Optical Nanowires Research Articles

Efficient coupling of single photons from chromium-vacancy centers in nanodiamonds into end-to-end aligned optical nanowires

Abstract We report a numerical simulation on the coupling of chromium-vacancy centers in nanodiamond (CrV-ND) with end-to-end aligned optical nanowires (ONWs) structure. The structure is designed using finite-difference time-domain simulations to maximize the bidirectional coupling of spontaneous emission from a CrV-ND into ONW-guided modes. We systematically analyze the dependence of spontaneous emission characteristics on the ONW and CrV-ND dimension, quantum emitter (QE) position, and polarization. We show that coupling efficiency as high as 62% can be achieved into the guided modes from a CrV-ND placed at the center of ONWs, which is twice as compared to a CrV-ND placed on an ONW surface. The degree of polarization of single photons from CrV-ND is also estimated to be as high as 64%. This simple device can be reconfigured for various QEs. This present fiber inline platform may open new avenues in quantum photonics.

Nanowire integration in silica based integrated optical circuits: Limitations and challenges towards quantum computing

Optical integrated circuits suggest a very promising platform for the development of robust and efficient quantum computers. A critical issue to their development and scalability is the integration of multiple single photon sources in the circuits. One major category of single photon sources is based on quantum dots that are embedded in semiconductor optical nanowires (NWQD) that allow their accurate handing and deterministic integration. Successful integrations of such nanowires have been reported in high index platforms like silicon or silicon nitride, with adequate coupling efficiency due the modal characteristics compatibility. On the other hand, Silica-on-Silicon is a major integration platform, which combined with new fabrication approaches like direct laser writing, for the definition of optical structures with refractive index modification, can provide the fabrication of highly optimized and tailor-made circuits by rapid prototyping. Considering for the first time the integration scenario of NWQD in laser written silica-based waveguides it is shown that the low (∼10-3) achievable refractive index contrast imposes strict limitations on the compatibility of such waveguides with NWQD resulting in general in low coupling efficiency. By considering several design and fabrication issues, suitable integration approaches with adequate efficiency are demonstrated, while also the limitations and challenges are revealed thus triggering new research directions.

Unidirectional Photon Coupling Using Asymmetric Diamond Emitters with Enhanced Spontaneous Emission

AbstractThe directional coupling of single photon emission from nitrogen vacancy (NV–) emitters to optical systems for various applications requires a suitable interface between NV– emitters and optical waveguides. Unidirectional coupling of photons may have significant importance in next generation quantum technology such as quantum and complex optical networks, quantum communication, and multi‐scale quantum photonics devices including quantum non‐linearity and quantum metrology. Here, a hybrid asymmetric structure of elliptically faceted (ELFA) diamond nanowire with Bragg grating (BG) containing negatively charged NV– center for efficient and unidirectional optical coupling to optical nanowire (waveguide) is proposed. The calculations indicate that the structure can provide coupling efficiency of ≈90% toward the elliptically facet direction and ≈1% in the opposite direction. Further, Purcell factor is enhanced due to the augmented electric field intensity in the ELFA structure. This structure is significantly robust against imperfect placement and rotational alignment of dipole from its intended position.

Bidirectional coupling of diamond emitters to optical nanowire: tunable and efficient

Negatively charged nitrogen vacancy ( N V − ) centers in diamond are required to be coupled to optical systems for various applications. A slowly varied tapered waveguide displays a near-unity power transfer from an optical fiber to on-chip photonic devices. This physical situation refers to an adiabatic transition of photons from a highly effective confinement mode to a lower effective confinement mode or vice versa. Here, we report tunable bidirectional coupling with enhanced efficiency in a hybrid structure of elliptically faceted (ELFA) diamond nanowire with N V − centers integrated to optical nanowire. Initiating from diabatic transition to adiabatic transition, corresponding to smaller length to longer wire length, respectively, the coupling efficiency oscillates and asymptotically saturates to a maximum value. Our calculations indicate coupling efficiencies of 85% and 84% for azimuthal and radial dipole configurations for the hybrid structure, respectively. The structure with optimum geometry provides similar coupling efficiency of ∼ 81 % for radial and azimuthal dipole configurations. By excitation of one of few dipoles placed strategically at various locations in the cylindrical region of the diamond nanowire will allow one to tune coupling efficiency in the two directions. Tailored size and tunable bidirectional coupling of ELFA diamond nanowire with enhanced efficiency will enable its wide-field applications including multi-scale quantum photonics devices.

(Invited) Top-Down Etching of III-Nitride Nanostructures

Chemical etch processes for III-nitride (AlGaInN) materials and devices are significantly underdeveloped due to its apparent inertness to common wet etchants. To fully realize the potential of the III-nitrides in new electronic and photonic nano and micro device concepts, a more complete knowledge and development of anisotropic and three-dimensional top-down etch techniques are needed. Here, we explore the etch characteristics of GaN and AlGaN based materials using the general geometric principles of crystallographic dissolution processes to enable the prediction of facet-determined etch structures, including high aspect ratio nanowires and nanowalls. We perform the first complete nonpolar orientation-dependent etch rate measurements for GaN, and use them to predict the faceting of pillar structures with good agreement with experiment. We use these developments in wet etching to fabricate functional III-nitride-based optical nanowire and microcavity structures with control over the cross-sectional shape, where faceting enables sidewalls with near atomic-smoothness. Finally, we discuss a new fabrication method, quantum-size controlled photoelectrochemical etching, for the etching of InGaN quantum dots with high size uniformity from planar InGaN films.

High optical quality long ultrafine ZnO nanowires by low-temperature oxidation of sputtered nanostructured Zn templates

In this study, we show that by applying appropriate deposition conditions, Zn nanostructured templates for the growth of zinc oxide (ZnO) nanowires can be fabricated, from which ultrafine high optical quality nanowires can be grown by means of post-deposition low-temperature oxidation. By identifying and optimizing the appropriate parameters, we successfully fabricated long ultrafine ZnO nanowires up to 30 microns in length and 50 nm in diameter. Our report contradicts the commonly held paradigm that sputter deposition can only be used to fabricate thin films with no significant nanostructure morphology and provides a low cost, high throughput method of fabricating different ZnO nanostructures. The studies of photoluminescence (PL) of the nanowires showed their high optical quality with band edge dominated emission with small defect-related input.

Rapid and Effective Electrical Conductivity Improvement of the Ag NW-Based Conductor by Using the Laser-Induced Nano-Welding Process

To date, the silver nanowire-based conductor has been widely used for flexible/stretchable electronics due to its several advantages. The optical nanowire annealing process has also received interest as an alternative annealing process to the Ag nanowire (NW)-based conductor. In this study, we present an analytical investigation on the phenomena of the Ag NWs’ junction and welding properties under laser exposure. The two different laser-induced welding processes (nanosecond (ns) pulse laser-induced nano-welding (LINW) and continuous wave (cw) scanning LINW) are applied to the Ag NW percolation networks. The Ag NWs are selectively melted and merged at the junction of Ag NWs under very short laser exposure; these results are confirmed by scanning electron microscope (SEM), focused-ion beam (FIB), electrical measurement, and finite difference time domain (FDTD) simulation.

Emerging interconnects: a state-of-the-art review and emerging solutions

ABSTRACTAluminium was a primary material for interconnection in integrated circuits (ICs) since their inception. Later, copper was introduced as interconnect material which has better metallic conductivity and resistance to electromigration. As the aggressive technology scaling continues, the copper resistivity increased because of size effects, which causes increase in delay, power dissipation and electromigration. The need to reduce the resistor-capacitor​​​​​​​ delay, dynamic power utilisation and the crosstalk commotion is as of now the fundamental main impetus behind the presentation of new materials. The purpose of this paper is to do a survey of interconnect material used in IC from introduction of ICs to till date. This paper studies and reviews new materials available for interconnect application which are optical interconnects, carbon nanotube (CNT), graphene nanoribbons (GNRs) and silicon nanowires which are alternatives to copper. While doing a survey of interconnect material, it is found that multiwalled CNTs, multilayer GNR and mixed CNT bundles are promising candidates and are ultimate choice that can strongly address the problems faced by copper but on integration basis copper would last for coming years.

Tailoring the dispersion behavior of optical nanowires with intercore-cladding lithium niobate thin film.

The dispersion properties of silica and silicon subwavelength-diameter wires with intercore-cladding uniaxial dielectric lithium niobate thin film has been studied numerically in detail. The waveguide dispersion shifts centered around 1550-nm wavelength have been investigated. It shows that the dispersion of optical nanowires with intercore-cladding lithium niobate thin film is highly sensitive to fiber geometry. Moreover, with applied electric field, considerable dispersion shifts without changing its geometric structure can be obtained. Our work may provide an inroad for developing miniaturized functional optoelectronic devices.

Hot Brownian thermometry and cavity-enhanced harmonic generation with nonlinear optical nanowires

The non-linear response of nanoscale materials is affected by phase-matching conditions that are critically dependent on temperature. It has remained a persistent challenge to measure the temperature of individual nonlinear optical nanostructures in situ. Here, we measure the temperature of individual KNbO3 nanowires in an optical trap through analysis of the Brownian dynamics. Additionally, we show Fabry–Pérot-type cavity enhancement of second harmonic generation in one-dimensional potassium niobate nanowires (KNNW) using a tunable, continuous wave (CW), near-infrared (NIR) trapping laser. A second co-aligned CW NIR laser is used to extend the range of visible emission through sum frequency generation.

Noise and Information Capacity in Silicon Nanophotonics

Modern computing and data storage systems increasingly rely on parallel architectures. The necessity for high-bandwidth data links has made optical communication a critical constituent of modern information systems and silicon the leading platform for creating the necessary optical components. While silicon is arguably the most extensively studied material in history, one of its most important attributes, i.e., an analysis of its capacity to carry optical information, has not been reported. The calculation of the information capacity of silicon is complicated by nonlinear losses, which are phenomena that emerge in optical nanowires as a result of the concentration of optical power in a small geometry. While nonlinear loss in silicon is well known, noise and fluctuations that arise from it have never been considered. Here, we report fluctuations that arise from two-photon absorption, plasma effect, cross-phase modulation, and four-wave mixing and investigate their role in limiting the information capacity of silicon. We show that these fluctuations become significant and limit the capacity well before nonlinear processes affect optical transmission. We present closed-form analytical expressions that quantify the capacity and provide an intuitive understanding of the underlying physics.

Strain-Induced Large Exciton Energy Shifts in Buckled CdS Nanowires

Strain engineering can be utilized to tune the fundamental properties of semiconductor materials for applications in advanced electronic and photonic devices. Recently, the effects of large strain on the properties of nanostructures are being intensely investigated to further expand our insights into the physics and applications of such materials. In this Letter, we present results on controllable buckled cadmium-sulfide (CdS) optical nanowires (NWs), which show extremely large energy bandgap tuning by >250 meV with applied strains within the elastic deformation limit. Polarization and spatially resolved optical measurements reveal characteristics related to both compressive and tensile regimes, while microreflectance spectroscopy clearly demonstrates the effect of strain on the different types of excitons in CdS. Our results may enable strained NWs-based optoelectronic devices with tunable optical responses.

High birefringence triangular optical nanowire in suspended-core fiber for temperature sensing

Triangular nanowires that present a high birefringence and a very strong confinement were fabricated by tapering suspended-core fibers (SCFs) down to core diameters below 1000 nm. Each nanowire presented a high birefringence with an order of magnitude of 10 −3 . As the spectra of the SCF tapers inserted in fiber loop mirrors can be used to generate a sinusoidal interference pattern from the two main modes (fast and slow axis), a nanowire was employed as a sensing element in a Sagnac interferometer for measuring temperature. Temperature sensitivity was determined to be - 56.2 pm / K using a triangular nanowire of 810 nm in-circle diameter when compared with that of a conventional untapered SCF whose temperature sensitivity is − 2.1 pm / K .

Self-healing, an intrinsic property of biomineralization processes

The sponge siliceous spicules are formed enzymatically via silicatein, in contrast to other siliceous biominerals. Originally, silicatein had been described as a major structural protein of the spicules that has the property to allow a specific deposition of silica onto their surface. More recently, it had been unequivocally demonstrated that silicatein displays a genuine enzyme activity, initiating and maintaining silica biopolycondensation at low precursor concentrations (<2 mM). Even more, as silicatein becomes embedded into the biosilica polymer, formed by the enzyme, it retains its functionality to enable a controlled biosilica deposition. The protection of silicatein through the biosilica mantel is so strong that it conserves the functionality of the enzyme for thousands of years. The implication of this finding, the preservation of the enzyme function over such long time periods, is that the intrinsic property of silicatein to display its enzymatic activity remains in the biosilica deposits. This self-healing property of sponge biosilica can be utilized to engineer novel hybrid materials, with silicatein as a functional template, which are more resistant toward physical stress and fracture. Those hybrid materials can even be used for the fabrication of silica dielectrics coupled to optical nanowires.

Full vectorial analysis of polarization effects in optical nanowires

We develop a full theoretical analysis of the nonlinear interactions of the two polarizations of a waveguide by means of a vectorial model of pulse propagation which applies to high index subwavelength waveguides. In such waveguides there is an anisotropy in the nonlinear behavior of the two polarizations that originates entirely from the waveguide structure, and leads to switching properties. We determine the stability properties of the steady state solutions by means of a Lagrangian formulation. We find all static solutions of the nonlinear system, including those that are periodic with respect to the optical fiber length as well as nonperiodic soliton solutions, and analyze these solutions by means of a Hamiltonian formulation. We discuss in particular the switching solutions which lie near the unstable steady states, since they lead to self-polarization flipping which can in principle be employed to construct fast optical switches and optical logic gates.

Relating localized nanoparticle resonances to an associated antenna problem

We conceptually unify the description of resonances existing at metallic nanoparticles and optical nanowire antennas. To this end the nanoantenna is treated as a Fabry-Perot resonator with arbitrary semi-nanoparticles forming the terminations. We show that the frequencies of the quasi-static dipolar resonances of these nanoparticles coincide with the frequency where the phase of the complex reflection coefficient of the fundamental propagating plasmon polariton mode at the wire termination amounts to $\pi$. The lowest order Fabry-Perot resonance of the optical wire antenna occurs therefore even for a negligible wire length. This approach can be used either to easily calculate resonance frequencies for arbitrarily shaped nanoparticles or for tuning the resonance of nanoantennas by varying their termination.

Dispersion Study of Optical Nanowire Microcoil Resonators

The dispersion characteristics of an optical nanowire microcoil resonator (ONMR) are investigated, which show remarkable influence on their performances at a high bitrate. In addition to the waveguide dispersion it was found that the adjacent ring coupling also has notable contribution to the total dispersion. If the nanowire diameter's range is from 1.5 to 5 μm, the waveguide dispersion and coupling dispersion could almost be canceled with each other so that the total dispersion is well suppressed, which is desired in high-speed (10 Gb/s and beyond) optical systems. On the other hand, both positive and negative dispersion could be obtained by adjusting suitable ONMR parameters. Even a tailored dispersion curve could be designed theoretically. These results are helpful for ONMR-based device design in communication and sensing applications.

Nonlinear polarization bistability in optical nanowires

Using the full vectorial nonlinear Schrödinger equations that describe nonlinear processes in isotropic optical nanowires, we show that there exist structural anisotropic nonlinearities that lead to unstable polarization states that exhibit periodic bistable behavior. We analyze and solve the nonlinear equations for continuous waves by means of a Lagrangian formulation and show that the system has bistable states and also kink solitons that are limiting forms of the bistable states.

Light Enhancement Within Nanoholes in High Index Contrast Nanowires

We present systematic predictions for light enhancement in optical nanowires with nanoscale air holes in the core through numerical modeling. We show that the light intensity within such holes is strongly dependent on the hole size and refractive index of the host material and that light enhancement becomes significant only when the hole size is less than a critical value: 70 nm for silica and F2 nanowires and 50 nm for a As2S3 nanowire. High index As2S3 nanowires exhibit nearly eight times higher average mode intensity than silica glass for hole sizes of less than 10 nm. Such intensity enhancements open up new device opportunities; for example, filling nanoscale holes within silicon nanowires with silicon nanocrystals enables 30% enhancement of the nonlinear coefficient.

Fluorescence-based sensing with optical nanowires: a generalized model and experimental validation

A model for the fluorescence sensing properties of small-core high-refractive-index fibers (optical nanowires) is developed and compared quantitatively with experiment. For the first time, higher-order modes and loss factors relevant to optical nanowires are included, which allows the model to be compared effectively with experiment via the use of fluorophore filled suspended optical nanowires. Numerical results show that high-index materials are beneficial for fluorescence-based sensing. However, both numerical and experimental results show that the fluorescence signal is relatively insensitive to core size, except for low concentration sensing where nanoscale fiber cores are advantageous due to the increased evanescent field power.