Pump Light Intensity Research Articles

Dual-mode multifunctional switchable polarization conversion metasurface based on VO2, Si, and graphene in THz region

In this paper, a dual-mode multifunctional switching terahertz metasurface polarization converter based on vanadium dioxide (VO2), photosensitive semiconductor silicon (Si) and graphene is designed. The converter can switch between transmission and reflection modes by adjusting the state of VO2. In the insulating state of VO2, the converter operates in transmission mode and performs linear-to-linear polarization conversion (LTL-PC) with a relative bandwidth of 166.0 %. Conversely, when VO2 is in the metallic state, the converter works in reflection mode. By adjusting the pump light intensity to modulate the state of Si, linear-to-circular polarization conversion (LTC-PC) and LTL-PC function can be achieved. Moreover, the Fermi level of graphene can be adjusted via electrostatic gate to further enhance the working bandwidth of the converter. In reflection mode, the relative bandwidths for LTL-PC, linear to left-circularly and right-circularly polarization conversion are 98.1 %, 87.0 % and 4.0 %, respectively. The metasurface polarization converter proposed in this paper has a variety of functions and a variety of control modes, and has the advantages of high polarization conversion rate and wide operating band, showing great potential in switching and tunable wideband terahertz converters and related applications.

Doping engineering of NiO–Zn nanofilms for modulation of morphological structure and ultrafast nonlinear optical response at 515 nm

In this work, to investigate the metal doping on the third-order nonlinear optical properties of NiO thin films under femtosecond pulsed laser excitation, pure NiO and 2w,4w,6w,8w Zn doped-NiO nanofilms were prepared by DC-RF co-sputtering technique. Then, the nonlinear absorption properties and nonlinear refractive properties of the five samples were investigated using the femtosecond laser at a wavelength of 515 nm. The experimental results show that the nonlinear absorption and nonlinear refractive properties of NiO are enhanced by the doped metal as well as the effect of defects. In addition, by varying the pump light intensity, we observed the transition of nonlinear absorption from the coexistence of saturated absorption and reverse saturated absorption to reverse saturated absorption. We established a multi-energy level system to interpret the nonlinear absorption and nonlinear refractive results, which broadens the application of NiO thin films in the field of nonlinear optics.

Quantification and suppression of optical non-orthogonality and light intensity noise in all-optical hot atomic sensing systems

All-optical hot atomic sensing systems such as optical pumping magnetometers (OPM) and comagnetometers, nuclear magnetic resonance (NMR) magnetometers and gyroscopes, have the ability to sense magnetic field, anomalous field and inertial rotation, thus having a wide potential for applications in precision measurement, fundamental physics research and inertial navigation. System noise limits the minimum value of the signal that can be measured and there is coupling effect between the noises. Therefore, the quantification and suppression of various noise are challenging and should be investigated. As one of the major noises, the light intensity fluctuation of the probe light and the pump light affects the sensitivity of the all-optical hot atomic sensing systems. However, there has been no in-depth research on the quantification of the light intensity noise. In this paper, a method of measuring the light intensity noise is proposed, which can be applied to kinds of all-optical hot atomic sensing systems. For verification, the effects of probe and pump light intensity fluctuations on system noise are simulated and experimentally verified in a spin exchange relaxation free (SERF) comagnetometer. Besides, it is found that the optical non-orthogonality amplifies the probe light intensity noise by more than 3.4 times and a quantification method of optical non-orthogonality is proposed. Finally, a suppression method of the light intensity noise is presented by suppressing the optical non-orthogonality and the noise is suppressed by 59%. A universal light intensity noise quantification method is proposed for all-optical hot atomic sensing systems, and a light intensity noise suppression method is proposed for SERF comagnetometers, which can be applied to the suppression of noises coupled with atomic polarization.

Enhanced terahertz third-harmonic generation based on the graphene-assisted meta-grating structure

The nonlinear response of optical materials provides a valuable approach for optical switching and nonlinear frequency conversion. However, the nonlinear responses of bulk crystals are often very weak, requiring higher pump energy for pumping. In this work, we investigate the significant enhancement of third harmonic generation (THG) at terahertz frequencies using an ultra-thin nonlinear meta-grating metasurface assisted by graphene strips. In this structure, the incident pump light experiences strong confinement and enhancement along the graphene surface caused by the generation of strongly localized surface plasmon resonances. Utilizing the significant third-order nonlinear conductivity of graphene, it becomes possible to achieve high THG conversion efficiency even at low pump intensities. The research results indicate that, at low incident intensity of 10 kW/cm2, the third harmonic wave conversion efficiency can reach approximately 2.8 %. We also thoroughly investigate the impact of meta-grating structural parameters, graphene Fermi level, incident angle, and pump light intensity on THG operation characteristics. Our study findings provide an effective approach for greatly boosting the THG conversion efficiency in the terahertz frequency bands, opening new avenues in the development of novel compact frequency converters and modulators that are emerging in terahertz chip integration and nonlinear devices.

Preparation of Bose-Einstein condensate of dysprosium atoms based on demagnetization cooling

In magnetic atomic gases, the dipolar relaxation process couples the system spin and kinetic degrees of freedom. When the average kinetic energy is significantly lower than the Zeeman splitting, the atoms predominantly occupy the lowest Zeeman state. As the Zeeman splitting approaches the average kinetic energy, some atoms transfer to adjacent Zeeman states through dipolar relaxation, converting kinetic energy into Zeeman energy. By utilizing optical pumping, atoms transferred to higher spin states can be repumped to the ground state, thereby achieving a continuous cooling cycle and effectively lowering the system’s temperature. As the energy removed in a single cooling cycle is much larger than the energy of scattered photons, this demagnetization cooling scheme significantly enhances cooling effciency and reduces atomic loss. In this work, we establish state-coupled equations that incorporate dipolar relaxation and optical pumping to analyze the demagnetization cooling process, modeling the evolution of atom number and temperature during the cooling of <inline-formula><tex-math id="M1">\begin{document}$^{164}{\text{Dy}}$\end{document}</tex-math></inline-formula> atoms. We develop a strategy to generate an optimal magnetic field waveform by maximizing the demagnetization rate. Based on this strategy, we investigate the influence of crucial experimental parameters on demagnetization cooling and determine their specific ranges for producing large atom number of BEC, including the optical dipole trap frequency, as well as the intensity and polarization purity of the optical pumping light. The results indicate that demagnetization cooling enables the direct preparation of a large number of dysprosium BEC with sub-second timescales, reducing the cooling time by an order of magnitude compared to conventional methods for dysprosium atoms. Furthermore, it could achieve a cooling effciency of <inline-formula><tex-math id="M2">\begin{document}$\chi \approx 44.92$\end{document}</tex-math></inline-formula>, an order of magnitude higher than that of traditional evaporative cooling.

Reconfigurable three multi-mode terahertz metasurface.

We designed a three reconfigurable multi-mode terahertz metasurface based on a concentric elliptical ring structure. The proposed unit cell is a concentric elliptical ring composed of copper, vanadium oxide and photosensitive silicon from the inside ring to the outside ring. The conductivity of photosensitive silicon and vanadium oxide can be adjusted by changing the external operating temperature and pump light intensity. The same unit cell can reconstruct three kinds of states with different properties, and they have completely different transmission characteristics in various terahertz bands. By encoding the arrangement, through changing external stimulus and operating frequencies, the reconfigurable terahertz metasurface can achieve multiple functions including terahertz focusing with adjustable focal length, vortex beam with different topological charge, and near-field imaging with different patterns. It provides what we believe to be a new idea for the field of information security and the design of multifunctional and multifrequency terahertz devices.

Analysis and Optimization of Signal Detection Noise of Single-Beam Modulated Atomic Magnetometer Array

Highly sensitive atomic magnetometers (AMs) have been wildly used to form magnetocardiography (MCG) or magnetoencephalography (MEG) devices recently. However, signal detection noise, including light noise due to pump light fluctuations and electrical noise introduced by the detection circuit, will inevitably affect the sensitivity of the AM. Therefore, this article proposes a novel method to suppress signal detection noise and improve the sensitivity of AM. We first propose a low-noise preamplifier circuit to suppress the electrical noise of the signal detection part. Then, based on the equivalent magnetic noise amplitude, a method to optimize the pump light intensity is proposed. The experimental setup has been established. Both the analysis and experimental results can be evaluated that the magnetometer sensitivity can always be maintained at a relatively high-sensitivity level (around 24 fT/Hz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{1/2}$ </tex-math></inline-formula> ) over a wide range of light intensities. We hope that the method proposed in this article can contribute to the improvement of sensor sensitivity in array AM systems.

Optically end-pumped scheme in rubidium vapor: Theory and experiment

The optimal detuning of the pump light at the rubidium D2 line is calculated and experimentally validated. A three-level optically end-pumped model in a hot rubidium vapor cell is demonstrated, which takes into account the hyperfine structure of the ground state, various collisional processes, and the nonlinear absorption of the pump light along the cell length. By developing a constrained optimization model, we calculate the optimal detuning of the pump light that maximizes the average excited state atom concentration in the cell. Experimentally, a 1529 nm probe light at higher levels, which is attenuated to the weak-probe limit, is introduced to accurately characterize the excited state atom concentration. Coupling with the experimental results, the variation of the pump light optimal detuning with the temperature and the pump light intensity at the unsaturated pumping is analyzed. At the optimal detuning point, the theoretical decay function of the pump light percentage along the vapor cell length almost coincides regardless of the temperature and the pump light intensity changes. Moreover, the theoretical fitted pump light decay function at optimal detuning is predicted. The fitted pump light percentage exiting the vapor cell is 4.75%. A fast implementation method to achieve the maximal average excited state atom concentration in the experiment is demonstrated.

Creation of a high-resolution atomic frequency comb and optimization of the pulse sequence for high-efficiency quantum memory in 167Er:Y2SiO5

We created high-resolution atomic frequency combs (AFCs) and optimized the pulse sequence used in AFC memory to realize highly efficient AFC quantum memory using the 167Er3+:Y2SiO5 hyperfine level system. As a result, the formation and relaxation times of AFCs were revealed, and AFCs with a resolution of 800 kHz were created by using an appropriate pump light intensity. An efficiency of 0.16% with a memory time of 0.5 µs was achieved even under a zero magnetic field and a 10 ppm Er concentration sample. This result paves the way for realizing highly efficient communication-wavelength AFC quantum memory.

Effect of Disperse Red 1 Azobenzene Dye Doping and Annealing on the Thermomechanical and Photomechanical Properties of PMMA Fibers

This work studies the effect of azobenzene dye Disperse Red 1 (DR1) doping and annealing on the thermomechanical and photomechanical properties of poly(methyl methacrylate) (PMMA) fibers. The mechanical properties are measured as a function of temperature, pump light intensity, and polarization. We find that doping with DR1 increases the stiffness and the glass transition temperature (Tg) of the PMMA fibers. Moreover, annealing below Tg decreases Young’s modulus and increases Tg. Finally, the photothermal heating contribution to the photomechanical response and the length change during laser exposure are determined in both unannealed and annealed plain PMMA and DR1-doped PMMA fibers. We find that photothermal heating is the dominant mechanism and the effect of photoisomerization is negligible. The temperature-dependent photomechanical efficiencies are also determined.

Optomagnonically induced RoF chaotic synchronization

Optomagnonics is a good platform for the interplay of radio frequency and optical signals, which are the primary communication carriers in the present day. On the basis of optomagnonics, we provide a multi-scale method for analyzing its behavior, including frequency comb and RoF chaotic synchronization at the microwave scale. Adjusting the pump light intensity permits the transition of RoF signals between harmonics, frequency combs, and chaotic movements. The dual optomagnonical device enables the synchronization of RoF signals between different cavities. Our study will contribute to the use of multiscale electromagnetic wave coupling in both conventional and quantum information applications.

Efficient Enhancement of Second Harmonic Generation via Noninvasive Modulation

Second harmonic generation has been widely applied in various fields. High second harmonic intensity can facilitate optical imaging, signal sensing, and detection. Thus, enhancing the intensity of the second harmonic is a significant work. However, changing the external character of crystal or increasing the pump light intensity to improve the intensity of the second harmonic is not always advisable in some applications, such as bioimaging, biopsies, etc. Here, we implemented a noninvasive method that constructs a specific spatial distribution field via a scattering medium to realize a high enhancement of second harmonic intensity. We studied that different scattering mediums exerted the influence on the optimal enhancement effect of second harmonic. It was found that choosing an appropriate scattering medium can greatly enhance the intensity of the second harmonic. The results can offer a helpful value for second harmonic applications such as bioimaging, sensing, and optical frequency conversion.

Second harmonic generation from GaAs-Au subwavelength relief gratings

We study numerically the second-harmonic generation (SHG) from subwavelength GaAs relief gratings with and without Au nano-stripes on the bottom of grating troughs. The SHG efficiency is calculated by finite element method as a function of grating amplitude, Au-stripes thickness, depth of trough for TE and TM polarized fundamental harmonic. Our results show that a deep subwavelength grating without Au stripes can be considered as a homogeneous film when the grating period is 3.5 times shorter than the wavelength. Such a grating with a certain amplitude enhances the SHG efficiency by about 20 times compared with the efficiency of a crystal with flat surface. The Au-stripes at the grating troughs significantly increase the electromagnetic field concentration in the crystal ridges of the grating due to plasmonic resonance. In this case, the SHG efficiency enhancement is more than 500 times, while the absolute value of SHG efficiency reaches 1.2 × 10−3% when the intensity of pump light is 1 GW/cm2.

Time response of spin-polarized rubidium thermal gas with radio-frequency pulse driving

The time evolution of the polarization of a rubidium atom spin ensemble driven by a resonant radio-frequency (RF) magnetic field is analyzed based on the rate equation. A simple optical pumping experimental system is constructed and the time response of the rubidium atomic ensemble is demonstrated by recording the transmitted intensity of pumping light. In the steady-state response, the polarization difference between the optical pumping steady state and the magnetic resonance steady state depends on the optical pumping power and RF magnetic intensity. We can obtain the optimal power value corresponding to the maximum polarization difference. In terms of transient response, where the intensity of RF magnetic field is too weak to observe Rabi oscillations, two decay processes between magnetic resonance and optical pumping steady states are monitored. The decay time from magnetic resonance steady state to optical pumping steady state depends on the optical pumping rate and the spin relaxation rate. The decay time from optical pumping steady state to magnetic resonance steady state depends on the optical pumping rate, the RF driving rate, and the spin relaxation rate. The scale factor of pumping rate to pumping power is obtained, in addition to that of RF driving rate to the RF magnetic field. It can provide an intuitive understanding of the spin dynamic evolution of the polarized atomic ensemble.

Terahertz topological photonic waveguide switch for on-chip communication

Terahertz (THz) topological photonic structures are promising for last-centimeter communication in intra/interchip communication systems because they support bit-error-free THz signal transmission with topological robustness. Active and dynamically tunable THz topological photonic components have not yet been experimentally realized. Here, we experimentally demonstrate a THz topological switch (270–290 GHz) based on a valley Hall photonic crystal structured high-resistivity silicon substrate, in which the THz waves can be dynamically turned on/off by an external 447 nm continuous-wave laser. Our device exhibited an on/off ratio of 19 dB under a pumping light intensity of 240 mW / mm 2 . The 3 dB switching bandwidth was ∼ 60 kHz .

Nonlinear optical properties of polydiacetylene nanofibers modified with Ag nanoparticles

Polydiacetylene (PDA) nanofibers (NFs) have been successfully modified with different amount of silver (Ag) nanoparticles (NPs) by using polydopamine as both adhesion layer and reducing agent. The enhancement factor of absorbance in PDA NFs by localized surface plasmon resonance (LSPR) effect has been evaluated from the transient transmittance spectra. The absorption of PDA NFs modified with suitable amount of Ag NPs was clearly enhanced by LSPR effect, while reduced considerably with further increasing adsorbed Ag NPs owing to saturation of concentrated light intensity and shielding of pump light by Ag NPs.

Dual bound states in the continuum enhanced second harmonic generation with transition metal dichalcogenides monolayer

The emergence of two dimensional (2D) materials has opened new possibilities for exhibiting second harmonic generation (SHG) at the nanoscale, due to their remarkable optical response related to stable excitons at room temperature. However, the ultimate atomic-scale interaction length with light makes the SHG of Transition Metal Dichalcogenides (TMDs) monolayers naturally weak. Here, we propose coupling a monolayer of TMDs with a photonic grating slab that works with doubly resonant bound states in the continuum (BIC). The BIC slabs are designed to exhibit a pair of BICs, resonant with both the fundamental wave (FW) and the second harmonic wave (SHW). Firstly, the spatial mode matching can be fulfilled by tilting FW's incident angle. We theoretically demonstrate that this strategy leads to more than four orders of magnitude enhancement of SHG efficiency than a sole monolayer of TMDs, under a pump light intensity of 0.1 GW/cm². Moreover, we demonstrate that patterning the TMDs monolayer can further enhance the spatial overlap coefficient, which leads to an extra three orders of magnitude enhancement of SHG efficiency. These results demonstrate remarkable possibilities for enhancing SHG with nonlinear 2D materials, opening many opportunities for chip-based light sources, nanolasers, imaging, and biochemical sensing.

Optimized gas pressure of an Rb vapor cell in a single-beam SERF magnetometer.

We present a theoretical and experimental study of a single-beam spin-exchange relaxation-free magnetometer in 87Rb vapor cells under different nitrogen gas pressures. The spin relaxation rate is a key component to limit the magnetic sensitivity, and the zero-field resonance method was used to measure the spin relaxation rates of different alkali metal cells. Simultaneously, in a single-beam spin-exchange-relaxation-free (SERF) magnetometer, we demonstrated that the fundamental magnetic field sensitivity was also limited by the pumping light intensity. Based on our theoretical analysis and experimental results, we determined the optimal pumping light intensity and optimal gas pressure. We experimentally demonstrated that the magnetic field sensitivity was 8.89 fT/Hz in the single-beam configuration, with an active measurement volume of 3 × 3 × 3~mm3.

Photoluminescence Switching Effect in a Two-Dimensional Atomic Crystal.

Two-dimensional materials are an emerging class of materials with a wide range of electrical and optical properties and potential applications. Single-layer structures of semiconducting transition metal dichalcogenides are gaining increasing attention for use in field-effect transistors. Here, we report a photoluminescence switching effect based on single-layer WSe2 transistors. Dual gates are used to tune the photoluminescence intensity. In particular, a side-gate is utilized to control the location of ions within a solid polymer electrolyte to form an electric double layer at the interface of electrolyte and WSe2 and induce a vertical electric field. Additionally, a back-gate is used to apply a second vertical electric field. An on-off ratio of the light emission up to 90 was observed under constant pump light intensity. In addition, a blue shift of the photoluminescence line up to 36 meV was observed. We attribute this blue shift to the decrease of exciton binding energy due to the change of nonlinear in-plane dielectric constant and use it to determine the third-order off-diagonal susceptibility χ(3) = 3.50 × 10-19 m2/V2.

Dynamically tunable plasmon-induced transparency in waveguide based on Dirac semimetal

We report a numerical and theoretical study of the plasmon-induced transparency (PIT) effect in bulk Dirac semimetal (BDS) waveguide. This plasmon waveguide system is composed of a bus waveguide side coupled with two stub photoactive silicon resonators. The coupled mode theory (CMT) is introduced to analyze this system, and the theoretical results are perfectly consistent with the numerical calculation results. The numerical results show that the spectral characteristics of the structure depend on the coupling distance between the two stub photoactive silicon resonators. Intriguingly, the resonance characteristics of the transparent window can be actively controlled by adjusting the Fermi energy of the BDS or the intensity of the incident pump light. Finally, the group time delay of the PIT window can reach 3.7 ps, which is more suitable for applications with slow-light devices. Such a simple and multi-functional plasmon system is more conducive to the potential applications of composite high-performance devices.