Low-loss Metal Research Articles

Low-loss liquid metal interconnects for superconducting quantum circuits

Building a modular architecture with superconducting quantum computing chips is one of the means to achieve qubit scalability, allowing the screening, selection, replacement, and integration of individual qubit modules into large quantum systems. However, the nondestructive replacement of modules within a compact architecture remains a challenge. Liquid metals, specifically gallium alloys, can be alternatives to solid-state galvanic interconnects. This is motivated by their self-healing, self-aligning, and other desirable fluidic properties, potentially enabling the nondestructive replacement of modules at room temperatures, even after operating the entire system at millikelvin regimes. In this study, we present coplanar waveguide resonators (CPWRs) interconnected by gallium alloy droplets, achieving high internal quality factors up to nearly one million and demonstrating performance on par with the continuous solid-state CPWRs. Leveraging the desirable fluidic properties of gallium alloys at room temperature and their compact design, we envision a modular quantum system enabled by liquid metals.

10-GHz Low-Loss Liquid Metal SIW Phase Shifter for Phased Array Antennas

10-GHz Low-Loss Liquid Metal SIW Phase Shifter for Phased Array Antennas

A Corrugated Planar-Goubau-Line Termination for Terahertz Waves

The planar Goubau line is a promising low-loss metal waveguide for terahertz interconnects. To enable advanced system architectures and multi-port measurements based on planar Goubau lines, there is a strong need for broadband impedance-matched loads, which can be used to absorb the energy and minimize standing waves in a system. In this work, we propose a matched load for planar Goubau lines based on an exponentially-tapered corrugated line, gradually increasing conductor losses while maintaining small reflections. The corrugation density is high enough to increase conductor losses without requiring an auxiliary low-conductivity material, simplifying the fabrication process. A 400-$\mu$m long planar Goubau line load was fabricated on a 10-$\mu$m thick silicon substrate suspended in the air. Electromagnetic simulations of the load show excellent agreement with calibrated reflection measurements in the frequency range 0.5 THz - 1.1 THz. Above the cut-off frequency of around 580 GHz, the measured reflections are much less than -19 dB, below the noise floor of the characterization setup.

Loss-coupled DFB nano-ridge laser monolithically grown on a standard 300-mm Si wafer.

We present a loss-coupled distributed feedback microlaser, monolithically grown on a standard 300-mm Si wafer using nano-ridge engineering. The cavity is formed by integrating a metallic grating on top of the nano-ridge. This allows forming a laser cavity without etching the III-V material, avoiding damaged interfaces and the associated carrier loss. Simulations, supported by experimental characterisation of the modal gain of the nano-ridge devices, predict an optimal duty cycle for the grating of ~0.4, providing a good trade-off between coupling strength and cavity loss for the lasing mode. The model was experimentally verified by characterising the lasing threshold and external efficiency of devices exhibiting gratings with varying duty cycle. The high modal gain and low threshold obtained prove the excellent quality of the epitaxial material. Furthermore, the low loss metal grating might provide a future route to electrical injection and efficient heat dissipation of these nanoscale devices.

High-efficiency narrow-band plasmonic hot electron conversion from nanoscale sodium–silicon heterostructures

Plasmonic harvesting of hot electrons has stimulated intensive research activities for applications ranging from sub-bandgap photodetection to photocatalysis. Both high photoelectric conversion efficiency and tunable spectral response are pursued by manipulating resonant metal–semiconductor (M–S) nanostructures. Although noble plasmonic metals have been exclusively employed in hot electron conversion studies, exploring new materials may offer an additional degree of freedom to manipulate the hot electron generation, transport, and emission processes. In this paper, we propose to employ the low-loss alkali metal sodium as an alternate plasmonic material for developing a narrow-band resonant hot electron device. Based on a backside-illumination (BSI) configuration where plasmonic hot electrons generate locally at the M–S interface, the transport loss can be significantly suppressed. Thanks to its ultralow imaginary part of the permittivity, bringing Na into the BSI design allows for efficient shrinking of the resonant linewidth down to sub-20 nm. Another intriguing feature is that Na has more preferred electron density of state distribution for facilitating hot electron emission at the M–S junction. The optimized Na BSI device can yield a photocurrent responsivity up to 50 mA/W at a wavelength of 1400 nm as predicted by our electromagnetic simulation and theoretical model. Our study highlights that the alkali metal could be a promising alternative material for the development of high-Q resonant hot electron devices for near-infrared wavelengths.

Device architectures for low voltage and ultrafast graphene integrated phase modulators.

The atomic layer thin geometry and semi-metallic band diagram of graphene can be utilized for significantly improving the performance matrix of integrated photonic devices. Its semiconductor-like behavior of Fermi-level tunability allows graphene to serve as an active layer for electro-optic modulation. As a low loss metal layer, graphene can be placed much closer to active layer for low voltage operation. In this work, we investigate hybrid device architectures utilizing semiconductor and metallic properties of the graphene for ultrafast and energy efficient electro-optic phase modulators on semiconductor and dielectric platforms. (1) Directly contacted graphene-silicon heterojunctions. Without oxide layer, the carrier density of graphene can be modulated by the directly contact to silicon layer, while silicon intrinsic region stays mostly depleted. With doped silicon as electrodes, carrier can be quickly injected and depleted from the active region in graphene. The ultrafast carrier transit time and small RC constant promise ultrafast modulation speed (3dB bandwidth of 67 GHz) with an estimated Vπ·L of 1.19 V·mm. (2) Graphene integrated lithium niobite modulator. As a transparent electrode, graphene can be placed close to integrated lithium niobate waveguide for improving coupling coefficient between optical mode profile and electric field with minimal additional loss (4.6 dB/cm). Numerical simulation indicates 2.5× improvement of electro-optic field overlap coefficient, with estimated V π of 0.2 V.

Kerr nonlinearity induced four-wave mixing of CMOS-compatible PECVD deposited ultra-Si-rich-nitride

Benefitting from low propagation loss and complementary metal–oxide–semiconductor compatibility, Si3N4 is heavily explored for the applications of nonlinear optical signal processing. However, the Si3N4 waveguide is limited by its low optical Kerr nonlinearity. In this work, we introduce highly nonlinear ultra-Si-rich-nitride (USRN) deposited by the plasma enhanced chemical vapor deposition system. The measured linear refractive index of USRN is 3.09 at a wavelength of 1550 nm, while the Kerr nonlinearity of the USRN waveguide is experimentally extracted with a value of 2.25×10−17 m2/W. Moreover, a broadband wavelength conversion ranging from S-band to L-band by a four-wave-mixing experiment is realized via designed USRN waveguide with a relatively short length of 3 mm. The measured bandwidth is 190 nm with a continuous-wave pump laser located at 1530 nm. The conversion efficiency is measured approximately −48 dB under a relatively low pump power of 7 dBm.

Conductivitylike Gilbert Damping due to Intraband Scattering in Epitaxial Iron.

Confirming the origin of Gilbert damping by experiment has remained a challenge for many decades, even for simple ferromagnetic metals. Here, we experimentally identify Gilbert damping that increases with decreasing electronic scattering in epitaxial thin films of pure Fe. This observation of conductivitylike damping, which cannot be accounted for by classical eddy-current loss, is in excellent quantitative agreement with theoretical predictions of Gilbert damping due to intraband scattering. Our results resolve the long-standing question about a fundamental damping mechanism and offer hints for engineering low-loss magnetic metals for cryogenic spintronics and quantum devices.

The preparation, characterization and application of ultra-smooth, low-loss plasmonics noble metal films

Since its discovery, plasmonics has become one of the most popular topics in the field of nano-optics due to its unprecedented capability of harvesting and localizing electromagnetic fields down to diffraction-unlimited length scales. However, the application of plasmonic effects has long been hindered by the difficulties in controlling the surface morphology and reducing the optical loss of noble metal films, which not only hinders the progress of its experimental research, but also cast a shadow on its application prospects in many fields. With the fast development of nanofabrication technologies, the surface roughness and optical loss of noble metal films is getting closer the theoretical expectation, while the preparation technology is becoming precisely controllable, which lets it play an important role not only in the traditional field, but in may emerging fields as well. In this review, we systematically summarize the recent progress in preparation, characterization and application of ultra-smooth, low-loss noble metal films.

Quantum Engineering of Atomically Smooth Single-Crystalline Silver Films

There is a demand for ultra low-loss metal films with high-quality single crystals and perfect surface for nanophotonics and quantum information processing. Many researches are devoted to alternative materials, but silver is by far theoretically the most preferred low-loss material at optical and near-IR frequencies. Usually, epitaxial growth is used to deposit single-crystalline silver films, but they still suffer from unpredictable losses and well-known dewetting effect that strongly limits films quality. Here we report the two-step approach for e-beam evaporation of atomically smooth single-crystalline metal films. The proposed method is based on the thermodynamic control of film growth kinetics at atomic level, which allows depositing state-of-art metal films and overcoming the film-surface dewetting. Here we use it to deposit 35–100 nm thick single-crystalline silver films with the sub-100pm surface roughness and theoretically limited optical losses, considering an ideal material for ultrahigh-Q nanophotonic devices. Utilizing these films we experimentally estimate the contribution of grain boundaries, material purity, surface roughness and crystallinity to optical properties of metal films. We demonstrate our «SCULL» two-step approach for single-crystalline growth of silver, gold and aluminum films which open fundamentally new possibilities in nanophotonics, biotechnology and superconductive quantum technologies. We believe it could be readily adopted for the synthesis of other extremely low-loss single-crystalline metal films.

Efficient analysis method of light scattering by a grating of plasmonic nanorods

PurposeThe purpose of this paper is to develop a rigorous self-contained formulation for analyzing electromagnetic scattering by grating of plasmonic nanorods. The authors investigate this structure from the viewpoint of the practical application as a refractive index plasmonic sensor. The presented rigorous formulation is accompanied with a neat implementation providing a high computation efficiency and could be considered as an important tool for designing and optimizing compact sensors.Design/methodology/approachScattering of an incident plane wave by grating made of a periodic arrangement of metal-coated dielectric nanocylinders on a dielectric slab is rigorously investigated using the recursive algorithm combined with the lattice sums technique. As a dielectric slab, the authors consider glass material, which is widely used in experiments, whereas silver (Ag) is used as a low loss metal suitable to excite plasmon resonances. The main advantage of the developed self-contained formulation is that first the authors extract the reflection and transmission matrices of a single planar array from a separate analysis of the grating and the slab and then obtain the scattering characteristics of the whole structure by using a recursive formula. The method is computationally fast.FindingsDependence of the surface plasmon resonance wavelength on the refractive index of the surrounding medium is carefully investigated. The resonance peaks are red-shifted with respect to an increasing refractive index of surrounding medium showing an almost linear behavior. Near field distributions are analyzed at the resonance wavelengths of the spectral responses. Special attention is paid to the formation of the dual-absorption bands because of the excitation of the localized surface plasmons. The authors give physical insight to the coupling between grating and the glass slab. The authors found that a strong enhancement of the field inside the slab occurs when the scattered wave of the grating is phase-matched to the guided modes supported by the slab.Originality/valueIn the authors’ formulation, they do not use any approximation and it is rigorous and accurate. The authors use their original method. The method is based on the lattice sums technique and uses the recursive algorithm to calculate the generalized reflection and transmission characteristics by grating. Such fast and accurate method is an effective tool apt for designing and optimizing tailored sensors, for e.g. advanced biomedical applications.

Broadband In-Plane Light Bending With a Doublet Silicon Nanopost Array

The in-plane tailoring of light propagation is significant in on-chip optical interconnections. Recently, an in-plane negative-angle refraction was realized with a thin line of silicon nanoposts, which are at resonance in the first angular momentum channel. This advancement is different from metasurface research, which mostly focuses on out-of-plane operations. In this paper, we experimentally demonstrate that a thin array of doublet silicon nanoposts, in which each unit comprises two tangent nanoposts functioning as an upright interface, remarkably improve efficiency for molding light. The designed upright interface exhibits a broadband response featuring high efficiency in a negative-angle light bending in the wavelength of 1480–1600 nm. The broadband, compactness, low loss, and complementary metal–oxide semiconductor (CMOS) compatibility enable the use of the subwavelength array as an alternative component for on-chip optical control.

Short-range plasmonic nanofocusing within submicron regimes facilitates in situ probing and promoting of interfacial reactions.

In this study, a simple configuration, based on high-index dielectric nanoparticles (NPs) and plasmonic nanostructures, is employed for the nanofocusing of submicron-short-range surface plasmon polaritons (SPPs). The excited SPPs are locally bound and focused at the interface between the dielectric NPs and the underlying metallic nanostructures, thereby greatly enhancing the local electromagnetic field. Taking advantage of the surface properties of the dielectric NPs, this system performs various functions. For example, the nanofocusing of submicron-short-range SPPs is used to enhance the Raman signals of gas molecules adsorbed on the dielectric NPs. In addition, the presence of the local strong electromagnetic field accelerates the rates of interfacial reactions on the surfaces of the dielectric NPs. Therefore, the proposed nanofocusing configuration can both promote and probe interfacial reactions simultaneously. Herein, the promotion and probing of the desorption of EtOH vapor are described, as well as the photodegradation of methylene blue. Moreover, the nanofocusing of SPPs is demonstrated on an aluminum surface in both the visible and UV regimes, a process that has not been achieved using conventional tapered waveguide nanofocusing structures. Therefore, the nanofocusing of submicron-short-range SPPs by dielectric NPs on plasmonic nanostructures is not limited to low-loss noble metals. Accordingly, this system has potential for use in light management and on-chip green devices and sensors.

Strong-Field-Enhanced Spectroscopy in Silicon Nanoparticle Electric and Magnetic Dipole Resonance near a Metal Surface

Strong-field-enhanced spectroscopy in a hybrid dipole resonance system composed of a low-loss semiconductor nanoparticle and metal film is proposed and demonstrated. This hybrid Si nanoparticle on silver system is featuring extraordinary near-field enhancement and large field confinement. Extensive numerical calculations are carried out to investigate the influence of the gap size, particle diameter, and metal substrate on the near-field enhancement response in the Si particle–metal gap in order to properly model their hybridization. Our analysis reveals that this near-field enhancement originates from the strong gap magnetic resonance response by the Si nanoparticle dipole interaction with metal mirror image and metal film surface plasmon effects. We further demonstrate the strong enhanced Raman spectroscopy of a single silicon nanoparticle over Ag film with a precisely sized molecular spacer layer between them. These results illustrate the capacity and tunability of the low-loss silicon particle on the ...

Efficient and broadband polarization conversion with the coupled metasurfaces.

Coupled metasurfaces may refer to a composite plasmonic structure, which consists of multilayered but usually different metasurfaces. A pair of orthogonal plasmonic polarizers, which represents one of such systems, can induce a transmission of light and 90-degree polarization rotation. We explored the effect systematically and found that such effect may be highly efficient and broadband in the near-infrared region. By combining the low-loss metal (silver), the longer operating wavelength, and a work style using propagating waveguide mode, conversion efficiency more than 80% has been suggested near the telecom wavelength. We also suggested that, by overlapping the internal surface-plasmon (2, 0) and (1, 1) modes, an efficient and wideband polarization rotation can be realized. The maximal efficiency is 83% around the wavelength 1340 nm, and the working bandwidth reaches 300 nm. Similar effect has also been revealed in the THz band. The results are useful for constructing compact and high-performance polarization rotators.

Analysis of low-loss hybrid silicon plasmon waveguide for telecommunications applications

In this paper, a numerical method based upon finite-element method has been utilized to analyze of low-loss hybrid silicon plasmonics and metal plasmonics waveguides. Using silicon instead of noble metals (e.g. Au and Ag) is an advantage in designing of optical waveguides due to have low-loss in comparison to noble at telecom bandwidth. The presented results show that the new model has smaller structures and have better performance in the terms mode confinement and 90% lower propagation loss that guide the waves five times longer.

Surface plasmon polariton enhanced ultrathin nano-structured CdTe solar cell

We demonstrate numerically that two-dimensional arrays of ultrathin CdTe nano-cylinders on Ag can serve as an effective broadband anti-reflection structure for solar cell applications. Such devices exhibit strong absorption properties, mainly in the CdTe semiconductor regions, and can produce short-circuit current densities of 23.4 mA/cm(2), a remarkable number in the context of solar cells given the ultrathin dimensions of our nano-cylinders. The strong absorption is enabled via excitation of surface plasmon polaritons (SPPs) under plane wave incidence. In particular, we identified the key absorption mechanism as enhanced fields of the SPP standing waves residing at the interface of CdTe nano-cylinders and Ag. We compare the performance of Ag, Au, and Al substrates, and observe significant improvement when using Ag, highlighting the importance of using low-loss metals. Although we use CdTe here, the proposed approach is applicable to other solar cell materials with similar absorption properties.

Coupling plasmons and dyakonons

We study the coupling of plasmons and Dyakonov surface waves propagating at the interfaces between isotropic-birefringent-metal layered structures. Efficient coupling is shown to occur with a proper choice of the crystal birefringence, the refractive index of the isotropic medium, and the light propagation direction relative to the crystal optical axis. In the case of low-loss metals, coupling efficiencies as high as 90% are predicted to be possible.

Photonic surface states in plasmonic crystals of metallic nanoshells

We report on the occurrence and properties of photonic surface states in fcc crystals of metallic nanoshells, by means of full-electrodynamic calculations using the layer-multiple-scattering method, properly extended. Detailed dispersion diagrams of the surface states associated with the $(001)$ and $(111)$ surfaces are calculated for such semi-infinite crystals and corresponding finite slabs, and convergence by increasing the slab thickness is discussed. It is shown that these states can be tuned over a broad frequency range by varying the shell thickness and can be characterized, along high-symmetry directions, according to their symmetry. Absorption in the metallic material limits the propagation length which can, however, be as long as several tens of lattice constants for low-loss metals and relatively broad bands.

On the cubic zero-order solution of electromagnetic waves. I. Periodic slabs with lossy plasmas

Electromagnetic waves are considered for periodic structures consisting of lossy plasmonic components and dielectric host media. For the plasmonic components, not only low-loss metals but also high-loss gas plasmas are taken into consideration. For small filling fractions of the plasmonic components, the intercell interactions are kept to a minimum. In this way, the zero-order solution to the dispersion relation is solved by focusing on its cubic nonlinearity in frequency. Analysis shows that there are two types of solutions: propagating waves and stationary states, depending on the magnitudes of the temporal attenuation rates. Depending on the relative strengths of the material loss of the plasmonic component and its filling fraction, several key critical parameters for the transitions between these two solution types are thus identified. In the following companion paper of Paper II, the cubic nonlinearities in frequency of the dispersion relations stem from different origins. Notwithstanding, they lead to strikingly similar features such as the transitions in wave types and Hopf bifurcations.