Using quasi-bound states in the continuum in an all-dielectric metasurface array to enhance terahertz fingerprint sensing.

The terahertz absorption fingerprint spectrum is crucial for qualitative spectral analysis, revealing the rotational or vibrational energy levels of numerous biological macromolecules and chemicals within the THz frequency range. However, conventional sensing in this band is hindered by weak interactions with trace analytes, leading to subtle signals. In this Letter, an all-dielectric metasurface array is proposed to enhance the absorption fingerprint spectrum using quasi-bound states in the continuum (BIC) resonance. The observable quasi-BIC resonance is achieved by breaking the symmetry of the C2v structure. The periodic dimensions of the structure are adjusted to excite quasi-BIC resonances at different frequencies, thereby enhancing the fingerprint spectra of four different substances. By exploiting the correlation between the Q-factor and absorption across different frequencies, calibration of the molecular absorption fingerprint spectrum obtained through metasurface sensing yields precise enhanced absorption fingerprint spectra for various substances within the 0.55-1.6 THz range. Our Letter introduces a novel, to the best of our knowledge, strategy for trace sensing in the THz frequency range, demonstrating the promising potential for enhanced absorption fingerprint spectrum sensing.

Coupling-Based Multiple Bound States in the Continuum and Grating-Assisted Permittivity Retrieval in the Terahertz Metasurface.

The terahertz (THz) metasurfaces that support bound states in the continuum (BICs) provide a promising platform for various applications due to their high Q-factor resonance. In this study, we experimentally demonstrate multiple BICs with different resonance symmetries in the THz metasurface based on mode coupling. The proposed metasurface is composed of 2 × 2 split ring resonators (SRRs) metamolecules. The SRRs of two different gap angles in the metamolecule lattice provide intrinsic resonance with different frequencies, and the coupling between them exhibits high transmission quasi-BIC resonance, which can be tuned by varying the gap angle. The arrangement of SRRs in the 2 × 2 metamolecule lattice determines the types of coupling that govern the resonance symmetry of quasi-BIC. More interestingly, the multiple quasi-BICs enabled by different couplings can be simultaneously achieved in a metasurface. Apart from tuning the gap angles, the permittivity in the vicinity of SRRs also changes the resonance frequency. Consequently, quasi-BIC can be artificially formed by deliberately constructing the permittivity difference of the dielectric environment on the SRRs. In view of this, we introduce the scheme of permittivity retrieval for the dispersive analyte, assisted by the fixed-permittivity gratings. In addition, we experimentally demonstrate the metasurface in combination with the microfluidic chip for the sensing of the glucose solution concentration. Our findings offer a possible strategy for the existing manufactured metasurface to achieve quasi-BIC resonance and provide a promising candidate for approaching the spectral analysis of the biochemical.

The generation of a fourth-harmonic probe and its application in Nomarski interferometry at Shengguang-II.

In this work, a design for the generation of a 4ω (263-nm) probe converted from a 1ω (1053-nm) laser is presented. The design is based on a beta-barium borate and potassium dihydrogen phosphate two-step frequency-conversion process. A suitable configuration for Nomarski interferometry based on the 4ω probe is proposed, for measuring the electron density of laser-produced plasmas. The signal-to-noise ratio of the output 4ω probe to 1ω and 2ω light after frequency quadrupling and harmonic separation is 103 with a 0.5 GW/cm2 1ω input but decreases to ∼102 at intensities below 0.1 GW/cm2. Additional noise suppression by a factor of 104 is achieved using filters before the interferometer recording camera. The spatial resolution of the diagnostic can reach 5.2 µm for a 10% modulation transfer function. An experiment validating the probe diagnostic system is conducted at the Shengguang-II laser facility. A clear interferogram of an aluminum plasma is obtained with 0.1 GW/cm2 input, suggesting a maximal electron density of about 2.5 × 1020 cm-3 as retrieved through an inverse-Abel transform. The design proposed in this paper is appropriate for a small laser device or a large laser facility that lacks a separate diagnostic beam, and it is an inexpensive solution as it requires small-aperture 1ω input at a relatively low intensity. All the key parameters necessary to implement the design are provided in detail, making it straightforward to reproduce or transplant the system for specific uses.

Observation of the bound states in the continuum supported by mode coupling in a terahertz metasurface.

Metasurface supporting bound states in the continuum (BIC) provides a unique approach for the realization of intense near-field enhancement and high quality factor (Q-factor) resonance, which promote the advancement of various applications. Here we experimentally demonstrate a Friedrich-Wintgen BIC based on the mode coupling in the terahertz metasurface, which produces BIC by the coupling of the LC mode and dipole mode resonances. The transition from ideal BIC to quasi-BIC is caused by the mismatch of the coupling, and the mode decay rate during this process is analyzed by temporal coupled mode theory. The Q-factor and the electric field enhancement of the quasi-BIC resonance are significantly increased, which provides enormous potential in sensing, nonlinear optics, and topological optics.

High-peak-power picosecond deep-UV laser sources.

Ultrafast deep-UV laser sources have extensive applications across a wide number of fields, whether biomedicine, photolithography, industrial processing, or state-of-the-art scientific research. However, it has been challenging to obtain deep-UV laser sources with high conversion efficiency and output peak power. Here, we simultaneously demonstrated high-peak-power picosecond deep-UV laser sources at two typical wavebands of 263.2 and 210.5 nm via the efficient fourth- and fifth-harmonic generation. The highest peak power of 263.2 and 210.5 nm laser radiations were up to 2.13 GW (6.72 ps) and 1.38 GW (5.08 ps). The overall conversion efficiencies from the fundamental wave to the fourth and fifth harmonic were up to 42.9% and 28.8%, respectively. The demonstrated results represent the highest conversion efficiencies and output peak powers of picosecond deep-UV laser sources at present to our knowledge. Additionally, we also systematically characterized the deep-UV optical properties of typical birefringent and nonlinear borate crystals, including α-BaB2O4, ß-BaB2O4, LiB3O5, and CsLiB6O10 crystals. The experiments and obtained numerous new optical data in this work will contribute to the generation of ultrahigh-peak-power deep-UV and vacuum-UV laser sources and crucial applications in both science and industry, such as high-energy-density physics, material science, and laser machining.

Correlation between C7-T1 Intervertebral Foramen Area and Sagittal Parameters in Patients with Cervical Spondylotic Myelopathy.

OBJECTIVE: Cervical spondylotic myelopathy (CSM) is an incomplete spinal cord injury characterized with pain and stiffness in the neck and motor and sensory dysfunction. This study aims to determine whether C7-T1 intervertebral foramen area could be used as a parameter to evaluate the sagittal curvature of cervical spine. METHODS: Patients with clinical manifestations of spinal cord compression were hospitalized in our hospital from September 2018 to August 2019. All patients were diagnosed with CSM by nuclear magnetic imaging and other imaging methods. C2-C7 Cobb angle and T1 slop (T1S) were measured on the sagittal, T2-weighted magnetic resonance image of cervical spine, and C7-T1 intervertebral foramen area were measured using oblique cervical spine X-rays. Patients were divided into two groups according to the value of C2-C7 Cobb angle, including lordosis group (C2-C7 Cobb angle >10°, n = 45) and straight group (C2-7 Cobb angle ≤10°, n = 55). The reliability of the data was evaluated by intraclass correlation coefficient (ICC), and the correlation of the imaging parameters was analyzed by Pearson correlation. RESULTS: A total of 100 patients diagnosed with CSM hospitalized in our department were included. The ICC of the cervical parameters was 0.73. C7-T1 intervertebral foramen area was 40.69 ± 11.44 and 39.95 ± 10.94 mm2 in lordosis and straight group, respectively. The results showed that C7-T1 intervertebral foramen area was positively correlated with both C2-C7 Cobb angle (r = 0.23, p = 0.02) and T1S (r = 0.21, p = 0.03). In lordosis group, there was a positive correlation between C7 and T1 intervertebral foramen area and C2-C7 Cobb angle (r = 0.69, p < 0.01) and T1S (r = 0.34, p = 0.02). However, in straight group, C7-T1 intervertebral foramen area was not correlated with either C2-C7 Cobb angle or T1S. CONCLUSION: C7-T1 intervertebral foramen area measured by oblique X-ray could be an effective method to evaluate the sagittal balance of cervical vertebrae for CSM patients with cervical lordosis.

Modulation of laser damage by temporal shaping of double picosecond pulses.

We propose a temporally shaped double-picosecond-pulse train at a sub-nanosecond scale to control the damage dynamics of optical glass. Both damage threshold and morphology are significantly modulated by pulse-train shaping. The ramp-up-shaped train effectively increases its damage threshold and decreases the damage density and size, which clearly shows that a pump pulse with optimized fluence has a strong positive modification of damage precursors. Furthermore, the temporal evolution of damage modulation is experimentally revealed by varying the interval of pump-probe pulses, and after pump exposure with optimized fluence, enhancement of the probe threshold reaches the maximum at a delay of about 260 ps.

Noncritical phase-matching fourth- and fifth-harmonic generation of 1077 nm laser using KDP-family crystals.

We systematically demonstrated the angular and temperature acceptances of noncritical phase-matching (NCPM) fourth- and fifth-harmonic generation (FHG and FiHG) of a 1077 nm laser in NH4H2PO4 (ADP), KH2PO4 (KDP), and KD2PO4 (DKDP) crystals. In this work, a new, to the best of our knowledge, laser frequency with a wavelength of 1077 nm was generated by optical parametric amplification, in which the pump light (526.3 nm) was generated by the frequency doubling of a Nd:YLF laser (1052.7 nm), and the signal light was a Yb:YAG laser (1029.5 nm). Subsequently, the 1077 nm laser was used as the fundamental wave for FHG and FiHG to obtain a deep-ultraviolet laser source. For ADP and DKDP crystals, NCPM FHG of a 1077 nm laser was realized at 74.0∘C and 132.5∘C, respectively, and large angular acceptances of 59.8 and 61.6 mrad were measured. For the FiHG, NCPM was realized in a KDP crystal at 48.5∘C with an angular acceptance of 56.4 mrad. The results pave the way for high-energy and high-power deep-ultraviolet laser generation using KDP-family crystals under noncryogenic conditions.

Vanadium Dioxide-Based Terahertz Metamaterial Devices Switchable between Transmission and Absorption.

Terahertz metamaterial plays a significant role in the development of imaging, sensing, and communications. The function of conventional terahertz metamaterials was fixed after fabrication. They can only achieve a single function and do not have adjustable characteristics, which greatly limits the scalability and practical application of metamaterial. Here, we propose a vanadium dioxide-based terahertz metamaterial device, which is switchable between being a transmitter and an absorber. The transmission and absorption characteristics and temperature tunable properties of phase change metamaterials in the terahertz band were investigated. As the temperature of vanadium dioxide is varied between 20 °C and 80 °C, the device can switch between transmission and quad-band resonance absorption at the terahertz frequency range, with a high transmission rate of over 80% and a peak absorbance of 98.3%, respectively. In addition, when the device acts as an absorber, the proposed metamaterial device is tunable, and the modulation amplitude can reach 94.3%; while the device is used as a transmissive device, the modulation amplitude of the transmission peak at 81%. The results indicate that the proposed metamaterial device can promote the applications of terahertz devices, such as switching, modulation, and sensing.

High-contrast OPCPA front end in high-power petawatt laser facility based on the ps-OPCPA seed system.

A high-energy, high-beam-quality, high-contrast picosecond optical parametric chirped-pulse amplification (ps-OPCPA) laser system was demonstrated. The pulse from a femtosecond oscillator was stretched to 4 ps, after which it was amplified from 140 pJ to 600 µJ by an 8 ps/6 mJ pump laser in two non-collinear OPCPA stages. The total gain was >106, and the root mean square of the energy stability of the laser system was 1.6% in 10 h. The contrasts of the solid and fiber mode-locked femtosecond oscillator-seeded ps-OPCPA systems were compared, and a signal-to-noise ratio of >1011 was achieved. Using this system, the contrast of the front end in high-power picosecond petawatt laser facility was improved by ∼40 dB to >1011, beyond ∼200 ps ahead of the main pulse with an output level of 60 mJ.

Multi-band terahertz resonant absorption based on an all-dielectric grating metasurface for chlorpyrifos sensing.

Perfect metasurface absorbers play a significant role in imaging, detecting, and manipulating terahertz radiation. We utilize all-dielectric gratings to demonstrate tunable multi-band absorption in the terahertz region. Simulation reveals quad-band and tri-band absorption from 0.2 to 2.5 THz for different grating depths. Coupled-mode theory can explain the absorption phenomenon. The absorption amplitude can be precisely controlled by changing the pump beam fluence. Furthermore, the resonant frequency is sensitive to the medium's refractive index, suggesting the absorber may be of great potential in the sensor detection field. The experimental results exhibit a high detectivity of pesticides.

Electro-optic coefficient measurement of a K(H1-xDx)2PO4 crystal based on χ(2) nonlinear optical technology.

We present a novel method utilizing the χ(2) nonlinear optical technology, which can realize high precision measurement of linear electro-optic (EO) coefficients of nonlinear materials. By applying the linear EO effect to the nonlinear optical process, the theoretical model of this measurement method was established, and the calculation formula of the linear EO coefficient was given. In the proof-of-principle experiment, by introducing an external electric field into the fourth harmonic generation (FHG) process, we comprehensively obtained the linear EO coefficients of K(H1-xDx)2PO4 crystals and revealed the relationship between deuterium content (x) and EO coefficient (γ63): γ63 = -9.789 - 16.53x. Meanwhile, the stability of FHG was greatly improved, and the angular range of efficiency stability was increased to 4.4 times in maximum. This work not only systematically demonstrates the FHG characteristics of KDP-family crystals, which provides a good reference for the deep ultraviolet laser generation, but also offers a new way to measure the basic parameters of nonlinear optical materials.

Excitation of Surface Plasmon Resonance on Multiwalled Carbon Nanotube Metasurfaces for Pesticide Sensors.

With the rapid advances in functional optoelectronics, the research on carbon-based materials and devices has become increasingly important at the terahertz frequency range owing to their advantages in terms of weight, cost, and freely bendable flexibility. Here, we report an effective material and device design for a terahertz plasmonic metasurface sensor (PMS) based on carbon nanotubes (CNTs). CNT metasurfaces based on silicon wafers have been prepared and obvious resonant transmission peaks are observed experimentally. The enhanced resonant peaks of transmission spectra are attributed to the surface plasmon polariton resonance, and the transmission peaks are further well explained by the Fano model. Furthermore, the different concentration gradients of pesticides (2,4-dichlorophenoxyacetic and chlorpyrifos solutions) have been detected by the designed PMSs, showing the lowest detection mass of 10 ng and the sensitivities of 1.38 × 10-2/ppm and 2.0 × 10-3/ppm, respectively. Good linear relationships between transmission amplitude and pesticide concentration and acceptable reliability and stability have been obtained. These materials and device strategies provide opportunities for novel terahertz functional devices such as sensors, detectors, and wearable terahertz imagers.

Correlation between C7 slope and cervical lordosis in patients after expansive open-door laminoplasty.

Purpose of the article: To investigate the correlation between C7 slope and cervical lordosis in patients after expansive open-door laminoplasty (EOLP).Material and methods: We retrospectively analyzed 57 patients who underwent EOLP between June 2013 and January 2017 in the Department of Spinal Surgery of our hospital. The operation time, intraoperative blood loss and follow-up time were recorded. The C7 slope, C2-7 sagittal vertical axis, and C2-7 Cobb angle were measured anteroposterior radiograph of the cervical spine preoperatively and postoperatively. All patients were divided into two groups according to the preoperative C7 slope (C7 slope ≤20° group and C7 slope >20° group).Results: The amount of intraoperative bleeding was 220.2 ± 180.9ml, and the operation time was 143.4 ± 51.2min. The average follow-up time was 24.9 ± 10.3months (range12-48 months). The C2-7 Cobb angle was 13.49 ± 10.46°at the final follow-up, which was significantly lower than that preoperatively (p = .026). But, The C7 slope and C2-7 sagittal vertical axis showed no significant difference between preoperatively and postoperatively. Preoperative and postoperative C7 slope and C2-7 Cobb angle were positively correlated to age and significant difference was observed. In the group of C7 slope >20°, significant difference was observed in term of the change of the C2-7 Cobb angle and C2-7SVA postoperatively (p = .009 and p= .020). However, there was no statistically significant difference detected in these two parameters in the group of C7 slope ≤20°.Conclusion: This study indicated that C7 slope could be used as an indicator of the change in the curvature of the cervical spine after EOLP. The loss of cervical curvature after surgery was prone to occur when C7 slope was greater than 20°, which should be noted in clinical practice.

All-Dielectric Terahertz Plasmonic Metamaterial Absorbers and High-Sensitivity Sensing.

Two types of plasmonic metamaterial absorbers (PMAs) formed from patterned all-dielectric resonators are designed and demonstrated experimentally in the terahertz (THz) range. Both PMAs use a simple grating design on highly N-doped silicon. The first shows broadband absorption with near-perfect peak absorbance at 1.45 THz and a bandwidth of 1.05 THz for 90% absorbance, while the second is a dual-band absorber. Experiments show that the second absorber has two distinct absorption peaks at 0.96 and 1.92 THz with absorption rates of 99.7 and 99.9%, respectively. A fundamental cavity mode coupled to coaxial surface plasmon polaritons is responsible for the characteristics of both PMAs. Additionally, the optically tunable responses of these all-dielectric absorbers demonstrate that the absorption behavior can be modified. The quality factor (Q) values of the dual-band resonances are 4.6 and 7.8 times larger than those of the broadband PMAs, respectively, which leads to a better sensing performance. As an example, the two proposed PMAs act as high-sensitivity sensors and demonstrate considerable potential for chlorpyrifos detection. These results show that these PMAs can be used as sensors that can detect the presence of trace pesticides in adsorption analyses, among other practical applications.

Development of frequency-tunable multiple-band terahertz absorber based on control of polarization angles.

Controlling and tuning the spectral absorption response of metamaterial absorbers fabricated by arranging a set of resonators in a regular array is challenging. Polarization tunable multi-band terahertz resonant absorbers were developed using anisotropic microstructure arrays. The unit cell consisted of four pairs of H-shaped resonators of different sizes and a metallic ground plane separated by a dielectric layer. Discrete operating frequency shifts and dynamic amplitude tuning were observed by changing the polarization angle. The effect of the polarization angle on the absorption amplitude was evaluated. This work provides a concise approach to realize tunable absorption characteristics, which can be applied in sensors, detectors, and switches.

Multiband terahertz absorber and selective sensing performance.

Multiband terahertz absorbers are essential photonic components for responding to, manipulating, and modulating terahertz waves. In this work, improved electric split resonant ring arrays are used to demonstrate multiband terahertz wave absorption. The proposed design strategy is simple, practical, and significant. Experiments and simulations reveal perfect absorption at 0.918 THz and 1.575 THz for the transverse magnetic (TM) polarization and at 0.581, 1.294, and 1.556 THz for the transverse electric (TE) polarization. In addition, the weak resonant peaks that occurred in the experiments in both polarization states have been verified by the simulations. Furthermore, five concentration gradients of 2, 4-dichlorophenoxyacetic acid solutions and six concentration gradients of chlorpyrifos have been detected using the absorber. The lowest detectable concentration that could be monitored was 0.1 ppm. The absorption, intensity, and frequency shift values for the different solution concentrations at the resonant peaks were analyzed. The highest linear regression coefficients were 0.9862 and 0.9565 for the TE and TM polarizations, respectively. This multi-band absorber was demonstrated to be highly efficient in detecting pesticides for food safety applications.

Tailoring terahertz surface plasmon wave through free-standing multi-walled carbon nanotubes metasurface.

Surface plasmons have a fundamental role in the dynamics of photon-electron interactions and in optical metamaterials. Terahertz (THz) time-domain spectroscopy was used to characterize the complex dielectric constant, index of refraction, and conductivity of super-aligned, free-standing, multi-walled carbon nanotube films over the range 0.2-2.5 THz. These complex parameters were in excellent agreement with Maxwell-Garnett and Drude-Lorentz models. In addition, surface plasmon excitations in engineered, subwavelength, multi-walled carbon nanotube metasurfaces were examined. The observed surface plasmon resonances, reproduced by simulation, could be changed over the THz frequency range by altering the lattice constant of the arrays. The THz transmission was enhanced at the resonance peak. Overall, the results indicate potential applications for THz metasurfaces based on super-aligned, free-standing multi-walled carbon nanotubes.

[Correlation analysis of preoperative T 1 slope in MRI and physiological curvature loss after expansive open-door laminoplasty].

Objective: To investigate whether preoperative T 1 slope (T 1S) in MRI can predict the changes of cervical curvature after expansive open-door laminoplasty (EOLP) in patients with cervical spondylotic myelopathy, so as to make up for the shortcomings of difficult measurement in X-ray film. Methods: The clinical data of 36 patients with cervical spondylotic myelopathy who underwent EOLP were retrospectively analysed. There were 21 males and 15 females with an average age of 55.8 years (range, 37-73 years) and an average follow-up time of 14.3 months (range, 12-24 months). The preoperative X-ray films at dynamic position, CT, and MRI of cervical spine before operation, and the anteroposterior and lateral X-ray films at last follow-up were taken out to measure the following sagittal parameters. The parameters included C 2-C 7 Cobb angle and C 2-C 7 sagittal vertical axis (C 2-C 7 SVA) in all patients before operation and at last follow-up; preoperative T 1S were measured in MRI, and the patients were divided into larger T 1S group (T 1S>19°, group A) and small T 1S group (T 1S≤19°, group B) according to the median of T 1S, and the preoperative T 1S, C 2-C 7 Cobb angle, C 2-C 7 SVA, and the C 2-C 7 Cobb angle and C 2-C 7 SVA at last follow-up, difference in axial distance (the difference of C 2-C 7 SVA before and after operation), postoperative curvature loss (the difference of C 2-C 7 Cobb angle before and after operation), the number of patients whose curvature loss was more than 5° after operation, and the number of patients whose kyphosis changed (C 2-C 7 Cobb angle was less than 0° after operation). Results: The C 2-C 7 Cobb angle at last follow-up was significantly decreased when compared with preoperative value ( t=8.000, P=0.000), but there was no significant difference in C 2-C 7 SVA between pre- and post-operation ( t=-1.842, P=0.074). The preoperative T 1S was (19.69±3.39)°; there were 17 cases in group A and 19 cases in group B with no significant difference in gender and age between 2 groups ( P>0.05). The preoperative C 2-C 7 Cobb angle in group B was significantly lower than that in group A ( t=-2.150, P=0.039), while there was no significant difference in preoperative C 2-C 7 SVA between 2 groups ( t=0.206, P=0.838). At last follow-up, except for the curvature loss after operation in group B was significantly lower than that in group A ( t=-2.723, P=0.010), there was no significant difference in the other indicators between 2 groups ( P>0.05). Conclusion: Preoperative larger T 1S (T 1S>19°) in MRI had a larger preoperative lordosis angle, but more postoperative physiological curvature was lost; preoperative T 1S in MRI can not predict postoperative curvature loss, but preoperative larger T 1S may be more prone to kyphosis.

Sparstolonin B attenuates spinal cord injury­induced inflammation in rats by modulating TLR4­trafficking.

The present study used a spinal cord injury (SCI) model to evaluate whether sparstolonin B was able to prevent SCI, and to investigate the underlying signaling mechanism. Sparstolonin B attenuated the SCI­induced Batto, Beattie and Bresnahan score and water content in rats. Sparstolonin B attenuated the mRNA expression of proinflammatory cytokines interleukin (IL)­18, IL­6, IL­1ß, and IL­23, decreased the levels of tumor necrosis factor­α and interferon­Î³, and decreased caspase­3 activity and apoptosis regulator Bax protein expression in SCI rats. Similarly, sparstolonin B inhibited monocyte chemoattractant protein­1 mRNA levels, and Toll­like receptor (TLR) 4, myeloid differentiation primary response protein MyD88 (MyD88) and nuclear factor (NF)­κB protein levels in SCI rats. The present results suggested that sparstolonin B may attenuate SCI­induced inflammation and apoptosis in rats by modulating the TLR4/MyD88/NF­κB signaling pathway.