Generation of the stable propagation Bessel beam and the axial multifoci beam with pure phase elements.

A recently proposed method is upgraded to convert two amplitude phase modulation systems (APMSs) to pure phase elements (PPEs), for generating the stable propagation Bessel beam and the axial multifoci beam, respectively. Phase functions of the PPEs are presented analytically. Numerical simulations by the complete Rayleigh-Sommerfeld method demonstrate that the converted PPE has implemented the same optical functionalities as the corresponding APMS, in either the longitudinal or the transverse direction. Compared with the traditional APMS, the converted PPE possesses many advantages such as fabrication process simplification, system complexity reduction, production cost conservation, alignment error avoidance, and experimental precision enhancement. These inherent advantages position the PPE as an ideal choice and driving force behind further advancements in optical system technology.

Terahertz rewritable wavefront modulator based on indium oxide and DMSO-doped PEDOT:PSS.

An optically rewritable and electrically erasable terahertz (THz) wavefront modulator based on indium oxide (In2O3) and DMSO-doped PEDOT:PSS is proposed. The modulator has a three-layer structure of In2O3/PEDOT:PSS/quartz, which can weaken the THz transmission under the action of light excitation. Optically written THz Fresnel plates, which can focus the input Gaussian beam into a point, were realized. After optical excitation, the function of the device reduces slowly if it is stored in the room environment. However, the function can be stored for a long time if it is encapsulated in the nitrogen environment. If a bias voltage of 22 V is applied on the device, the function of the device can be erased in 10 seconds. The new function can be written into the device after wiping. Experiments on THz rewritable holographic devices are carried out to show the validity of this approach. This method can provide new devices for THz wavefront modulation and develop tunable optical imaging elements.

Ideal suppression of Bessel beam intensity oscillations with a vortex phase plate.

We propose what we believe to be a new kind of diffractive phase element, i.e., vortex phase plate (VPP) with phase singularities along the azimuth direction. Phase function of the proposed VPP is given analytically. Axial intensity oscillations of propagating Bessel beams are ideally suppressed by using the proposed VPP. Compared with the traditional amplitude mask, the proposed VPP takes such advantages as a simpler fabrication procedure and a lower cost.

Generation of stable propagation Bessel beams and axial multifoci beams with binary amplitude filters.

The binary amplitude filter (BAF) is employed to generate stable propagation Bessel beams and axial multifoci beams, rather than the traditional continuous amplitude filter (CAF). We introduce a parameter along the azimuth direction, i.e., angular order of the BAF, to weaken transverse intensity asymmetry. Numerical simulations reveal that the BAF implements the same optical functionalities as the CAF. The BAF holds advantages over the traditional CAF: a simpler fabrication process, a lower cost, and a higher experimental accuracy. It is believed that the BAF should have many practical applications in future optical systems.

Multimodal remote sensing application for weed competition time series analysis in maize farmland ecosystems.

Although weeds cause serious harm to crops through competition for resources, they also have ecological functions. We need to study the change law of competition between crops and weeds, and achieve scientific farmland weed management under the premise of protecting weed biodiversity. In the research, we perform a competitive experiment in Harbin, China, in 2021, with five periods of maize as the study subjects. Comprehensive competition indices (CCI-A) based on maize phenotypes were used to describe the dynamic processes and results of weeds competition. The relation between in structural and biochemical information of maize and weed competitive intensity (Levels 1-5) at different periods and the effects on yield parameters were analyzed. The results showed that the differences of maize plant height, stalk thickness, and N and P elements among different competition levels (Levels 1-5) changed significantly with increasing competition time. This directly resulted in 10%, 31%, 35% and 53% decrease in maize yield; and 3%, 7%, 9% and 15% decrease in hundred grain weight. Compared to the conventional competition indices, CCI-A had better dispersion in the last four periods and was more suitable for quantifying the time-series response of competition. Then, multi-source remote sensing technologies are applied to reveal the temporal response of spectral and lidar information to community competition. The first-order derivatives of the spectra indicate that the red edge (RE) of competition stressed plots biased in short-wave direction in each period. With increasing competition time, RE of Levels 1-5 shifted towards the long wave direction as a whole. The coefficients of variation of canopy height model (CHM) indicate that weed competition had a significant effect on CHM. Finally, the deep learning model with multimodal data (Mul-3DCNN) is created to achieve a large range of CCI-A predictions for different periods, and achieves a prediction accuracy of R2 = 0.85 and RMSE = 0.095. Overall, this study use of CCI-A indices combined with multimodal temporal remote sensing imagery and DL to achieve large scale prediction of weed competitiveness in different periods of maize.

Smart grating coupled whispering-gallery-mode microcavity on tip of multicore optical fiber with response enhancement.

The potential of whispering-gallery-modes (WGMs) microcavities in sensing applications has been being released continuously with improvements from various aspects. Introducing smart materials and structures into the WGMs microcavities based sensing systems are an effective approach to promote their applications in real world. Here, we propose a smart grating as the coupling setup to a WGMs microcavity of polystyrene microsphere to enhance the responses to chemical and thermal stimulations. The changes of the coupling distance due to the deformation of the smart grating induce additional increments to the intrinsic wavelength shifts of the WGMs of the microcavity, which is proved to be the mechanism of the response enhancements. We use two-photon lithography based "lab on fiber" technology to realize the device and the demonstration of the response enhancements. Our results may be of great significance to the design of the WGMs microcavity based chemical and temperature sensors.

"Optical tentacle" of suspended polymer micro-rings on a multicore fiber facet for vapor sensing.

We designed a new type of gas sensor, an optical tentacle, made of highly integrated polymer micro-ring resonators in three-dimensional space on the tiny end-facet of a multicore optical fiber. Two pairs of three polymer micro-ring resonators were hung symmetrically on both sides of three suspended micro-waveguides as the sensing units. The micro-waveguides interlace to form a three-layer nested configuration, which makes the multicore optical fiber a "tentacle" for vapors of volatile organic compounds. Both experiments and theoretical simulation confirmed that the symmetrical coupling of multiple pairs of rings with the micro-waveguide had better resonance than the single ring setup. This is because the symmetrical light modes in the waveguides couple with the rings separately. All the optical micro-components were fabricated by the two-photon lithography technology on the end facet of multicore optical fiber. The optical tentacle shows good sensitivity and reversibility. This approach can also be adopted for sensor array design on a chip. Furthermore, optical sensors that can sense vapors with multiple constituents may be achieved in the future by adding selective sensitive materials to or on the surface of the rings.

Contribution of the optical rectification in terahertz radiation driven by two-color laser induced plasma.

In the terahertz (THz) generation driven by two-color laser pulses, the THz wave radiated from the BBO crystal as the effect of the optical rectification is always assumed to be less and negligible. In this paper, the contribution of the optical rectification in the THz radiation driven by two-color laser pulses has been determined quantitatively, by the crucial factors including BBO crystal rotation angle, the pump power of laser, and the numerical aperture of lens. The experimental and simulation results show that the above related factors have dramatically affected the intensity ratio of the THz waves from the plasma and BBO crystal. It is helpful for understanding the mechanism of THz generation from air plasma.

Terahertz image reconstruction based on compressed sensing and inverse Fresnel diffraction.

The introduction of compressed sensing (CS) effectively pushes the development of single-pixel THz imaging due to reducing the experimental time and avoiding raster scanning. In this work, a CS method based on photoinduced dynamic masks is employed to recover a THz diffraction field in the time domain, and an inverse Fresnel diffraction (IFD) integral is adopted to remove the influence of the diffraction and reconstruct the sharp THz spectral image in a single-pixel THz imaging system. The compatibility of the CS and IFD algorithms are validated on the simulation and experiment. Besides, the reconstruction effects are also systematically analyzed by reducing the measurement number and varying the diffraction distance, respectively. This work supplies a novel thinking for improving the practicability of single-pixel THz imaging.

Modulation of terahertz radiation from graphene surface plasmon polaritons via surface acoustic wave.

We present a theoretical study of terahertz (THz) radiation induced by surface plasmon polaritons (SPPs) on a graphene layer under modulation by a surface acoustic wave (SAW). In our gedanken experiment, SPPs are excited by an electron beam moving on a graphene layer situated on a piezoelectric MoS2 flake. Under modulation by the SAW field, charge carriers are periodically distributed over the MoS2 flake, and this causes periodically distributed permittivity. The periodic permittivity structure of the MoS2 flake folds the SPP dispersion curve back into the center of the first Brillouin zone, in a manner analogous to a crystal, leading to THz radiation emission with conservation of the wavevectors between the SPPs and the electromagnetic waves. Both the frequency and the intensity of the THz radiation are tuned by adjusting the chemical potential of the graphene layer, the MoS2 flake doping density, and the wavelength and period of the external SAW field. A maximum energy conversion efficiency as high as ninety percent was obtained from our model calculations. These results indicate an opportunity to develop highly tunable and integratable THz sources based on graphene devices.

Vectorial diffraction properties of THz vortex Bessel beams.

A vortex Bessel beam combines the merits of an optical vortex and a Bessel beam, including a spiral wave front and a non-diffractive feature, which has immense application potentials in optical trapping, optical fabrication, optical communications, and so on. Here, linearly and circularly polarized vortex Bessel beams in the terahertz (THz) frequency range are generated by utilizing a THz quarter wave plate, a spiral phase plate, and Teflon axicons with different opening angles. Taking advantage of a THz focal-plane imaging system, vectorial diffraction properties of the THz vortex Bessel beams are comprehensively characterized and discussed, including the transverse (Ex, Ey) and longitudinal (Ez) polarization components. The experimental phenomena are accurately simulated by adopting the vectorial Rayleigh diffraction integral. By varying the opening angle of the axicon, the characteristic parameters of these THz vortex Bessel beams are exhibited and compared, including the light spot size, the diffraction-free range, and the phase evolution process. This work provides the precise experimental and theoretical bases for the comprehension and application of a THz vortex Bessel beam.

Flattening axial intensity oscillations of a diffracted Bessel beam through a cardioid-like hole.

We present a new feasible way to flatten the axial intensity oscillations for diffraction of a finite-sized Bessel beam, through designing a cardioid-like hole. The boundary formula of the cardioid-like hole is given analytically. Numerical results by the complete Rayleigh-Sommerfeld method reveal that the Bessel beam propagates stably in a considerably long axial range, after passing through the cardioid-like hole. Compared with the gradually absorbing apodization technique in previous papers, in this paper a hard truncation of the incident Bessel beam is employed at the cardioid-like hole edges. The proposed hard apodization technique takes two advantages in suppressing the axial intensity oscillations, i.e., easier implementation and higher accuracy. It is expected to have practical applications in laser machining, light sectioning, or optical trapping.

Polarization determination based on the longitudinal field of a converging terahertz wave.

Based on coherently measuring the longitudinal field of a converging terahertz (THz) wave, a polarization determination method to the THz radiation is proposed. By utilizing the method, the arbitrary uniform polarization state of a THz field can be effectively identified in a single measurement. By using the vector diffraction integral, the principle of the method is theoretically discussed in detail. The feasibility of the method is validated experimentally by measuring a THz wire grating polarizer and a THz quarter-wave plate. The method offers a powerful technical support for developing the THz polarization spectroscopy.

Terahertz vortex beam generator based on a photopatterned large birefringence liquid crystal.

A terahertz (THz) q-plate is proposed and demonstrated to generate THz vortex beams. It is composed of a large birefringence liquid crystal (LC) with spatially-varying director distribution sandwiched by two pieces of fused silica glass. A polarization-sensitive alignment agent is photopatterned to carry out the specific LC director distribution. THz vortex beams with different topological charges are characterized with a THz digital holographic imaging system. The intensity and phase distributions consistent with theoretical analyses are obtained. Besides, an eight-lobed intensity distribution is observed corresponding to the vertical polarization component of a cylindrical vector beam. This work may inspire novel THz applications.

Active modulation of the terahertz spectra radiated from two air plasmas.

A simple and energy-saving method has been proposed to actively modulate the spectra of terahertz (THz) waves radiated from two serial plasmas, which uses the background light to generate one plasma to make full use of the energy of the femtosecond laser. With this method, the modulation of the central frequency, spectral bandwidth, and spectral profile of the output THz waves have been observed. The shifting of the amplitude dip has been manipulated by changing the distance of the two serial plasmas. The manipulation results agree with the ones simulated by the transition-Cherenkov model. This proposed method provides a useful tool for getting the modulated THz spectra that can be used in the THz remote sensing.

Tailoring axial intensity of laser beams with a heart-shaped hole.

We demonstrated a simple heart-shaped hole to tailor the axial intensity of a collimated laser beam. This hole is transformed from a soft-boundary one, which avoids the difficulty in fabricating the soft-boundary mask designed by the apodization method, as well as the interference problem caused by the pixel structure of the spatial light modulator. When a collimated light passes through this hole, its axial intensity oscillates less than 11% within a certain distance, while the fluctuation after the circular aperture is up to 200%. We compared the propagation of beams after this hole and a circular aperture experimentally and theoretically. The results show that this hole is a useful tool to get the laser beam with uniform axial intensity.

Observation of dehydration dynamics in biological tissues with terahertz digital holography [Invited].

A terahertz (THz) digital holographic imaging system is utilized to investigate natural dehydration processes in three types of biological tissues, including cattle, mutton, and pork. An image reconstruction algorithm is applied to remove the diffraction influence of THz waves and further improve clarity of THz images. From THz images of different biological specimens, distinctive water content as well as dehydration features of adipose and muscle tissues are precisely distinguished. By analyzing THz absorption spectra of these samples, temporal evolution characteristics of the absorbances for adipose and muscle tissues are described and compared in detail. Discrepancies between water retention ability of different animal tissues are also discussed. The imaging technique provides a valuable measurement platform for biological sensing.

Longitudinal field characterization of converging terahertz vortices with linear and circular polarizations.

Linearly and circularly polarized terahertz (THz) vortex beams are generated by adopting a THz quarter wave plate and spiral phase plates with topological charges 1 and 2. Taking advantage of a THz digital holographic imaging system, longitudinal components of THz vortices with different polarizations and topological charges are coherently measured and systemically analyzed in a focusing condition. The application potential of circularly polarized THz vortex beams in microscopy is experimentally demonstrated and the transformation between the spin angular momentums and orbital angular momentums of THz waves is also checked. Modified Richards-Wolf vector diffraction integration equations are applied to successfully simulate experimental phenomena.

Spin-selected focusing and imaging based on metasurface lens.

Spin of light provides a route to control photons. Spin-based optical devices which can manipulate photons with different spin states are imperative. Here we experimentally demonstrated a spin-selected metasurface lens based on the spin-orbit interaction originated from the Pancharatnam-Berry (PB) phase. The optimized PB phase enables the light with different spin states to be focused on two separated points in the preset plane. Furthermore, the metasurface lens can perform the spin-selected imaging according to the polarization of the illuminating light. Such a spin-based device capacitates a lot of advanced applications for spin-controlled photonics in quantum information processing and communication based on the spin and orbit angular momentum.

[Terahertz Spectrum Modulation with Liquid Crystal Spatial Light Modulator].

Terahertz wave with modulation spectrum is valuable in many fields. Terahertz spectrum has been modulated with a pure-phase liquid crystal spatial light modulator by shaping the femtosecond laser beam profile. In the experiment, terahertz wave is generated by femtosecond laser pulses in mode of Optical Rectification and then its signals are detected by a terahertz time-domain spectroscopy system. Phase maps obtained with the GS algorithm are loaded to get the shaped beam profile. Therefore, Terahertz spectrum has been successfully modulated by changing the detection range and beam profile parameters. Simultaneously, the simulations have been performed with the Fresnel diffraction, and they agree with the experimental results well. The results show that the modulation of the THz spectrum with this method is feasible.