Waveguide Architecture Research Articles

75‐4: Late‐News Paper: Metalens‐Integrated Augmented Reality (AR) Waveguides for Eye‐tracking: A Proof of Concept

We demonstrate a proof‐of‐concept metalens‐integrated AR waveguide for imaging and eye‐tracking. The 4.5 mm metalens is stacked onto a diffractive exit‐pupil‐expander structure to provide aberration‐free imaging. The combined system achieves a 17×16.8‐degree field of view and an 11×9 mm eye box, currently limited by the waveguide's architecture and efficiency.

Slow-light silicon modulator with 110-GHz bandwidth.

Silicon modulators are key components to support the dense integration of electro-optic functional elements for various applications. Despite numerous advances in promoting the modulation speed, a bandwidth ceiling emerges in practices and becomes an obstacle toward Tbps-level throughput on a single chip. Here, we demonstrate a compact pure silicon modulator that shatters present bandwidth ceiling to 110 gigahertz. The proposed modulator is built on a cascade corrugated waveguide architecture, which gives rise to a slow-light effect. By comprehensively balancing a series of merits, the modulators can benefit from the slow light for better efficiency and compact size while remaining sufficiently high bandwidth. Consequently, we realize a 110-gigahertz modulator with 124-micrometer length, enabling 112 gigabits per second on-off keying operation. Our work proves that silicon modulators with 110 gigahertz are feasible, thus shedding light on its potentials in ultrahigh bandwidth applications such as optical interconnection and photonic machine learning.

Substrate-Integrated Hybrid Metallo-Dielectric Waveguide Architecture for Millimeter-Wave and Terahertz Applications

In millimeter-wave (mmW) and terahertz (THz) applications, the transmission behavior along circuit structures and components, such as radiation and leakage losses, is critical for their overall performances. It is imperative to develop low-loss interconnect and transmission techniques that should be used for the construction of building circuit blocks and elements. In this work, a hybrid metallo-dielectric (MD) waveguide architecture is proposed and studied for the first time. The scheme is made of mixed substrate-integrated dielectric waveguide (SIDW) and substrate-integrated nonradiative dielectric (SINRD) waveguide, which are deployed for the design of specific building parts in consideration of respective transmission properties of the two waveguides. A back-to-back WR3-band prototype based on this hybrid scheme is investigated theoretically and validated experimentally. The hybrid MD waveguide architecture with SINRD is found to outperform the hybrid MD waveguide architecture with substrate-integrated waveguide (SIW) in terms of transmission performance and manufacturing complexity because of the advantageous features of metallized-via-free structure. The presented hybrid MD waveguide architecture shows its potential for developing low-loss highly integrated mmW and THz circuits and systems.

Recent progress on femtosecond laser micro-/nano-fabrication of functional photonic structures in dielectric crystals: A brief review and perspective

Femtosecond (Fs) laser micro-/nano-fabrication technology allows direct definition of on-demand nanostructures with three-dimensional (3D) geometric features and tailored photonic functionalities in a facile manner. In addition, such a strategy is widely applicable to various material families, including dielectrics, semiconductors, and metals. Based on diverse dielectric crystals, fs-laser direct writing of optical waveguides with flexible geometries and functional waveguide-based photonic devices have been well-developed. Beyond waveguide architectures, the combination of 3D nanofabrication of fs lasers and the multi-functionalities of dielectric crystals has also lighted up the future development of novel photonic structures with features even beyond the optical diffraction limit. In this article, promising research topics on domain engineering for nonlinear optics, color centers and waveguides for integrated quantum photonics, and surface processing for integrated photonics enabled by fs laser micro-/nano-fabrication in dielectric crystals are briefly overviewed. We highlight recent progress on these research topics and stress the importance of optical aberration correction during laser fabrication, followed by a discussion of challenges and foreseeing the future development of fs laser defined nanostructures in dielectric crystals toward multi-functional photonics.

GHz operation of a quantum point contact using stub-impedance matching circuit

Quantum point contacts (QPC) are the building blocks of quantum dot qubits and semiconducting quantum electrical metrology circuits. QPCs also make highly sensitive electrical amplifiers with the potential to operate in the quantum-limited regime. Though the inherent operational bandwidth of QPCs can eclipse the THz regime, the impedance mismatch with the external circuitry limits the operational frequency to a few kHz. Lumped-element impedance-matching circuits are successful only up to a few hundreds of MHz in frequency. QPCs are characterised by a complex impedance consisting of quantized resistance, capacitance, and inductance elements. Characterising the complex admittance at higher frequencies and understanding the coupling of QPC to other circuit elements and electromagnetic environments will provide valuable insight into its sensing and backaction properties. In this work, we couple a QPC galvanically to a superconducting stub tuner impedance matching circuit realised in a coplanar waveguide architecture to enhance the operation frequency into the GHz regime and investigate the electrical amplification and complex admittance characteristics. The device, operating at ∼ 1.96GHz, exhibits a conductance sensitivity of 2.92×10−5(e2/h)/Hz with a bandwidth of 13MHz. Besides, the RF reflected power unambiguously reveals the complex admittance characteristics of the QPC, shining more light on the behaviour of quantum tunnel junctions at higher operational frequencies.

Design, Fabrication and Characterisation of Multi-Parameter Optical Sensors Dedicated to E-Skin Applications

For many years there has been a strong research interest in soft electronics for artificial skin applications. However, one challenge with stretchable devices is the limited availability of high performance, stretchable, electrical conductors and semiconductors that remain stable under strain. Examples of such electronic skin require excessive amounts of wires to address each sensing element-compression force and strain-in a conventional matrix structure. Here, we present a new process for fabricating artificial skin consisting of an optical waveguide architecture, enabling wide ranging sensitivity to external mechanical compression and strain. The manufacturing process allows design of a fully stretchable polydimethylsiloxane elastomer waveguide with embedded gratings, replicated from low cost DVD-Rs. This optical artificial skin allows the detection of compression forces from 0 to 3.8 N with controllable sensitivity. It also permits monitoring of elongation deformations up to 135%. This type of stretchable optical sensor is highly robust, transparent, and presents a large sensing area while limiting the amount of wires connecting to the sensor. Thus, this optical artificial skin presents far superior mechanical properties compared to current electronic skin.

Multifunctional nanopore electrode array method for characterizing and manipulating single entities in attoliter-volume enclosures

Structurally regular nanopore arrays fabricated to contain independently controllable annular electrodes represent a new kind of architecture capable of electrochemically addressing small collections of matter—down to the single entity (molecule, particle, and biological cell) level. Furthermore, these nanopore electrode arrays (NEAs) can also be interrogated optically to achieve single entity spectroelectrochemistry. Larger entities such as nanoparticles and single bacterial cells are investigated by dark-field scattering and potential-controlled single-cell luminescence experiments, respectively, while NEA-confined molecules are probed by single molecule luminescence. By carrying out these experiments in arrays of identically constructed nanopores, massively parallel collections of single entities can be investigated simultaneously. The multilayer metal–insulator design of the NEAs enables highly efficient redox cycling experiments with large increases in analytical sensitivity for chemical sensing applications. NEAs may also be augmented with an additional orthogonally designed nanopore layer, such as a structured block copolymer, to achieve hierarchically organized multilayer structures with multiple stimulus-responsive transport control mechanisms. Finally, NEAs constructed with a transparent bottom layer permit optical access to the interior of the nanopore, which can result in the cutoff of far-field mode propagation, effectively trapping radiation in an ultrasmall volume inside the nanopore. The bottom metal layer may be used as both a working electrode and an optical cladding layer, thus, producing bifunctional electrochemical zero-mode waveguide architectures capable of carrying out spectroelectrochemical investigations down to the single molecule level.

Enhanced Light–Tellurium Interaction through Evanescent Wave Coupling for High Speed Mid‐Infrared Photodetection

AbstractMid‐infrared (MIR) waveguide‐integrated photodetector is essential for various applications in the fields of sensing and optical communications. However, it is challenging to integrate traditional MIR photoactive materials such as HgCdTe or III−V compounds with complementary metal‐oxide‐semiconductor (CMOS)‐compatible silicon platform due to the lattice mismatch. Tellurium (Te), a novel van der Waals (vdW) material with a narrow bandgap, high carrier mobility, and great air stability, is a promising candidate for high‐performance MIR detection. Here, high‐quality Te nanosheets are synthesized using a hydrothermal method and their carrier dynamics are characterized by transient reflection spectrum. The effect of mobility anisotropy on response speed is investigated intuitively by a free space phototransistor. Combining the strong evanescent wave of a waveguide architecture with the synergy effect between the carrier collection path and the highest mobility crystal orientation in Te, an integrated Te photodetector with enhanced light–matter interaction and reduced carrier transit time is achieved. For the first time, the MIR waveguide‐integrated Te photodetector with a responsivity of 2.3 A W−1 and a bandwidth of 4 GHz at 2015 nm is realized, which is the highest speed MIR photodetector based on narrow bandgap vdW materials to date.

Multidirectional Polymer Waveguide Lattices for Enhanced Ultrawide-Angle Light Capture in Silicon Solar Cells.

We report the synthesis and characterization of a polymer thin-film structure consisting of two intersecting broadband optical waveguide lattices, and its performance in wide-angle optical energy collection and conversion in silicon solar cells. The structures are synthetically organized via the concurrent irradiation of photoreactive polymer blends by two arrays of intersecting, microscale optical beams transmitted through the medium. Through optical beam-induced photopolymerization and photopolymerization-induced phase separation, well-organized lattices are produced comprising of cylindrical core–cladding waveguide architectures that intersect one another. The optical waveguide properties of the lattices transform the transmission characteristics of the polymer film so that incident optical energy is collected and transmitted along the waveguide axes, rather than their natural directions dictated by refraction, thereby creating efficient light-collecting capability. The embedded structures collectively impart their wide-angle acceptance ranges to enable the film to efficiently collect and interact with light over a large angular range (±70°). When employed as the encapsulant material for a commercial silicon solar cell, the novel light collection and transmission properties result in greater wide-angle conversion efficiency and electrical current density, compared to a single vertically aligned waveguide array. The sustained and greater conversion of light afforded by the encapsulating optical material promises to increase solar cell performance by enabling ultrawide-angle solar energy conversion.

Non-classical mechanical states guided in a phononic waveguide

The ability to create, manipulate and detect non-classical states of light has been key for many recent achievements in quantum physics and for developing quantum technologies. Achieving the same level of control over phonons, the quanta of vibrations, could have a similar impact, in particular on the fields of quantum sensing and quantum information processing. Here we present a crucial step towards this level of control and realize a single-mode waveguide for individual phonons in a suspended silicon microstructure. We use a cavity–waveguide architecture, where the cavity is used as a source and detector for the mechanical excitations while the waveguide has a free-standing end to reflect the phonons. This enables us to observe multiple round trips of phonons between the source and the reflector. The long mechanical lifetime of almost 100 μs demonstrates the possibility of nearly lossless transmission of single phonons over, in principle, tens of centimetres. Our experiment demonstrates full on-chip control over travelling single phonons strongly confined in the directions transverse to the propagation axis, potentially enabling a time-encoded multimode quantum memory at telecommunications wavelength and advanced quantum acoustics experiments. Non-classical vibrations are generated and transmitted along a mechanical waveguide, providing a platform for distributing quantum information and realizing hybrid quantum devices using phonons in a solid-state system.

A new double-line waveguide architecture for photonic applications using fs laser writing in Nd3+ doped GeO2-PbO glasses

A new double-line waveguide architecture produced in Nd3+ doped GeO2-PbO glasses is presented for photonic applications. The waveguides are written directly into Nd3+ doped GeO2-PbO glasses using a Ti:Sapphire femtosecond (fs) laser, operating at 800 nm, delivering 30 fs pulses at 10 kHz repetition rate and writing speed of 0.5 mm/s. Two parallel lines form a dual-waveguide each line being a result of either 4 or 8 superimposed lines. Results of propagation losses, M2 beam quality factor at 632 and 1064 nm, refractive index change, and relative gain at the signal wavelength (1064 nm) are presented. Structural changes, due to laser writing process were investigated by Raman spectroscopy. The observed near-field pattern image showed good waveguiding quality, consisting of a single, circular lobe. X,y-symmetrical guiding for both waveguides was observed. The relative gain reached 4.5 and 6.0 dB/cm for 4 and 8 superimposed lines, respectively, for 420 mW of 808 nm pumping. Propagation losses were 0.89 and 0.44 dB/cm, for 4 and 8 superimposed lines, respectively, leading to positive internal gain of 3.6 and 5.56 dB/cm. The results obtained in the present work demonstrate that this new double line architecture for Nd3+ doped GeO2-PbO glasses is promising for the fabrication of integrated amplifiers, lossless components and lasers.

4-Bit All-Optical Serial-to-Parallel Converter With Sub-dB/cm Delay Lines Based on Rib Waveguides

This paper extends our previous work on the microring resonator-type serial-to-parallel conversion scheme aimed for all-optical label processing on the silicon photonic platform. In the previously proposed scheme, the architecture of the delay lines was silicon wires. This resulted in considerable propagation loss, confining multibit operation of serial-to-parallel conversion to a label packet of 2 bits. However, during a practical application, propagation loss at the delay lines has to be improved to support label packets comprising of a greater number of bits. The purpose of this article is twofold. First, we confirm that propagation loss can be lowered to sub-dB/cm values by introducing rib waveguides via simulations and experiment. Second, we demonstrate multibit serial-to-parallel conversion on a 4-bit label “1101” by utilizing a cascaded microring resonator device of which the straight sections of the delay lines are of rib waveguide architecture. In order to focus on free-carrier dispersion effect of silicon during operation, we left out a temporal delay between the pump pulses and probe bits to be extracted. As a result, serial-to-parallel conversion was viable even at peak pump power levels as low as ∼100 mW. We hereby verify that the proposed serial-to-parallel conversion scheme incorporated with rib waveguide delay lines is a realistic solution in realizing a photonics-based solution to all-optical label processing.

RF Analysis of a Sub-GHz InP-Based 1550 nm Monolithic Mode-Locked Laser Chip

We report a monolithic sub-GHz repetition rate mode-locked laser with record low pulse-to-pulse RMS timing jitter of 3.65 ps in the passive mode locking regime. We analyse the optical pulse generation in passive and hybrid mode-locking operating regimes, finding narrower RF tone linewidth in the passive regime, attributed to the improved contact structure of the gain sections. The noise performance is also characterized in passive and hybrid regimes, showing RMS integrated timing jitter of approximately 600 fs. For hybrid modelocking, the repetition rate can be varied over a large range from 880 to 990 MHz. We observe broad pulse widths of few hundred picoseconds attributed to the (long folded) waveguide architecture and on-chip multimode interference mirrors. This device subjects a stand-alone, ultra-compact, mode-locking based clock source to realize frequency synthesizers operating over a frequency range from sub-GHz up to approximately 15 GHz.

Design and Simulation Investigation of Si3N4 Photonics Circuits for Wideband On-Chip Optical Gas Sensing around 2 µm Optical Wavelength.

We theoretically explore the potential of Si3N4 on SiO2 waveguide platform toward a wideband spectroscopic detection around the optical wavelength of 2 μm. The design of Si3N4 on SiO2 waveguide architectures consisting of a Si3N4 slot waveguide for a wideband on-chip spectroscopic sensing around 2 μm, and a Si3N4 multi-mode interferometer (MMI)-based coupler for light coupling from classical strip waveguide into the identified Si3N4 slot waveguides over a wide spectral range are investigated. We found that a Si3N4 on SiO2 slot waveguide structure can be designed for using as optical interaction part over a spectral range of interest, and the MMI structure can be used to enable broadband optical coupling from a strip to the slot waveguide for wideband multi-gas on-chip spectroscopic sensing. Reasons for the operating spectral range of the system are discussed.

Femtosecond laser direct writing of depressed cladding waveguides in Nd:YAG with "ear-like" structures: fabrication and laser generation.

Low-loss depressed cladding waveguide architecture is highly attractive for improving the laser performance of waveguide lasers. We report on the design and fabrication of the "ear-like" waveguide structures formed by a set of parallel tracks in neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal via femtosecond laser writing. The obtained "ear-like" waveguides are with more symmetric mode profiles and lower losses by systematically comparing the guiding properties of two kinds of normal cladding waveguide. Efficient waveguide lasers are realized based on the designed structure in both continuous wave and pulsed regimes. Combined the high-gain from cladding waveguide and special "ear-like" structure, a passively fundamentally Q-switched laser with the narrow pulse width and the high repetition rate has been obtained by using tin diselenide (SnSe2) as saturable absorber.

Efficient leaky-wave antennas at terahertz frequencies generating highly directional beams

Due to their frequency-dependent angular emission, leaky-wave antennas have been recently introduced in the terahertz band to tackle many of the challenges associated with THz wireless communications. Most previous works have exploited conventional leaky-wave waveguide architectures developed for the microwave region. In this paper, we study in detail the emission characteristics of leaky-wave antennas at THz frequencies. We show that, at these high frequencies, the wavelength-scale interaction with the aperture causes a nonuniform electric field distribution at the slot interface, which is a unique regime that is not typically encountered at lower frequencies. This effect is even more pronounced as the slot width increases to a point where the sides of the slot act as secondary leaking structures, and the well-known frequency–angle relationship is not obeyed as the energy at a given frequency is radiated in a broad range of angles. Therefore, to exploit the phase matching condition, which couples frequency to emission angle, one must use very thin rectangular slots d≪λ, at the expense of device efficiency. To address this problem, we explore an alternate slot aperture design, in which the slot width increases linearly along its length (i.e., a trapezoidal shape). We show that this preserves the phase-matching constraint while allowing higher output coupling efficiencies. Moreover, since a wider effective aperture is used, the radiated beam is narrow in both angular directions, allowing the generation of true pencil-like THz beams.

Slim-panel holographic video display

Since its discovery almost 70 years ago, the hologram has been considered to reproduce the most realistic three dimensional images without visual side effects. Holographic video has been extensively researched for commercialization, since Benton et al. at MIT Media Lab developed the first holographic video systems in 1990. However, commercially available holographic video displays have not been introduced yet for several reasons: narrow viewing angle, bulky optics and heavy computing power. Here we present an interactive slim-panel holographic video display using a steering-backlight unit and a holographic video processor to solve the above issues. The steering-backlight unit enables to expand the viewing angle by 30 times and its diffractive waveguide architecture makes a slim display form-factor. The holographic video processor computes high quality holograms in real-time on a single-chip. We suggest that the slim-panel holographic display can provide realistic three-dimensional video in office and household environments.

Optical Gain in Ultrathin Self‐Assembled Bi‐Layers of Colloidal Quantum Wells Enabled by the Mode Confinement in their High‐Index Dielectric Waveguides

This study demonstrates an ultra-thin colloidal gain medium consisting of bi-layers of colloidal quantum wells (CQWs) with a total film thickness of 14nm integrated with high-index dielectrics. To achieve optical gain from such an ultra-thin nanocrystal film, hybrid waveguide structures partly composed of self-assembled layers of CQWs and partly high-index dielectric material are developed and shown: in asymmetric waveguide architecture employing one thin film of dielectric underneath CQWs and in the case of quasi-symmetric waveguide with a pair of dielectric films sandwiching CQWs. Numerical modeling indicates that the modal confinement factor of ultra-thin CQW films is enhanced in the presence of the adjacent dielectric layers significantly. The active slabs of these CQW monolayers in the proposed waveguide structure are constructed with great care to obtain near-unity surface coverage, which increases the density of active particles, and to reduce the surface roughness to sub-nm scale, which decreases the scattering losses. The excitation and propagation of amplified spontaneous emission (ASE) along these active waveguides are experimentally demonstrated and numerically analyzed. The findings of this work offer possibilities for the realization of ultra-thin electrically driven colloidal laser devices, providing critical advantages including single-mode lasing and high electrical conduction.

High-Power 1.5 μm Tapered Distributed Bragg Reflector Laser Diodes for Eye-Safe LIDAR

A high-power InAlGaAs/InP tapered distributed Bragg reflector laser diode with narrow linewidth emission at $1.5~\mu \text{m}$ is reported. The laser has a monolithic waveguide architecture comprising a third-order grating section for longitudinal mode selection, an index-guided gain section for lateral mode filtering, and a gain-guided tapered section for power scaling. An output power of 770 mW is reported for continuous wave operation at room temperature. In pulsed mode, the laser delivered a peak power of 4.6 W with a full width at half-maximum spectral linewidth of only 250 pm. In addition to the narrow linewidth and high-power features, the emission wavelength exhibits a temperature dependent shift of only 0.1 nm/°C. The parameters achieved suggest that these laser diodes would enable the realization of compact LIDAR systems with improved signal-to-noise ratio, owing to the high output power and the possibility to use narrow passband filters at receiver side, which is enabled by the narrow and temperature-stable emission spectrum. The wavelength range around $1.5~\mu \text{m}$ also enables LIDAR systems with high output powers while maintaining eye safety, ultimately leading to improved system performance.

Enhancing light‐matter interaction in 2D materials by optical micro/nano architectures for high‐performance optoelectronic devices

AbstractTwo‐dimensional materials are a promising solution for next‐generation electronic and optoelectronic devices due to their unique properties. Owing to the atomic thickness of 2D materials, the light‐matter interaction length in 2D materials is much shorter than that in bulk materials, which limits the performance of optoelectronic devices composed of 2D materials. To improve the light‐matter interactions, optical micro/nano architectures have been introduced into 2D material optoelectronic devices. In this review, we present a concise introduction and discussion of various strategies for the enhancement of light‐matter interaction in 2D materials, namely, the plasmonic effect, waveguide, optical cavity, and reflection architecture. We have outlined the current advances in high‐performance 2D material optoelectronic devices (eg, photodetectors, electro‐optic modulators, light‐emitting diodes, and molecular sensors) assisted by these enhancement strategies. Finally, we have discussed the future challenges and opportunities of micro/nano photonic structure designs in 2D material devices.image