Small Mode Area Research Articles

Ultra-low threshold deep ultraviolet generation in a hollow-core fiber.

Tunable ultrashort pulses in the ultraviolet spectral region are in great demand for a wide range of applications, including spectroscopy and pump-probe experiments. While laser sources capable of producing such pulses exist, they are typically very complex. Notably, resonant dispersive-wave (RDW) emission has emerged as a simple technique for generating such pulses. However, the required pulse energy used to drive the RDW emission, so far, is mostly at the microjoule level, requiring complicated and expensive pump sources. Here, we present our work on lowering the pump energy threshold for generating tuneable deep ultraviolet pulses to the level of tens of nanojoules. We fabricated a record small-core antiresonant fiber with a hollow-core diameter of just 6 μm. When filled with argon, the small mode area enables higher-order soliton propagation and deep ultraviolet (220 to 270 nm) RDW emission from 36 fs pump pulses at 515 nm with the lowest pump energy reported to date (tens of nanojoules). This approach will allow the use of low-cost and compact laser oscillators to drive nonlinear optics in gas-filled fibers for the first time to our knowledge.

Numerical Investigation of Optical Properties in Carbon Tetrachloride-Filled Photonic Crystal Fibers

In this work, we design circular lattice photonic crystal fibers using Lumerical Mode Solution software, its hollow core filled with carbon tetrachloride. Optical properties including effective refractive index, chromatic dispersion, nonlinearity, and fiber loss were investigated numerically in detail based on solving Maxwell's wave equations. The small effective mode areas of only a few µm2, and confinement loss as low as 14.799 dB/m at specific pump wavelengths were obtained. It was found that near-zero ultra-flattened chromatic dispersion with fluctuations of ±0.44 ps/nm.km spans from 1300 to 1830 nm. Three photonic crystal fibers with optimal structure and features were suggested for broadband supercontinuum generation orientation.

Design of a high-sensitivity polymer double-slot waveguide sensor for point-of-care biomedical applications

Silicon photonics is a rapidly developing field that offers cost-effective biosensors with improved sensitivity and the potential for interconnecting with other electronic devices for instant disease diagnosis. The Mach Zehnder interferometer architecture (MZI) is a key technology for biosensors, as it detects changes in refractive index (RI) caused by the presence of biomolecules. In this study, a silicon-polymer double-slot waveguide-based MZI was designed, with a small mode area and a large evanescent field to enhance light-analyte interaction. The waveguide was optimized by converting a normal slot waveguide into a double-slot waveguide with varying slot widths. In transmission spectrum, the wavelength shift was measured for both normal and disease samples. Additionally, the loss at a specific wavelength was analyzed to understand the impact of the biomolecule on the sensor performance. The results show that this sensor has a high sensitivity of 2.39 X10^5 nm/RIU, making it a promising candidate for biosensing applications.

Design of Single-Mode Single-Polarization Large-Mode-Area Multicore Fibers.

In laser science and industry, considerable effort is directed toward designing fibers for fiber laser and fiber amplifier applications, each of which offers a particular advantage over the others. Evanescently coupled multicore fibers, however, have been studied less extensively due to the relatively small mode area in the single-mode regime. Here, by proposing a new structure with stress-applying parts in a 37-core fiber and optimizing this structure through a comprehensive framework, we present 21 solutions characterized by large-mode-area and high beam quality in the single-mode, single-polarization regime. Different fiber designs are optimal for different output parameters. In one design, the mode area can significantly increase to above 880 μm2, which is comparable with that of photonic-crystal fibers. Moreover, besides the single-mode operation, the beam quality factor (M2 factor) of the fundamental mode is considered an output parameter in the bent state and is improved up to 1.05 in another design. A comprehensive tolerance analysis is then performed to assess the performance of the designs under deviations from normal conditions. Moreover, in spite of the shifts in the loss of modes, the proposed high beam quality LMA fibers maintain single-polarization, single-mode operation across a wide range of core pitches, bending orientation angles, and bending radius deviations. Our results highlight the potential of multicore fibers for the efficient operation of fiber lasers and amplifiers.

Highly confined low-loss light transmission in linear array-enabled hybrid plasmonic waveguides

Deep subwavelength highly confined and long-range optical propagation is vital for photonics integration. However, the performance of the guided mode could be improved by the trade-off between light confinement and loss. Here, we demonstrate a high-performance hybrid waveguide consisting of a high-index nanowire separated from a linear array by a low-index dielectric gap. The array significantly achieves the plasmonic platform optimization based on a linear combination of few-layer graphene (FLG) and hexagonal boron nitride (hBN) layers. Through the hybridization of graphene plasmon polaritons and hyperbolic phonon polaritons mode, the resulting hybrid waveguide shows at least double times larger propagation distance and smaller mode area than the multilayer waveguide. Further, modulated by altering material configuration and geometric effects, the mode properties reveal that it is more flexible to adjust the optical transmission, along with a strong deep-subwavelength mode with low loss. Because of highly confined low-loss propagation, the hybrid waveguide is expected to be an excellent building block for various mid-infrared photonic integrated circuits. The present structure also has the potential to be extended to other FLGs, like magic-angle twisted bilayer graphene and trilayer graphene/hBN moiré superlattice.

Simplified design of a hybrid plasmonic waveguide to maximize propagation length

We design a waveguide for maximizing propagation length using the COMSOL environment based on the finite element method. The proposed design aims to achieve a high propagation distance and a constrained normalized TM mode. The range of propagation length (5−12)mm and the normalized TM mode area range can be achieved in the range of gap thickness (2−17)nm, silver thickness (1−6)nm, and arc radius 50nm. A trade-off will be between having a small mode area and a long propagation length in a successful hybrid plasmonic waveguide (HPW) design. The findings show that, with such a small mode area and long propagation length of SPPs at the wavelength 1.55μm, the suggested waveguide may avoid the unavoidable loss-confinement trade-off, which is desirable for integrated circuits. Balancing the arc radius and gap thickness at the widely used wavelength of 1.55 µm typically yields the required results. A substantial confinement coefficient has been achieved, essential in the HPW field.

Supercontinuum generation in second-order topological fibers composed of hexagonal chalcogenide photonic crystals

Topological photonics has attracted much attention in the community of photonics due to the fundamental and practical significances. Recently, topological photonic crystal fibers (PCFs) have been proposed as a new category of photonic topological systems, while the investigations in this direction are still at an early stage, only dealing with linear effects. This work theoretically demonstrates supercontinuum (SC) generation with waveguiding corner modes in topological fibers composed of hexagonal holey photonic crystals of chalcogenide material. The proposed topological PCF has high nonlinearity due to the small mode area and high material nonlinearity, enabling octave-spanning SC generation with low pump energy at the central wavelength of 1550 nm. The topological PCFs exhibit the dispersion characteristics and SC spectra which are immune to the structural defects, contrary to the non-topological ones. The results presented in this work pave the way towards nonlinear fiber optics with structural defects-immune performances.

Investigations on Mode Analysis of Distributed Bragg Reflector based Hybrid Plasmonic Terahertz Waveguide

Terahertz (THz) technology is emerging field and is fascinating to the researchers who are working with photonics integrated circuits. With technological advancements and developments for various applications, a novel distributed Bragg reflector (DBR) based hybrid plasmonic terahertz waveguide (HPTW) is proposed and designed to operate at 3 THz with Graphene, Gallium Arsenide (GaAs), High-density Polyethylene (HDPE) and Aluminium Gallium Arsenide (AlGaAs) is discussed in this work. To bring the confinement of the THz wave in a small mode area efficiently, a stripe is introduced between graphene and DBR in HDPE. The normalized mode area is decreased to about 3.733 x 10-3 µm2 by reducing the stripe’s width from 20 to 14 µm with a constant of stripe’s height of 1 µm. The Figure of Merit (FOM) and Propagation Length (PL) increases inversely in correspondence with the stripe’s width from 1.4 to 4.8 and 6 to 16.8 µm respectively.

(INVITED)Dispersion-shifted tellurite fibers for nonlinear frequency conversion

We report a detailed numerical investigation of step-index tellurite fiber properties based on the careful experimental characterization of refractive indices for two tellurite-glass systems employed in fiber manufacturing. More specifically, our study focuses on two typical step-index configurations, namely weak and strong index differences between core and cladding glasses. We reveal that a wide range of dispersion-shifted features for tellurite fibers can be obtained in the 2–3 μm range combined with small or large effective mode areas. Our work also outlines the potential application of such dispersion-engineered fibers for nonlinear wavelength converters between near- and mid-infrared regions.

Phase modulators in hybrid silicon and lithium niobate thin films

The hybridization of mono-crystalline silicon and lithium niobate thin films (Si-LNOI) can combine the physical properties of Si and the optical properties of LN, and it has emerged as a new material platform for integrated photonics. In this paper, phase modulators in Si-LNOI were demonstrated. First, the phase modulator was designed. According to the simulation, the Si loading strip waveguide had a small mode area due to the large refractive index of Si. This allowed a small electrode gap that resulted in enhancement of the overlap of the optical field and the electrostatic field, and the VπL of the phase modulator could be as small as 3.3 V·cm. Second, phase modulators with a Si loading strip waveguide with a top width of 0.5 μm were fabricated by plasma etching, and the VπL of the phase modulator was measured to be 4.74 V·cm. This study provides useful information for the devices in the Si-LNOI.

On-chip ultrafast pulse generation based on graphene-silicon hybrid waveguides

On-chip ultrafast mode-locking lasers are basic building blocks for the realization of a chip-based optical frequency comb. In this paper, an ultrafast saturable absorber made up of a graphene pad on top of a silicon waveguide is applied to implement an ultrafast pulse laser. Benefiting from the small mode area of the graphene/silicon hybrid waveguide, the saturable pulse energy is reduced by two orders of magnitude compared with the fiber. A mode-locked pulse with a duration of 542 fs and a repetition rate of 54.37 MHz is realized. Pump–probe measurement shows that the carrier relaxation process of free carrier recombination with atomic-thin graphene/silicon junctions is three orders of magnitude faster than silicon, which plays a fundamental role in pulse narrowing. The chip-scale silicon ultrafast laser lays a foundation for a new class of nonlinear devices, in which a combination with multiple functional silicon photonic circuits enables efficient nonlinear interaction at the micrometer scale and less than 1 W of power consumption.

Design of a photonic crystal fiber for optical communications application

The desire for photonic crystal fibers (PCF) with small mode areas that can be used in nonlinear optics and optical communications is a never-ending and a continuously evolving research quest. By deliberately varying the diameters of air-holes and spacing between holes, a PCF with possible application in optical communications and nonlinear optics is proposed. The zero-dispersion wavelength (ZDW) is easily tailored to be located around 1.55μm. A design of a PCF with high nonlinearity, reduced dispersion and reduced loss due to improper confinement of mode is investigated. The proposed PCF is designed and demonstrated through the commercially available FemSIM software which employs the full-vectorial finite element method. Among the properties demonstrated and analyzed are the; ZDW which is shown to be around 1.55μm, an effective mode area of 1.7256μm2 at 1.55μm are obtained.

A hybrid plasmonic terahertz waveguide with ridge structure base on Bulk-Dirac-semimetal

We propose a hybrid plasmonic waveguide (HPW) base on Bulk-Dirac-semimetal (BDS). By inserting the dual-ridge (DR) structure into the low-index gap of the conventional HPW, the terahertz wave can be confined efficiently and then a small mode area can be brought. Based on the finite element method (FEM), the results of the proposed DR HPW are compared with that of the single-ridge hybrid plasmonic waveguide (SR HPW), which show the advanced performance of the proposed design. Besides, the BDS plasmon particularly differs from plasmon of metal in that the propagation loss can be tuned by adjusting the Fermi energy level. With a reasonable Fermi energy level, over 10 times improvement of the Figure of merit (FoM) can be obtained.

Efficient ion-photon qubit SWAP gate in realistic ion cavity-QED systems without strong coupling.

We present a scheme for deterministic ion-photon qubit exchange, namely a SWAP gate, based on realistic cavity-QED systems with 171Yb+, 40Ca+ and 138Ba+ ions. The gate can also serve as a single-photon quantum memory, in which an outgoing photon heralds the successful arrival of the incoming photonic qubit. Although strong coupling, namely having the single-photon Rabi frequency be the fastest rate in the system, is often assumed essential, this gate (similarly to the Duan-Kimble C-phase gate) requires only Purcell enhancement, i.e. high single-atom cooperativity. Accordingly, it does not require small mode volume cavities, which are challenging to incorporate with ions due to the difficulty of trapping them close to dielectric surfaces. Instead, larger cavities, potentially more compatible with the trap apparatus, are sufficient, as long as their numerical aperture is high enough to maintain small mode area at the ion's position. We define the optimal parameters for the gate's operation and simulate the expected fidelities and efficiencies, demonstrating that efficient photon-ion qubit exchange, a valuable building block for scalable quantum computation, is practically attainable with current experimental capabilities.

Optical Wave Guiding and Spectral Properties of Micro/Nanofibers Used for Quantum Sensing and Quantum Light Generation

Subwavelength optical micro/nanofibers have been widely used as basic building blocks in the field of quantum sensing and quantum light source by virtue of their properties which include pronounced evanescent field, large surface area, and small optical mode area. This paper presents theoretical studies on the propagation properties of the guided optical wave and the spectral properties of entangled photons from spontaneous four-wave mixing in micro/nanofibers. We first analyze numerically single-mode propagation, field distribution, fraction of power, and group-velocity-dispersions by solving Maxwell’s equations with boundary conditions in cylindrical coordinates. Then, optical wave guiding properties of micro/nanofibers are applied to estimate the spectral properties such as central wavelengths and bandwidths of the created photons via spontaneous four-wave mixing that can be tailored by controlling diameter and length of micro/nanofibers. This theoretical work provides useful guidelines to design micro/nanofiber-based quantum sensing and quantum light sources for quantum technologies.

Generation of broadband correlated photon-pairs in short thin-film lithium-niobate waveguides.

An efficient source of quantum-correlated photon-pairs that is integrable with existing silicon-electronics fabrication techniques is desirable for use in quantum photonic integrated circuits. Here we demonstrate signal-idler photon pairs with high coincidence-to-accidental count ratios of over 103 on a coarse wavelength-division-multiplexing grid that spans 140 nm by using a 300-μm-long poled region in a thin-film periodically-poled lithium-niobate ridge waveguide bonded to silicon. The pairs are generated via spontaneous parametric downconversion pumped by a continuous-wave tunable laser source. The small mode area of the waveguide allows for efficient interaction in a short length of the waveguide and, as a result, permits photon-pair generation over a broad range of signal-idler wavelengths.

Large on-chip Brillouin net amplification in silicon-based nano-photonics

Recent developments in on-chip forward Brillouin scattering open up potential applications such as RF photonic signal processing, on-chip Brillouin amplification, and on-chip Brillouin lasers. The stimulated Brillouin scattering gain coefficients become significant with a small optical mode area, and the Brillouin net amplification has been believed to be strong with a small mode area, too. However, here, we present a theoretical study of higher net amplification with a large optical mode area than that with a small mode area and explain this counter-intuitive phenomenon by examining the contribution of various optical forces to Brillouin gain coefficients for various optical waveguide dimensions. The simulation results show that a waveguide with large optical waveguide dimensions can yield significant net amplification by high Brillouin gain coefficients and low optical losses at high pump power even if the Brillouin gain coefficients are lower than that with a small waveguide dimension. Therefore, it is necessary to optimize the optical waveguide dimensions to achieve maximum net amplification for the development of Brillouin lasers and amplifiers in silicon-based nanophotonics.

Solid-core bismuth–tellurite glass fiber with low propagation loss and high nonlinearity

We have fabricated a single-mode bismuth-doped tellurite solid-core glass fiber using a multi-stage fiber fabrication method. Owing to the optimization of the glass composition and rod-in-tube fiber drawing technology, this fiber features a small mode area of 24.63 μm2 and low propagation loss of 0.9 dB/m at 1550 nm. By using the continuous-wave self-phase modulation method, the effective nonlinear parameter γ of this bismuth-doped tellurite glass fiber is estimated to be 417 W−1 km−1 at 1550 nm, which is the highest to date among all known solid-core tellurite glass fibers. Thus, we have developed an unprecedented high nonlinearity and low loss tellurite glass fiber. © 2019 Elsevier B.V. All rights reserved.

Guiding and controlling light at nanoscale in field effect transistor

Metal–oxide–semiconductor field-effect transistor (MOSFET) and tunnel field-effect transistor (TFET) are proposed to guide and control the light at nanoscale utilizing the hybridization of plasmonic and optical modes. The hybrid plasmonic (HP) mode is confined in the dielectric sandwiched between the metal gate and semiconductor channel, which results from the coupling of surface plasmonic polariton mode at the metal–dielectric interface with the optical mode in the dielectric. Conventional conductivity modulations in the channel through gate and drain–source voltages are utilized to control the guided light. A long propagation length of 74 μm and a very small mode area of $$\lambda ^2/96$$ are reported for field-effect transistor at an operating wavelength of 1550 nm which are useful to realize low loss and compact optoelectronic devices. The charge-carrier dynamics along with the plasma dispersion effect in the silicon channel, through voltages applied on the gate and source–drain, result in the optical phase modulation in MOSFET and TFET. Phase shift of $$\pi$$ radian at a length of 1.2 mm and 0.21 mm is obtained in MOSFET and TFET, respectively. The proposed concept has the potential to enable multifunctionality of the mature field effect transistors.

Low-loss TeO2-coated Si3N4 waveguides for application in photonic integrated circuits.

We report on high-quality tellurium oxide waveguides integrated on a low-loss silicon nitride wafer-scale platform. The waveguides consist of silicon nitride strip features, which are fabricated using a standard foundry process and a tellurium oxide coating layer that is deposited in a single post-processing step. We show that by adjusting the Si3N4 strip height and width and TeO2 layer thickness, a small mode area, small bend radius and high optical intensity overlap with the TeO2 can be obtained. We investigate transmission at 635, 980, 1310, 1550 and 2000 nm wavelengths in paperclip waveguide structures and obtain low propagation losses down to 0.6 dB/cm at 2000 nm. These results illustrate the potential for compact linear, nonlinear and active tellurite glass devices in silicon nitride photonic integrated circuits operating from the visible to mid-infrared.