shed soliton and multiple secondary peaks of the asymmetric pulse give the mechanism to stimulate various resonant frequencies. The whole process can be precisely controlled by simply adjusting the spectral phase modulation structure without the need to change the fiber length. This radiation is either reduced or enhanced by the Raman effect depending on whether the third-order dispersion is positive or negative. Our results provide a novel strategy to manipulate DWs emission and particularly to boost the DWs energy in optical fibers that may be relevant in the development of novel electromagnetic sources for broadband supercontinuum and on-chip frequency-comb generation.

A stabilization scheme is proposed for the dynamics of breather waves induced by the coherent-seed modulation instability based on manipulation of phase-space trajectory. Theoretical and numerical analysis show that carefully dispersion- and nonlinearity-managed cascades of fiber configuration allows the system evolution to be stabilized around an elliptic center point, forming stable pulse trains with ultrahigh contrast efficiently. We also demonstrate that the scheme proposed works equally well for near-separatrix dynamics. Our results provide an alternative means to control the unsteady nonlinear waves by abruptly changing the waveguide properties.

Polarization topological charge (PTC) of a vector beam can be defined as the repetition number of polarization state change along the azimuthal axis, with its sign standing for the rotating direction of the polarization. In this paper, we present a design for PTC sorting of vector beams with polarization conjugated-phase component (PCC). The components, made of birefringent crystals, can induce conjugated spatial phase modulations for the orthogonal polarizations. Our simulations show, using a pair of PCC with specific design in a 4-f optical system, the system focuses vector beam modes on the locations representing the PTC of corresponding mode, and sorts PTC with separation efficiency as high as 77%. Our designs have the potential applications in modal characterization, communication, encryption, and so on.

We propose a novel laser configuration that can output 3.5-μm nanosecond laser pulses based on a simple and monolithic fiber structure. Cascade-gain-switching (CGS) converts the wavelength of nanosecond pulses from 1.55 to 3.5 μm by two successive gain-switching processes. CGS eliminates the requirement of using bulky free-space modulators for Q-switching. With a well-established theoretical model, we investigated the feasibility of this novel configuration and thoroughly explored its characteristics. In single-shot regime, the pulse width of the 1.55-μm pump has major impact on the temporal shape of the intermediate 1.97-μm pulse while has neglected influence on the generated 3.5-μm pulse. On the other hand, increasing the continuous-wave (CW) pump power can significantly improve the output peak power and shorten the pulse when the pump power is less than ~4 W. In the repetitive-pulse regime, we found the maximum repetition rate is positively correlated the CW pump power. With a typical CW pump power of 5 W, the 3.5-μm pulse train can be stably outputted when the repetition rate is

Luminescent materials that are capable of binary temporal coding are desirable for multilevel anti-counterfeiting. Here, the authors engineer nanoparticles that produce binary color codes on different timescales by combining the long-lived luminescence of Mn2+ with the relatively short-lived emission of lanthanides.

Topological effects known from condensed matter physics have recently also been explored in photonic systems. Here, the authors directly observe topological surface-state arcs in momentum space by near-field scanning the surface of a chiral hyperbolic metamaterial.

Abstract Singular beams which possess helical phase wavefront or spatially inhomogeneous polarization provide new freedom for optical field manipulation. However, conventional schemes to produce the singular beams have difficulty in realizing the flexible switch between different singular beams. In this work, we have experimentally demonstrated the capability of dielectric metasurfaces to generate three types of singular beams and switch between them at working wavelength of 1550 nm. We have shown vortex beam and cylindrical vector beam generation with single metasurface and cylindrical vector vortex beam generation with two cascaded metasurfaces. Moreover, experimental demonstration on switching cylindrical vector beam into vortex beam has also been done by combining one quarter-wave plate and a Glan laser polarizer. The experimental results match well with the analysis from the Jones matrix calculations. The average conversion efficiency of cylindrical vector beam to vortex beam was estimated to be 47.7%, which was about 2.3% lower than the theoretical prediction.

Cylindrical vector vortex (CVV) beam, which possesses both helical phase front and spatially inhomogeneous polarization, is a promising structured light for its various applications ranging from optical communication to optical field manipulation and optical microscopy. However, approaches to generate CVV beams with switchable and tunable polarization order and topological charge are still immature, which hinders the wide application of CVV beams. In this paper, we have experimentally demonstrated that order-controllable CVV beams can be produced by using spatial light modulator (SLM) and equivalent q-plate system at wavelength of 1550.8 nm. It is shown that the topological charge of the CVV beam can be switched by directly programming the SLM. We have also demonstrated that the polarization order of the CVV beam can be tuned to as high as eight by employing an equivalent q -plate system, which consists of two cascaded metasurfaces and a half-wave plate. To further verify the helical phase of the CVV beam, we have proposed a novel measurement method based on first removing the vector property and then interfering the remaining helical phase with plane wave or spherical wave.

Beams carrying orbital angular momentum (OAM) are very helpful in enhancing the information carrying capacity in free-space optical communications. However, atmospheric turbulence and energy attenuation will seriously affect communication quality and signal transmissions. Here, a novel coherent separation detection technology for OAM mode division multiplexing is proposed, and numerical simulation work is conducted. With the proposed structure, two light beams with different OAM states, each encoded with 16-quadrature-amplitude modulation orthogonal frequency-modulated (QAM-OFDM) signals, can be demultiplexed and allow two orders of magnitude bit error rate (BER) lower than direct separation detection. Moreover, we show the scalability of multiplexing four OAM beams, achieving a 20 m free-space transmission with BER below 3.8 × 10–3 for all channels at signal-noise ratio (SNR) 18 dB. Our results show that coherent separation detection has excellent antinoise performance and can effectively extend the communication distance. It also paves the way for entirely new coherent optical OAM communications.

The identification of hybrid orbital angular momentum (OAM) modes with high-accuracy and -speed is always a challenge in practically applying optical vortex beams (VBs). In this paper, we propose and investigate a deep feedforward neural network (FNN) method for identifying the hybrid OAM modes of VBs. The FNN model with 15 input neurons and 7 hidden layers was constructed. And an improved1-dimension fork grating (1-D FG) was designed to diffract VBs and produce feature parameters which are used as the input of the FNN. After supervised training, the deep FNN model can identify arbitrarily combined hybrid OAM modes with a wide detection range owing to its non-linear operations to neurons and massive iterations. Besides, this FNN model has better robustness to atmospheric turbulence. The results show that the identification accuracy reaches 97% with five superimposed modes. Under the influence of atmospheric turbulence with Cn2=1×10-15m-2/3, the accuracy still exceeds 98% at the transmission distance of 1000 m. With an Intel Core i5-4590 CPU and NVIDIA GeForce GTX 750 GPU, this identification process takes only 0.09 ms. Furthermore, we constructed a 120-ary OAM shift-keying communication link, and the signals were demodulated by the FNN model with only 4.3×10-3 pixels error rate. It is anticipated that the FNN might pave an effective way to detect hybrid OAM modes, which may have great potentials in increasing communication capacity and modulation ability. Keywords: Vortex beams, Feedforward neural network, Hybrid orbital angular momentum modes, 1-dimension fork grating

Abstract Herein, a core–shell tellurium–selenium (Te–Se) nanomaterial with polymer‐tailed and lateral heterojunction structures is developed as a photothermal absorber in a bionic solar‐evaporation system. It is further revealed that the amorphous Se shell surrounds the crystalline Te core, which not only protects the Te phase from oxidation but also serves as a natural barrier to life entities. The core (Te)–shell (Se) configuration thus exhibits robust stability enhanced by 0.05 eV per Se atom and excellent biocompatibility. Furthermore, high energy efficiencies of 90.71 ± 0.37% and 86.14 ± 1.02% and evaporation rates of 12.88 ± 0.052 and 1.323 ± 0.015 kg m−2 h−1 are obtained under 10 and 1 sun for simulated seawater, respectively. Importantly, no salting out is observed in salt solutions, and the collected water under natural light irradiation possesses extremely low ion concentrations of Na+, K+, Ca2+, and Mg2+ relative to real seawater. Considering the tunable electronic structures, biocompatibilities, and modifiable broadband absorption of the solar spectrum of lateral heterojunction nanomaterials of Te–Se, the way is paved to engineering 2D semiconductor materials with supporting 3D porous hydrophilic materials for application in solar desalination, wastewater treatment, and biomedical ventures.

A novel long-range surface plasmon with graphene is proposed to enhance the sensitivity and detection accuracy (DA) of the biosensor. Compared with the conventional long-range surface plasmon resonance (LRSPR) structure with Au, the coating of the metal surface with graphene is employed to increase the biomolecules adsorption, prevent oxidation, and enhance the sensitivity and DA. Furthermore, we demonstrated that the sensitivity has a nearly tenfold improvement with a huge increasing DA in the proposed LRSPR biosensor compared with the SPR biosensor. Finally, we discuss the influence of the refractive index of sensing medium on the LRSPR biosensor and find that the sensitivity is changing with the refractive index of the sensing medium and that an optimal sensitivity can be obtained at a suitable refractive index. We believe that this scheme could find potential applications in chemical examination, medical diagnosis, and biological detection.

In this paper, we present the theoretical modeling and characterization of the gain spectra at ~1600-2100 nm from thulium-doped fiber amplifiers (TDFAs) for the first time. Our theoretical model was validated through the comparison with the experiments on the widely tunable TDFAs and thulium-doped fiber lasers using in-band pumping (3H6 →3F4) and 79 χ-nm pumping (3H6 →3H4) schemes. Different emission and absorption cross sections of the commercial thulium-doped fibers were employed for comparison and comprehensive demonstration. We then calculated the performance of the in-band-pumped TDFAs under different spectral cross section profiles and amplifier configurations. It shows, without the restriction of the loss induced by the fiber-optic components, that high optical gains (> 20 dB in small signal regime and > 3 dB in saturation regime) can be achieved over an ultrabroad spectrum ranging from ~1580 to ~2080 nm. We also demonstrate, although it is rarely used in the experiments of the broadband TDFAs, that the 79x-nm cladding pumping scheme can effectively avoid the strong reabsorption at short wavelengths when long gain fibers are used.

In the conventional surface plasmon resonance biosensor, the sensitivity using angular interrogation is low. Graphene/MoS2 has been attached on the metal surface to improve the sensitivity; however, the ability to improve sensitivity is limited. Here, by sandwiching the MoS2 sheets between two gold films in the Kretschmann configuration, we demonstrate an ultrasensitive surface plasmon resonance sensor. By optimizing the structure of sensor, we find that the sensitivity as high as 182°/RIU can be realized with 4-layers MoS2 and a monolayer graphene. Moreover, it is indicated that the sensitivity can be tuned and controlled by changing the layer numbers of graphene and MoS2.

By applying nonequilibrium Green’s function in combination with the density-functional theory, we investigate the electronic transport properties of boron or nitrogen doped zigzag phagraphene nanoribbon junctions. Our calculated results show that the negative differential resistance effect could be observed in zigzag phagraphene nanoribbon junction. Moreover, the peak to valley ratio of the negative differential resistance significantly increase from 1.6 to 111.32, when the junction is doped by boron atoms. We rationalize the mechanism leading to negative differential resistance by providing a detailed analysis of transmission spectra and transmission pathways.

We experimentally demonstrate a stable passively mode-locked Raman fiber laser delivering high-energy pulses that can be switched between the regime of hundreds of nanoseconds and that of picoseconds by the nonlinear polarization rotation technique. Maximum average output power values of 304 and 53 mW are obtained, respectively, for the two typical mode-locking states with the pulse duration of 500 ns and 180 ps at the fundamental repetition rate of 275 kHz. The corresponding single pulse energy is as much as 1.1 μJ and 193 nJ, respectively. To the best of our knowledge, this is the highest pulse energy achieved from mode-locked Raman fiber lasers reported so far.

We propose a graphene/planar waveguide hybrid structure, and demonstrate total absorption in the visible wavelength range by means of attenuated total reflectance. The excitation of planar waveguide mode, which has strong near field enhancement and increased light interaction length with graphene, plays a vital role in total absorption. We analyze the origin and physical insight of total absorption theoretically by using an approximated reflectance, and show how to design such hybrid structure numerically. Utilizing the tunability of doped graphene, we discuss the possible application in optical modulators. We also achieve broadband absorption enhancement in near-IR range by cascading multiple graphene-waveguide hybrid structures. We believe our results will be useful not only for potential applications in optical devices, but also for studying other two-dimension materials.

By using optical heterodyne technique, we demonstrated the stable emission of sub-terahertz wave with the frequency ranging from 88 GHz to 101 GHz, which can operate as microwave source for nonlinear response measurement system. Mutual frequency beating of two well-separated sideband signals at a 0.1 THz photo-detector (PD) allows for the generation of sub-terahertz signal. Based on this approach, we have achieved the radiation of 0.1 THz wave with power up to 4 mW. By transmittance measurement, two-dimensional nanomaterial topological insulator (TI: Bi2Te3) shows saturable absorption behaviors with normalized modulation depth of 47% at 0.1 THz. Our results show that optical heterodyne technique could be developed as an effective microwave source generation for nonlinear measurement at sub-terahertz, even terahertz band.

In this study, laser sintering of TiO2 nanoparticle films on plastic substrates was conducted in order to improve the incident photon-to-electron conversion efficiency (IPCE) of flexible dye-sensitized solar cells (DSCs). Lasers with different wavelengths (355 nm and 1064 nm) were used to process the TiO2 electrodes. With an optimized processing parameter combination, the 1064 nm laser can sinter 13 μm thick TiO2 films uniformly, but the uniform sintering cannot be achieved by the 355nm ultraviolet (UV) laser, since the films possess a high absorption ratio at 355 nm. The experimental results demonstrate that the near-infrared laser sintering can enhance the electrical connection between TiO2 nanoparticles without destroying the flexible plastic substrate, reduce the transmission impedance of electrons and increase the absorption rate of incident light. Furthermore, the charge collection efficiency, fill factor, and short-circuit current have all been improved to some extent, and the solar conversion efficiency increased from 4.6% to 5.7%, with an efficiency enhancement reaching 23.9%.

Density functional theory calculations of the layer (L)-dependent electronic band structure, work function and optical properties of β-InSe have been reported. Owing to the quantum size effects (QSEs) in β-InSe, the band structures exhibit direct-to-indirect transitions from bulk β-InSe to few-layer β-InSe. The work functions decrease monotonically from 5.22 eV (1 L) to 5.0 eV (6 L) and then remain constant at 4.99 eV for 7 L and 8 L and drop down to 4.77 eV (bulk β-InSe). For optical properties, the imaginary part of the dielectric function has a strong dependence on the thickness variation. Layer control in two-dimensional layered materials provides an effective strategy to modulate the layer-dependent properties which have potential applications in the next-generation high performance electronic and optoelectronic devices.

An in-band pumped 1.645-μm Er:YAG ceramic laser passively Q-switched by a topological insulator: Bi2Te3 saturable absorber is reported. The average output power could reach up to 210 mW, corresponding to a pulsewidth, a pulse repetition rate, and a per-pulse energy of 6.3 μs, 40.7 kHz, and 5.3 μJ, respectively. This result indicates that the topological insulator: Bi2Te3 could be a promising saturable absorber such as graphene, with potential applications for solid-state lasers.