Direct growth of single-chiral-angle tungsten disulfide nanotubes using gold nanoparticle catalysts.

Transition metal dichalcogenide (TMD) nanotubes offer a unique platform to explore the properties of TMD materials at the one-dimensional limit. Despite considerable efforts thus far, the direct growth of TMD nanotubes with controllable chirality remains challenging. Here we demonstrate the direct and facile growth of high-quality WS2 and WSe2 nanotubes on Si substrates using catalytic chemical vapour deposition with Au nanoparticles. The Au nanoparticles provide unique accommodation sites for the nucleation of WS2 or WSe2 shells on their surfaces and seed the subsequent growth of nanotubes. We find that the growth mode of nanotubes is sensitive to the temperature. With careful temperature control, we realize ~79% WS2 nanotubes with single chiral angles, with a preference of 30° (~37%) and 0° (~12%). Moreover, we demonstrate how the geometric, electronic and optical properties of the synthesized WS2 nanotubes can be modulated by the chirality. We anticipate that this approach using Au nanoparticles as catalysts will facilitate the growth of TMD nanotubes with controllable chirality and promote the study of their interesting properties and applications.

Giant Photoluminescence Enhancement of Monolayer WSe2 Using a Plasmonic Nanocavity with On-Demand Resonance.

Monolayer transition metal dichalcogenides (TMDs) are considered promising building blocks for next-generation photonic and optoelectronic devices, owing to their fascinating optical properties. However, their inherent weak light absorption and low quantum yield severely hinder their practical applications. Here, we report up to 18000-fold photoluminescence (PL) enhancement in a monolayer WSe2-coupled plasmonic nanocavity. A spectroscopy-assisted nanomanipulation technique enables the assembly of a nanocavity with customizable resonances to simultaneously enhance the excitation and emission processes. In particular, precise control over the magnetic cavity mode facilitates spectral and spatial overlap with the exciton, resulting in plasmon-exciton intermediate coupling that approaches the maximum emission rate in the hybrid system. Meanwhile, the cavity mode exhibits high radiation directivity, which overwhelmingly directs surface-normal PL emission and leads to a 17-fold increase in the collection efficiency. Our approach opens up a new avenue to enhance the PL intensity of monolayer TMDs, facilitating their implementation in highly efficient optoelectronic devices.

Room-Temperature Two-Dimensional InSe Plasmonic Laser.

Two-dimensional (2D) semiconductors, owing to their strong excitonic emission, are emerging as efficient gain media for constructing the ultimate nanolaser. The further integration of 2D semiconductors with plasmonic devices holds promise for realizing the thinnest laser. However, the implementation of 2D semiconductor plasmonic lasing is severely hindered by the limited cavity feedback and low gain resulting from insufficient plasmon-exciton interactions. Here, we report the realization of a room-temperature 2D semiconductor plasmonic laser by embedding an InSe nanoflake into a plasmonic Fabry-Perot (F-P) cavity. This plasmonic F-P cavity shows an exceptional ability to recycle the leaked dark surface plasmon, resulting in >2-fold enhancement of feedback compared to that of conventional metal-insulator-semiconductor nanolasers. Moreover, via combination of field enhancement and orientation matching, this cavity facilitates optimized plasmon-exciton coupling to ensure sufficient gain for sustaining room-temperature lasing. Our work may open up the possibilities for multifunctional photonic devices based on 2D materials.

Monolithic PMN-39PT nanograting-assisted second harmonic generation enhancement.

Second harmonic generation plays a vital role in frequency conversion which mutually promotes the laser technology and allows the wavebands extension of new coherent source. The monolithic crystals are supposed to be a superior choice for harmonic generation due to long interaction distance, however, the phase-mismatch brought a sharp reduction in the conversion efficiency. Although birefringent phase-matching and quasi-phase-matching techniques are commonly utilized to fill the phase gap in monolithic crystals, these techniques are limited by the natural refractive index of crystal and the domain engineering, respectively. In recent years, subwavelength structures evolve as a flexible scheme to realize phase matching by engineering the geometry features of crystals. Here, structured nanogratings are designed and fabricated on a monolithic PMN-39PT (Pb(Mg1/3Nb2/3)O3-0.39PbTiO3) substrate, a novel ferroelectric crystal with promising optical prospect, for enhancing second harmonic generation, where birefringent or quasi phase-matching is hard to achieve. The nanograting-assisted second harmonic generation enhancement is observed which is not limited by the availability of thin crystalline films. Meanwhile, a boost in the second harmonic signal synchronously promotes the cascading third harmonic generation. This method may provide an alternative solution for enhanced harmonic generation on monolithic substrates and develop potential nonlinear optical materials for frequency conversion.

Strong enhancement of third harmonic generation from a Tamm plasmon multilayer structure with WS2.

Improving the conversion efficiency is particularly important for the generation and applications of harmonic waves in optical microstructures. Herein, we propose to enhance the efficiency of third harmonic generation by integrating a monolayer WS2 with the metal/dielectric/photonic crystal multilayer structure. The numerical simulations show that the multilayer structure enables to generate the Tamm plasmon mode between the metal film and photonic crystal around the telecommunication wavelength, which is consistent with the experimental result. By measuring with a self-built nonlinear optical micro-spectroscopy system, we find that the third harmonic signal can be reinforced by 16-fold through inserting the monolayer WS2 in the dielectric spacer. This work will provide a new way for improving nonlinear optical response, especially THG in multilayer photonic microstructures.

Microcavity induced by a few-layer GaSe crystal on a silicon photonic crystal waveguide for efficient optical frequency conversion.

We demonstrate the post-induction of high-quality microcavities on a silicon photonic crystal (PC) waveguide by integrating a few-layer GaSe crystal, which promises efficient on-chip optical frequency conversions. The integration of GaSe shifts the dispersion bands of the PC waveguide mode into the bandgap, resulting in localized modes confined by the bare PC waveguides. Thanks to the small contrast of refractive index at the boundaries of the microcavity, it is reliable to obtain quality factors exceeding 104. With the enhanced light-GaSe interaction by the microcavity modes and GaSe's high second-order nonlinearity, remarkable second-harmonic generation (SHG) and sum-frequency generation (SFG) are achieved with continuous-wave (CW) lasers.

Self-Powered Programmable van der Waals Photodetectors with Nonvolatile Semifloating Gate.

Tunable photovoltaic photodetectors are of significant relevance in the fields of programmable and neuromorphic optoelectronics. However, their widespread adoption is hindered by intricate architectural design and energy consumption challenges. This study employs a nonvolatile MoTe2/hexagonal boron nitride/graphene semifloating photodetector to address these issues. Programed with pulsed gate voltage, the MoTe2 channel can be reconfigured from an n+-n to a p-n homojunction and the photocurrent transition changes from negative to positive values. Scanning photocurrent mapping reveals that the negative and positive photocurrents are attributed to Schottky junction and p-n homojunction, respectively. In the p-n configuration, the device demonstrates self-driven, linear, rapid response (∼3 ms), and broadband sensitivity (from 405 to 1500 nm) for photodetection, with typical performances of responsivity at ∼0.5 A/W and detectivity ∼1.6 × 1012 Jones under 635 nm illumination. These outstanding photodetection capabilities emphasize the potential of the semifloating photodetector as a pioneering approach for advancing logical and nonvolatile optoelectronics.

Van der Waals Nonlinear Photodetector with Quadratic Photoresponse.

With unique electronic and optical attributes and dangling-bond-free surface, two-dimensional (2D) materials have broadened the functionalities of photodetectors. Here, we report a quadratically nonlinear photodetector (QNPD) composed of a van der Waals (vdW) stacked GaSe/InSe heterostructure. Compared with the reported 2D material-based photodetectors, the extra second-harmonic generation (SHG) process in GaSe/InSe leads to the quadratically nonlinear function between photocurrent and optical intensity, extending the photodetection wavelength from 900 to 1750 nm. The QNPD is highly sensitive to the variation of optical intensity with improved spatial resolution. With the light-light interaction in SHG converted into electrical signal directly, we also demonstrate the QNPD as an autocorrelator for measuring ultrafast pulse widths and an optoelectronic mixer of two modulated pulses for signal processings. The simultaneous involvement of light-light interaction and photoelectric conversion in the vdW stacked QNPD promises its potential to simplify the optoelectronic systems.

Ultrahigh thermal-efficient all-optical silicon photonic crystal nanobeam cavity modulator with TPA-induced thermo-optic effect.

We propose and experimentally demonstrate an on-chip all-optical silicon photonic crystal nanobeam cavity (PCNBC) modulator. With the advantages of the strong two-photon absorption (TPA)-induced thermo-optic (TO) effect, ultrahigh thermal-efficient tuning with π phase shift temperature difference ΔTπ of 0.77°C and power Pπ of 0.26 mW is implemented. Moreover, the all-optical modulation is carried out by a pulsed pump light with an average switching power of 0.11 mW. The response times for the rising and falling edges are 7.6 µs and 7.4 µs, respectively. Such a thermal-efficient modulator is poised to be the enabling device for large-scale integration optical signal control systems.

Strong Second Harmonic Generation from Bilayer Graphene with Symmetry Breaking by Redox-Governed Charge Doping.

Missing second-order nonlinearity in centrosymmetric graphene overshadows its intriguing optical attribute. Here, we report redox-governed charge doping could effectively break the centrosymmetry of bilayer graphene (BLG), enabling a strong second harmonic generation (SHG) with a strength close to that of the well-known monolayer MoS2. Verified from control experiments with in situ electrical current annealing and electrically gate-controlled SHG, the required centrosymmetry breaking of the emerging SHG arises from the charge-doping on the bottom layer of BLG by the oxygen/water redox couple. Our results not only reveal that charge doping is an effective way to break the inversion symmetry of BLG despite its strong interlayer coupling but also indicate that SHG spectroscopy is a valid technique to probe molecular doping on two-dimensional materials.

Integrating a Nanowire Laser in an on-Chip Photonic Waveguide.

We report a simple and facile integration strategy of a laser source in passive photonic integrated circuits (PICs) by deterministically embedding semiconductor nanowires (NWs) in waveguides. InP NWs laid on a SiN slab are buried by a polymer layer which also acts as an electron-beam resist. With electron-beam lithography, hybrid polymer-SiN waveguides are formed with precisely embedded NWs. The lasing behavior of the waveguide-embedded NWs is confirmed, and more importantly, the NW lasing mode couples into the hybrid waveguide and forms an in-plane guiding mode. Multiple waveguide-embedded NW lasers are further integrated in complex photonic structures to illustrate that the waveguiding mode supplied by the NW lasers could be manipulated for on-chip signal processing, including power splitting and wavelength-division multiplexing. This integration strategy of an on-chip laser is applicable to other PIC platforms, such as silicon and lithium niobate, and the top cladding layer could be changed by depositing SiN or SiO2, promising its CMOS compatibility.

Suspended-core fiber with embedded GaSe nanosheets for second harmonic generation.

We report an all-fiber scheme for the second harmonic generation (SHG) by embedding gallium selenide (GaSe) nanosheets into a suspended-core fiber (SCF). Based on modes analysis and theoretical calculations, the phase-matching modes from multiple optional modes in the SHG process and the optimal SCF length are determined by calculating the effective refractive index and balancing the SHG growth and transmission loss. Due to the long-distance interaction between pumped fundamental mode and GaSe nanosheets around the suspended core, an SHG signal is observed under a milliwatt-level pump light, and exhibits a quadratic growth with the increased pump power. The SHG process is also realized in a broad wavelength range by varying the pump in the range of 1420∼1700 nm. The SCF with the large air cladding and suspended core as an excellent platform can therefore be employed to integrate low-dimensional nonlinear materials, which holds great promise for the applications of all-fiber structures in new light source generating, signal processing and fiber sensing.

Femtosecond laser plasmonic nano-printing metasurfaces for multiple-dimensional manipulation of light fields.

Metasurfaces enable the multidimensional manipulation of light fields in a subwavelength scale. However, the low-cost preparation of large-area metasurfaces is still a challenge. In this Letter, we first, to the best of our knowledge, use the laser plasmonic nano-printing technique to efficiently manufacture metasurfaces with multidimensional manipulation capability. By utilizing a phase-polarization mapping method, we fabricated a silicon-based metasurface for color display, and indium tin oxide-based metasurfaces for decoupled near- and far-field holographic displays. This flexible and efficient laser plasmonic nano-printing method has great potential in the preparation of large-area metasurfaces, and is of great significance to promote the practical application of metasurfaces.

Room-Temperature Ferroelectricity in 1T

van der Waals materials possess an innate layer degree of freedom and thus are excellent candidates for exploring emergent two-dimensional ferroelectricity induced by interlayer translation. However, despite being theoretically predicted, experimental realization of this type of ferroelectricity is scarce at the current stage. Here, we demonstrate robust sliding ferroelectricity in semiconducting 1T^{'}-ReS_{2} multilayers via a combined study of theory and experiment. Room-temperature vertical ferroelectricity is observed in two-dimensional 1T^{'}-ReS_{2} with layer number N≥2. The electric polarization stems from the uncompensated charge transfer between layers and can be switched by interlayer sliding. For bilayer 1T^{'}-ReS_{2}, the ferroelectric transition temperature is estimated to be ∼405 K from the second harmonic generation measurements. Our results highlight the importance of interlayer engineering in the realization of atomic-scale ferroelectricity.

Phase fluctuation cancellation for coherent-detection BOTDA fiber sensors based on optical subcarrier multiplexing.

The employment of coherent detection in a Brillouin optical time domain analysis (BOTDA) fiber sensor brings benefits, including signal-to-noise ratio enhancement, non-local effect reduction, and sensing speed improvement. Recently, it was found that the performance of a coherent-detection BOTDA fiber sensor suffers from phase fluctuations introduced by the fiber group delay jitter. Here, we propose a phase fluctuation cancellation approach based on optical subcarrier multiplexing. In a proof-of-concept experiment, the phase stability for in-phase/quadrature demodulation reaches a standard deviation value of as small as 0.4 mrad. The variations in the Brillouin gain and phase spectra caused by the phase fluctuation are then effectively alleviated, resulting in an enhancement of the Brillouin frequency shift measurement certainty along the whole sensing fiber.

Dynamic strain measurement based on ultrafast Brillouin collision in the correlation domain.

The Brillouin collision rate in a Brillouin optical correlation domain analysis fiber sensor is ultimately limited by the sensing fiber length, which restricts the single point sampling rate in dynamic strain measurement. Here, a time-gated long-phase-sequence-coded pump method is proposed to overcome this limitation. The Brillouin collision rate is limited only by the phonon lifetime, since it governs the building and decaying time of the acoustic wave. For a sensing fiber length of ∼1km, a Brillouin collision rate as high as 1 MHz is experimentally realized. This further results in a single-point sampling rate of 1 kHz for dynamic strain sensing with a spatial resolution of ∼2cm and a measurement uncertainty of <33.5µÎµ.

Continuous-wave pumped frequency upconversions in an InSe-integrated microfiber.

We report the achievement of continuous-wave (CW)-pumped second-harmonic generation (SHG) and sum frequency generation (SFG) in a layered indium selenide (InSe)-integrated microfiber. As a result of the strong interaction between the InSe nanosheets and the evanescent field, the second-order nonlinear processes are greatly enhanced in the InSe-integrated microfiber pumped by a few milliwatt CW lasers. The experimental results reveal that the intensities of SHG and SFG are quadratic and linear dependencies with the incident pump power, respectively, which is consistent with theoretical predictions. Additionally, the SHG intensity is strongly polarization-dependent on the nonaxisymmetrical distribution of the InSe nanosheets around the microfiber, providing the possibility of the SHG-polarized manipulation. The proposed device has the potential to be integrable into all-fiber systems for nonlinear applications.

Optical Transmission Properties of Si3N4 Add-Drop Micro-Ring Resonator Induced by a Fabry-Perot Resonance Effect.

To resolve the problem of miscellaneous peaks and improve the accuracy of data processing in micro-ring resonators (MRRs), we propose an optical transmission model based on a Fabry-Perot (F-P) resonance effect in a Si3N4 add-drop MRR-waveguide structure, which is analyzed using a coupled mode theory and stationary interference method. The analysis indicates the experimentally obtained miscellaneous peaks are mainly induced by the multiple reflections between the two end grating couplers, which form a F-P cavity. In addition, an anti-reflection film on the interface of the grating couplers is proposed to reduce the F-P resonance effect. This work could be useful to analyze optical transmission properties of other MRR-based structures.

Photonic thermometer with a sub-millikelvin resolution and broad temperature range by waveguide-microring Fano resonance.

Fano resonance theoretically is an effective approach for sensitivity enhancement in photonic sensing applications, but the reported methods suffer from complicated structure and fabrication, narrow dynamic range, etc. In this article, we propose a photonic thermometer with sub-millikelvin resolution and broad temperature measurement range implemented by a simple waveguide-microring Fano structure. An air hole is introduced at the center of the coupling region of the waveguide of an all-pass microring resonator. The effective refractive index theory is used to design its equivalent phase shift and therefore the lineshape of the Fano resonance. Experimental results showed that the quality factor and the Fano parameter of the structure were invariant in a broad temperature range. The wavelength-temperature sensitivity was 75.3 pm/℃, the intensity-temperature sensitivity at the Fano asymmetric edge was 7.49 dB/℃, and the temperature resolution was 0.25 mK within 10℃ to 90℃.

Near-infrared photodetection with plasmon-induced hot electrons using silicon nanopillar array structure.

A plasmon-induced hot-electron photodetector based on silicon nanopillar array is developed. The nanostructure is fabricated by reactive ion etching with a monolayer of self-assembled polystyrene nanosphere in hexagonal close-packed lattice as the mask. Light absorption and hot-electron generation are mainly enhanced by the surface plasmon polaritons formed at the surface of the gold film on the nanopillar sidewalls. The photoresponse spans two telecom wavebands, viz. the range of 1250-1600 nm, and has a value of 2.5 mA W-1 at 1310 nm. The proposed silicon nanopillar-based hot-electron infrared detector has great potentials for device integration in silicon photonics relying on the economic large-area fabrication process.