Interest in the nonlinear properties of multi-mode optical waveguides has seen a recent resurgence on account of the large dimensionality afforded by the platform. The large volume of modes in these waveguides provides a new spatial degree of freedom for phase matching nonlinear optical processes. However, this spatial dimension is quantized, which narrows the conversion bandwidths of intermodal processes and constrains spectral and temporal tailoring of the light. Here we show that by engineering the relative group velocity within the spatial dimension, we can tailor the phase-matching bandwidth of intermodal parametric nonlinearities. We demonstrate group-velocity-tailored parametric nonlinear mixing between higher-order modes in a multi-mode fiber with gain bandwidths that are more than an order of magnitude larger than that previously thought possible for intermodal four-wave mixing. As evidence of the technological utility of this methodology, we seed this process to generate the first high-peak-power wavelength-tunable all-fiber quasi-CW laser in the Ti:sapphire wavelength regime. More generally, with the combination of intermodal interactions, which dramatically expand the phase-matching degrees of freedom for nonlinear optics, and intermodal group-velocity engineering, which enables tailoring of the bandwidth of such interactions, we showcase a platform for nonlinear optics that can be broadband while being wavelength agnostic.

Graphene microfibers are burgeoning modulators with great potential in all-optical communication. One of the critical issues that remains to be understood is the dynamic mechanism of light–graphene interaction. Here, we propose a power dependent modulation by using 980 nm pump light and 1064 nm signal light via graphene microfiber, and the results show a strong transmission reduction and frequency blue shift with the increase of pump power. The experimental observation is attributed to a stimulated Brillouin scattering process induced by the pump light. Power and frequency variations are a result of energy transition of the scattered phonon in the fiber. This work reveals the nonlinear effect process in the light–graphene interaction and provides a new method for power and frequency control with graphene all-optical modulation.

We propose and demonstrate a widely tunable passively Q-switched Ho3+/Pr3+-codoped ZrF4-BaF2-LaF3-AlF3-NaF fiber laser operating in the 2.8 μm mid-infrared (MIR) waveband based on a single-walled carbon nanotube (SWCNT) saturable absorber (SA). The SWCNTs have diameters ranging from 1.4 to 1.7 nm. The modulation depth and saturation intensity of the SWCNT SA measured at 2850 nm are 16.5% and 1.66 MW/cm2, respectively. Stable Q-switched pulses with the shortest pulse duration of 1.46 μs and the maximum pulse energy of 0.43 μJ are achieved at a launched pump power of 445.6 mW. The combined use of a broadband SWCNT SA and a plane ruled grating ensures a broad continuously tuning range of 55.0 nm from 2837.6 to 2892.6 nm. The output powers, emission spectra, repetition rates, and pulse durations at different tuning wavelengths are also characterized and analyzed. Our results indicate that SWCNTs can be excellent broadband SAs in the 3 μm MIR region. To the author’s knowledge, this is the first demonstration of a widely tunable carbon-nanotube-enabled passively Q-switched fiber laser operating in the 2.8 μm MIR waveband.

High-dimensional entanglement is a valuable resource for secure and efficient quantum information processing. A major challenge for practical use of multidimensional quantum systems is the establishment of controls over arbitrary superposition states in realistic conditions. This work demonstrates spatially entangled photon pairs propagating through two separate four-core optical fibers with the amplitudes and phases of the superposition being independently controllable. Using quantum state analyzers that can detect arbitrary multicore superposition states, Bell-type CGLMP inequalities in two, three, and four dimensions are directly tested. Enhanced violation of the inequality by slight nonmaximality of entanglement is also demonstrated.

We experimentally demonstrate self-trapping of light, as a result of plasmonic resonant optical nonlinearity, in both aqueous and organic (toluene) suspensions of gold nanorods. The threshold power for soliton formation is greatly reduced in toluene as opposed to aqueous suspensions. It is well known that the optical gradient forces are optimized at off-resonance wavelengths at which suspended particles typically exhibit a strong positive (or negative) polarizability. However, surprisingly, as we tune the wavelength of the optical beam from a continuous-wave (CW) laser, we find that the threshold power is reduced by more than threefold at the plasmonic resonance frequency. By analyzing the optical forces and torque acting on the nanorods, we show theoretically that it is possible to align the nanorods inside a soliton waveguide channel into orthogonal orientations by using merely two different laser wavelengths. We perform a series of experiments to examine the transmission of the soliton-forming beam itself, as well as the polarization transmission spectrum of a low-power probe beam guided along the soliton channel. It is found that the expected synthetic anisotropic properties are too subtle to be clearly observed, in large part due to Brownian motion of the solvent molecules and a limited ordering region where the optical field from the self-trapped beam is strong enough to overcome thermodynamic fluctuations. The ability to achieve tunable nonlinearity and nanorod orientations in colloidal nanosuspensions with low-power CW laser beams may lead to interesting applications in all-optical switching and transparent display technologies.

The ultrafast fiber laser has attracted a great deal of research interest due to its low cost, high efficiency, and simple maintenance. Optical passive devices are vital parts of a fiber laser. In order to obtain a fiber laser with high quality, optical passive devices with high performance are required. Here, we demonstrate a highly integrated optical device with the combination of a saturable absorber (SA), coupler, isolator, wavelength division multiplexer, and erbium-doped fiber. The built-in SA has a modulation depth of 7% and can withstand high pump power due to the unique structure of the proposed device. The proposed device is applied to an ultracompact fiber laser, which greatly simplifies the laser structure and reduces the size of the proposed laser. The central wavelength, pulse duration, repetition rate, and signal-to-noise ratio of the output soliton are 1560 nm, 1.06 ps, 25.8 MHz, and 50 dB, respectively. The proposed device has great potential for application in high-power and high-frequency fiber lasers. The proposed ultracompact fiber laser has important applications in optical communication, optical sensing, optical frequency combs, and micromachining.

We experimentally demonstrate two kinds of all few-mode fiber (FMF) ring lasers with high-order mode (HOM) oscillation in the laser cavity. One kind is a switchable-wavelength all-FMF HOM laser with an output of tunable optical vortex beams (OVBs); the other is a Q-switched all-FMF HOM laser with an output of pulsed cylindrical vector beams (CVBs). The lasers are composed of all-FMF components and few-mode erbium-doped fiber. A Sagnac interferometer made of a 3 dB FMF coupler functions as the wavelength selector, and switchable multiwavelength tunable OVBs are experimentally realized. Carbon nanotube-based saturable absorbers and the nonlinear polarization rotation technique are used to achieve Q-switched CVB lasers. This is the first report, to our knowledge, on the generation of switchable-wavelength and Q-switched HOM beams in all-FMF laser cavities.

We develop a hybrid optofluidic microcavity by placing a microsphere with a diameter ranging from 1 to 4 μm in liquid-filled plano-plano Fabry–Perot (FP) cavities, which can provide an extremely low effective mode volume down to 0.3–5.1 μm3 while maintaining a high Q-factor up to 1×104–5×104 and a finesse of ～2000. Compared to the pure plano-plano FP cavities that are known to suffer from the lack of mode confinement, diffraction, and geometrical walk-off losses as well as being highly susceptible to mirror misalignment, our microsphere-integrated FP (MIFP) cavities show strong optical confinement in the lateral direction with a tight mode radius of only 0.4–0.9 μm and high tolerance to mirror misalignment as large as 2°. With the microsphere serving as a waveguide, the MIFP is advantageous over a fiber-sandwiched FP cavity due to the open-cavity design for analytes/liquids to interact strongly with the resonant mode, the ease of assembly, and the possibility to replace the microsphere. In this work, the main characteristics of the MIFP, including Q-factor, finesse, effective mode radius and volume, and their dependence on the surrounding medium’s refractive index, mirror spacing, microsphere position inside the FP cavity, and mirror misalignment, are systematically investigated using a finite-element method. Then, by inserting dye-doped polystyrene microspheres of various sizes into the FP cavity filled with water, we experimentally realize single-mode MIFP optofluidic lasers that have a lasing threshold as low as a few microjoules per square millimeter and a lasing spot radius of only ～0.5 μm. Our results suggest that the MIFP cavities provide a promising technology platform for novel photonic devices and biological/chemical detection with ultra-small detection volumes.

A hybrid no-core fiber (NCF)–graded index multimode fiber (GIMF) structure is used as a saturable absorber (SA) for mode-locked laser operation. Such an SA supports various types of soliton outputs. By changing the cavity parameters, not only the spatiotemporal mode-locking states with a stable single pulse but also tightly and loosely bound solitons are generated. Single 35.5 pJ solitons centered at 1568.5 nm have a 4 nm spectral full-width at half-maximum and an 818 fs temporal duration. Tightly bound soliton pairs with continuously tunable wavelength from 1567.48 nm to 1576.20 nm, featured with an ～700 fs pulse train with a separation of 2.07 ps, have been observed by stretching the NCF-GIMF structured device. Meanwhile, several different pulse separations from 37.57 ps to 56.46 ps of loosely bound solitons have also been realized. The results provide help in understanding the nonlinear dynamics in fiber lasers.

Optical trapping techniques hold great interest for their advantages that enable direct handling of nanoparticles. In this work, we study the optical trapping effects of a diffraction-limited focal field possessing an arbitrary photonic spin and propose a convenient method to manipulate the movement behavior of the trapped nanoparticles. In order to achieve controllable spin axis orientation and ellipticity of the tightly focused beam in three dimensions, an efficient method to analytically calculate and experimentally generate complex optical fields at the pupil plane of a high numerical aperture lens is developed. By numerically calculating the optical forces and torques of Rayleigh particles with spherical/ellipsoidal shape, we demonstrate that the interactions between the tunable photonic spin and nanoparticles lead to not only 3D trapping but also precise control of the nanoparticles’ movements in terms of stable orientation, rotational orientation, and rotation frequency. This versatile trapping method may open up new avenues for optical trapping and their applications in various scientific fields.

Polarization manipulation is a significant issue for artificial modulation of the electromagnetic (EM) wave, but general mechanisms all suffer the restriction of inherent symmetric properties between opposite handedness. Herein, a strategy to independently and arbitrarily manipulate the EM wave with orthogonal circular polarizations based on a metasurface is proposed, which effectually breaks through traditional symmetrical characteristics between orthogonal handedness. By synthesizing the propagation phase and geometric phase, the appropriate Jones matrix is calculated to obtain independent wavefront manipulation of EM waves with opposite circular polarizations. Two transmissive ultra-thin meta-deflectors are proposed to demonstrate the asymmetrical refraction of transmitted circularly polarized waves in the microwave region. Simulated transmitted phase front and measured far-field intensity distributions are in excellent agreement, indicating that the transmitted wave with different polarizations can be refracted into arbitrary and independent directions within a wide frequency band (relative bandwidth of 25%). The results presented in this paper provide more freedom for the manipulation of EM waves, and motivate the realizations of various polarization-independent properties for all frequency spectra.

Millimeter and terahertz wave photodetectors have a wide range of applications. However, the state-of-the-art techniques lag far behind the urgent demand due to the structure and performance limitations. Here, we report sensitive and direct millimeter and terahertz wave photodetection in compact InGaAs-based subwavelength ohmic metal–semiconductor–metal structures. The photoresponse originates from unidirectional transportation of nonequilibrium electrons induced by surface plasmon polaritons under irradiation. The detected quantum energies of electromagnetic waves are far below the bandgap of InGaAs, offering, to the best of our knowledge, a novel direct photoelectric conversion pathway for InGaAs beyond its bandgap limit. The achieved room temperature rise time and noise equivalent power of the detector are 45 μs and 20 pW·Hz 1/2, respectively, at the 0.0375 THz (8 mm) wave. The detected wavelength is tunable by mounting different coupling antennas. Room temperature terahertz imaging of macroscopic samples at around 0.166 THz is also demonstrated. This work opens an avenue for sensitive and large-area uncooled millimeter and terahertz focal planar arrays.

As one of the ideal tools for monitoring the formation and evolution of solar storms, the vacuum ultraviolet (VUV) detector should have both a fast temporal response and an array structure that enables image formation. Here, by combining a nontraditional graphene processing technique with traditional metal organic chemical vapor deposition epitaxy technology, we created hybrid heterostructure (HH) arrays of p-Gr/AlN/p-Si with VUV photovoltaic response capability and silicon integration potential. The HH arrays not only exhibit ultrafast temporal response (rise time of only 120 ns) and an extremely high Ion/Ioff ratio of 107, but also achieve the imaging demonstration of a VUV pattern for the first time. The HH technique provides a possible new path for the development of VUV imaging devices.