Routing to mid-infrared microcomb via near-infrared direct pump.

Mid-infrared (MIR) microcomb provides a new way into the "molecular fingerprint" region. However, it remains rather a challenge to realize the broadband mode-locked soliton microcomb, which is often limited by the performance of available MIR pump sources and coupling devices. Here, we propose an effective approach towards broadband MIR soliton microcombs generation via a direct pump in the near-infrared (NIR) region, through full utilization of the second- and third-order nonlinearities in a thin-film lithium niobate microresonator. The optical parametric oscillation process contributes to conversion from the pump at 1550 nm to the signal around 3100 nm, and the four-wave mixing effect promotes spectrum expansion and mode-locking process. While the second-harmonic and sum-frequency generation effects facilitate simultaneous emission of the NIR comb teeth. Both the continuous wave and pulse pump sources with relatively low power can support a MIR soliton with a bandwidth over 600 nm and a concomitant NIR microcomb with a bandwidth of 100 nm. This work can provide a promising solution for broadband MIR microcombs by breaking through the limitation of available MIR pump sources, and can deepen the understanding of the physical mechanism of the quadratic soliton assisted by the Kerr effect.

The correlation of temporal changes of neutrophil-lymphocyte ratio with seizure severity and the following seizure tendency in patients with epilepsy.

Background: Changes in the neutrophil-lymphocyte ratio (NLR) has been reported to be associated with epilepsy. Here we aim to investigate the correlation of temporal changes of NLR level with seizure severity and the follow-up seizure attacks in patients with epilepsy (PWE). Methods: We performed a retrospective analysis of the laboratory data including leukocyte count and NLR within 24 h of acute seizure and during the follow-up period of 5-14 days after acute seizure (NLR1, NLR2, respectively) in 115 PWE, and 98 healthy individuals were included as controls in this study. The correlation of laboratory data with seizure types, etiology of epilepsy, anti-seizure drugs (ASDs), seizure severity, and the follow-up seizure attacks in PWE was studied. Results: Leukocyte count (P < 0.001) and NLR level (P < 0.001) were found significantly different between PWE and controls. On the other hand, a multivariable logistic regression analysis showed that NLR1 level (OR = 2.992, P = 0.001) and admission leukocyte (OR = 2.307, P = 0.002) were both independently associated with acute epileptic seizures. Especially, higher NLR1 level was significantly associated with status epileptics (P = 0.013) and recurrent seizures after admission (P < 0.001). Furthermore, the multivariable logistic regression analysis indicated that higher NLR1 was a predictor for the tendency of the following recurrent seizure attacks (OR = 1.144, P = 0.002). NLR2 was inversely correlated with ASDs taken (P = 0.011). Levels of NLR1 (r = 0.441, P < 0.001) and NLR2 (r = 0.241, P = 0.009) were both positively correlated with seizure severity. Conclusions: Seizures were correlated with the alterations of systemic inflammation reflected by leukocyte and NLR. NLR1 and admission leukocyte were both independently associated with acute epileptic seizures. Higher NLR1 was associated with status epilepticus and independently predicted the tendency of the following epileptic seizures. NLR2 was significantly associated with ASDs taken. Besides, NLR may be used as a biomarker for seizure severity.

The First Case of Familiar Anti-leucine-rich Glioma-Inactivated1 Autoimmune Encephalitis: A Case Report and Literature Review.

Anti-leucine-rich glioma-inactivated1 (Anti-LGI1) autoimmune encephalitis is a rare autoimmune disease discovered in recent years. It is generally not defined as an inherited disease, though its etiology is still unclear. Herein, we report the first case of adult patients with familial anti-LGI1 encephalitis. Two biological siblings who worked in different regions were successively diagnosed with anti-LGI1 encephalitis in their middle age. The two patients had similar clinical manifestations including imaging results. Their clinical symptoms improved after immunotherapy and antiepileptic therapy. Given that some unique human leukocyte antigen (HLA) subtypes appear at a high frequency, multiple recent studies have revealed that anti-LGI1 encephalitis is associated with genetic susceptibility. One of the patients underwent HLA genotyping and whole-exome sequencing (WES), revealing the same HLA typing as in previous studies and two rare HLA variants. Therefore, further studies involving larger samples and more populations should be conducted to explore the possibility of other influencing factors such as environmental impacts.

Simultaneous generation of a broadband MIR and NIR frequency comb in a GaP microring.

Midinfrared (MIR) optical frequency combs are of great significance as broadband coherent light sources used in extensive areas such as coherent communications and molecule detections. Conventional MIR combs are usually restricted in size and power, while most microcombs are focused in the near-infrared (NIR) region because of the limited accessible Q-factor of microrings and the poor performances of available pumps. In this paper, we numerically demonstrate the simultaneous generation of a broadband MIR and NIR comb in a GaP microring with an additive waveguide. The achieved octave-spanning (1890-4050 nm) MIR microcomb at a low pump power of 34 mW can be effectively converted to the second-harmonic NIR comb covering 1120-1520 nm with separate dispersion optimization of the ring cavity and straight waveguide. The proposed system has the advantage of simple structure and low power threshold, which could find potential in highly integrated MIR optical sources and related applications.

Mid-infrared optical parametric oscillation spanning 3.4-8.2µm in a MgF2microresonator.

Mid-infrared optical parametric oscillators (OPOs) offer a compelling route for accessing the 'molecular fingerprint' region and, thus, can find intensive applications such as precision spectroscopy and trace gas detection. Yet it still remains rather a challenge to realize broadband mid-infrared OPOs within a single cavity, usually limited by strict phase-matching conditions for wide spectral coverage and available pump power for adequate frequency generation. Here, we report the mid-infrared parametric oscillation spanning from 3.4 to 8.2µm, based on four-wave mixing in a high-QMgF2microresonator with optimized dispersion. The center wavelength at 4.78µm is determined by the continuous tunable quantum cascade laser source, which contributes to effective expansion towards longer wavelength, as well as systemic miniaturization with smaller pump module. Such results could not only shed light on new ultimates of crystal and other microresonators, but also inspire explorations on their growing potentials in near future.

Impact of third-order dispersion and three-photon absorption on mid-infrared time magnification via four-wave mixing in Si0.8Ge0.2 waveguides.

We investigate the influence of third-order dispersion of dispersive elements, three-photon absorption and free-carrier effects on mid-infrared time magnification via four-wave mixing (FWM) in ${{\rm Si}_{0.8}}{{\rm Ge}_{0.2}}$Si0.8Ge0.2 waveguides. It is found that the magnified waveform is seriously distorted by these factors, and conversion efficiency is decreased, mainly because of nonlinear absorption. A time lens based on FWM in ${{\rm Si}_{0.8}}{{\rm Ge}_{0.2}}$Si0.8Ge0.2 waveguides is proposed for time magnification of mid-infrared ultrashort pulses, in which the low-distortion, high-magnification in the time domain could be obtained by optimizing system parameters. These results make it possible to analyze the transient dynamic process through oscilloscopes and detectors with gigahertz bandwidth and have important applications in ultrafast process analysis, optical pulse sampling, and optical communications.

Mid-infrared dual-comb generation via the cross-phase modulation effect in a normal-dispersion microcavity.

We numerically demonstrate orthogonally polarized dual-comb generation in a single microcavity with normal dispersion assisted by the cross-phase modulation (XPM) effect. It is found that the XPM effect facilitates the emission of a secondary polarized comb with different temporal properties in a wide existence range covering the blue- to red-detuned regime and thus releases the requirements for delicate control on the detuned region of pump frequency. Also, the energy transfer between two polarization components together with the normal-dispersion property contributes to a more balanced intensity difference and significantly increased conversion efficiency from the pump light into the comb operation. This work could provide a route to a low-cost and compact mid-infrared dual-comb system with a lower power requirement as well as an effective approach to higher comb teeth power with improved efficiency for practical applications.

Low-threshold optical bistability in field-enhanced nonlinear guided-mode resonance grating nanostructure.

We have numerically studied the optical bistability in guided-mode resonance-assisted nonlinear grating nanostructure. A low-index slot is introduced to significantly improve the confinement of light in nonlinear material. In this way, the proposed novel configuration possesses low-threshold optical switching intensity (∼3 MW/cm2), which is about 58 times lower than that of typical nonlinear grating nanostructure without the low-index slot. This bistability study provides an effective method to reduce the threshold of optical switching intensity and thus can be applied in optical logic, optical computation, and all-optical memory.

Influences of high-order dispersion on temporal and spectral properties of microcavity solitons.

We theoretically and numerically investigate the effects of high-order dispersion (HOD) on microcavity solitons, both in time and frequency domain with an extended normalized Lugiato-Lefever equation (LLE). The observed temporal drift of bright and dark solitons is shown to originate from high-odd-order dispersion, while the sign determines the direction of soliton movement and the amplitude decides the drift speed. HOD can also be introduced to stabilize the breathing bright and dark cavity solitons. In spectral domain, the nonlinear symmetry breaking is mainly introduced by third-order dispersion, whereas both third- and fourth-order dispersion can introduce dispersive wave accompanied by soliton tail oscillation. This work could give insight for exploring detailed intracavity pulse dynamics and spectral characteristics of Kerr combs influenced by HOD, as well as provide a viable route to delicate control of Kerr comb generation through tailoring the dispersion parameters.

Robust soliton crystals in a thermally controlled microresonator.

We demonstrate robust soliton crystals generation with a fixed frequency pump laser through a thermoelectric-cooler-based thermal-tuning approach in a butterfly-packaged complementary-metal-oxide-semiconductor-compatible microresonator. Varieties of soliton crystal states, exhibiting "palm-like" optical spectra that result from the strong interactions between the dense soliton ensembles and reflect their temporal distribution directly, are experimentally observed by sweeping one cavity resonance across the pump frequency from the blue-detuned side by reducing the operating temperature of the resonator. Benefitting from the tiny intra-cavity energy change, repeatable interconversion between the chaotic modulation instability and stable soliton crystal states can be successfully achieved via simple tuning of the temperature or pump power, showing the easy accessibility and excellent stability of such soliton crystals. This work could facilitate microresonator-based optical frequency combs towards a portable, adjustable, and low-cost system while avoiding the requirements of delicate frequency-sweeping pump techniques.

Broadband self-collimating phenomenon in a low-loss hybrid plasmonic photonic crystal.

A 2D planar self-collimating photonic crystal, based on a dielectric square lattice and a hexagonal lattice, is proposed. We demonstrate that the proposed structure can support the propagation of a hybrid surface plasmon polarition (SPP) mode with a loss of -0.017 dB/µm, and the mode size is only 0.33 µm. The defined figure of merit is one order of magnitude higher than that of the dielectric-metal structure. In addition, the self-collimating angle of more than 10° can be tuned with a silica index change of 0.08. The proposed structure possesses broad operation bandwidth of 88 nm and 58 nm for a dielectric square lattice and a hexagonal lattice, respectively. These two kinds of photonic crystals promise potential applications in photonic modulators and SPP photonic devices.

Pulse noise-hidden image reconstruction and visualization via stochastic resonance.

We investigate the nanosecond pulse noise-hidden image reconstruction and visualization using stochastic resonance implemented by modulation instability. In particular, this dynamical stochastic resonance holds with coupling between the pulse incoherent noise and pulse coherent signal, and provides a substantial enhancement of the signal-to-noise ratio and cross-correlation. This means that the pulse noise-hidden image can be effectively reconstructed with high visibility and fidelity via stochastic resonance at appropriate system parameters. Such a simple and convenient method has potential applications in image processing under noisy environment.

Reduction of phase noise to amplitude noise conversion in silicon waveguide-based phase-sensitive amplification.

We use a vector phase sensitive amplification (PSA) scheme, which can eliminate the inherent phase noise (PN) to amplitude noise (AN) conversion in a conventional PSA process. A dispersion-engineered silicon strip waveguide is used to investigate the vector PSA scheme at the telecom wavelengths. The phase-dependent gain and phase-to-phase transfer functions as well as constellation diagram at different signal polarization states (SPSs) are numerically analyzed. It is found that the PN to AN conversion is completely suppressed when the SPS is identical to one of the pump polarization states. Moreover, the binary phase shift keying signal is regenerated by the proposed vector PSA scheme, and the error vector magnitude is calculated to assess the regeneration capacity. Our results have potential application in all-optical signal processing.

Numerical analysis of pulse signal restoration by stochastic resonance in a buckled microcavity.

A novel scheme is proposed to restore weak pulse signals immersed in noise by stochastic resonance based on photothermal-effect-induced optical bistability in a buckled dome microcavity. The bistable properties of the dome microcavity are analyzed with different initial detuning wavelengths and effective cavity lengths, and bistable transmission can be obtained for input powers in submilliwatt range. A theoretical model is derived to interpret the nonlinear process of pulse signal recovery through double-well potential theory. The cross-correlation coefficient between output signals and pure input pulses is calculated to quantitatively analyze the influence of noise intensity on stochastic resonance. A cross-correlation gain of 7 is obtained, and the noise-hidden signal can be recovered effectively though the buckled dome microcavity with negligible distortion. The simulation results show the potential of using this structure to restore low-level or noise-hidden pulse signals in all-optical integrated systems.

Influence of absorption on stability of terahertz difference frequency generation.

This work presents numerical studies of the stability feature of terahertz difference frequency generation (THz-DFG) with a ZnGeP(2) crystal using two pump wavelengths. We found that the maximum output of a THz wave is located in the unstable output region because of the competitive equilibrium between the absorption and the gain. Furthermore, the output stability is dependent on the pump stability. Different from the results at the pump wavelength of 9.588 µm, there is neither an appropriate stable output region nor gain saturation region at the pump wavelength of 1.064 µm for a larger absorption coefficient. This work demonstrates that the stable output region of the THz wave is difficult to obtain when the pump absorption is excessively large in DFG.

Extracting nanosecond pulse signals via stochastic resonance generated by surface plasmon bistability.

A technology is investigated to extract nanosecond pulse noise hidden signals via stochastic resonance, which is based on surface plasmon bistability. A theoretical model for recovering nanosecond pulse signals is derived to describe the nonlinear process. It is found that the incident angle, polarization state, medium properties, and input noise intensity all determine the efficiency and fidelity of the output signal. The bistable behavior of the output intensity can be accurately controlled to obtain a cross-correlation gain larger than 6 in a wide range of input signal-to-noise ratio from 1∶5 to 1∶30. Meanwhile, the distortion in the time domain induced by phase shift can be reduced to a negligible level. This work provides a potential method for detecting low-level or hidden pulse signals in various communication fields.

Nonlinear restoration of pulse and high noisy images via stochastic resonance.

We propose a novel scheme for restoring pulse and high noisy images using stochastic resonance, which is based on the modulation instability and provides a cross-correlation gain higher than 8. As opposed to previously reported designs, this unique approach employs a continuous noise and pulse signal for the generation of modulation instability. The visibility and quality of output images can be improved by appropriately adjusting the system parameters. This provides a simple and feasible method for detecting low-level or hidden pulse images in various imaging applications.

Supercontinuum generation in strip/slot hybrid waveguide with flat and low dispersion.

A strip/slot hybrid waveguide with double horizontal silicon nanocrystals slots is proposed to achieve flat and low dispersion with four zero dispersion wavelengths. By tuning structural parameters of the waveguide, dispersion tailoring is fully characterized. The flat dispersion varying between -13 and 14 ps/(nm·km) is obtained over an 845 nm bandwidth. A broadband supercontinuum spectrum, spanning from 1.15 to 3.65 µm in the -15 dB level, was generated in this waveguide pumped by a femtosecond pulse at 1.86 µm. Results indicate that the waveguide has great potential in near- and mid-infrared nonlinear applications.

Reconstruction of pulse noisy images via stochastic resonance.

We investigate a practical technology for reconstructing nanosecond pulse noisy images via stochastic resonance, which is based on the modulation instability. A theoretical model of this method for optical pulse signal is built to effectively recover the pulse image. The nanosecond noise-hidden images grow at the expense of noise during the stochastic resonance process in a photorefractive medium. The properties of output images are mainly determined by the input signal-to-noise intensity ratio, the applied voltage across the medium, and the correlation length of noise background. A high cross-correlation gain is obtained by optimizing these parameters. This provides a potential method for detecting low-level or hidden pulse images in various imaging applications.

Metamaterial terahertz switch based on split-ring resonator embedded with photoconductive silicon.

In this paper, a metamaterial terahertz (THz) switch based on a split-ring resonator embedded with photoconductive silicon is presented and numerically investigated. Simulation results show that the switch works at two different resonant modes with different pump light powers and that the response time of the switch is less than 1 ps. By defining the switching window as the frequency range where the transmission magnitude of the ON state is one order of magnitude higher than the OFF state, a switching window ranging from 1.26 to 1.49 THz is obtained. The large modulation depth of the switch is due to the large separations of the maximum and minimum transmissions, which are 0.89 and 0.01, respectively. Particularly, the switch is frequency tunable by changing the thickness and permittivity of the dielectric layer.