The widespread use of multifunctional metasurfaces has started to revolutionize conventional electromagnetic devices due to their unprecedented capabilities and exceedingly low losses. Specifically, geometric metasurfaces that utilize spatially varied single-celled elements to impart arbitrary phase modulation under circularly polarized (CP) waves have attracted more attention. However, the geometric phase has intrinsically opposite signs for two spins, resulting in locked and mirrored functionalities for the right-handed and left-handed CP beams. Additionally, the demonstrated geometric metasurfaces so far have been limited to operating in either transmission or reflection modes at a single wavelength. Here, we propose a double-layered metasurface composed of complementary elliptical and reversal ring resonator structures to achieve simultaneous and independent control of the reflection and transmission of CP waves at two independent terahertz frequencies, which integrates three functions of reflected beam deflection, reflected Bessel beam generation, and transmitted beam focusing on the whole space. The high efficiency and simple design of our metasurface will open new avenues for integrated terahertz metadevices with advanced functionalities.

For a conventional cascaded metasurface, the combination channel and each single channel are mutually dependent because the phase modulation of a cascaded metasurface is the sum of each single one. Here we propose a cascaded metasurface that can independently encode information into multiple channels. Based on the orientation degeneracy of anisotropic metasurfaces, each single metasurface can produce a quick-response (QR) image in the near field, governed by the Malus law, while the combined channel can produce a holographic image in the far field, governed by geometric phase. The independent and physically separated trichannel design makes information encryption safer.

In this paper, an optical pulse amplitude modulation with 4 levels (PAM-4) using a fiber combiner is proposed to enhance the data rate of a field-programmable gate-array-based long-distance real-time underwater wireless optical communication system. Two on–off keying signals with different amplitudes are used to modulate two pigtailed laser diodes, respectively, and the generated optical signals are superimposed into optical PAM-4 signals by a fiber combiner. The optical PAM-4 scheme can effectively alleviate the nonlinearity, although it reduces the peak-to-peak value of the emitting optical power by 25%. A real-time data rate of 187.5 Mbit/s is achieved by using the optical PAM-4 with a transmission distance of 50 m. The data rate is increased by about 25% compared with the conventional electrical PAM-4 in the same condition.

To overcome the unbalanced signal-to-noise ratio (SNR) among data-carrying subcarriers (SCs) induced by the imperfect frequency response of optoelectronic devices and various interferences, a channel-independent partial data-carrying SCs precoding (PDSP) method based on orthogonal circular matrix transform (OCT) is proposed and experimentally investigated in an intra-symbol frequency average (ISFA)-enabled discrete multi-tone (DMT) visible light communication (VLC) system. After transmission over 1.9 m free space, at the optimal bias current of 100 mA, the experimental results show that the bit error ratio (BER) performance can be improved by up to an order of magnitude with conventional full data-carrying SCs precoding (FDSP) and PDSP scheme, compared to that without a precoding scheme. Moreover, the BER performance can further be enhanced when the ISFA algorithm with optimal taps is employed. Compared with the FDSP scheme, the proposed PDSP scheme owns a similar BER performance and a significant reduction in required multiplication and addition operations, and it may be a good option to efficiently combat the unbalanced impairments of DMT-VLC transmission systems.

In this paper, we propose a temperature-sensing scheme utilizing a passively mode-locked fiber laser combined with the beat frequency demodulation system. The erbium-doped fiber is used in the laser ring cavity to provide the gain and different lengths of single-mode fibers inserted into the fiber ring cavity operate as the sensing element. Different temperature sensitivities have been acquired in the experiment by monitoring the beat frequency signals at different frequencies. The experimental results indicate that the beat frequency shift has a good linear response to the temperature change. The sensitivity of the proposed sensor is about -44 kHz/°C when the monitored beat frequency signal is about 10 GHz and the ratio of the sensing fiber to the overall length of the laser cavity is 10 m/17.5 m, while the signal-to-noise ratio (SNR) of the monitored signal is approximately 30 dB. The proposed temperature-sensing scheme enjoys attractive features such as tailorable high sensitivity, good reliability, high SNR, and low cost, and is considered to have great potential in practical sensing applications.

We report a wavelength-tunable multi-point pump scheme of the semiconductor disk lasers (SDLs). By designing an external cavity of SDL with an intra-cavity transmission grating, multiple pump gain regions share the same resonator. The effect of the intra-cavity grating on the output laser power, wavelength, and beam quality was investigated. The emission wavelength could be tuned over a bandwidth of ∼18nm. With multi-point pumping, we achieve the laser output power with almost no loss, and further improvement is limited by the thermal effect. The changes in the beam are due to the mode selectivity by the intra-cavity grating.

MXene V₂CT_x has great practicability because it is not easy to degrade under ambient conditions. In this paper, a V₂CT_x saturable absorber (SA) was firstly applied to a passively Q-switched (PQS) laser, to the best of our knowledge. The V₂CT_x-SA was prepared by the spin-coating method. The linear absorption of the V₂CT_x-SA in the 1000–2200 nm region and the nonlinear absorption near 2 µm were studied. With the V₂CT_x-SA, a typical PQS operation at 1.94 µm was realized in a Tm:YAlO3 laser. The minimum pulse width produced by the PQS laser was 528 ns, and the peak power, repetition rate, and average output power were 10.06 W, 65.9 kHz, and 350 mW, respectively. Meanwhile, the maximum pulse energy was 6.33 µJ. This work demonstrates that the V₂CT_x can be used as an effective SA to obtain nanosecond pulses with high peak power and high repetition rate simultaneously.

We present a continuously tunable high-power continuous wave (CW) single-frequency (SF) Nd:YAlO3/lithium triborate (Nd:YAP/LBO) laser with dual-wavelength output, which is implemented by combining an optimized and locked etalon with an intracavity nonlinear loss. The obtained output powers of the stable SF 1080 and 540 nm lasers are 2.39 and 4.18 W, respectively. After the etalon is locked to an oscilating mode of the laser, the wideband continuous frequency tuning and long-term stable single-longitudinal-mode operation of the laser are successfully realized, which can be well used for the applications of quantum information and quantum computation. To the best of our knowledge, this is the first realization of the continuously tunable high-power CW SF 1080/540 nm dual-wavelength laser.

We fabricate a pair of fiber Bragg gratings (FBGs) by a visible femtosecond laser phase mask scanning technique on passive large-mode-area double-cladding fibers for multi-kilowatt fiber oscillators. The bandwidth of high-reflection (HR) and low-reflection (LR) FBG is ∼1.6nm and 0.3 nm, respectively. The reflection of the HR-FBG is higher than 99%, and that of the LR-FBG is about 10%. A bidirectional pumped all-fiber oscillator is constructed using this pair of FBGs, a record output power of 5027 W located in the signal core is achieved with a slope efficiency of ∼82.1%, and the beam quality factor M² is measured to be ∼1.6 at the maximum power. The FBGs are simply fixed on a water cooling plate without a special package, and the thermal efficiency of the HR-FBG and the LR-FBG is 2.76°C/kW and 1°C/kW, respectively. Our research provides an effective solution for robust high-power all-fiber laser oscillators.

In this paper, we report on a wide wavelength tuning optical vortex carrying orbital angular momentum (OAM) of ±ħ, from a thulium-doped yttrium aluminum perovskite (YAP) laser employing a birefringent filter. The OAM is experimentally found to be well maintained during the whole wavelength tuning process. The Laguerre–Gaussian (LG0,+1) mode with a tuning range of 58 nm from 1934.8 to 1993.0 nm and LG0,-1 mode with a range of 76 nm from 1920.4 to 1996.6 nm, are, respectively, obtained. This is, to the best of our knowledge, the first experimental implementation of wavelength tuning for a scalar vortex laser in the 2 µm spectral range, as well as the broadest tuning range ever reported from the vortex laser cavity. Such a vortex laser with robust structure and straightforward wavelength tuning capability will be an ideal light source for potential applications in the field of optical communication with one additional degree of freedom.

An all-fiberized random distributed feedback Raman fiber laser (RRFL) with LP11 mode output at 1134 nm has been demonstrated experimentally, where an intracavity acoustically induced fiber grating is employed for modal switching. The maximum output power of LP11 mode is 93.8 W with the modal purity of 82%, calculated by numerical mode decomposition technology based on stochastic parallel-gradient descent algorithm. To our best knowledge, this is the highest output power with high purity of LP11 mode generated from the RRFL. This work may pave a path towards advanced fiber lasers with special temporal and spatial characteristics for applications.

Based on the Rydberg cascade electromagnetically induced transparency, we propose a simultaneous dual-wavelength locking method for Rydberg atomic sensing at room temperature. The simplified frequency-locking configuration uses only one signal generator and one electro-optic modulator, realizing real-time feedback for both lasers. We studied the effect of the different probe and coupling laser powers on the error signal. In addition, the Allan variance and a 10 kHz amplitude-modulated signal are introduced to evaluate the performance of the laser frequency stabilization. In principle, the laser frequency stabilization method presented here can be extended to any cascade Rydberg atomic system.

We designed a femtosecond (fs) + picosecond (ps) double-pulse sequence by using a Mach–Zehnder-like apparatus to split a single 120 fs pulse into two sub-pulses, and one of them was stretched to a width of 2 ps by a four-pass grating system. Through observing the ripples induced on the ZnO surface, we found the ionization rate appeared to be higher for the sequence in which the fs pulse arrived first. The electron rate equation was used to calculate changes of electron density distribution for the sequences with different delay times. We suggest that using a temporally shaped fs+ps pulse sequence can achieve nonlinear ionization control and influence the induced ripples.

Frequency conversion based on three-wave mixing is a critical nonlinear optic application, extending the frequency range of existing lasers and realizing frequency-transduced detectors in a wavelength range that lacks an effective detector. Phase matching is vital for effective frequency conversion. The advantages of quasi-phase matching (QPM) over birefringent phase matching are a lack of walk-off effect, a maximum nonlinear coefficient, and phase matching in the entire transparency window. Herein, using different types and orders of QPM, four kinds of effective frequency doubling processes are realized in a periodically poled potassium titanyl phosphate (KTP) crystal with a single period, and three kinds of frequency doubling processes are experimentally verified. We also show a feasible way to construct an RGB color generator based on two different QPM processes. This study significantly expands the feasible frequency conversion of existing lasers to different wavelengths, providing an effective method for multi-color laser generation based on periodically poled KTP crystals.

We investigate the nonlinear optical limiting effect of novel graphene dispersions and graphene dispersions in carbon tetrachloride at a wavelength of 3.8 µm. The transmittances of graphene dispersions in carbon tetrachloride under two different concentrations (0.004 and 0.008 mg/mL) and two different solution thicknesses (10 and 20 mm) are measured. The influences of concentration and thickness on the optical limiting effect of graphene dispersion are analyzed. Theoretical analysis of the experimental data shows that the main optical limiting mechanism of the new graphene dispersions is Mie scattering. The limiting capacity coefficient of the new graphene dispersion in carbon tetrachloride reaches 0.405 and the minimum transmittance reaches 0.292 when the concentration is 0.004 mg/mL and the thickness is 20 mm. The graphene dispersions in carbon tetrachloride can be used to achieve good nonlinear optical limiting effects at the wavelength of 3.8 µm.

In this Letter, we presented a flexible omnidirectional reflective film made of polymer substrates and multiple alternating layers of two chalcogenide glasses for full-angle CO2 laser protection. The structure parameters of the device were simulated for theoretical prediction of best device structure. The reflector was fabricated by alternate thermal evaporation of two chalcogenide glasses with large refractive index contrast. The reflectivity was greater than 78% at 10.6 µm. The flexible reflective film can provide an effective solution for full-angle CO2 laser protection of the moving targets, such as laser operators and mobile optical components, with potential applications for wearable laser protective clothing.

Considerable progress has been made in organic light-emitting diodes (OLEDs) to achieve high external quantum efficiency, among which dipole orientation has a remarkable effect. In most cases, the radiation of the dipoles in OLEDs is theoretically predicted with only one orientation parameter to match with corresponding experiments. Here, we develop a new theory with three orientation parameters to fully describe the relationship between dipole orientation and power density. Furthermore, we design an optimal test structure for measuring all three orientation parameters. All three orientation parameters could be retrieved from non-polarized spectra. Our theory provides a universal plot of dipole orientations in OLEDs, paving the way for designing more complicated OLED devices.

We present a non-contact optical investigation of laser-induced plasma at moderate Ar pressure ranging from 1 to 100 Pa. The significant shock front and spatial fractionation among the different charged ions are demonstrated at the pressure of 20 Pa. The collisions between Si IV ions and ambient Ar atoms generate distinct and excited Ar II ions, fresh Si III ions, and electrons at the dense layer. The electron density peaks at the position of the shock front, indicating that the collision that yields electrons is dominant over the recombination process in the region of the shock layer and its immediate vicinity.

In this paper, a simple theoretical model combining Monte Carlo simulation with the enthalpy method is provided to simulate the damage resistance of B₄C/Si-sub mirror under X-ray free-electron laser irradiation. Two different damage mechanisms are found, dependent on the photon energy. The optimum B₄C film thickness is determined by studying the dependence of the damage resistance on the film thickness. Based on the optimized film thickness, the damage thresholds are simulated at photon energy of 0.4–25 keV and a grazing incidence angle of 2 mrad. It is recommended that the energy range around the Si K-edge should be avoided for safety reasons.

Wide field-of-view (FOV) optics are widely used in various imaging, display, and sensing applications. Conventional wide FOV optics rely on complicated lens assembly comprising multiple elements to suppress coma and other Seidel aberrations. The emergence of flat optics exemplified by metasurfaces and diffractive optical elements (DOEs) offers a promising route to expand the FOV without escalating complexity of optical systems. To date, design of large FOV flat lenses has been relying upon iterative numerical optimization. Here, we derive, for the first time, to the best of our knowledge, an analytical solution to enable computationally efficient design of flat lenses with an ultra-wide FOV approaching 180°. This analytical theory further provides critical insights into working principles and otherwise non-intuitive design trade-offs of wide FOV optics.

The metasurface is a platform with a small footprint and abundant functionalities. With propagation phase and geometric phase, polarization multiplexing is possible. However, different response behaviors of propagation phase and geometric phase to wavelength have not been fully employed to widen the capabilities of metasurfaces. Here, we theoretically demonstrate that metasurfaces can achieve near-field and far-field decoupling with the same polarization at two wavelengths. First, we found a set of pillars whose propagation phase difference between two wavelengths covers the full range of 2π. Then, by rotating pillars to control the geometric phase, the phase at both wavelengths can cover the full range of 2π. Finally, by means of interference principle, arbitrary independent coding for the near field and far field of dual wavelengths is realized. In addition, when the far-field function is focusing, the focused spot is close to the diffraction limit, and, when the NA of the lens is very small, the final output focal length is four times of initial input focal length. This work circumvents the strong wavelength-dependent limitation of planar devices and paves the way toward designing multi-wavelength and multi-functional metadevices for scenarios such as AR applications, fluorescence microscopy, and stimulated emission depletion microscopy.

Metasurfaces have exhibited great capabilities to control electromagnetic waves, and many multifunctional metasurfaces were recently proposed. However, although angle-multiplexed meta-devices were successfully realized in reflection geometries, their transmission-mode counterparts are difficult to achieve due to the additional requirements. Here, we design and fabricate a transmissive angle-multiplexed meta-polarizer in the microwave regime based on a multilayer metasurface. Coupled-mode-theory analyses reveal that the device exhibits distinct angle-dependent transmissive responses under excitations with different polarizations, and such differences are further enhanced by multiple scatterings inside the device. Microwave experimental results are in good agreement with numerical simulations and theoretical analyses.

Terahertz (THz) NH₃ lasing with optical pumping by electron-beam-sustained discharge “long” (∼100µs) CO₂ laser pulses was obtained. The NH₃ laser emission pulses and the “long” pulses of the CO₂ pump laser were simultaneously measured with nanosecond response time. The NH₃ lasing duration and its delay with respect to the pump pulse were measured for various CO₂ laser pulse energies. For the CO₂ laser pump line 9R(30), three wavelengths of 67.2, 83.8, and 88.9 µm were recorded. For the CO₂ laser pump line 9R(16), only a single NH₃ laser line with a wavelength of 90.4 µm was detected.

Recently, wireless communication capacity has been witnessing unprecedented growth. Benefits from the optoelectronic components with large bandwidth, photonics-assisted terahertz (THz) communication links have been extensively developed to accommodate the upcoming wireless transmission with a high data rate. However, limited by the available signal-to-noise ratio and THz component bandwidth, single-lane transmission of beyond 100 Gbit/s data rate using a single pair of THz transceivers is still very challenging. In this study, a multicarrier THz photonic wireless communication link in the 300 GHz band is proposed and experimentally demonstrated. Enabled by subcarrier multiplexing, spectrally efficient modulation format, well-tailored digital signal processing routine, and broadband THz transceivers, a line rate of 72 Gbit/s over a wireless distance of 30 m is successfully demonstrated, resulting in a total net transmission capacity of up to 202.5 Gbit/s. The single-lane transmission of beyond 200 Gbit/s overall data rate with a single pair of transceivers at 300 GHz is considered a significant step toward a viable photonics-assisted solution for the next-generation information and communication technology (ICT) infrastructure.

We experimentally demonstrated the use of intelligent impairment equalization (IIE) for microwave downconversion link linearization in noncooperative systems. Such an equalizer is realized based on an artificial neural network (ANN). Once the training process is completed, the inverse link transfer function can be determined. With the inverse transformation for the detected signal after transmission, the third-order intermodulation distortion components are suppressed significantly without requiring any prior information from an input RF signal. Furthermore, fast training speed is achieved, since the configuration of ANN-based equalizer is simple. Experimental results show that the spurious-free dynamic range of the proposed link is improved to 106.5 dB · Hz^2/3, which is 11.3 dB higher than that of a link without IIE. Meanwhile, the training epochs reduce to only five, which has the potential to meet the practical engineering requirement.