Low-loss Devices Research Articles

High-performance optical phased array for LiDARs demonstrated by monolithic integration of polymer and SiN waveguides.

Optical phased array (OPA) beam scanners for light detection and ranging (LiDAR) are proposed by integrating polymer waveguides with superior thermo-optic effect and silicon nitride (SiN) waveguides exhibiting strong modal confinement along with high optical power capacity. A low connection loss of only 0.15 dB between the polymer and SiN waveguides was achieved in this work, enabling a low-loss OPA device. The polymer-SiN monolithic OPA demonstrates not only high optical throughput but also efficient beamforming and stable beam scanning. This novel integrative approach highlights the potential of leveraging heterogeneous photonic materials to develop advanced photonic integrated circuits with superior performance.

High-Performance Monolithic Integration of III-V QD Lasers on Si Substrates

With the development of digital processes, the speed and efficiency of information exchange has increased to a great extent. Silicon-based photonics allows for large-scale photonic integration through CMOS manufacturing processes, the advantages of which lie in the low cost, low energy consumption and high quality of such processes. Currently, silicon-based monolithic integrated quantum dot lasers have achieved lifetimes comparable to those of heterogeneous integrated lasers and are even available for commercial applications. The silicon photonic platform also offers low-loss passive devices, as well as high-speed optical modulators and photodetectors. However, device performance suffers due to differences in physical properties between group III-V lasers and substrate materials, an important factor limiting the development of silicon-based monolithic integrated quantum dot lasers. This paper reviews the respective characteristics of silicon-based monolithic and heterogeneous integration. The advantages of monolithically integrated III-V quantum dot lasers over heterogeneous integration are highlighted, as well as the challenges, solutions and recent developments.

One-way propagation of topologically non-conventional bulk transverse elastic waves in infinite and finite superlattices: Application to low-loss acoustic wave devices

Static superlattices that do not break time-reversal symmetry can support robust topologically protected elastic waves with non-zero amplitude in the forward propagating direction but zero amplitude in the opposite direction. We form a prototypical acoustic wave device by sandwiching a finite superlattice that supports one-way propagating waves between input and detector layers. Compared to conventional elastic waves, topologically protected waves provide a significant benefit for reducing the return loss of the prototypical device. Superlattices supporting topologically protected acoustic waves provide attractive and disruptive solutions for designing the next-generation of low-loss acoustic wave devices for telecommunication or sensing.

Label-Free Detection of Ultra-Low Weight Molecules Based on Fiber Optic Sensors With Low Loss Dielectric Nanostructures

Fiber optic (FO) sensors have great potential for low-cost, microsample testing and compact medical diagnostics. In this work, a FO sensor based on dielectric nanostructure is proposed and demonstrated based on the unique characteristics of dielectric materials, such as low ohmic loss and good biocompatibility. Specifically, we fabricate a SPR-FO sensor by inserting a sleeve with a large-scale, highly ordered dielectric nanodisk array at the end of the fiber probe. And we systematically investigate the sensing performance of the FO sensor including bulk and surface sensitivities. It is found that due to very low absorption losses, the FO sensor based on dielectric nanodisk exhibits a narrow resonance, thus leading to a sensing figure-of-merit almost twice larger than that of based on the metal plasmonic nanodisk. Also, the FO sensor shows a linear response to adsorbate layer thickness changes, which is clearly advantageous for quantitative analysis. Due to the highly localized electromagnetic fields, it can detect small molecules with a concentration of 1 μM, in which a molecular layer with thickness less than 1 nm can produce resonance changes as large as 6 nm. Our work could open a new opportunity for future bio-compatible, robust and low-loss medical diagnostic devices.

Evolutionary selection growth of silver films for low-loss nanophotonic devices

When talking about nanophotonics, quantum computing and sensing high-quality factor plasmonic devices are playing crucial role. In most cases for making such devices one has to use non-lattice matched or transparent amorphous substrates. Plasmonic devices quality factor is mainly determined by ohmic losses, scattering losses at grain boundaries, and in-plane plasmonic scattering losses of a metal – substrate system. We demonstrate here the e-beam evaporation method to deposit ultralow-loss silver thin films on transparent lattice-mismatched substrates. This process corresponds to evolutionary selection growth mode. The key feature of our approach is a precise control during the whole process consisting of film deposition on a cold substrate, self-crystallization and subsequent annealing for stress relaxation. In particular, the stress relaxation leads to further grains growth. We are able to deposit 100 nm thick polycrystalline silver films with micrometer-scale grains, low roughness and ultralow optical losses. Finally, we show ultrahigh-quality factor plasmonic silver nanostructures on transparent lattice-mismatched substrate. This can be of the great interest for high performance and single-molecule optical sensors applications.

Resonant Bonding Dependence of the Phase Change Characteristics of a VO2-Sb2Se Film for Tunable Photonic Devices in the Mid-Infrared Region

Phase change material (PCM) has gained great attraction for being widely used in tunable photonic devices due to its nonvolatility, excellent scalability, high speed, and tunable properties. However, the unclear source of the phase-change characteristics limits PCM for true device-level integrated circuits with high configuration and multifunctional attributes, especially in the mid-infrared (MIR) region. In this paper, the inherent dependences of phase-change characteristics on both the microstructure and bonding environments of the (VO2)x(Sb2Se)100–x film are disclosed for low-loss and nonvolatile tunable photonic devices. (VO2)x(Sb2Se)100–x films possess a tendency that the crystallization temperature, 10 year data-retention temperature, and optical band gap of (VO2)x(Sb2Se)100–x films increase with the increase of VO2 content. The improvement in thermal stability of (VO2)x(Sb2Se)100–x is ascribed to the complex bonding environment consisting of Se–V and Sb–V bonds with the large bonding enthalpy, which disturbs the crystallization. The symbolic value for the contribution of resonant bonding ζ is derived by the real part ε1 and imaginary part ε2 of dielectric functions with a wavelength ranging from 1.7 to 25 μm. With increasing VO2 content, a large structural distortion in the VO2-rich phase leads to a reduction in the level of resonant bonding, which accounts for a decrease in the optical refractive index contrast. These results shed light on intrinsic material science and engineering to the ongoing search for other phase-change materials for tunable photonic devices in the MIR region.

Controllable Tunneling of Single Flux Quanta Mediated by Quantum Phase Slip in Disordered Superconducting Loops

Quantum phase-slip (QPS) is the exact dual to the well-known Josephson effect. Although there are numerous proposals for applications of QPS devices, experimental work to develop these remains in the relatively early stages. Significant barriers to exploiting QPS nanowires for useful technologies still exist, such as establishing robust nanowire fabrication methods that allow coupling to low-loss circuits, and demonstrating control over the QPS process with an experimenter-controlled external bias. Here we report experiments which show that both of these barriers have been overcome. We present measurements at 300 mK of NbN coplanar waveguide (CPW) resonators embedded with nanowires fabricated using a neon focused ion-beam. The internal quality factor exceeds $2\times10^{4}$ -- significantly higher than previously reported in comparable experiments. The resonator frequency tunes periodically with an applied magnetic field, revealing tunnelling of the order parameter that always occurs at half-integer values of the applied flux. In contrast to previous studies of single QPS, the order-parameter tunnelling is shown to be adiabatic, demonstrating improved control over energy dissipation in nanowire QPS circuits. Our results highlight a promising pathway towards realising low-loss nanowire-based QPS devices.

Coexistence of quantum spin and valley hall effect in two-dimensional dielectric photonic crystals

The location and transmission of light is the core of modern photonic integrated device, and the proposal of topological photonics provides a new way of implementing optical manipulation. Topological photonic structures based on the quantum spin hall effect or quantum valley hall effect have the properties of immunity to defects and suppress backscattering, so they play a key role in designing novel low-loss photonic devices. In this work, we design a two-dimensional dielectric photonic crystal with time-reversal symmetry to achieve the coexistence of the quantum spin hall effect and the quantum valley hall effect in a photonic crystal. The design can be likened to an electronic system in which two pairs of Kramers simplex pairs are constructed to achieve a quadruple simplex pair in a photonic crystal. First, based on the method of shrinking and expanding the silicon pillars arranged in the honeycomb structure, the quadruple degeneracy point at the <i>Γ</i> point of the first Brillouin zone is opened, and the corresponding topologically trivial or non-trivial photonic band gap is formed,thereby realizing quantum spin hall effect. The expanded honeycomb lattice evolves into a Kagome structure, and then positive and negative perturbations are added to the Kagome lattice, breaking the spatial inversion symmetry of the Photonic crystal. When mirror symmetry is broken, different chiral photonic crystals can be created,leading the degeneracy point of the non-equivalent valleys <i>K</i> and <i>K'</i> in the Brillouin zone to be opened and a complete band gap to appear, thus realizing the Quantum valley hall effect. In the common band gap, topologically protected edge states are induced by nontrivial valley Chern number at the interface between two photonic crystals with opposite chirality. The numerical calculations show that unidirectional transport and bending-immune topological boundary states can be realized at the interface composed of topologically trivial (non-trivial) and positively (negatively) perturbed photonic crystals. Finally, a four-channel system based on the coexistence of the two effects is designed, The system is a novel electromagnetic wave router that can be selectively controlled by pseudospin degree of freedom or valley degree of freedom. This system provides a potential method for realizing the optical encoding and robust signal transmission, thereby providing greater flexibility for manipulating electromagnetic waves.

Excellent surface plasmon and hot carrier properties of transition metal nitride at different temperatures

Plasmonic materials that exhibit excellent plasmonic behavior at high temperatures would greatly expand existing applications, but the search for such materials is ongoing. Transition metal nitrides with good conductivity and high-temperature stability are promising candidates for temperature-dependent plasmonic applications. Here, we systematically investigate the temperature-dependent surface plasmon and hot carrier properties of TiN and VN. Significantly, high temperatures significantly affect the phonon-assisted intraband transition processes. Meanwhile, TiN and VN exhibit efficient optical absorption and low surface plasmon loss at different temperatures, which are comparable to or even higher than Ag. In particular, the surface plasmon response of TiN extends to visible light, making it an excellent candidate for low-loss and broadband plasmonic applications at high temperatures. In contrast, VN exhibits a much narrower plasmonic window, which is mainly suitable for infrared low-loss plasmonic devices. For hot carriers, the energy and probability distributions of hot holes in TiN and hot electrons in VN exhibit robustness to temperature. Meanwhile, transport properties indicate that the temperature effect on hot holes is significantly higher than that on hot electrons in TiN and VN. Therefore, we expect that the reported results will provide theoretical guidance for the design of next-generation high-temperature plasmonic devices.

All-dielectric Si metamaterials with electromagnetically induced transparency and strong gap-mode electric field enhancement

We designed all-dielectric π-shaped Si metamaterials and numerically demonstrated their electromagnetically induced transparency (EIT) behavior. Each unit of the metamaterial comprises one horizontal Si nanobar and two vertical Si nanobars with feed-nanogaps between them. A narrow EIT peak with a transmittance of 99% and a Q-factor of 2436 is observed due to the destructive interference between the horizontal bright and vertical dark mode resonators. The EIT can be tunable by changing the nanogap width, and the narrowest EIT window with the largest Q-factor is obtained as the nanogap width is 20 nm. Strong electric field enhancement exceeding 104 in magnitude is also realized inside the 20-nm nanogap. Theoretical fittings according to the coupled Lorentz oscillator model exhibit a well consistency with the simulation spectra, validating the accuracy of the numerical simulation. The designed metamaterial is sensitive to the surround refractive index change with a figure-of-merit of 460, which far exceeds Fano-resonant plasmonic metal sensors. The designed Si metamaterial with a sharp EIT response and a strong electric field enhancement is an ideal platform for applications in optical sensing, low-loss slow-light devices, fluorescence enhancement, and nonlinear optics.

Electrical modulation of excitonic transition in monolayer tungsten disulfide on periodically poled ferroelectric substrates

Although the spectral investigation of monolayer two-dimensional transition metal dichalcogenide (1L-TMD) has been extensively performed for identifying modulation of excitonic transition, study of excitonic transition in ferroelectric/TMD hybrid structure is essential to understand and design high speed and low loss devices based on in-plane junctions. Here, we demonstrate electrical modulation of excitonic transition by transferring 1L-tungsten disulfide (WS2) onto a ferroelectric domain surface, a periodically poled lithium niobate (PPLN). We observe that exciton (A0) and trion (A−) emissions of 1L-WS2 are spatially modulated depending on the sign of the phase domain, such that A0 and A− emission is stronger at the positive (D+) and negative (D−) domain, respectively. In addition, different trends of excitonic transition of 1L-WS2 between D+ and D− domain are demonstrated by applying a back-gate bias voltage. The spatial modulation of electrical and optical characteristics of 1L-TMDs will help to design the homo- or hetero p-n junctions based on TMDs.

Design and analysis of a two-dimensional large-scale silicon-photonic optical phased array

Currently, two-dimensional (2D) optical phased arrays (OPAs) have broad development prospects in many emerging fields. However, a traditional 2D-OPA sets a phase shifter for each antenna, causing large antenna spacing, high power consumption, and complex wiring. In this study, we first introduce a theoretical model of an N × N OPA by separating the phase shifter from the antenna element; only 2N phase shifters are used to realize 2D beam scanning, effectively reducing the power consumption and wiring complexity. To reduce the antenna spacing, we next use inverse design and particle swarm optimization to design compact, low-loss, and high-performance silicon-photonic devices, including cross waveguides, Y branches, directional couplers, and grating antennas. For the thermo-optic phase shifter (TOPS), we introduce the semiconductor material AlN that enables the modulation speed to reach 45 kHz. The length of the TOPS is 30 μm and phase-shifting efficiency (Pπ) is only 20 mW. Finally, for proof of concept, we built an 8 × 8 OPA architecture model with an antenna spacing of 8.5 μm. The simulation results show that the OPA achieves 10.5° × 10.5° 2D-beam steering at the 1.55-μm wavelength using only 16 TOPSs, and we use genetic algorithm to realize the sparse uniform array and achieve the side-lobe rejection ratio of 18 dB in the field of view.

(Invited) Rare-Earth-Doped Tellurium Oxide Light Emitting Nanophotonic Devices

Tellurium oxide is a promising material for passive, nonlinear and rare-earth-doped active photonic devices because of its high transparency, high refractive index, high nonlinearity and unique structure allowing for high rare-earth solubility. In this talk I present on our recent progress on tellurite glass on-chip light emitting nanophotonic devices. Low-loss passive devices including high-Q-factor microdisks and microring resonators will be discussed. In addition, rare-earth-doped active devices, including erbium-doped and thulium-doped waveguide amplifiers and microlasers will be presented. Using similar structures, we demonstrate nonlinear light emission via four-wave-mixing, supercontinuum generation and third harmonic generation. These tellurium oxide integrated nanophotonic devices are highly promising for compact and low-cost passive, active and nonlinear photonic integrated circuits for applications in communications, computing, and sensing.

Exceeding the classical time-bandwidth product in nonlinear time-invariant systems

The classical “time-bandwidth” limit for linear time-invariant (LTI) devices in physics and engineering asserts that it is impossible to store broadband propagating waves (large $${\Delta }\omega$$ ’s) for long times (large Δt’s). For standing (non-propagating) waves, i.e., vibrations, in particular, this limit takes on a simple form, $${\Delta }t \,{\Delta }\omega = 1$$ , where $${\Delta }\omega$$ is the bandwidth over which localization (energy storage) occurs, and $${\Delta }t$$ is the storage time. This is related to a well-known result in dynamics, namely that one can achieve a high Q-factor (narrowband resonance) for low damping, or small Q-factor (broadband resonance) for high damping, but not simultaneously both. It thus remains a fundamental challenge in classical wave physics and vibration engineering to try to find ways to overcome this limit, not least because that would allow for storing broadband waves for long times, or achieving broadband resonance for low damping. Recent theoretical studies have suggested that such a feat might be possible in LTI terminated unidirectional waveguides or LTI topological “rainbow trapping” devices, although an experimental confirmation of either concept is still lacking. In this work, we consider a nonlinear but time-invariant mechanical system and demonstrate experimentally that its time-bandwidth product can exceed the classical time-bandwidth limit, thus achieving values both above and below unity, in an energy-tunable way. Our proposed structure consists of a single-degree-of-freedom nonlinear oscillator, rigidly coupled to a nondispersive waveguide. Upon developing the full theoretical framework for this class of nonlinear systems, we show how one may control the nonlinear flow of energy in the frequency domain, thereby managing to disproportionately decrease (increase) $${\Delta }t$$ , the storage time in the resonator, as compared with an increase (decrease) of the system’s bandwidth $${\Delta }\omega$$ . Our results pave the way toward conceiving and harnessing hitherto unattainable broadband and simultaneously low-loss wave-storage devices, both linear and nonlinear, for a host of key applications in wave physics and engineering.

High-sensitivity measurements of the nonlinear absorption coefficient of wide bandgap oxide thin films with the Z-scan method

The nonlinear response of wide bandgap oxide thin films gradually emerges and attracts attention with the development of an ultra-short and ultra-intense laser. In Z-scan technique, due to the extremely lower nonlinear response of thin film compared with the common substrate, it isn’t easy to measure the multiphoton absorption coefficient of wide bandgap oxide thin films. In this study, a method is proposed to suppress the substrate impact and improve the thin film measurement sensitivity. To make the thin film nonlinear intensity dominate the total intensity, including unwanted substrate impact, material and thickness of the substrate are analyzed. Considering the nonlinear effects of different substrates and the adhesion between the substrate and the thin film, 50 μm MgF2 and quartz glass are selected as the substrate for deposition. The nonlinear intensity of substrate is suppressed to at least 80% of the whole element or can even be ignored so that the normalized transmittance of the thin film can be obtained effectively. The two-photon and three-photon absorption coefficients of HfO2, Al2O3, and SiO2 thin film are measured at different wavelengths. The nonlinear absorption response measurements of wide bandgap oxide thin films can advance the design and fabrication of low-loss photonic devices in ultra-fast lasers.

The anisotropic broadband surface plasmon polariton and hot carrier properties of borophene monolayer

Abstract Borophene monolayer with its intrinsic metallic and anisotropic band structures exhibits extraordinary electronic, optical, and transport properties. Especially, the high density of Dirac electrons enables promising applications for building low-loss broadband SPP devices. However, a systematic characterization of the surface plasmon polariton (SPP) properties and hot carriers generated from the inevitable SPP decay in borophene has not been reported so far. Most importantly, the mechanism for SPP losses remains obscurely quantified. In this work, from a fully first-principles perspective, we explicitly evaluate the main loss effects of SPP in borophene, including the Drude resistance, phonon-assisted intraband and direct interband electronic transitions. With this knowledge, we further calculate the frequency- and polarization-dependent SPP response of borophene, and evaluate some typical application-dependent figure of merits of SPP. On the other hand, we evaluate the generation and transport properties of plasmon-driven hot carriers in borophene, involving energy- and momentum-dependent carrier lifetimes and mean free paths, which provide deeper insight toward the transport of hot carriers at the nanoscale. These results indicate that borophene has promising applications in next-generation low-loss optoelectronic devices and photocatalytic reactors.

Octave-spanning low-loss mid-IR waveguides based on semiconductor-loaded plasmonics

Plasmonic waveguides are crucial building blocks for integrated on-chip mid-infrared (mid-IR) sensors, which have recently attracted great interest as a sensing platform to target enhanced molecular sensing. However, while hosting a wide range of applications from spectroscopy to telecommunication, the mid-IR lacks suitable broadband solutions that provide monolithic integration with III-V materials. This work reports a novel concept based on hybrid semiconductor-metal surface plasmon polariton waveguides, which result in experimentally demonstrated low loss and broadband devices. Composed of a thin germanium slab on top of a gold layer, the waveguiding properties can be directly controlled by changing the geometrical parameters. The measured losses of our devices are as low as 6.73 dB/mm at 9.12 µm and remain <15 dB/mm in the mid-IR range of 5.6–11.2 µm. The octave-spanning capability of the waveguides makes them ideal candidates for combination with broadband mid-IR quantum cascade laser frequency combs and integrated spectroscopic sensors.

Low Subthreshold Slope AlGaN/GaN MOS-HEMT with Spike-Annealed HfO2 Gate Dielectric.

AlGaN/GaN metal-oxide semiconductor high electron mobility transistors (MOS-HEMTs) with undoped ferroelectric HfO2 have been investigated. Annealing is often a critical step for improving the quality of as-deposited amorphous gate oxides. Thermal treatment of HfO2 gate dielectric, however, is known to degrade the oxide/nitride interface due to the formation of Ga-containing oxide. In this work, the undoped HfO2 gate dielectric was spike-annealed at 600 °C after the film was deposited by atomic layer deposition to improve the ferroelectricity without degrading the interface. As a result, the subthreshold slope of AlGaN/GaN MOS-HEMTs close to 60 mV/dec and on/off ratio>109 were achieved. These results suggest optimizing the HfO2/nitride interface can be a critical step towards a low-loss high-power switching device.

All-optical binary computation based on inverse design method

Abstract The development of information technology urgently requires ultrafast, ultra-low energy consumption and ultra-high-capacity data computing abilities. Traditional computing method of electronic chips is limited by the bottleneck of Moore’s Law. All-optical computing of photonic chips provides a promising way to realize such high-performance data computing abilities. Until now, it is still a huge challenge to realize all-optical four arithmetic operations at the same time on a photonic chip. Here, we propose a new encoding scheme for all-optical binary computation, including n-bit addition, subtraction, multiplication and division. We theoretically present n-bit calculation and experimentally demonstrate 1 bit calculation. The computation part includes a half binary adder and a shifter, whose feature sizes are only 2 μm × 19.5 μm and 4 μm × 9 μm, respectively. The half binary adder and shifter consist of three low-loss basic devices through inverse design method. The distance between two adjacent basic devices is smaller than 1.5 μm, within wavelength magnitude scale. The response time is the propagation time of the signal light in a single device, within 100 fs. The threshold energy consumption is within 10 fJ/bit. Our results provide a new method to realize ultrafast, ultra-low energy consumption and ultra-high-capacity data processing abilities all-optical n-bit binary computing.

(Invited) High-Speed Plasma Etching of SiC Wafer Toward Backside Thinning

The semiconductor silicon carbide (SiC) is a promising material for low-loss power devices. The development of a highly efficient strain-free process for the backside thinning of vertical power transistors is required due to its hard and brittle properties. Thus, plasma etching using high-pressure SF6 plasma was proposed, and it was found that a high-speed etching of the entire surface of a 2-inch wafer at approximately 15 μm/min was possible. It was also demonstrated that the thickness of the commercially available 2-inch wafer could be thinned to approximately 100 μm with only 20-min plasma etching, and larger-diameter wafers could be thinned at the same rate using the same rf power per unit area. Further, from the evaluation using commercial SiC MOSFET chips, it was confirmed that the backside thinning via this method does not affect the frontside devices.