High harmonic generation in solids: particle and wave perspectives.

High harmonic generation (HHG) from gas-phase atoms (or molecules) has opened up a new frontier in ultrafast optics, where attosecond time resolution and angstrom spatial resolution are accessible. The fundamental physical pictures of HHG are always explained by the laser-induced recollision of particle-like electron motion, which lay the foundation of attosecond spectroscopy. In recent years, HHG has also been observed in solids. One can expect the extension of attosecond spectroscopy to the condensed matter if a description capable of resolving the ultrafast dynamics is provided. Thus, a large number of theoretical studies have been proposed to understand the underlying physics of solid HHG. Here, we revisit the recollision picture in solid HHG and show some challenges of current particle-perspective methods, and present the recently developed wave-perspective Huygens-Fresnel picture for understanding dynamical systems within the ambit of strong-field physics.

Flexible Hollow Core Fiber Photoacoustic Gas Sensor Based on Embedded Acoustic Resonant Structure.

In this paper, we demonstrate a flexible leaky hollow core fiber (LHCF) photoacoustic (PA) gas sensor based on an embedded acoustic resonant structure. The sensor employs a part of a gas conduit as the buffer chamber to construct an equivalent T-type half-open PA cell. The LHCF is installed inside of the gas conduit and the LHCF is hence replaceable. Also, the flexibility of the LHCF and the gas conduit make the gas sensor flexible to reduce spatial size. The inner diameter and length of the LHCF are 1.6 mm and 70 mm, respectively. The inner diameter and length of the gas conduit are 4 mm and 210 mm, respectively. The total gas volume of the sensor is only ∼2.6 mL. Trace acetylene (C2H2) is selected as the target gas to evaluate the performance of the PA gas sensor. A near-infrared distributed feedback (DFB) laser is utilized to generate the PA signal, and an electrical micro-electro-mechanical system (MEMS) microphone is employed to extract the PA signal. The experimental results show that the minimum detection limit (MDL) can be as low as 21.1 ppb when the lock-in integration time is 200 s. And the normalized noise equivalent absorption coefficient (NNEA) is calculated to be 5.7 × 10-9·W/cm-1·Hz-1/2.

Low-frequency Resonant Photoacoustic Gas Sensor by Employing Hollow Core Fiber-Based O-Shaped Multipass Cells.

A low-frequency flexible resonant photoacoustic (PA) gas sensor using an O-shaped multipass cell is demonstrated. The PA sensor employed a flexible gradually tapered leaky hollow core fiber (LHCF). The LHCF was bent to be an end-to-end structure to make full use of the incident light. Additionally, the two ends of the LHCF were put inside a single buffer chamber, yielding an equivalent H-type acoustic resonator. The geometric size was reduced thanks to the bending structure. The geometric length of the LHCF was 500 mm. A micro-electro-mechanical-systems electrical microphone was installed at the center of the resonant tube to detect the PA signal. The proposed PA gas sensor exhibited a first-order longitudinal resonance frequency of 408 Hz. Trace acetylene (C2H2) was used as the target gas. The minimum detectable limit was calculated to be 25.8 parts-per-billion (ppb) with an average time of 400 s, which was 1.93 times higher than that of a single-pass PA gas sensor. The normalized noise-equivalent absorption coefficient and the PA cell constant were calculated to be 9.6 × 10-9 W·cm-1·Hz-1/2 and 8295 Pa/W·cm-1, respectively. The PA gas sensor owns a low resonance frequency and can be used for detection of most of the polar gaseous molecules, especially suitable for gas molecules with a long V-T relation time, such as carbon monoxide and carbon dioxide.

Long-range surface plasmon resonance-based hollow fiber temperature sensor with ultrahigh sensitivity and tunable detection range.

A dielectric/Ag-coated hollow fiber (HF) temperature sensor based on long-range surface plasmon resonance (LRSPR) is proposed and experimentally demonstrated. The structural parameters, including the dielectric material and layer thicknesses, are optimized through comprehensive theoretical analysis to achieve the best performance. By filling it with a high refractive index (RI) thermosensitive liquid, the GK570/Ag-coated HF temperature sensor with optimal structural parameters is fabricated. Due to the high sensitivity of the LRSPR sensor and the optimized design, the fabricated sensor achieves a temperature sensitivity of 3.6∼20.5 nm/°C, which is almost the highest among the optical fiber temperature sensors based on surface plasmon resonance reported experimentally. Moreover, the detection range of the proposed sensor can be easily tuned up to 170°C by varying the RI of the filled thermosensitive liquid, and the sensor performance remains stable. Considering that most temperature sensors using polydimethylsiloxane have a fixed detection range, this is an outstanding advantage that could expand the application field of the optical fiber temperature sensor.

Highly sensitive photoacoustic gas sensor with micro-embedded acoustic resonator for gas leakage detection.

In this work, a photoacoustic (PA) gas sensor with a micro-embedded acoustic resonator for gas leakage detection was demonstrated. The micro-embedded acoustic resonator was fabricated by putting a leaky hollow-core fiber (L-HCF) into a cylindrical buffer chamber. The L-HCF was utilized as the PA cavity and the light transmission media simultaneously. The optimal inner diameter of the L-HCF was 1.7 mm. The embedded acoustic resonator was experimentally proven to be equivalent to a T-type half-open acoustic resonator, but the structure became much more compact. The volume of the amount of gas in the cell was only ∼0.3 mL, and the gas diffusion time to fill the sensor under room temperature (25°C) and ambient pressure (101 kPa) was ∼44 s. Trace acetylene (C2H2) in pure nitrogen (N2) was chosen as the target gas, and the minimum detectable limit (MDL) reached 29 ppb when the lock-in integration time was 1 s. The normalized noise equivalent absorption (NNEA) coefficient was calculated to be 3.0 × 10-9 W·cm-1·Hz-1/2. The micro-resonant PA gas sensor, with merits of compactness, low gas consumption, and low cost, has the potential to be a remote gas sensing scheme in fields of environmental protection, industrial process monitoring, and so on.

CH4, C2H6, and CO2 Multi-Gas Sensing Based on Portable Mid-Infrared Spectroscopy and PCA-BP Algorithm.

A multi-gas sensing system was developed based on the detection principle of the non-dispersive infrared (NDIR) method, which used a broad-spectra light source, a tunable Fabry-Pérot (FP) filter detector, and a flexible low-loss infrared waveguide as an absorption cell. CH4, C2H6, and CO2 gases were detected by the system. The concentration of CO2 could be detected directly, and the concentrations of CH4 and C2H6 were detected using a PCA-BP neural network algorithm because of the interference of CH4 and C2H6. The detection limits were achieved to be 2.59 ppm, 926 ppb, and 114 ppb for CH4, C2H6, and CO2 with an averaging time of 429 s, 462 s, and 297 s, respectively. The root mean square error of prediction (RMSEP) of CH4 and C2H6 were 10.97 ppm and 2.00 ppm, respectively. The proposed system and method take full advantage of the multi-component gas measurement capability of the mid-infrared broadband source and achieve a compromise between performance and system cost.

Surface-Enhanced Raman Scattering in Silver-Coated Suspended-Core Fiber.

In this paper, the silver-coated large-core suspended-core fiber (LSCF) probe was fabricated by the dynamic chemical liquid phase deposition method for surface-enhanced Raman scattering (SERS) sensing. The 4-mercaptophenylboronic acid (4-MPBA) monolayer was assembled in the LSCF as the recognition monolayer. Taking advantage of the appropriate core size of the LSCF, a custom-made Y-type optical fiber patch cable was utilized to connect the semiconductor laser, Raman spectrometer, and the proposed fiber SERS probe. The SERS signal is propagated in the silver-coated air channels, which can effectively reduce the Raman and fluorescence background of the silica core. Experiments were performed to measure the Raman scattering spectra of the 4-MPBA in the silver-coated LSCF in a non-enhanced and enhanced case. The experiment results showed that the Raman signal strength was enhanced more than 6 times by the surface plasmon resonance compared with the non-enhanced case. The proposed LSCF for SERS sensing technology provides huge research value for the fiber SERS probes in biomedicine and environmental science. The combination of SERS and microstructured optical fibers offers a potential approach for SERS detection.

[Advances in traditional Chinese medicine treatment of non-alcoholic fatty liver disease via farnesoid X receptor].

Non-alcoholic fatty liver disease(NAFLD) is a chronic metabolic condition with rapidly increasing incidence, becoming a public health issue of worldwide concern. Studies have shown that farnesoid X receptor(FXR)-based modulation of downstream targets can improve liver function and metabolic status in the patients with NAFLD and may be a potential drug target for treating this di-sease. Great progress has been achieved in the development of drugs targeting FXR for the treatment of NAFLD. A number of studies have explored the traditional Chinese medicine and their active ingredients for the treatment of NAFLD via FXR considering the high safety and efficacy and mild side effects. This paper systematically describes the mechanism of traditional Chinese medicines in the treatment of NAFLD via FXR and the downstream targets, aiming to provide precise targets for the drug development and clinical treatment of NAFLD.

Pivotal interplays between fecal metabolome and gut microbiome reveal functional signatures in cerebral ischemic stroke.

BACKGROUND: Integrative analysis approaches of metagenomics and metabolomics have been widely developed to understand the association between disease and the gut microbiome. However, the different profiling patterns of different metabolic samples in the association analysis make it a matter of concern which type of sample is the most closely associated with gut microbes and disease. To address this lack of knowledge, we investigated the association between the gut microbiome and metabolomic profiles of stool, urine, and plasma samples from ischemic stroke patients and healthy subjects. METHODS: We performed metagenomic sequencing (feces) and untargeted metabolomics analysis (feces, plasma, and urine) from ischemic stroke patients and healthy volunteers. Differential analyses were conducted to find key differential microbiota and metabolites for ischemic stroke. Meanwhile, Spearman's rank correlation and linear regression analyses were used to study the association between microbiota and metabolites of different metabolic mixtures. RESULTS: Untargeted metabolomics analysis shows that feces had the most abundant features and identified metabolites, followed by urine and plasma. Feces had the highest number of differential metabolites between ischemic stroke patients and the healthy group. Based on the association analysis between metagenomics and metabolomics of fecal, urine, and plasma, fecal metabolome showed the strongest association with the gut microbiome. There are 1073, 191, and 81 statistically significant pairs (P < 0.05) in the correlation analysis for fecal, urine, and plasma metabolome. Fecal metabolites explained the variance of alpha-diversity of the gut microbiome up to 31.1%, while urine and plasma metabolites only explained the variance of alpha-diversity up to 13.5% and 10.6%. Meanwhile, there were more significant differential metabolites in feces than urine and plasma associated with the stroke marker bacteria. CONCLUSIONS: The systematic association analysis between gut microbiome and metabolomics reveals that fecal metabolites show the strongest association with the gut microbiome, followed by urine and plasma. The findings would promote the association study between the gut microbiome and fecal metabolome to explore key factors that are associated with diseases. We also provide a user-friendly web server and a R package to facilitate researchers to conduct the association analysis of gut microbiome and metabolomics.

Surface plasmon resonance temperature sensor with tunable detection range based on a silver-coated multi-hole optical fiber.

A novel surface plasmon resonance (SPR) temperature sensor based on a silver-coated multi-hole optical fiber (SMHOF) is presented. The central and surrounding air holes of the SMHOF are filled with two kinds of thermosensitive liquid with high and low refractive index (RI), respectively. Two separated resonance dips, which are related to the high and low RI filled liquid respectively, are observed at different wavelength in the transmission spectrum. Advantageously, the two dips move towards opposite direction with the temperature variation. The interval between the two SPR dips is measured under different environmental temperature and exhibits a good linearity. The proposed sensor with different detection range is fabricated by changing the RIs of the filled thermosensitive liquids. The temperature sensitivity of 7.72 nm/°C and -7.81 nm/°C is obtained in the range of 20-60 °C and -20-20 °C, respectively. Owing to the high temperature sensitivity and tunable detection range, the proposed sensor is expected to find potential applications in biomedicine, health care and environmental monitoring.

Design, fabrication, and characterization of a single-polarization single-mode flexible hollow waveguide for low loss millimeter wave propagation.

A flexible metallic waveguide with elliptical core that achieves single-polarization single-mode (SPSM) propagation at millimeter wave was designed, fabricated, and characterized. In order to achieve SPSM propagation, optimization of the lengths of major/minor axes of elliptical core was conducted to cut off one of the two orthogonally polarized fundamental modes and all high-order modes. A one-meter long hollow elliptical waveguide (HEW) with major/minor axis length of 1.5/2.7 mm was fabricated. The substrate tube was a flexible elliptical polycarbonate (PC) tube, which was fabricated through glass-draw technique. Silver film was then coated on the inner surface of the tube. Simulation results show that the 1.5/2.7 mm HEW maintains SPSM propagation in the frequency band from 66.5 to 114 GHz. The SPSM operation was experimentally discussed in detail at 100 GHz. The measured loss of 2.58 dB/m and the output polarization ratio of 99.9% was obtained after propagating one meter. Furthermore, the waveguide was robust to bending and twisting. The additional loss was as small as 0.2 dB/m even when the waveguide was coiled into a circle. The potential application of HEWs as polarizers was demonstrated by using a 10 cm long waveguide for polarization detection and extinction ratio of 22.3 dB was achieved at 100 GHz.

Control of the Geometric Phase and Nonequivalence between Geometric-Phase Definitions in the Adiabatic Limit.

If the time evolution of a quantum state leads back to the initial state, a geometric phase is accumulated that is known as the Berry phase for adiabatic evolution or as the Aharonov-Anandan (AA) phase for nonadiabatic evolution. We evaluate these geometric phases using Floquet theory for systems in time-dependent external fields with a focus on paths leading through a degeneracy of the eigenenergies. Contrary to expectations, the low-frequency limits of the two phases do not always coincide. This happens as the degeneracy leads to a slow convergence of the quantum states to adiabaticity, resulting in a nonzero finite or divergent contribution to the AA phase. Steering the system adiabatically through a degeneracy provides control over the geometric phase as it can cause a π shift of the Berry phase. On the other hand, we revisit an example of degeneracy crossing proposed by AA. We find that, at suitable driving frequencies, both geometric-phase definitions give the same result and the dynamical phase is zero due to the symmetry of time evolution about the point of degeneracy, providing an advantageous setup for manipulation of quantum states.

Fingerprint of the Interbond Electron Hopping in Second-Order Harmonic Generation.

We experimentally explore the fingerprint of the microscopic electron dynamics in second-order harmonic generation (SHG). It is shown that the interbond electron hopping induces a novel source of nonlinear polarization and plays an important role even when the driving laser intensity is 2 orders of magnitude lower than the characteristic atomic field. Our model predicts anomalous anisotropic structures of the SHG yield contributed by the interbond electron hopping, which is identified in our experiments with ZnO crystals. Moreover, a generalized second-order susceptibility with an explicit form is proposed, which provides a unified description in both the weak and strong field regimes. Our work reveals the nonlinear responses of materials at the electron scale and extends the nonlinear optics to a previously unexplored regime, where the nonlinearity related to the interbond electron hopping becomes dominant. It paves the way for realizing controllable nonlinearity on an ultrafast time scale.

Artesunate induces ferroptosis via modulation of p38 and ERK signaling pathway in glioblastoma cells.

Ferroptosis is implicated in various tumors, including glioblastoma. Artesunate (ART), an anti-malarial drug, exerted antitumor properties in several cancer types. However, the role of ferroptosis in the inhibiting effect of artesunate on glioblastoma remains unclear. The purpose of this study was to investigate the effects of ART on the ferroptosis of glioblastoma and to elucidate the underlying mechanisms. We found that ART inhibited the proliferation of glioblastoma cells in vitro and glioblastoma tumorigenesis in vivo. Characteristic changes of ferroptosis were observed in ART group, including GSH depletion, lipid peroxidation and iron overload. Meanwhile, the protein level of GPX4 were lower in ART group than that in control group. Ferrostatin-1, a ferroptosis inhibitor, could rescue the cell death induced by ART in U251 cells. Further examination of the mechanism revealed that the effect of ART on ferroptosis was partially governed by regulating iron homeostasis and p38 and ERK signaling pathway. These findings support that ART triggers ferroptosis in glioblastoma and might be a potential therapeutic agent for glioblastoma treatment.

Identification and functional analysis of novel oncogene DDX60L in pancreatic ductal adenocarcinoma.

BACKGROUND: Pancreatic ductal adenocarcinoma (PDAC) is a lethal cancer. Approximately 80% of patients initially diagnosed with locally advanced or metastatic disease survive only 4-11 months after diagnosis. Tremendous efforts have been made toward understanding the biology of PDAC. RESULTS: In this study, we first utilized next-generation sequencing technique and existing microarray datasets to identify significant differentially expressed genes between PDAC and non-tumor adjacent tissue. By comparing top significant survival genes in PDAC Gene Expression Profiling Interactive Analysis database and PDAC transcriptome data from patients, our integrated analysis discovered five potential central genes (i.e., MYEOV, KCNN4, FAM83A, S100A16, and DDX60L). Subsequently, we analyzed the cellular functions of the potential novel oncogenes MYEOV and DDX60L, which are highly expressed in PDAC cells. Notably, the knockdown of MYEOV and DDX60L significantly inhibited the metastasis of cancer cells and induced apoptosis. Further RNA sequencing analyses showed that massive signaling pathways, particularly the TNF signaling pathway and nuclear factor-kappa B (NF-κB) signaling pathway, were affected in siRNA-treated cancer cells. The siDDX60L and siMYEOV significantly inhibited the expression of chemokine CXCL2, which may potentially affect the tumor microenvironment in PDAC tissues. CONCLUSIONS: The present findings identified the novel oncogene DDX60L, which was highly expressed in PDAC. Transcriptome profiling through siRNA knockdown of DDX60L uncovered its functional roles in the PDAC in humans.

Dielectric layer thickness insensitive EVA/Ag-coated hollow fiber temperature sensor based on long-range surface plasmon resonance.

A novel hollow fiber temperature sensor (HFTS) based on long-range surface plasmon resonance is presented. The HFTS consists of a dielectric/Ag-coated hollow fiber filled with the thermosensitive liquid and two multimode fibers connected at both ends. By measuring the transmission spectra under different temperatures, the performances, including sensitivity and figure of merit (FOM) of the sensors with different structural parameters, such as thermosensitive liquid property, ethylene-vinyl acetate (EVA) and silver layer thicknesses, were investigated experimentally. The results shows that the sensitivity of the optimized HFTS is 1.60nm/°C to 5.21nm/°C in the range from 20°C to 60°C, and the FOM is up to 0.0453°C-1. Both performances are higher than most reported optical fiber temperature sensors based on surface plasmon resonance. Moreover, the performance of the HFTS is not sensitive to the dielectric layer thickness, which greatly reduces the difficulty of fabrication.

Transmission and imaging characteristics of flexible gradually tapered waveguide at 0.3 THz.

Flexible gradually tapered metal waveguides (GTMWs) are fabricated by an inner plating silver film in a polycarbonate (PC) capillary for the transmission and imaging at 0.3 THz. It was demonstrated theoretically and experimentally that GTMWs have lower transmission losses and smaller additional losses of bending, comparing with thin constant bore metal waveguides (CBMWs). Measured losses of 1.95 dB and 2.45 dB were obtained for a 1 m long GTMW with bore size varying from 2.6 mm to 1.6 mm under straight and one circle bending configuration. Measured losses were 4.48 dB/m and 7.78 dB/m for 1.6 mm bore CBMW under the same straight and bend configurations. Owing to higher energy concentration at the output, a larger penetration ability of output wave can be achieved by GTMW, which is beneficial for imaging application. A scanning imaging system was established using fabricated waveguides as the probes. Measured results show that the air slits of the order of wavelength can be clearly distinguished. An imaging system with a GTMW probe also has better performances due to lower bending loss and improved coupling efficiency.

Enhancement of the photocurrents injected in gapped graphene by the orthogonally polarized two-color laser field.

We theoretically investigate the photocurrents injected in gapped graphene by the orthogonally polarized two-color laser field. Depending on the relative phase, the photocurrents can be coherently controlled by deforming the electron trajectory in the reciprocal space. Under the same field strength, the peak photocurrent in the orthogonally polarized two-color field is about 20 times larger than that for linearly polarized light, and about 3.6 times for elliptically polarized light. The enhancement of the photocurrent can be attributed to an obvious asymmetric distribution of the real population in the reciprocal space, which is sensitive to the waveform of the laser field and related to the quantum interference between the electron trajectories. Our work provides a noncontact method to effectively enhance the injected current in graphene.

Fiber polarizer based on selectively silver-coated large-core suspended-core fiber.

A tunable fiber polarizer based on the selectively silver-coated large-core suspended-core fiber (LSCF) was proposed. A thin silver layer was coated on the inner surface of two opposite air holes of the LSCF by the chemical liquid-phase deposition method. The $y$-polarized light (parallel to the two silver-coated air holes) will excite surface plasmon resonance and experience large transmission loss, while the $x$-polarized light does not, resulting in a fiber polarizer. By varying the liquid filled in the microchannels of the LSCF, the operating wavelength can be tuned in the visible and near infrared region along with the surface plasmon resonance wavelength. The dependence of the polarization characteristics on the fiber length was experimentally investigated. The maximum polarization extinction ratio (PER) of 20.1 dB, 19.6 dB, and 18.3 dB and insertion loss (IL) of 2.24 dB, 2.56 dB, and 2.08 dB are achieved with the optimal fiber length of 16 cm at the operating wavelengths of 565.4 nm, 626.7 nm, and 739.7 nm, respectively. Compared with the multimode fiber-based polarizers reported previously, the proposed selectively silver-coated LSCF polarizer exhibits higher PER and lower IL.

Huygens-Fresnel Picture for High Harmonic Generation in Solids.

High harmonic generation (HHG) is usually described by the laser-induced recollision of particlelike electrons, which lies at the heart of attosecond physics and also inspires numerous attosecond spectroscopic methods. Here, we demonstrate that the wavelike behavior of electrons plays an important role in solid HHG. By taking an analogy to the Huygens-Fresnel principle, an electron wave perspective on solid HHG is proposed by using the wavelet stationary-phase method. From this perspective, we have explained the deviation between the cutoff law predicted by the particlelike recollision model and the numerical simulation of semiconductor Bloch equations. Moreover, the emission times of HHG can be well predicted with our method involving the wave property of electrons. However, in contrast, the prediction with the particlelike recollision model shows obvious deviations compared to the semiconductor Bloch equations simulation. The wavelike properties of the electron motion can also be revealed by the HHG in a two-color field.