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.

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.

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.

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.

Terahertz near-field microscopy based on an air-plasma dynamic aperture.

Terahertz (THz) near-field microscopy retains the advantages of THz radiation and realizes sub-wavelength imaging, which enables applications in fundamental research and industrial fields. In most THz near-field microscopies, the sample surface must be approached by a THz detector or source, which restricts the sample choice. Here, a technique was developed based on an air-plasma dynamic aperture, where two mutually perpendicular air-plasmas overlapped to form a cross-filament above a sample surface that modulated an incident THz beam. THz imaging with quasi sub-wavelength resolution (approximately λ/2, where λ is the wavelength of the THz beam) was thus observed without approaching the sample with any devices. Damage to the sample by the air-plasmas was avoided. Near-field imaging of four different materials was achieved, including metallic, semiconductor, plastic, and greasy samples. The resolution characteristics of the near-field system were investigated with experiment and theory. The advantages of the technique are expected to accelerate the advancement of THz microscopy.

Development and Validation of a Predictive Scoring System for In-hospital Death in Patients With Intra-Abdominal Infection: A Single-Center 10-Year Retrospective Study.

Objective: To develop and validate a scoring system to predict the risk of in-hospital death in patients with intra-abdominal infection (IAI). Materials and Methods: Patients with IAI (n = 417) treated at our hospital between June 2010 and May 2020 were retrospectively reviewed. Risk factors for in-hospital death were identified by logistic regression analysis. The regression coefficients of each risk factor were re-assigned using the mathematical transformation principle to establish a convenient predictive scoring system. The scoring system was internally validated by bootstrapping sample method. Results: Fifty-three (53/417, 12.7%) patients died during hospitalization. On logistic regression analysis, high APACHE II score (P = 0.012), pneumonia (P = 0.002), abdominal surgery (P = 0.001), hypoproteinemia (P = 0.025), and chronic renal insufficiency (P = 0.001) were independent risk factors for in-hospital death. On receiver operating characteristic curve analysis, the composite index combining these five risk factors showed a 62.3% sensitivity and 80.2% specificity for predicting in-hospital death (area under the curve: 0.778; 95% confidence interval: 0.711-0.845, P < 0.001). The predictive ability of the composite index was better than that of each independent risk factor. A scoring system (0-14 points) was established by re-assigning each risk factor based on the logistic regression coefficient: APACHE II score (10-15 score, 1 point; >15 score, 4 points); pneumonia (2 points), abdominal surgery (2 points), hypoproteinemia (2 points), and chronic renal insufficiency (4 points). Internal validation by 1,000 bootstrapping sample showed relatively high discriminative ability of the scoring system (C-index = 0.756, 95% confidence interval: 0.753-0.758). Conclusions: The predictive scoring system based on APACHE II score, pneumonia, abdominal surgery, hypoproteinemia, and chronic renal insufficiency can help predict the risk of in-hospital death in patients with IAI.

"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.

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.

Vector measurement and performance tuning of a terahertz bottle beam.

A terahertz (THz) bottle beam is realized by adopting the combination of a Teflon axicon and a silicon lens. By using a THz imaging system with a focal-plane array, the vector characteristics of the THz bottle beam are coherently measured and detailedly analyzed, including the transverse (Ex) and longitudinal (Ez) components. The experimental phenomena vividly reveal the distribution characteristics and the formation origin of the THz optical barrier. A vectorial diffraction integral algorithm of a focusing optical system are utilized to exactly simulate the measured results. Besides, the features of the THz bottle beam are effectively tuned by varying the parameters of the Teflon axicon and the silicon lens. This work gives a full view to understand the evolution characteristics of the THz bottle beam and provide a solid experimental foundation for guiding the future applications of this type of THz beam.

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.

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.

High-efficiency terahertz devices based on cross-polarization converter.

Metasurface-based devices have been investigated intensively because of their attractive properties but these devices generally suffer from low efficiency. Here we demonstrate several high-efficiency terahertz (THz) devices based on cross-polarization converters that is composed of bilayer metasurface-based structures. The converter can transfer the polarization states of transmitted THz waves from the x-direction into the y-direction with an experimental conversion efficiency of 85%. This high-efficiency transfer mechanism is investigated in detail. Furthermore, this kind of devices can be fabricated easily. A THz metalens is designed and fabricated and its focusing and imaging properties are investigated experimentally. A pure phase THz hologram that can generate different images on different propagation planes is also designed and the image reconstruction capabilities of the phase holograms are demonstrated experimentally. The performance levels of all designed devices show excellent agreement between the theoretical expectations and the corresponding experimental results. This technology may pave the way towards practical applications of such metasurface devices.

New design model for high efficiency cylindrical diffractive microlenses.

A new model, i.e., the decreasing thickness model (DTM) is proposed and employed for designing the cylindrical diffractive microlenses (CDMs). Focal performances of the designed CDMs are theoretically investigated by solving Maxwell's equations with the boundary element method. For comparison, the CDMs designed by the traditional equal thickness model (ETM) are also studied. Theoretical simulations demonstrate that focal performances of the designed CDMs are improved a lot via replacing the traditional ETM with the proposed DTM. Concretely, the focal efficiency is heightened and the focal spot size is shrunk. Experimental measurements verify the theoretical simulations well. Especially, the above-mentioned improvements become more prominent for the CDM with a higher numerical aperture.

Vector characterization of zero-order terahertz Bessel beams with linear and circular polarizations.

As a kind of special beams, Bessel beams are always a research hot spot in optics due to its non-diffractive and self-healing properties. Here, zero-order terahertz (THz) Bessel beams with linear and circular polarizations are generated by using a THz quarter wave plate and Teflon axicons with different opening angles. By applying a THz digital holographic imaging system, the evolutions of the transverse (E x , E y ) and longitudinal (E z ) electric fields are coherently measured and analyzed during the propagation processes of the THz Bessel beams. The vectorial Rayleigh diffraction integral is used to accurately reproduce the amplitude, phase, and non-diffractive feature of each polarization component for the THz Bessel beams. With varying opening angles of the axicons, the focal spots, diffraction-free ranges, and Gouy phase shifts of the THz Bessel beams are compared and discussed. The experiment and simulation results provide a comprehensive view for exactly understanding peculiar features of THz Bessel beams.

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.