Nanoscale Waveguides Research Articles

Highly compact refractive index sensor based on stripe waveguides for lab-on-a-chip sensing applications.

In this paper we report the design and experimental realisation of a novel refractive index sensor based on coupling between three nanoscale stripe waveguides. The sensor is highly compact and designed to operate at a single wavelength. We demonstrate that the sensor exhibits linear response with a resolution of 6 × 10−4 RIU (refractive index unit) for a change in relative output intensity of 1%. Authors expect that the outcome of this paper will prove beneficial in highly compact, label-free and highly sensitive refractive index analysis.

Terahertz Generation in Nonlinear Plasmonic Waveguides

In this paper, terahertz (THz) generation in nonlinear plasmonic waveguides is proposed and thoroughly investigated. The proposed plasmonic waveguide is composed of metal–insulator–metal nanostructure filled with lithium niobate. The THz generation is conducted by means of difference-frequency generation of surface plasmon-polariton (SPP) modes. The SPP modes are designed to be tightly confined inside the plasmonic waveguide, while the generated THz waves diffract rapidly to propagate outside the plasmonic waveguide. It then follows that the THz absorption is minimized. It is shown that viable THz generation can be achieved, despite the SPP losses, by properly choosing the SPP wavelengths and the waveguide dimension. Our numerical evaluations show viable THz generation over the entire frequency band from 1 to 10 THz. The proposed structure introduces a new modality of having the SPP modes totally confined inside a nanoscale waveguide, while most of the generated THz waves are (and thus interacting) with the surrounding medium of the waveguide. These properties promise potential applications in nanocommunication systems and body-centric nanonetworks.

Polarization contrast of nanoscale waveguides in high harmonic imaging

The optical polarization response of a structured material is one of its most significant properties, carrying information about microscopic anisotropies as well as chiral features and spin orientations. Polarization analysis is therefore a key element of imaging and spectroscopy techniques throughout the entire spectrum. In the case of extreme ultraviolet (EUV) radiation, however, both the preparation and detection of well-defined polarization states remain challenging. As a result, polarization-sensitive EUV microscopy based on table-top sources has not yet been realized, despite its great potential, for example, in nanoscale magnetic imaging. Here, we demonstrate polarization contrast in coherent diffractive imaging using high harmonic radiation and investigate the polarization properties of nanoscale transmission waveguides. We quantify the achievable polarization extinction ratio for different waveguide geometries and wavelengths. Our results demonstrate the utility of slab waveguides for efficient EUV polarization control and illustrate the importance of considering polarization contrast in the imaging of nanoscale structures.

Brillouin resonance broadening due to structural variations in nanoscale waveguides

We study the impact of structural variations (that is slowly varying geometry aberrations and internal strain fields) on the width and shape of the stimulated Brillouin scattering (SBS) resonance in nanoscale waveguides. We find that they lead to an inhomogeneous resonance broadening through two distinct mechanisms: firstly, the acoustic frequency is directly influenced via mechanical nonlinearities; secondly, the optical wave numbers are influenced via the opto-mechanical nonlinearity leading to an additional acoustic frequency shift via the phase-matching condition. We find that this second mechanism is proportional to the opto-mechanical coupling and, hence, related to the SBS-gain itself. It is absent in intra-mode forward SBS, while it plays a significant role in backward scattering. In backward SBS increasing the opto-acoustic overlap beyond a threshold defined by the fabrication tolerances will therefore no longer yield the expected quadratic increase in overall Stokes amplification. Finally, we illustrate in a numerical example that in backward SBS and inter-mode forward SBS the existence of two broadening mechanisms with opposite sign also opens the possibility to compensate the effect of geometry-induced broadening. Our results can be transferred to other micro- and nano-structured waveguide geometries such as photonic crystal fibres.

Magnetoplasmonic isolators utilizing the nonreciprocal phase shift.

We propose a long-range magnetoplasmonic waveguide structure, based on cerium-substituted yttrium iron garnets, that is capable of achieving a useful π/2 nonreciprocal phase shift within one propagation length. Incorporating this plasmonic geometry into a Mach-Zehnder interferometer allows for the implementation of an efficient, integrable isolator scheme with an insertion loss of 2.51 dB and an extinction ratio of 22.82 dB. With a small footprint of 6.4×10(-3) mm(2) and nanoscale waveguide dimensions, we envision this device to be a key building block in the development of nanoplasmonic integrated circuitry.

Manipulative Dual-Microsphere Lasing Based on Nanosilica Waveguide Integrated in a Simplified Hollow-Core Microstructured Optical Fiber

Manipulative dual-microsphere lasing based on a nanosilica waveguide integrated in a simplified hollow-core microstructured optical fiber (SHMOF) is proposed and demonstrated. The SHMOF, which has a silica-branch structure, provides a nanoscale waveguide array and offers a versatile platform for selective lasing. Emissions at 600 and 640 nm are produced by excitation of different dye-coated microspheres, enabling a laser array with an ultralow nJ-level threshold. Because the transitive energy is bridged by the silica ring, the microsphere lasing could be controlled by another microsphere laser because of the interaction between and dependence of their thresholds. In addition, semidirectional lasing is also achieved by deliberately embedding a microsphere against the hexagonal silica wall.

Influence of optical forces on nonlinear optical frequency conversion in nanoscale waveguide devices.

We theoretically investigate the influence of optical gradient forces on nonlinear frequency conversion in a typical nanoscale optomechanical system, which consists of two parallel, suspended waveguides. The waveguides deform with the input power and the phase-matching wavelength changes along the waveguides. Utilizing the spread of deformations collectively allows phase matching over a wider range of pump wavelengths. The third harmonic phase-matching wavelength shift can be as large as 3.6 nm/mW when the waveguide length is 100 μm and the initial gap is 150 nm. It is analogous to chirping the poling period of quasi-phase-matched devices to extend their bandwidths, and allows broad third harmonic to be generated for uses such as biological spectroscopy. Finally, we discuss the conversion efficiency and the optimal phase-matching wavelength with a single-frequency pump.

Fabrication of free-standing lithium niobate nanowaveguides down to 50 nm in width

Nonlinear optical nanoscale waveguides are a compact and powerful platform for efficient wavelength conversion. The free-standing waveguide geometry opens a range of applications in microscopy for local delivery of light, where in situ wavelength conversion helps to overcome various wavelength-dependent issues, such as biological tissue damage. In this paper, we present an original patterning method for high-precision fabrication of free-standing nanoscale waveguides based on lithium niobate, a material with a strong second-order nonlinearity and a broad transparency window covering the visible and mid-infrared wavelength ranges. The fabrication process combines electron-beam lithography with ion-beam enhanced etching and produces nanowaveguides with lengths from 5 to 50 μm, widths from 50 to 1000 nm and heights from 50 to 500 nm, each with a precision of few nanometers. The fabricated nanowaveguides are tested in an optical characterization experiment showing efficient second-harmonic generation.

Polarization Engineering in Nanoscale Waveguides Using Lossless Media

A device that achieves controllable rotation of the state of polarization by rotating the orientation of the eigenmodes of a waveguide by 45$^{\circ}$ is introduced and analyzed. The device can be implemented using lossless materials on a nanoscale and helps circumvent the inherent polarization dependence of photonic devices realized within the silicon on insulator platform. We propose and evaluate two novel polarization rotator-based schemes to achieve polarization engineering functions: (1) A multi-purpose device, with dimensions on the order of a few wavelengths which can function as a polarization splitter or an arbitrary linear polarization state generator. (2) An energy efficient optical modulator that utilizes eigenmode rotation and epsilon near zero (ENZ) effects to achieve high extinction ratio, polarization insensitive amplitude modulation without the need to sweep the device geometry to match the TE and TM mode attributes. By using indium tin oxide (ITO) as an example for a tunable material, the proposed modulator provides polarization insensitive operation and can be realized with a modulation bandwidth of 112 GHz, a length of 1800 nm an energy per bit of 7.5 $fJ$ and an optical bandwidth of 210 nm.

An Integrated Photonic Gas Sensor Enhanced by Optimized Fano Effects in Coupled Microring Resonators With an Athermal Waveguide

Microresonator-based photonic gas sensors exhibit unique advantages of building a CMOS-compatible, cost-effective, and portable sensing system fully integrated on a single chip. In this paper, we propose gas sensors featured by nanoscale waveguide layers using titanium dioxide to achieve athermal device operation, at close to visible wavelengths. Sensitivity and limit of detection are enhanced by Fano effects in coupled-microresonator devices (also called photonic molecules). Sharp Fano lineshapes are obtained by optimizing the dual-ring structures in terms of coupling coefficients and resonance offsets, with minimized cavity loss including material absorption, scattering and bending losses, and substrate leakage. Various types of dual-ring resonator configurations have been comprehensively compared. Overall performance of the on-chip sensing systems including integrated light sources and detectors is carefully analyzed by taking into account the imperfections of each component, such as relative intensity noise of laser, laser linewidth and frequency jitter, and detector noise. We outline the similarity and difference of three dual-ring systems and provide design guidelines to optimize the sensor with balanced consideration between device performance and its tolerance to fabrication imperfections.

Giant gain enhancement in surface‐confined resonant Stimulated Brillouin Scattering

AbstractThe notion that Stimulated Brillouin Scattering (SBS) is primarily defined by bulk material properties has been overturned by recent work on nanoscale waveguides. It is now understood that boundary forces of radiation pressure and electrostriction appearing in such highly confined waveguides can make a significant contribution to the Brillouin gain. Here, this concept is extended to show that gain enhancement does not require nanoscale or subwavelength features, but generally appears where optical and acoustic fields are simultaneously confined near a free surface or material interface. This situation routinely occurs in whispering gallery resonators (WGRs), making gain enhancements much more accessible than previously thought. To illustrate this concept, the first full‐vectorial analytic model for SBS in WGRs is developed, including optical boundary forces, and the SBS gain in common silica WGR geometries is computationally evaluated. These results predict that gains 104 times greater than the predictions of scalar theory may appear in WGRs even in the 100 μm size range. Further, trapezoidal cross‐section microdisks can exhibit very large SBS gains approaching 102 m−1W−1. With resonant amplification included, extreme gains on the order of 1012 m−1W−1 may be realized, which is 108 times greater than the highest predicted gains in linear waveguide systems. image

Diffraction grating polarization beam splitter using nano optical slits

In this work a polarization sensitive diffraction grating is proposed that has an effective density of lines being different according to the polarization of the input. This results in diffracting different polarizations at separate angles. The structure of the proposed diffraction grating consists of an arrangement of nanoscale waveguides in a metal slab. The structure is simulated using the finite difference time domain technique. The diffraction pattern from the structure is calculated using an integral expression for the free space propagation after the diffraction grating. A first order diffraction at an angle of 10.3° with an extinction ratio of 22.8 dB was obtained for one polarization and an angle of 15.7° with an extinction ratio of 23.4 dB was obtained for the second polarization.

Non-plasmonic nanoantennas for surface enhanced spectroscopies with ultra-low heat conversion.

Nanoplasmonics has recently revolutionized our ability to control light on the nanoscale. Using metallic nanostructures with tailored shapes, it is possible to efficiently focus light into nanoscale field ‘hot spots'. High field enhancement factors have been achieved in such optical nanoantennas, enabling transformative science in the areas of single molecule interactions, highly enhanced nonlinearities and nanoscale waveguiding. Unfortunately, these large enhancements come at the price of high optical losses due to absorption in the metal, severely limiting real-world applications. Via the realization of a novel nanophotonic platform based on dielectric nanostructures to form efficient nanoantennas with ultra-low light-into-heat conversion, here we demonstrate an approach that overcomes these limitations. We show that dimer-like silicon-based single nanoantennas produce both high surface enhanced fluorescence and surface enhanced Raman scattering, while at the same time generating a negligible temperature increase in their hot spots and surrounding environments.

Fabrication of nanoscale lithium niobate waveguides for second-harmonic generation.

Nanoscale waveguides are basic building blocks of integrated optical devices. Especially, waveguides made from nonlinear optical materials, such as lithium niobate, allow access to a broad range of applications using second-order nonlinear frequency conversion processes. Based on a lithium niobate on insulator substrate, millimeter-long nanoscale waveguides were fabricated with widths as small as 200 nm. The fabrication was done by means of potassium hydroxide-assisted ion-beam-enhanced etching. The waveguides were optically characterized in the near infrared wavelength range showing phase-matched second-harmonic generation.

Optical waveguide beam splitters based on hybrid metal–dielectric–semiconductor nanostructures

Miniature integration is desirable for the future photonics circuit. Low-dimensional semiconductor and metal nanostructures is the potential building blocks in compact photonic circuits for their unique electronic and optical properties. In this work, a hybrid metal–dielectric–semiconductor nanostructure is designed and fabricated to realizing a nano-scale optical waveguide beam splitter, which is constructed with the sandwiched structure of a single CdS nanoribbon/HfO2 thin film/Au nanodisk arrays. Micro-optical investigations reveal that the guided light outputting at the terminal end of the CdS ribbon is well separated into several light spots. Numerical simulations further demonstrate that the beam splitting mechanism is attributed to the strong electromagnetic coupling between the Au nanodisks and light guided in the nanoribbon. The number of the split beams (light spots) at the terminal end of the nanoribbon is mainly determined by the number of the Au nanodisk rows, as well as the distance of the blank region between the nanodisks array and the end of the CdS ribbon, owing to the interference between the split beams. These optical beam splitters may find potential applications in high-density integrated photonic circuits and systems.

Nanoplasmonic waveguides: towards applications in integrated nanophotonic circuits

The properties of propagating surface plasmon polaritons (SPPs) along one-dimensional metal structures have been investigated for more than 10 years and are now well understood. Because of the high confinement of electromagnetic energy, propagating SPPs have been considered to represent one of the best potential ways to construct next-generation circuits that use light to overcome the speed limit of electronics. Many basic plasmonic components have already been developed. In this review, researches on plasmonic waveguides are reviewed from the perspective of plasmonic circuits. Several circuit components are constructed to demonstrate the basic function of an optical digital circuit. In the end of this review, a prototype for an SPP-based nanochip is proposed, and the problems associated with building such plasmonic circuits are discussed. A plasmonic chip that can be practically applied is expected to become available in the near future. The prospects for creating sophisticated nanophotonic circuits by harnessing the opportunities provided by plasmonics are exciting. Yurui Fang and Mengtao Sun from the Beijing National Laboratory for Condensed Matter Physics review recent progress in the development of various components and devices for generating, manipulating and detecting surface plasmon polaritons — tightly confined waves that can be excited by light at the interface between a metal and a dielectric. Their small size offers opportunities for constructing photonic devices that are smaller than the wavelength of light, a major barrier to the miniaturization of optical integrated circuits. Fang and Sun describe how nanoscale waveguides, multiplexers, lasers, high-speed modulators, logic gates and detectors for surface plasmon polaritons are all coming to fruition. They also consider the future prospects of this technology.

Excitation of bound plasmons along nanoscale stripe waveguides: a comparison of end and grating coupling techniques

In this paper we excite bound long range stripe plasmon modes with a highly focused laser beam. We demonstrate highly confined plasmons propagating along a 50 µm long silver stripe 750 nm wide and 30 nm thick. Two excitation techniques were studied: focusing the laser spot onto the waveguide end and focusing the laser spot onto a silver grating. By comparing the intensity of the out-coupling photons at the end of the stripe for both grating and end excitation we are able to show that gratings provide an increase of a factor of two in the output intensity and thus out-coupling of plasmons excited by this technique are easier to detect. Authors expect that the outcome of this paper will prove beneficial for the development of passive nano-optical devices based on stripe waveguides, by providing insight into the different excitation techniques available and the advantages of each technique.

A self-consistent simulation of nonlinear surface plasmon polaritons in a linear–metal/nonlinear slab waveguide

Soliton-like nonlinear SPPs have shown to posses intensity dependent propagation characteristics. Two different dispersion relations with arbitrary parameters being input field intensity and position of soliton-like peak were derived and coupled using self-consistent calculations to avoid arbitrary parameters in order to study exact propagation characteristics of nonlinear SPP supported by a linear–metal/nonlinear slab waveguide. It is observed that the position of soliton-like peak in nonlinear medium has significant contribution in determining dispersion characteristics of nonlinear SPP. The propagation properties and mode profiles of nonlinear SPP obtained could help in exploring further possibilities of exciting and guiding SPP efficiently in nanoscale waveguides using nonlinear substrate.

Above-Bandgap Surface-Emitting Frequency Conversion in Semiconductor Nanoribbons With Ultralow Continuous-Wave Pump Power

Semiconductors have large optical nonlinear susceptibilities especially in the spectral range above material bandgaps. However, optical frequency conversion encounters large absorption above bandgaps and difficulty using common phase-matching techniques. The frequency conversion bandwidths are thus limited. Here, frequency up-conversion far above the bandgaps using surface emissions from semiconductor nanoribbons is demonstrated, wherein nanoscale waveguiding tightly confines fundamental waves for decreasing pump powers, and above-bandgap absorption is greatly decreased in nanoscale waveguide thickness. By using CdSe nanoribbons, efficient 532- and 404-nm second-harmonic and 459-nm sum-frequency generations are obtained with the continuous-wave pump power less than 100 μW. The normalized efficiency of 532-nm second-harmonic generation is about 2 × 10 -5 mm -1 at pump power of 300 μW. Attractive features such as tunable spatial distribution and highly polarization are observed. A broadband emission with a full width half maximum of ~10 nm is attained by frequency summing a continuous-wave laser and an amplified spontaneous emission source.

Simulation of Self-Aligned Optical Coupling Between Micro- and Nano-Scale Devices Using Self-Organized Waveguides

Using the finite-difference time-domain method, we simulated the growth of self-organized waveguides between a 3-μm-wide micro-scale waveguide and a 600-nm-wide nano-scale waveguide, which has a luminescent target on its core edge. The two waveguides are placed together, with gap sizes ranging from 16 to 64 μm, in a photo-induced refractive-index increase-type material. When a 400 nm wavelength write beam is introduced from the micro-scale waveguide, luminescence is generated by the luminescent target. A waveguide is then gradually self-organized between the two waveguides, even when a lateral misalignment of 600 nm exists between them, and provides a self-aligned optical coupling with a coupling loss of 1.5–1.8 dB. This indicates that the self-organized waveguide can be used as an optical solder to connect a micro-scale waveguide in a multi-chip module or printed circuit board to a nano-scale waveguide in a large-scale integrated circuit. The optimum writing time required to attain the minimum coupling loss increases with increasing lateral misalignment. The dependence of the optimum writing time on the misalignment is reduced with increasing gap distance, and the dependence almost vanishes when the gap distance is 64 μm, thus enabling unmonitored optical solder formation.