Surface plasmon polariton enhanced by optical parametric amplification in nonlinear hybrid waveguide

Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces

Lithium niobate (LN) has been widely used for second-harmonic generation (SHG) from bulk crystals. Recent studies have reported improved SHG efficiency in LN micro-ring resonators and hybrid waveguiding structures, as well as in LN nanostructures supporting anapole modes and plasmon-assisted dipole resonances. Here we numerically demonstrate that high Q-factor resonances associated with symmetry-protected bound states in the continuum can lead to highly efficient frequency doubling in LN metasurfaces. Simulations show that the radiative Q-factor and on-resonance field enhancement factor observed in the metasurface are closely dependent on the asymmetric parameter α of the system. Furthermore, high Q-factor resonances boost the SH conversion process in the LN nanostructures. In particular, for a LN metasurface with a Q-factor of ∼8×10⁴, a 0.49% peak SH conversion efficiency is achieved at a pump intensity of 3.3kW/cm². This suggests that such high Q-factor LN metasurfaces may be good candidates for practical blue-ultraviolet light sources. Our work provides insight into the possible implementation of metadevices based on nanoengineering of conventional nonlinear crystals.

Recent progress, challenges, and prospects in emerging group-VIA Xenes: synthesis, properties and novel applications

The discovery of graphene (G) attracted considerable attention to the study of other novel two-dimensional materials (2DMs), which is identified as modern day "alchemy" since researchers are converting the majority of promising periodic table elements into 2DMs. Among the family of 2DMs, the newly invented monoelemental, atomically thin 2DMs of groups IIIA-VIA, called "Xenes" (where, X = IIIA-VIA group elements, and "ene" is the Latin word for nanosheets (NSs)), are a very active area of research for the fabrication of future nanodevices with high speed, low cost and elevated efficiency. Currently, any novel structure of 2DMs from the typical Xenes will probably be applicable in electronic technology. Analysis of their possible highly sensitive synthesis and characterization present opportunities for theoretically examining proposed 2D-Xenes with atomic precision in ideal circumstances, thus providing theoretical predictions for experimental support. Several theoretically predicted and experimentally synthesized 2D-Xene materials have been investigated for the group-VIA elements (tellurene (2D-Te), and selenene (2D-Se)), which are similar to topological insulators (TIs), thus potentially rendering them suitable materials for application in upcoming nanodevices. Although the investigation and device application of these materials are still in their infancy, theoretical studies and a few experiment-based investigations have proven that they are complementary to conventional (i.e., layered bulk-derived) 2DMs. This review focuses on the synthesis of novel group-VIA Xenes (2D-Te and 2D-Se) and summarizes the current development in understanding their basic properties, with the current advancement in signifying device applications. Lastly, the future research prospects, further advanced applications and associated shortcomings of the group-VIA Xenes are summarized and highlighted.

Theoretical study of bicharacteristic waveguide for fundamental-mode phase-matched SHG from MIR to NIR

In this paper, a bicharacteristic waveguide (BW) is proposed for fundamental-mode phase-matched second harmonic generation (SHG) from mid-infrared (MIR) to near-infrared (NIR). The required phase matching condition (PMC) is satisfied between the fundamental plasmonic mode at 3100 nm and the photonic mode at 1550 nm. With 1 W pump power, the SHG conversion efficiency of 4.173% can be obtained in 90.3 μm length waveguide. Moreover, the SHG conversion can be enhanced by using a microring resonator (MRR). By optimizing the MRR, the SHG conversion efficiency is increased to 8.30%. The proposed waveguide can also provide a promising platform for upconversion detection. By using an on-chip cascaded configuration, a gas sensor with the capability of MIR absorption and NIR detection is proposed. It is found that the detection limit (DL) can reach 1.04 nmol/L with 100 mW pump power, which shows significant enhancement compared with direct MIR absorption and detection.

Adjustable Propagation Length Enhancement of the Surface Plasmon Polariton Wave via Phase Sensitive Optical Parametric Amplification

The adjustable propagation length enhancement of the surface plasmon polariton (SPP) mode under the effects of the initial relative phase (ψ₀) between interacting waves in difference frequency generation (DFG) based optical parametric amplification (OPA) are numerically considered. The waveguide is a silver coated PPLN planar waveguide. Obtained results indicate ultra long propagation length for the SPP mode could be achieved via manipulation of ψ₀ in exact quasi phase matching (QPM) case up to 30 mm for initial pump intensity about 66 MW/cm for degenerate DFG (dDFG). For chirped QPM by mitigating the high depletion of the pump intensity, it is possible to enhance the SPP propagation length up to 43 mm for initial pump intensity about 135 MW/cm. In this case ψ₀ does not affect the SPP propagation length except around a narrow range of unsuitable phases. The unsuitable phase is [Formula: see text] for exact QPM but is pump dependent for chirped QPM case. Using this unsuitable phase is the key parameter to the SPP propagation length enhancement via controlling ψ₀. In this case with a high pump intensity, the pump and the SPP modes interact at longer distances which leads to the SPP propagation length enhancement.

Second harmonic generation in metal-LiNbO3-metal and LiNbO3 hybrid-plasmonic waveguides

Nanoplasmonic waveguides based on lithium niobate (LN) are shown to provide the light-matter interaction required for next-generation developments in nonlinear frequency-conversion nanostructures. Here, we numerically investigate second harmonic generation of a 1550 nm, 100 fs pulse in metal-LN-metal (MLNM) nanoplasmonic and LN hybrid-plasmonic (LNHP) waveguides. In comparison to a photonic LN waveguide, a 2.1 µm-long LNHP waveguide exhibits a conversion efficiency improvement of 11 times, whereas a 20 µm-long MLNM nanoplasmonic waveguide is shown to have a conversion efficiency of 1.1 × 10^-4. The MLNM nanoplasmonic and LNHP waveguides have the potential to operate as sources of optical radiation for on-chip photonic systems.

Ultralong propagation of a surface plasmon polariton wave within an ultrawide bandwidth via phase-sensitive optical parametric amplification

The propagation length enhancement of surface plasmon polariton (SPP) waves could lead to practical applications. This Letter proposes the numerically verified phase-sensitive nonlinear χ⁽²⁾-based optical parametric amplification (OPA) for ultralong propagation of a SPP wave within an ultrawide bandwidth. The strong nonlinear interaction between the SPP mode and the hybrid guided mode, which limits the length enhancement, is mitigated in a silver-coated linearly chirped periodically poled lithium niobate planar waveguide via slowly phase-matched OPA. Obtained results indicate an ultralong propagation length for a SPP mode of about 4 cm when a 135 MW/cm pump intensity is launched. The acceptance bandwidth of the amplified SPP shows its dependency on the pump intensity; for a pump intensity range between 70 and 135 MW/cm, the acceptance bandwidth is still ultrawide, varying from 28 to 18 nm, respectively.

Imaging the dark emission of spasers

Spasers are a new class of laser devices with cavity sizes free from optical diffraction limit. They are an emergent tool for various applications, including biochemical sensing, superresolution imaging, and on-chip optical communication. According to its original definition, a spaser is a coherent surface plasmon amplifier that does not necessarily generate a radiative photon output. However, to date, spasers have only been studied with scattered photons, and their intrinsic surface plasmon emission is a "dark" emission that is yet to be revealed because of its evanescent nature. We directly image the surface plasmon emission of spasers in spatial, momentum, and frequency spaces simultaneously. We demonstrate a nanowire spaser with a coupling efficiency to plasmonic modes of 74%. This coupling efficiency can approach 100% in theory when the diameter of the nanowire becomes smaller than 50 nm. Our results provide clear evidence of the surface plasmon amplifier nature of spasers and will pave the way for their various applications.

Efficient second harmonic generation in internal asymmetric plasmonic slot waveguide

We theoretically propose an internal asymmetric plasmonic slot waveguide (IAPSW), containing two different materials in the slot region. The IAPSW is used for second harmonic generation (SHG) at a wavelength of 1.55 μm. The required phase matching condition is satisfied between the 0th-order mode at the fundamental frequency and the 1st-order mode at the second harmonic frequency. By choosing appropriate slot geometry and materials, the mode field distribution is engineered to enhance the nonlinear coupling coefficient for SHG. With an 11 μm long IAPSW, a conversion efficiency of 24% (1.8 × 10⁵ W^-1cm^-2 normalized conversion efficiency) is predicted. Furthermore, the SHG efficiency is more pronounced in IAPSW with thinner slot.

Tuning the hybridization of plasmonic and coupled dielectric nanowire modes for high-performance optical waveguiding at sub-diffraction-limited scale

We report the realization of low-loss optical waveguiding at telecommunication wavelength by exploiting the hybridization of photonic modes guided by coupled all-dielectric nanowires and plasmon waves at planar metal-dielectric interfaces. The characteristics of the hybrid plasmon polaritons, which are yielded by the coupling between two types of guided modes, can be readily tuned through engineering key structural parameters of the coupled nanowires and their distances to the metallic surfaces. In addition to exhibiting significantly lower attenuations for similar degrees of confinement as compared to the conventional hybrid waves in single-dielectric-nanowire-based waveguides, these hybridized plasmonic modes are also capable of enabling reduced waveguide crosstalk for comparable propagation distances. Being compatible with semiconductor fabrication techniques, the proposed guiding schemes could be promising candidates for various integrated photonic devices and may lead to potential applications in a wide variety of related areas.

Third harmonic generation from mid-IR to near-IR regions in a phase-matched silicon-silicon-nanocrystal hybrid plasmonic waveguide

The conversion efficiency of third harmonic generation (THG) from mid-IR (3600 nm) to near-IR (1200 nm) regions in a silicon-silicon-nanocrystal hybrid plasmonic waveguide (SSHPW) was calculated. The required modal phase-matching condition (PMC) between the 0-th mode at fundamental wave (FW) and the 2-nd mode at third harmonic (TH) is achieved by carefully designing the waveguide geometry. Benefiting from the hybridized surface plasmon polariton (SPP) nature of the two guided modes, the SSHPW is capable of achieving both high THG nonlinear coefficient |I₆| and reasonable linear propagation loss, thereby resulting in large figure-of-merits (FOMs) for both FW and TH. According to our simulation, THG conversion efficiency up to 0.823% is achieved at 62.9 ����m SSHPW with pump power of 1 W.

Highly efficient phase-matched second harmonic generation using an asymmetric plasmonic slot waveguide configuration in hybrid polymer-silicon photonics

We theoretically investigate the possible increase of the second harmonic generation (SHG) efficiency in silicon compatible waveguides by considering an asymmetrical plasmonic slot waveguide geometry and a χ((2)) nonlinear polymer infiltrating the slot. The needed phase matching condition is satisfied between the fundamental waveguide mode at the fundamental frequency (FF) and second-order waveguide mode at the second harmonic frequency (SHF) by an appropriate design of the waveguide opto-geometrical parameters. The SHG signal generated in our starting waveguide is three orders of magnitude higher than those previously reported for a FF corresponding to λ = 1550 nm. Then, the SHG performance was further improved by increasing the asymmetry of the structure where nonlinear coupling coefficients as large as 292 psm(-1)W(-1/2) are predicted. The device length is shorter than 20 µm and the normalized SHG conversion efficiency comes up to more than 1 × 10(5) W(-1)cm(-2).

Efficient second harmonic generation from mid-infrared to near-infrared regions in silicon-organic hybrid plasmonic waveguides with small fabrication-error sensitivity and a large bandwidth

We theoretically investigate the quadratic nonlinear property of a silicon-organic hybrid plasmonic waveguide with a thin polymer layer deposited on top of a silicon slab and covered by a metal cap. Due to the hybridization property of the waveguide modes, efficient phase-matched second harmonic generation (SHG) from mid-infrared (IR) (~3.1 μm) to near-IR (~1.55 μm) wavelengths are achieved with a small fabrication-error sensitivity (225 nm ≤ tolerated waveguide width ≤ 378 nm) and a large bandwidth (Δλ=100 nm). The SHG yield is as large as 8.8% for a pumping power of 100 mW.

Phase regeneration of phase-shift keying signals in highly nonlinear hybrid plasmonic waveguides

Phase regeneration of phase-shift keying signals is theoretically proposed, we believe for the first time, based on the efficient optical parametric amplification (OPA) process in a highly nonlinear symmetric hybrid plasmonic waveguide. This optimized stacked waveguide with nonlinear organic materials has a relatively low loss of about 0.005 dB/μm and an effective nonlinear OPA coupling coefficient up to 60 ps/m/W(1/2). The phase-recovery process was achieved in this waveguide within a length as short as 150 μm.

Design of ultra-broadband terahertz polymer waveguide emitters for telecom wavelengths using coupled mode theory

We use coupled mode theory, adequately incorporating optical losses, to model ultra-broadband terahertz (THz) waveguide emitters (0.1-20 THz) based on difference frequency generation of femtosecond infrared (IR) optical pulses. We apply the model to a generic, symmetric, five-layer, metal/cladding/core waveguide structure using transfer matrix theory. We provide a design strategy for an efficient ultra-broadband THz emitter and apply it to polymer waveguides with a nonlinear core composed of a poled guest-host electro-optic polymer composite and pumped by a pulsed fiber laser system operating at 1567 nm. The predicted bandwidths are greater than 15 THz and we find a high conversion efficiency of 1.2 × 10(-4) W(-1) by balancing both the modal phase-matching and effective mode attenuation.

Second harmonic generation by modal phase matching involving optical and plasmonic modes

The feasibility of a modal phase matching scheme between optical modes and surface plasmonic modes is demonstrated: in fact, the high effective index of a plasmonic mode allows us to obtain phase matching even in semiconductors showing a large dispersion between fundamental and second harmonic wavelengths. We design a realistic device to obtain Type-II second harmonic generation in AlGaAs-based waveguides; whereas one of the two pumps is carried by a plasmonic mode, the generated second harmonic signal is guided inside the AlGaAs multilayer, and hence it is not hampered by high propagation losses.

Efficient second-harmonic generation in nonlinear plasmonic waveguide

We theoretically studied a nonlinear optical process in a hybrid plasmonic waveguide composed of a nonlinear dielectric waveguide and a metal film with a separation of a thin air gap. Owing to the hybridization effect of guided mode and surface plasmon polariton mode, this particular waveguide is able to confine the optical-field in a deep subwavelength scale together with low propagation loss. Based on this, efficient second-harmonic generations (SHG) were revealed at the fundamental wavelength of λ=1.55 μm with good field confinement. The SHG efficiency, as well as the coupling coefficient and mode area, were analyzed and discussed in detail with respect to the structural parameters.