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Dynamically encircling exceptional points (EPs) in non‐Hermitian systems has garnered considerable attention for enabling chiral mode switching, where the nature of the output mode is determined solely by the encircling handedness. However, typical chiral transmission devices primarily operate within the 1.55 µm waveband and encounter the wavelength‐dependent mode mismatch, which constrains operational range and degrades mode purity. In this study, we address these limitations by introducing metamaterial waveguides to tailor the requisite optical properties and achieve ultra‐broad‐bandwidth and ultra‐high‐purity chiral mode switching at 1.55 µm and beyond. Simulation results spanning the optical communication and 2 µm wavebands show near‐unity transmission efficiency and high mode purity (> 98.8%) across a record 500 nm bandwidth. Moreover, experimental results at 2 µm waveband reveal an average transfer efficiency of < 1 dB and high mode purity exceeding 95% within a setup‐limited bandwidth of 85 nm. This work paves the way for achieving high‐efficiency and high‐purity chiral transmission devices in the near/mid‐infrared ranges, presenting compelling prospects for future practical EP‐based devices and applications. [ABSTRACT FROM AUTHOR]

The mid-infrared (MIR, 2–20 μm) waveband is of great interest for integrated photonics in many applications such as on-chip spectroscopic chemical sensing, and optical communication. Thermo-optic switches are essential to large-scale integrated photonic circuits at MIR wavebands. However, current technologies require a thick cladding layer, high driving voltages or may introduce high losses in MIR wavelengths, limiting the performance. This paper has demonstrated thermo-optic (TO) switches operating at 2 μm by integrating graphene onto silicon-on-insulator (SOI) structures. The remarkable thermal and optical properties of graphene make it an excellent heater material platform. The lower loss of graphene at MIR wavelength can reduce the required cladding thickness for the thermo-optics phase shifter from micrometers to tens of nanometers, resulting in a lower driving voltage and power consumption. The modulation efficiency of the microring resonator (MRR) switch was 0.11 nm/mW. The power consumption for 8-dB extinction ratio was 5.18 mW (0.8 V modulation voltage), and the rise/fall time was 3.72/3.96 μs. Furthermore, we demonstrated a 2 × 2 Mach-Zehnder interferometer (MZI) TO switch with a high extinction ratio of more than 27 dB and a switching rise/fall time of 4.92/4.97 μs. A comprehensive analysis of the device performance affected by the device structure and the graphene Fermi level was also performed. The theoretical figure of merit (2.644 mW−1μs−1) of graphene heaters is three orders of magnitude higher than that of metal heaters. Such results indicate graphene is an exceptional nanomaterial for future MIR optical interconnects. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate the transmission spectral and surrounding refractive index (SRI) sensing performance of long-period fiber gratings (LPFGs) at 2 μm waveband. The cladding modes operating at 2 μm waveband show higher SRI sensitivity than 1.55 μm waveband. The influence of a little off resonance region of grating period, refractive index modulation and duty cycle of gratings are analyzed theoretically. Additionally, the LPFGs of LP08 mode near dispersion turning point operating at 2 μm waveband are investigated theoretically and experimentally. Mode splitting brings out with the increasing SRI. After the resonant dip splitting into two dips (dip A and dip A′), the SRI sensitivity of dip A are measured to be as high as −8495.2 nm/RIU in the SRI region of 1.445–1.457. The proposed 2-μm-waveband LPFGs makes it a potential in biochemical and environmental monitoring. [ABSTRACT FROM AUTHOR]

Silicon photonics at the 2 μm waveband, specifically the 1.9 μm wavelength region is strategically imperative. This is due to its infrastructural compatibility (i.e., thulium-doped fiber amplifier, hollow-core photonic bandgap fiber) in enabling communications, as well as its potential to enable a wide range of applications. While the conventional Silicon-on-Insulator platform permits passive/active functionalities, it requires stringent processing due to high-index contrast. On the other hand, SiN can serve to reduce waveguiding losses via its moderate-index contrast. In this work, by demonstrating SiN passives and Si-SiN interlayer coupler with favorable performance, we extend the Si-SiN platform to the 1.9 μm wavelength region. We report waveguide propagation loss of 2.32 dB/cm. Following, trends in radiation loss with regards to bending radius is analyzed. A high performance 3-dB power splitter with insertion loss and bandwidth of 0.05 dB and 55 nm (1935-1990 nm) respectively is introduced. Lastly, Si-SiN transition loss as low as 0.04 dB is demonstrated.

The “ $2~\mu \text{m}$ waveband”, specifically the $1.9~\mu \text{m}$ wavelength region, is playing an increasingly imperative role in photonics. Development into compact tunable light sources operating at the wavelength region can unlock numerous technological applications. Instances, while not exhaustive, include alleviating the capacity load in fiber communications, H2O spectroscopy, optical logic, signal processing as well as enabling the optical Kerr effect on silicon. Silicon photonics is a disruptive technology. Through mature silicon processing, recent developments suggest that silicon will emerge as the workhorse of integrated optics. While the realization of a monolithic light source has proved to be challenging, the hybrid/heterogenous Si platforms, consisting of silicon and III-V materials, has stepped to the fore. In this work, we present the study of Vernier-based hybrid silicon photonic wavelength-tunable lasers with an operating range of 1881–1947 nm (66 nm), subject to different coupling gaps (Gapmrr) between the silicon microring resonators (MRRs) and bus waveguide. Wavelength tuning functionality is enabled via the thermo-optic effect of MRRs. Gapmrr, being the smallest feature in the assemble, is highly influential to the characteristics of the SHREC. As such, trends in hybrid laser performance with respect to Gapmrr are measured and analyzed. Slope efficiency, laser output power and side-mode suppression ratio of 0.232 W/A, 28 mW and 42 dB respectively are obtained across the developed lasers. Through the design of the Vernier spectrum and Gapmrr, tuning of laser wavelength from 1881–1947 nm can be achieved by applying only 47.2 mW of thermal power to a single MRR. [ABSTRACT FROM AUTHOR]

The 2 $\mu \text{m}$ waveband, specifically near $1.9~\mu \text{m}$ , is an imperative resource that could possibly be exploited in future communications systems. This is due to the promising infrastructural developments at the wavelength region (hollow-core photonic bandgap fiber, thulium-doped fiber amplifier) near $1.9~\mu \text{m}$. In this work, we report the 1 $\times N$ selector switch based on Mach-Zehnder interferometers operating near the $1.9~\mu \text{m}$ wavelength region. As an elementary cell ($N $ = 2), an insertion loss as low as 1.1 dB, $\text{P}_{\pi }$ of 23 mW, 10-90% switching time of lower than 38 $\mu \text{s}$ and a crosstalk of lower than −25 dB from 1880 to 1955 nm has been determined. In order to prove scalability, the 1 $\times $ 8 switch $(N $ = 8) is demonstrated, indicating crosstalk as low as −21 dB, considering all possible switching configurations across the abovementioned wavelength region. Insertion loss levels are examined. [ABSTRACT FROM AUTHOR]

Silicon photonics at the 2 μm waveband, specifically the 1.9 μm wavelength region is strategically imperative. This is due to its infrastructural compatibility (i.e., thulium-doped fiber amplifier, hollow-core photonic bandgap fiber) in enabling communications, as well as its potential to enable a wide range of applications. While the conventional Silicon-on-Insulator platform permits passive/active functionalities, it requires stringent processing due to high-index contrast. On the other hand, SiN can serve to reduce waveguiding losses via its moderate-index contrast. In this work, by demonstrating SiN passives and Si-SiN interlayer coupler with favorable performance, we extend the Si-SiN platform to the 1.9 μm wavelength region. We report waveguide propagation loss of 2.32 dB/cm. Following, trends in radiation loss with regards to bending radius is analyzed. A high performance 3-dB power splitter with insertion loss and bandwidth of 0.05 dB and 55 nm (1935--1990 nm) respectively is introduced. Lastly, Si-SiN transition loss as low as 0.04 dB is demonstrated. [ABSTRACT FROM AUTHOR]

• The rectangular grooves are shallowly etched on the MMI waveguide to introduce the refractive-index perturbations on the MMI waveguide eigenmodes, leading to an effective refractive-index manipulation of the eigenmodes. • The refractive-index perturbation scheme provides a simple and scalable method to manipulate the four beat lengths of the two wavelengths at TE- or TM-polarization in a single waveguide via self-imaging effect, leading to a compact footprint. • The present refractive-index perturbation scheme realizes, for the first time, NIR/MIR wavelength demultiplexing and polarization independence simultaneously in an individual device. • A compact and polarization-independent wavelength diplexer for 1.31/2 μm is also realized by flexibly changing the dimensions, showing the flexibility and extensibility of our refractive-index perturbation scheme. Faced with the rapid growth of internet traffic demand, which has posed a huge challenge to the current fiber-based communications systems, expanding the telecommunications bands from traditional near-infrared (NIR) region to mid-infrared (MIR) region is emerging as an attractive solution. And on-chip wavelength division multiplexing (WDM) application for NIR/MIR wavelengths is an essential element in future optical telecommunication systems. Here, we propose a compact silicon-based polarization-independent wavelength diplexer for the NIR/MIR wavelengths of 1.55/2 μm, where multiple rectangular grooves are shallowly etched on the silicon MMI waveguide for introducing the refractive-index perturbations on the MMI waveguide eigenmodes. And this refractive-index perturbation scheme provides a simple and scalable method to manipulate the four beat lengths of the two wavelengths at TE- or TM-polarization in a single waveguide via self-imaging effect, leading to a compact footprint. By optimizing the structural parameters, both NIR/MIR wavelength demultiplexing and polarization independence, for the first time, are obtained simultaneously in the present single device. From the results, the proposed diplexer is only 21.6 μm in length, and shows a wide bandwidth of ∼ 100/120 nm around the wavelength of 1.55/2 μm for insertion loss (IL) < 1 dB and extinction ratio (ER) > 15 dB. Moreover, we also realize a compact and polarization-independent wavelength diplexer for 1.31/2 μm by flexibly changing the dimensions, showing the flexibility and extensibility of our refractive-index perturbation scheme. Besides, fabrication tolerances are analyzed and mode propagation profiles are also demonstrated. [ABSTRACT FROM AUTHOR]

Ternary chalcogenides possess a promising prospect for their superior nonlinear optical response, offering new opportunities in future photonic devices. Ta 2 NiS 5 and Ta 2 NiSe 5 nanoflakes fabricated by liquid-phase exfoliation were investigated. Based on Tm:YAG ceramic and Tm,Ho:Ca(Gd,Lu)AlO 4 crystal as gain media, the doubly passively Q-switched lasers with Ta 2 NiS 5 and Ta 2 NiSe 5 saturable absorbers were realized. The shortest pulse duration was 391 ns, which was realized in Tm,Ho:Ca(Gd,Lu)AlO 4 laser. And the highest peak power 17.73 W was obtained in Tm:YAG ceramic laser. The results demonstrate that the Ta 2 NiS 5 and Ta 2 NiSe 5 nanomaterials have the potential to generate a short-pulsed laser. Doubly Q-switching technology can compress pulse duration and enhance peak power. • The Ta 2 NiS 5 and Ta 2 NiSe 5 nanoflakes was fabricated by liquid-phase exfoliation. • Ternary chalcogenide nanomaterials performed well for pulsed laser generation. • The shortest pulse duration of 391 ns was realized in Tm,Ho:Ca(Gd,Lu)AlO 4 laser. • The highest peak power of 17.73 W was obtained in Tm:YAG ceramic laser. [ABSTRACT FROM AUTHOR]

• Operational bandwidth of the emerging 2 μm waveband. • Based on subwavelength grating structure. • Excellent performance of mode-order converters. • Ultra-high contrast ratio of reciprocal optical diodes. • Demonstration of 3 dB mode converter and splitters. We propose a silicon-based asymmetric directional coupler assisted by subwavelength grating anisotropic metamaterial operating at the emerging 2 μm waveband. The effective medium theory is utilized to explain the principle of the proposed structure. Three devices with mode-order conversions are presented. Firstly, TE0-to-TE1, TE0-to-TE2, and TE0-to-TE3 conversions with remarkably high transmittance (>−0.17 dB) and extremely high mode purity (>98.81%) are achieved at the central wavelength of 1985 nm. The fabrication tolerance analyses demonstrate reasonable robustness of the proposed structure. Secondly, such structure can realize a reciprocal optical diode with an exceedingly high contrast ratio (>34 dB) based on spatial symmetry breaking. Finally, we present a symmetric 3 dB mode converter and splitter just by adjusting the length of subwavelength grating and the gap of asymmetric directional coupler. The results reveal that the transmittances of both output ports approach −3 dB around 2 μm and the transmittance difference remains below −10 dB above 40 nm within the waveband from 1.95 μm to 2.05 μm. [ABSTRACT FROM AUTHOR]

In this paper, the Nb 2 CT x MXene nanosheets were fabricated and the corresponding microstructures were investigated. The nonlinear optical response was illustrated by open aperture Z-scan and I-scan methods. The ground and the excited state absorption cross-sections of 2D Nb 2 CT x MXene were also investigated. As the saturable absorber (SA), the Nb 2 CT x MXene was applied in the passively Q-switched Tm:YAP laser. 1.96 μ s Q-switched pulses with 3.97 W peak power were achieved at the repetition frequency of 80 kHz. Further theoretical model was built by using the coupled rate equations in simulating the dynamic process of the passively Q-switched Tm:YAP laser. The numerical simulation results are fundamentally in agreement with the experimental results, which proves the Nb 2 CT x MXene can be a good potential nanomaterial for further optoelectronic applications., (© 2021 IOP Publishing Ltd.)

Two-dimensional (2D) graphitic carbon nitride (g-C 3 N 4) was fabricated as saturable absorber (SA) and the nonlinear optical absorption properties were measured. A laser-diode (LD) pumped doubly Q-switched Tm-doped yttrium-aluminium perovskite YAlO 3 (Tm:YAP) laser at 2 μm with g-C 3 N 4 and acousto-optic modulator (AOM) is presented for the first time to the best of our knowledge. Under an absorbed pump power of 5.34 W, the minimum pulse width of 239 ns and the maximum peak power of 1146 W were obtained by the doubly Q-switched laser. In comparison with the singly Q-switched laser using g-C 3 N 4 SA or AOM, the dual-loss-modulated Q-switched laser could generate shorter pulse width and higher peak power. The maximum pulse width compression ratio was 4.48 and the highest peak power enhancement factor was 241. The experimental results indicated that 2D g-C 3 N 4 SA is potential in Q-switched laser at 2 μm and the doubly Q-switched technology is a useful way to compress the pulse width and improve the peak power. • 2D g-C 3 N 4 SA was fabricated and the nonlinear optical absorption properties were measured. • A laser-diode (LD) pumped doubly Q-switched Tm:YAP laser at 2 μm with g-C 3 N 4 SA and AOM is realized for the first time. • The pulse width of the Q-switched laser is compressed greatly. • The pulsed peak power is improved evidently. [ABSTRACT FROM AUTHOR]