Bilayer dispersion-flattened waveguides with four zero-dispersion wavelengths

On-chip octave-spanning flat supercontinuum in all-normal-dispersion silicon nitride waveguides

On-chip supercontinuum generators have emerged as an attractive optical source with small size, broad spectrum and high power efficiency. Nevertheless, there has long been a trade-off between spectral uniformity and bandwidth. We propose a novel silicon nitride waveguide with flat saddle-shaped all-normal dispersion, particularly for enhancing the nonlinear interactions over a wide band. By launching a 250-fs 30-kW input pulse, an ultra-flat (-6 dB) octave-spanning supercontinuum extending from 638 nm to 1477 nm can be generated. We analyze the performance of the supercontinuum generator in terms of spectral flatness and bandwidth under different input pulse conditions. Thanks to mature dispersion engineering, the pump wavelength can be flexibly selected within the flat dispersion region. The generated supercontinuum, therefore, can be applied to different spectral regions by shifting the center wavelength.

Highly coherent and multi-octave mid-infrared supercontinuum generations in a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths

In this paper, a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths (ZDWs) is designed. The corresponding three ZDWs are located at 3.74, 6.56, and 8.89 µm. The nonlinearity coefficient of the proposed reverse-strip AlGaAs waveguide is calculated as 2.09W^-1m^-1 at wavelength 4.9 µm. The effects of pump pulse parameters, waveguide length, and noise coefficient on the nonlinear dynamics of supercontinuum (SC) generation are investigated. When the hyperbolic secant pump pulse with a wavelength of 4.9 µm, peak power of 900 W, and duration of 100 fs is launched into the proposed waveguide and propagated after a 3 mm length, highly coherent and multi-octave mid-infrared (MIR) SC spanning from 2.2 to 14.5 µm (more than 2.7 octaves, at -40dB level) is generated. Finally, a possible fabrication process of the reverse-strip AlGaAs waveguide is introduced. Our research results have important applications in MIR photonics, MIR spectroscopy, optical precision measurement, and more.

Graphene Nanoribbon Gap Waveguides for Dispersionless and Low-Loss Propagation with Deep-Subwavelength Confinement

Surface plasmon polaritons (SPPs) have been attracting considerable attention owing to their unique capabilities of manipulating light. However, the intractable dispersion and high loss are two major obstacles for attaining high-performance plasmonic devices. Here, a graphene nanoribbon gap waveguide (GNRGW) is proposed for guiding dispersionless gap SPPs (GSPPs) with deep-subwavelength confinement and low loss. An analytical model is developed to analyze the GSPPs, in which a reflection phase shift is employed to successfully deal with the influence caused by the boundaries of the graphene nanoribbon (GNR). It is demonstrated that a pulse with a 4 μm bandwidth and a 10 nm mode width can propagate in the linear passive system without waveform distortion, which is very robust against the shape change of the GNR. The decrease in the pulse amplitude is only 10% for a propagation distance of 1 μm. Furthermore, an array consisting of several GNRGWs is employed as a multichannel optical switch. When the separation is larger than 40 nm, each channel can be controlled independently by tuning the chemical potential of the corresponding GNR. The proposed GNRGW may raise great interest in studying dispersionless and low-loss nanophotonic devices, with potential applications in the distortionless transmission of nanoscale signals, electro-optic nanocircuits, and high-density on-chip communications.

Pure quartic solitons in dispersion-engineered aluminum nitride micro-cavities

Pure quartic soliton (PQS) is a new class of solitons demonstrated in recent years and provides innovations in nonlinear optics and its applications. Generating PQSs in micro-cavities offers a novel way to achieve coherent microcombs, presenting a promising application potential. Here we numerically investigate the PQS generation in a dispersion-engineered aluminum nitride (AlN) micro-cavity. To support PQS, a well-designed shallow-trench waveguide structure is adopted, which is feasible to be fabricated. The structure exhibits a dominant fourth-order dispersion reaching up to -5.35×10^-3 ps⁴/km. PQSs can be generated in this AlN micro-cavity in the presence of all-order-dispersion and stimulated Raman scattering. Spectral recoil and soliton self-frequency shift are observed in the PQS spectrum. Furthermore, we find that due to the narrow Raman gain spectrum of crystalline AlN, the PQS evolves directly to chaos rather than turning into a breather. The threshold pump power with which the PQS turns into chaos is also theoretically calculated, which squares with the simulation results.

Advanced dispersion engineering of a III-nitride micro-resonator for a blue frequency comb

A systematic dispersion engineering approach is presented toward designing a III-nitride micro-resonator for a blue frequency comb. The motivation for this endeavor is to fill the need for compact, coherent, multi-wavelength photon sources that can be paired with, e.g., the ¹⁷¹Yb⁺ ion in a photonic integrated chip for optical sensing, time-keeping, and quantum computing applications. The challenge is to overcome the normal material dispersion exhibited by the otherwise ideal (i.e., low-loss and large-Kerr-coefficient) AlGaN family of materials, as this is a prerequisite for bright-soliton Kerr comb generation. The proposed approach exploits the avoided-crossing phenomenon in coupled waveguides to achieve strong anomalous dispersion in the desired wavelength range. The resulting designs reveal a wide range of dispersion response tunability, which is expected to allow access to the near-UV wavelength regime as well. Numerical simulations of the spatio-temporal evolution of the intra-cavity field under continuous-wave laser pumping confirm that such a structure is capable of generating a broadband blue bright-soliton Kerr frequency comb. The proposed micro-resonator heterostructure is amenable to the current state-of-the-art growth and fabrication methods for AlGaN semiconductors.

Dispersion control of silicon nanophotonic waveguides using sub-wavelength grating metamaterials in near- and mid-IR wavelengths

Controlling the group velocity dispersion of silicon nanophotonic waveguides has been recognized as a key ingredient to enhance the development of various on-chip optical applications. However, the strong wavelength dependence of the dispersion in waveguides implemented on the high index contrast silicon-on-insulator (SOI) platform substantially hinders their wideband operation, which in turn, limits their deployment. In this work, we exploit the potential of non-resonant sub-wavelength grating (SWG) nanostructures to perform a flexible and wideband control of dispersion in SOI waveguides. In particular, we demonstrated that the overall dispersion of the SWG-engineered metamaterial waveguides can be tailored across the transparency window of the SOI platform, keeping easy-to-handle single-etch step manufacturing. The SWG silicon waveguides overcladded by silicon nitride exhibit significant reduction of wavelength dependence of dispersion, yet providing intriguing and customizable synthesis of various attractive dispersion profiles. These include large normal up to low anomalous operation regimes, both of which could make a great promise for plethora of emerging applications in silicon photonics.

Mid-IR supercontinuum generated in low-dispersion Ge-on-Si waveguides pumped by sub-ps pulses

Ge-on-Si is an attractive material platform for mid-IR broadband sources on a chip because of its wide transparency window, high Kerr nonlinearity and CMOS compatibility. We present a low-loss Ge-on-Si waveguide with flat and low dispersion from 3 to 11 µm, which enables a coherent supercontinuum from 2 to 12 µm, generated using a sub-ps pulsed pump. We show that 700-fs pump pulses with a low peak power of 400 W are needed to generate such a wide supercontinuum, and the waveguide length is around 5.35 mm.

Fabrication-friendly subwavelength-structure-assisted waveguide for dispersion engineering

A subwavelength structure deposited on top of a silicon strip is utilized as a novel tool for dispersion engineering. The equivalent refractive index of the subwavelength structure can be tailored through adjusting its period and duty cycle. As finding suitable materials with both appropriate refractive index and fabrication compatibility is one of the main difficulties in dispersion engineering, the possibility of refractive index engineering is the most significant advantage of the proposed waveguide. It can be beneficial for controlling the properties of the fundamental quasi-TM mode and consequently its dispersion characteristics without any concern about material compatibility. Utilizing this waveguide geometry, a wide and flattened low-dispersion bandwidth can be achieved. Moreover, high anomalous and normal dispersion is realizable without any degradation in dispersion flatness over bandwidth. Therefore, the proposed waveguide structure is promising for dispersion tailoring in both linear and nonlinear applications.