Tm,Ho:YLF waveguide lasers at 2.05†µm.

In this work, we report on the first, to our knowledge, 2.05-µm laser based on femtosecond-laser direct written (FsLDW) Tm,Ho:YLF cladding waveguides. A channel waveguide with a 90-µm diameter "fiber-like" low-index cladding is fabricated in a 6 at. % Tm3+, 0.4 at. % Ho3+:LiYF4 crystal by FsLDW. Pumped by Ti:sapphire laser at 795.1 nm, the fabricated waveguide supports efficient lasing oscillation at 2050 nm with a maximum output power of 47.5 mW, a minimum lasing threshold of 181 mW, and a slope efficiency of 20.1%. The impacts of cavity conditions and polarizations of the pump light on the obtained lasing performance are well studied. The experimental results obtained in this study demonstrate the great potential of utilizing Tm,Ho:YLF and FsLDW for the development of durable mid-infrared lasers featuring compact designs.

Second harmonic generation of visible vortex laser based on a waveguide-grating emitter in LBO.

In this work, we propose a practical solution to visible vortex laser emission at 532 nm based on second harmonic generation (SHG) in a well-designed waveguide-grating structure. Such an integrated structure is fabricated by femtosecond laser direct writing (FsLDW) in an LBO crystal. Confocal micro-Raman spectroscopy is employed for detailed analysis of FsLDW-induced localized crystalline damage. By optical excitation at 1064 nm, the guiding properties, SHG performance, as well as vortex laser generation of the waveguide-grating hybrid structure are systematically studied. Our results indicate that FsLDW waveguide-grating emitter is a reliable design holding great promise for nonlinear vortex beam generation in integrated optics.

Broadband nonlinear optical response and sub-picosecond carrier dynamics in graphene-SnSe2 van der Waals heterostructures.

The van der Waals (vdWs) heterostructures, with vertical layer stacking structure of various two-dimensional (2D) materials, maintain the reliable photonic characteristics while compensating the shortcomings of the participating individual components. In this work, we combine the less-studied multilayer tin selenide (SnSe2) thin film with one of the traditional 2D materials, graphene, to fabricate the graphene-based vdWs optical switching element (Gr-SnSe2) with superior broadband nonlinear optical response. The transient absorption spectroscopy (TAS) measurement results verify that graphene acts as the recombination channel for the photogenerated carrier in the Gr-SnSe2 sample, and the fast recovery time can be reduced to hundreds of femtoseconds which is beneficial for the optical modulation process. The optical switching properties are characterized by the I-scan measurements, exhibiting a saturable energy intensity of 2.82 mJ·cm-2 (0.425 µJ·cm-2) and a modulation depth of 15.6% (22.5%) at the wavelength of 1030 nm (1980nm). Through integrating Gr-SnSe2 with a cladding waveguide, high-performance picosecond Q-switched operation in the near-infrared (NIR) and mid-infrared (MIR) spectral regions are both achieved. This work experimentally demonstrates the great potential of graphene-based vdWs heterostructures for applications in broadband ultrafast photonics.

Floquet parity-time symmetry in integrated photonics.

Parity-time (PT) symmetry has been unveiling new photonic regimes in non-Hermitian systems, with opportunities for lasing, sensing and enhanced light-matter interactions. The most exotic responses emerge at the exceptional point (EP) and in the broken PT-symmetry phase, yet in conventional PT-symmetric systems these regimes require large levels of gain and loss, posing remarkable challenges in practical settings. Floquet PT-symmetry, which may be realized by periodically flipping the effective gain/loss distribution in time, can relax these requirements and tailor the EP and PT-symmetry phases through the modulation period. Here, we explore Floquet PT-symmetry in an integrated photonic waveguide platform, in which the role of time is replaced by the propagation direction. We experimentally demonstrate spontaneous PT-symmetry breaking at small gain/loss levels and efficient control of amplification and suppression through the excitation ports. Our work introduces the advantages of Floquet PT-symmetry in a practical integrated photonic setting, enabling a powerful platform to observe PT-symmetric phenomena and leverage their extreme features, with applications in nanophotonics, coherent control of nanoscale light amplification and routing.

Block copolymer-templated surface-enhanced Raman scattering-active nanofibers for hydrogen sulfide detection.

Metabolic disorders involving endogenous H2S have been linked to a variety of serious human diseases, particularly cancer. In this study, we employed nanofibers with surface-enhanced Raman scattering (SERS) activity for the detection of H2S within live cells. These nanofibers were chosen for their minimal invasiveness, high spatial resolution, and enhanced SERS sensitivity. To improve the performance of SERS, highly sensitive core-shell multibranched-Au NPs (MBAuNP)@Ag NPs were decorated on the nanofibers as SERS tags for H2S detection. A SERS probe named MBN, embedded between the Au core and Ag shell, was utilized for quantitative detection. These nanofibers exhibited excellent reproducibility (relative standard deviation (RSD) within 5.7 %) and demonstrated a strong linear relationship with sulfide concentrations ranging from 50 nM to 1 µM, with an estimated detection limit of 0.12 nM. As a proof of concept, the aforementioned nanofibers were successfully applied to detect endogenous H2S in living cells, offering a potential analytical method in the related research of detection.

Q-switched vortex waveguide laser generation based on LNOI thin films with implanted Ag nanoparticles.

Lithium-niobate-on-insulator (LNOI) thin films have gained significant attention in integrated photonics due to their exceptional crystal properties and wide range of applications. In this paper, we propose a novel approach to realize a Q-switched vortex waveguide laser by incorporating integrated lithium niobate thin films with embedded silver nanoparticles (Ag:LNOI) as a saturable absorber. The saturable absorption characteristics of Ag:LNOI are investigated using a home-made Z-scan system. Additionally, we integrate Ag:LNOI as a saturable absorber into a Nd:YAG "ear-like" cladding waveguide platform, which is prepared via femtosecond laser direct writing. By combining this setup with helical phase plates for phase modulation in the resonator, we successfully achieve a passive Q-switched vortex laser with a high repetition rate and narrow pulse duration in the near-infrared region. This work demonstrates the potential applications of LNOI thin films towards on-chip integration of vortex waveguide laser sources.

Nd:YSAG waveguide-grating vortex laser: design and implementation.

In this work, we fabricate a hybrid waveguide-grating vortex laser in Nd:YSAG by using femtosecond laser direct writing (FsLDW). The detailed parameters of the hybrid structure are fixed by optical simulation. In experiments, an efficient vortex beam is produced in the passive operation at 1064 nm. Under optical pumping at 808 nm, a dual-wavelength waveguide laser at 1060/1062 nm as well as a waveguide-grating vortex laser at 1060 nm are obtained. The laser performance and diffraction properties of the generated vortex laser are detailed, studied, and discussed, providing meaningful reference results toward the practical applications of FsLDW and waveguide-grating structures in integrated photonics.

Tunable sum-frequency generation in modal phase-matched thin film lithium niobate rib waveguides.

In this work, we report a highly efficient and tunable on-chip sum-frequency generation (SFG) on a thin-film lithium niobate platform via modal phase matching (e + e→e). It provides on-chip SFG a solution with both high efficiency and poling-free by using the highest nonlinear coefficient d33 instead of d31. The on-chip conversion efficiency of SFG is approximately 2143%W-1 with a full width at half maximum (FWHM) of 4.4 nm in a 3-mm-long waveguide. It can find applications in chip-scale quantum optical information processing and thin-film lithium niobate based optical nonreciprocity devices.

1.8-µm laser operation based on femtosecond-laser direct written Tm:YVO4 cladding waveguides.

In this work, we have demonstrated tunable 1.8-µm laser operation based on a Tm:YVO4 cladding waveguide fabricated by means of femtosecond laser direct writing. Benefiting from the good optical confinement of the fabricated waveguide, efficient thulium laser operation, with a maximum slope efficiency of 36%, a minimum lasing threshold of 176.8 mW, and a tunable output wavelength from 1804 to 1830nm, has been achieved in a compact package via adjusting and optimizing the pump and resonant conditions of the waveguide laser design. The lasing performance using output couplers with different reflectivity has been well studied in detail. In particular, due to the good optical confinement and relatively high optical gain of the waveguide design, efficient lasing can be obtained even without using any cavity mirrors, thereby opening up new possibilities for compact and integrated mid-infrared laser sources.

Interrelated Roles of Multiple Key Structure-Directing Agents in Seed-Mediated Growth of Monodisperse and Multibranched Au Nanoparticles.

Detailed mechanistic investigations of the interrelated roles of multiple key structure-directing agents in the growth solution of Au nanoparticles (AuNPs) is required for the optimization of synthetic protocols. Here, we report a robust seed-mediated growth strategy for synthesizing multibranched NPs (MB-AuNPs) with monodispersed size distribution, and investigate the roles of Ag ions and 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES) based on an overgrowth synthesis approach. The intertwining roles of Ag+ , surface-capping stabilizers, and reducing agents were elucidated, and used to control the morphology of MB-AuNPs. The overgrowth of MB-AuNPs involves two distinct underlying pathways, namely, directional and anisotropic growth of Au branches on specific facets of Au seeds as well as an aggregation and growth mechanism governed by HEPES. In addition to Ag ions and HEPES, morphology tunability can also be achieved by pre-modification of the Au seeds with molecular probes. Optimized probe-containing MB-AuNPs prove to be excellent surface-enhanced Raman scattering (SERS) substrates and nanozymes. Taken together, the results of this work reveal the mechanistic evolution of nanocrystal growth which should stimulate the development of new synthetic strategies, improve the capabilities of tuning the optical, catalytic, and electronic properties of NPs, and further advance their applications in biolabeling, imaging, biosensing, and therapy.

A tailored 100%-efficient 532/1064-nm demultiplexer in y-cut LiNbO3 crystal.

In this Letter, we report a tailored 532/1064-nm demultiplexer based on a multimode interference (MMI) coupler with an efficiency of 100%. The device structure is designed according to the self-imaging principle, and the propagation and the wavelength division performance are simulated by the beam propagation method. The demultiplexer is fabricated in a y-cut LiNbO3 crystal by femtosecond laser direct writing (FLDW) combined with the ion implantation technique. The end-face coupling system is used to measure the near field intensity distribution, and the spectra collected from the output ports are obtained by spectrometers. The simulated and the experimental results indicate that the customized demultiplexer in the LiNbO3 crystal presents excellent wavelength division performance operating at 532 nm and 1064 nm. This work demonstrates the application potential of FLDW technology for developing miniaturized photonic components and provides a new strategy for fabricating high-efficiency integrated wavelength division devices on an optical monocrystalline platform.

Layer-dependent nonlinear optical properties of two-dimensional InSe and its applications in waveguide lasers.

The thickness-dependent third-order nonlinear optical properties of two-dimensional ß-InSe and its potential applications as a saturable absorber in pulsed laser generation are investigated. InSe sheets with different layers are prepared by the chemical vapor deposition. Using open-aperture femtosecond Z-scan technique at 1030 nm, the modulation depth and nonlinear absorption coefficient are obtained to be 36% and -1.6 × 104 cm·GW-1, respectively. The intrinsic mechanism of the layer-dependent energy band structure evolution is analyzed based on density functional theory, and the theoretical analysis is consistent with the experimental results. Based on a waveguide cavity, a Q-switched mode-locked laser at 1 µm with a repetition frequency of 8.51 GHz and a pulse duration of 28 ps is achieved by utilizing the layered InSe as a saturable absorber. This work provides an in-depth understanding of layer-dependent properties of InSe and extends its applications in laser technology for compact light devices.

Heterodyne detection of backscattering for whispering-gallery-mode sensors.

Whispering-gallery-mode (WGM) microcavities have shown significant applications in nanoparticle sensing for environmental monitoring and biological analysis. However, the enhancement of detection resolution often calls for active cavities or elaborate structural designs, leading to an increase of fabrication complexity and cost. Herein, heterodyne amplification is implemented in WGM microsensors based on backscattering detection mechanism. By interfering with an exotic reference laser, the reflecting light backscattered by perturbation targets can be strongly enlarged, yielding an easy-to-resolve and consequently sensitive microsensor. The dependence of detection laser frequency has also been characterized with the assistance of optothermal dynamics. We show that exploiting heterodyne interferometry boosts the detection of weak signals in microresonator systems and provides a fertile ground for optical microsensor development.

Self-Powered Lithium Niobate Thin-Film Photodetectors.

Thin-film lithium niobate platform, namely lithium-niobate-on-insulator (LNOI), brings new opportunities for integrated photonics, taking advantages from both outstanding crystalline properties and special structural features. The excellent properties of LNOI have triggered development of a variety of on-chip photonic devices for light generation and manipulation. However, as an indispensable component for photonic circuit with full functionalities, the thin-film photodetector lacks in portfolios of LNOI-based devices due to standing obstacles of low electrical conductivity and poor photoelectric conversion ability. Here, a self-powered broadband LNOI photodetector based on enhanced photovoltaic effect, benefitting from encapsulated plasmonic nanoparticles and doped silver ions, is reported. Maximum responsivity of 0.25 A W-1 and detectivity (1.56 × 1014 Jones) are achieved. First-principle calculations and electric-field simulation reveal intrinsic mechanisms and crucial roles of plasmonic nanoparticles and silver ions on photocurrent generation and collection. This work opens an avenue to develop high-performance on-chip LNOI photodetectors, offering a conceivable means toward multiple-functional photonic circuits.

A Plasmon-Enhanced SnSe2 Photodetector by Non-Contact Ag Nanoparticles.

The 2D layered materials are promising candidates for broadband, low-cost photodetectors. One deficiency of 2D materials is the relatively low absorbance of light, limiting the applications of the 2D photodetectors. Doping of plasmonic nanoparticles into 2D materials may enhance the optical absorbance owing to the localized surface plasmonic resonance (LSPR) effect; however, considerable defects may be introduced into the 2D materials at the same time, resulting in certain degradation of device performance. Here, a novel design of 2D photodetectors with enhanced photoresponsivity by non-contact plasmonic nanoparticles (NPs) is proposed, consisting of a hybrid structure of few-layer SnSe2 transferred a fused silica (SiO2 ) plate with embedded Ag NPs. The system of Ag NPs-in-SiO2 shows strong LSPR effect with significantly enhanced optical absorption, acting on SnSe2 in a non-contact configuration. Benefiting from well-preserved intrinsic features of SnSe2 and LSPR effect, the responsivity of the photodetector is enhanced by 881 times with the bias voltage of 0.1 V, which is superior to previously reported results of plasmon-enhanced 2D photodetectors. Moreover, the SiO2 with embedded Ag NPs is recyclable and can be easy to be recombined with different 2D materials. This work offers additional strategy for development of efficient, low-cost 2D photodetectors by using plasmonic NPs.

Surface-Enhanced Raman Scattering Optophysiology Nanofibers for the Detection of Heavy Metals in Single Breast Cancer Cells.

Mercury(II) ions (Hg2+) and silver ions (Ag+) are two of the most hazardous pollutants causing serious damage to human health. Here, we constructed surface-enhanced Raman scattering (SERS)-active nanofibers covered with 4-mercaptopyridine (4-Mpy)-modified gold nanoparticles to detect Hg2+ and Ag+. Experimental evidence suggests that the observed spectral changes originate from the combined effect of (i) the coordination between the nitrogen on 4-Mpy and the metal ions and (ii) the 4-Mpy molecular orientation (from flatter to more perpendicular with respect to the metal surface). The relative intensity of a pair of characteristic Raman peaks (at ∼428 and ∼708 cm-1) was used to quantify the metal ion concentration, greatly increasing the reproducibility of the measurement compared to signal-on or signal-off detection based on a single SERS peak. The detection limit of this method for Hg2+ is lower than that for the Ag+ (5 vs 100 nM), which can be explained by the stronger interaction energy between Hg2+ and N compared to Ag+ and N, as demonstrated by density functional theory calculations. The Hg2+ and Ag+ ions can be masked by adding ethylenediaminetetraacetate and Cl-, respectively, to the Hg2+ and Ag+ samples. The good sensitivity, high reproducibility, and excellent selectivity of these nanosensors were also demonstrated. Furthermore, detection of Hg2+ in living breast cancer cells at the subcellular level is possible, thanks to the nanometric size of the herein described SERS nanosensors, allowing high spatial resolution and minimal cell damage.

Femtosecond laser direct writing of depressed cladding waveguides in Nd:YAG with "ear-like" structures: fabrication and laser generation.

Low-loss depressed cladding waveguide architecture is highly attractive for improving the laser performance of waveguide lasers. We report on the design and fabrication of the "ear-like" waveguide structures formed by a set of parallel tracks in neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal via femtosecond laser writing. The obtained "ear-like" waveguides are with more symmetric mode profiles and lower losses by systematically comparing the guiding properties of two kinds of normal cladding waveguide. Efficient waveguide lasers are realized based on the designed structure in both continuous wave and pulsed regimes. Combined the high-gain from cladding waveguide and special "ear-like" structure, a passively fundamentally Q-switched laser with the narrow pulse width and the high repetition rate has been obtained by using tin diselenide (SnSe2) as saturable absorber.

Femtosecond laser direct writing of few-mode depressed-cladding waveguide lasers.

We report on mirrorless laser operation of Nd:YVO4 single- and double-cladding waveguides fabricated by femtosecond laser direct writing. Fundamental- (LP01) and high-order-mode (LP03, LP05) guiding and lasing have been observed in waveguides with different geometries and sizes. Double-cladding waveguides exhibit good guiding and lasing performance via inheriting advantages respectively from individual single cladding. As a result, continuous-wave lasing with a threshold as low as 59 mW is obtained, depending on the optical feedback provided only by Fresnel reflections at the waveguide end faces. By using few-layer graphene as saturable absorber, passively Q-switched operation in fabricated waveguides is also achieved.

Extremely broadband terahertz generation via pulse compression of an Ytterbium laser amplifier.

We present a system for extremely broadband terahertz (THz) generation based on an Ytterbium (Yb) amplified laser emitting 170-fs-long pulses centered at 1030 nm. The pulses are first spectrally broadened in an Ar-filled hollow-core capillary fiber (HCF) and then recompressed down to ∼18 fs with a chirped-mirror pair. Extreme broadband THz pulses of bandwidths up to 60 THz and peak electric field as high as 55 kV/cm are obtained via two-color plasma generation. The combination of high-power Yb laser systems with gas-filled HCF opens the path towards the realization of the next generation high-repetition-rate, extremely broadband, and intense-field THz time-domain spectroscopy systems.

Frequency comb up- and down-conversion in synchronously driven χ(2) optical microresonators.

Optical frequency combs are key to optical precision measurements. While most frequency combs operate in the near-infrared (NIR) regime, many applications require combs at mid-infrared (MIR), visible (VIS), or even ultra-violet (UV) wavelengths. Frequency combs can be transferred to other wavelengths via nonlinear optical processes; however, this becomes exceedingly challenging for high-repetition-rate frequency combs. Here it is demonstrated that a synchronously driven high-Q microresonator with a second-order optical nonlinearity can efficiently convert high-repetition-rate NIR frequency combs to VIS, UV, and MIR wavelengths, providing new opportunities for microresonator and electro-optic combs in applications including molecular sensing, astronomy, and quantum optics.