We report on diode-pumped Er:Y2O3 ceramic lasers at about 2.7 μm in the tunable continuous-wave, self-Q-switching and tungsten disulfide (WS2)-based passively Q-switching regimes. For stable self-Q-switched operation, the maximum output power reaches 106.6 mW under an absorbed power of 2.71 W. The shortest pulse width is measured to be about 1.39 μs at a repetition rate of 26.7 kHz at maximum output. Using a spin-coated WS2 as a saturable absorber, a passively Q-switched Er:Y2O3 ceramic laser is also realized with a maximum average output power of 233.5 mW (for the first time, to the best of our knowledge). The shortest pulse width decreases to 0.72 μs at a corresponding repetition rate of 29.4 kHz, which leads to a pulse energy of 7.92 μJ and a peak power of 11.0 W. By inserting an undoped YAG thin plate as a Fabry–Perot etalon, for the passive Q switching, wavelength tunings are also demonstrated at around 2710, 2717, 2727, and 2740 nm.

Computer-generated holography (CGH) is a technique for converting a three-dimensional (3D) object scene into a two-dimensional (2D), complex-valued hologram. One of the major bottlenecks of CGH is the intensive computation that is involved in the hologram generation process. To overcome this problem, numerous research works have been conducted with the aim of reducing arithmetic operations involved in CGH. In this paper, we shall review a number of fast CGH methods that have been developed in the past decade. These methods, which are commonly referred to as point-based CGH, are applied to compute digital Fresnel holograms for an object space that is represented in a point cloud model. While each method has its own strength and weakness, trading off conflicting issues, such as computation efficiency and memory requirement, they also exhibit potential grounds of synergy. We hope that this paper will bring out the essence of each method and provide some insight on how different methods may crossover into better ones.

Strong plasmonic focal spots, excited by radially polarized light on a smooth thin metallic film, have been widely applied to trap various micro- and nano-sized objects. However, the direct transmission part of the incident light leads to the scattering force exerted on trapped particles, which seriously affects the stability of the plasmonic trap. Here we employ a novel perfect radially polarized beam to solve this problem. Both theoretical and experimental results verify that such a beam could strongly suppress the directly transmitted light to reduce the piconewton scattering force, and an enhanced plasmonic trapping stiffness that is 2.6 times higher is achieved in experiments. The present work opens up new opportunities for a variety of research requiring the stable manipulations of particles.

Mode-locked fiber lasers that can simultaneously generate two asynchronous ultrashort pulse trains could play an attractive role as the alternative light sources for low-complexity dual-comb metrology applications. Although a few multiplexing schemes to realize such lasers have been proposed and demonstrated, here we investigate the lasing characteristics of a passively mode-locked fiber laser with a finite amount of intracavity birefringence. By introducing a section of polarization-maintaining (PM) fiber into the otherwise-non-PM-single-mode cavity, dual asynchronous pulses with nearly orthogonal states of polarization are generated. With a repetition rate difference of hundreds of hertz, the pulses have well-overlapped spectra and show typical features of polarization-locked vector solitons. It is demonstrated that under an anomalous or net normal dispersion regime, either dual vector solitons or dual dissipative vector solitons can be generated, respectively. Such polarization-multiplexed single single-cavity dual-comb lasers could find further uses in various applications in need of simple dual-comb system solutions.

Heterogeneously integrating III-V materials on silicon photonic integrated circuits has emerged as a promising approach to make advanced laser sources for optical communication and sensing applications. Tunable semiconductor lasers operating in the 2–2.5 μm range are of great interest for industrial and medical applications since many gases (e.g., CO2, CO, CH4) and biomolecules (such as blood glucose) have strong absorption features in this wavelength region. The development of integrated tunable laser sources in this wavelength range enables low-cost and miniature spectroscopic sensors. Here we report heterogeneously integrated widely tunable III-V-on-silicon Vernier lasers using two silicon microring resonators as the wavelength tuning components. The laser has a wavelength tuning range of more than 40 nm near 2.35 μm. By combining two lasers with different distributed Bragg reflectors, a tuning range of more than 70 nm is achieved. Over the whole tuning range, the side-mode suppression ratio is higher than 35 dB. As a proof-of-principle, this III-V-on-silicon Vernier laser is used to measure the absorption lines of CO. The measurement results match very well with the high-resolution transmission molecular absorption (HITRAN) database and indicate that this laser is suitable for broadband spectroscopy.

We propose an optical weighing technique with a sensitivity down to a single atom through the coupling between a surface plasmon and a suspended graphene nanoribbon resonator. The mass is determined via the vibrational frequency shift on the probe absorption spectrum while the atom attaches to the nanoribbon surface. We provide methods to separate out the signals of the ultralow frequency vibrational modes from the strong Rayleigh background, first based on the quantum coupling with a pump-probe scheme. Owing to the spectral enhancement in the surface plasmon and the ultralight mass of the nanoribbon, this scheme results in a narrow linewidth (～GHz) and ultrahigh mass sensitivity (～30 yg). Benefitting from the low noises, our optical mass sensor can be achieved at room temperature and reach ultrahigh time resolution.

We investigate the properties of spatial solitons in the fractional Schr dinger equation (FSE) with parity-time (PT)-symmetric lattice potential supported by the focusing of Kerr nonlinearity. Both one- and two-dimensional solitons can stably propagate in PT-symmetric lattices under noise perturbations. The domains of stability for both one- and two-dimensional solitons strongly depend on the gain/loss strength of the lattice. In the spatial domain, the solitons are rigidly modulated by the lattice potential for the weak diffraction in FSE systems. In the inverse space, due to the periodicity of lattices, the spectra of solitons experience sharp peaks when the values of wavenumbers are even. The transverse power flows induced by the imaginary part of the lattice are also investigated, which can preserve the internal energy balances within the solitons.

Tunable optical delay lines are one of the key building blocks in optical communication and microwave systems. In this work, tunable optical delay lines based on integrated grating-assisted contradirectional couplers are proposed and experimentally demonstrated. The device performance is comprehensively improved in terms of parameter optimization, apodization analysis, and electrode design. Tunable group delay lines of 50 ps at different wavelengths within the bandwidth of 12 nm are realized with a grating length of 1.8 mm. Under thermal tuning mode, the actual delay tuning range is around 20 ps at 7.2 V voltage. At last, a new scheme adopting an ultra-compact reflector for doubling group delay is proposed and verified, achieving a large group delay line of 400 ps and a large dispersion value up to 5.5×106 ps/(nm·km) within bandwidth of 12 nm. Under thermal tuning mode, the actual delay tuning range is around 100 ps at 8 V voltage.

We theoretically investigate dark dimer mode excitation and strong coupling with a nanorod dipole. Efficient excitation of a dark mode in a gold (Au) nanorod dimer using an electric dipole can be achieved by an optimal overlap between the dipole moment and dark modal field. By replacing the dipole emitter with an Au nanorod, a plane wave excited dipole mode in the nanorod can be effectively coupled to the dark dimer mode through near-field interaction. At a 10-nm separation of the nanorod and the dimer, plasmonic interaction between dipole-dark modes enters the strong coupling regime with a Rabi-like splitting of 219.2 meV, which is further evidenced by the anticrossing feature and Rabi-like oscillation of electromagnetic energy of the coupled modes. Our results propose an efficient approach to far-field activating dark modes in coupled nanorod dimers and exchanging plasmonic excitations at nanoscale, which may open new opportunities for nanoplasmonic applications such as nanolasers or nanosensors.

We report a passively Q-switched and mode-locked erbium-doped fiber laser (EDFL) based on PtSe2, a new two-dimensional material, as a saturable absorber (SA). Self-started Q-switching at 1560 nm in the EDFL was achieved at a threshold pump power of 65 mW, and at the maximum pump power of 450 mW, the maximum single Q-switched pulse energy is 143.2 nJ. Due to the polarization-dependent characteristics of the PtSe2-based SA, the laser can be switched from the Q-switched state to the mode-locked state by adjusting the polarization state. A mode-locked pulse train with a repetition rate of 23.3 MHz and a pulse width of 1.02 ps can be generated when the pump power increases to about 80 mW, and the stable mode-locked state is maintained until the pump power reaches its maximum 450 mW. The maximum single mode-locked pulse energy is 0.53 nJ. This is the first time to our knowledge that successful generation of stable Q-switched and mode-locked pulses in an Er-doped fiber laser has been obtained by using PtSe2 as a saturable absorber.

We experimentally demonstrate a long-term stable two-dimensional saturable absorption material under ambient conditions—multi-layer antimonene feasible for the mid-infrared spectral region—for the first time to our knowledge. The multi-layer antimonene material prepared using a liquid-phase exfoliation method was coated on a quartz/CaF2 for characterizations and an Au mirror as a reflection-type saturable absorber (SA) device. It has a modulation depth of 10.5%, a saturation peak intensity of 0.26 GW/cm2, and a non-saturation loss of 19.1% measured at 2868.0 nm using the typical power-dependent method. By introducing the SA device into a linear-cavity Ho3+/Pr3+-codoped fluoride fiber laser at 2865.0 nm, stable Q-switched pulses were obtained. It generated a maximum output power of 112.3 mW and pulse energy of 0.72 μJ, while the shortest pulse duration and largest repetition rate were 1.74 μs and 156.2 kHz, respectively. The long-term stability of the SA device was also checked using the same laser setup within 28 days. The results indicate that multi-layer antimonene is a type of promising long-term stable SA material under ambient conditions that can be applied in the mid-infrared spectral region.

We study modulation properties of two-element phased-array semiconductor lasers that can be described by coupled mode theory. We consider four different waveguide structures and modulate the array either in phase or out of phase within the phase-locked regions, guided by stability diagrams obtained from direct numerical simulations. Specifically, we find that out-of-phase modulation allows for bandwidth enhancement if the waveguide structure is properly chosen; for example, for a combination of index antiguiding and gain-guiding, the achievable modulation bandwidth in the case of out-of-phase modulation could be much higher than the one when they are modulated in phase. Proper array design of the coupling, controllable in terms of the laser separation and the frequency offset between the two lasers, is shown to be beneficial to slightly improve the bandwidth but not the resonance frequency, while the inclusion of the frequency offset leads to the appearance of double peak response curves. For comparison, we explore the case of modulating only one element of the phased array and find that double peak response curves are found. To improve the resonance frequency and the modulation bandwidth, we introduce simultaneous external injection into the phased array and modulate the phased array or its master light within the injection locking region. We observe a significant improvement of the modulation properties, and in some cases, by modulating the amplitude of the master light before injection, the resulting 3 dB bandwidths could be enhanced up to 160 GHz. Such a record bandwidth for phased-array modulation could pave the way for various applications, notably optical communications that require high-speed integrated photonic devices.

The denaturation of double-stranded deoxyribonucleic acid (ds-DNA) has been well known to break nucleobase bonds, resulting in single-stranded deoxyribonucleic acid (ss-DNA) in solutions, which can recombine to form ds-DNA in a reversible manner. We developed an efficient process to irreversibly maintain various DNA denaturation levels in thin solid films in order to investigate the impacts of the denaturation on the optical properties of DNA films. By adding NaOH in an aqueous solution of salmon testis DNA, we flexibly controlled the level of denaturation in the solution, which was then spin-coated on Si and silica substrates to irreversibly bind ss-DNAs in a thin solid film. The denaturation of DNA in thin solid films was experimentally confirmed by ultraviolet-visible and Fourier transform infrared spectroscopic investigations, whose level could be controlled by the NaOH content in the aqueous solution precursor. By this irreversible denaturation process, we developed a new method to flexibly vary the refractive index of DNA thin solid films in a wide range of Δn>0.02 in the visible to near-infrared range. Thermo-optic coefficients dn/dT of the films were also experimentally measured in the temperature range from 40°C to 90°C to confirm the significant impacts of denaturation. Detailed thin film processes and optical characterizations are discussed.

Mid-infrared (MIR) integrated photonics has attracted broad interest due to its promising applications in biochemical sensing, environmental monitoring, disease diagnosis, and optical communication. Among MIR integration platforms, germanium-based platforms hold many excellent properties, such as wide transparency windows, high refractive indices, and high nonlinear coefficients; however, the development of MIR germanium photonic devices is still in its infancy. Specifically, MIR high-Q germanium resonators with comparable performance to their silicon counterparts remain unprecedented. Here we experimentally demonstrate an MIR germanium nanocavity with a Q factor of ～18,000, the highest-to-date of reported nanocavities across MIR germanium-based integration platforms. This is achieved through a combination of a feasible theoretical design, Smart-Cut methods for wafer development, and optimized device fabrication processes. Our nanocavity, with its high Q factor and ultrasmall mode volume, opens new avenues for on-chip applications in the MIR spectral range.