There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

ZnS nanostructures: From synthesis to applications

Semiconducting quantum dots, whose particle sizes are in the nanometer range, have very unusual properties. The quantum dots have band gaps that depend in a complicated fashion upon a number of factors, described in the article. Processing-structure-properties-performance relationships are reviewed for compound semiconducting quantum dots. Various methods for synthesizing these quantum dots are discussed, as well as their resulting properties. Quantum states and confinement of their excitons may shift their optical absorption and emission energies. Such effects are important for tuning their luminescence stimulated by photons (photoluminescence) or electric field (electroluminescence). In this article, decoupling of quantum effects on excitation and emission are described, along with the use of quantum dots as sensitizers in phosphors. In addition, we reviewed the multimodal applications of quantum dots, including in electroluminescence device, solar cell and biological imaging.

https://www.mdpi.com/1996-1944/3/4/2260/pdf

Quantum Dots and Their Multimodal Applications: A Review

Recent developments in chip-based nonlinear photonics offer the tantalizing prospect of realizing many applications that can use optical frequency comb devices that have form factors smaller than 1 cm3 and that require less than 1 W of power. A key feature that enables such technology is the tight confinement of light due to the high refractive index contrast between the core and the cladding. This simultaneously produces high optical nonlinearities and allows for dispersion engineering to realize and phase match parametric nonlinear processes with laser-pointer powers across large spectral bandwidths. In this Review, we summarize the developments, applications and underlying physics of optical frequency comb generation in photonic-chip waveguides via supercontinuum generation and in microresonators via Kerr-comb generation that enable comb technology from the near-ultraviolet to the mid-infrared regime. This Review discusses the developments and applications of on-chip optical frequency comb generation based on two concepts—supercontinuum generation in photonic-chip waveguides and Kerr-comb generation in microresonators.

Photonic-chip-based frequency combs

Trapped ions are among the most promising systems for practical quantum computing (QC). The basic requirements for universal QC have all been demonstrated with ions, and quantum algorithms using few-ion-qubit systems have been implemented. We review the state of the field, covering the basics of how trapped ions are used for QC and their strengths and limitations as qubits. In addition, we discuss what is being done, and what may be required, to increase the scale of trapped ion quantum computers while mitigating decoherence and control errors. Finally, we explore the outlook for trapped-ion QC. In particular, we discuss near-term applications, considerations impacting the design of future systems of trapped ions, and experiments and demonstrations that may further inform these considerations.

Trapped-ion quantum computing: Progress and challenges

Lithium niobate (LN), dubbed by many as the silicon of photonics, has recently risen to the forefront of chip-scale nonlinear optics research since its demonstration as an ultralow-loss integrated photonics platform. Due to its significant quadratic nonlinearity (χ(2)), LN inspires many important applications such as second-harmonic generation (SHG), spontaneous parametric downconversion, and optical parametric oscillation. Here, we demonstrate high-efficiency SHG in dual-resonant, periodically poled z-cut LN microrings, where quasi-phase matching is realized by field-assisted domain engineering. Meanwhile, dual-band operation is accessed by optimizing the coupling conditions in fundamental and second-harmonic bands via a single pulley waveguide. As a result, when pumping a periodically poled LN microring in the low power regime at around 1617 nm, an on-chip SHG efficiency of 250,000%/W is achieved, a state-of-the-art value reported among current integrated photonics platforms. An absolute conversion efficiency of 15% is recorded with a low pump power of 115 μW in the waveguide. Such periodically poled LN microrings also present a versatile platform for other cavity-enhanced quasi-phase-matched χ(2) nonlinear optical processes.

Periodically poled thin-film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/W

ZnS nanowires with periodically alternating twins along the growth direction of wires have been synthesized via a thermal evaporation method. Structural characterizations reveal that both sphalerite (cubic) and wurtzite (hexagonal) phases coexist in the synthesized nanowires. The periodically twinned ZnS nanowires are formed through the vapor-solid mechanism. High quality of the nanowires is confirmed by the optical absorption and the photoluminescence observations combined with large ratio surface and interfaces induced by the twin modulations. The perfect crystal structure indicates their potential applications in nanoscience and nanotechnology.

Structure and optical properties of periodically twinned ZnS nanowires

Chip-scale mode-locked dissipative Kerr solitons have been realized on various materials platforms, making it possible to achieve a miniature, highly coherent frequency comb source with high repetition rates. Aluminum nitride (AlN), an appealing nonlinear optical material having both Kerr (χ3), and Pockels (χ2) effects, has immerse potential for comb self-referencing without the need for external harmonic generators. However, cavity soliton states have not yet been achieved in AlN microresonators. Here, we demonstrate mode-locked Kerr cavity soliton generation in a crystalline AlN microring resonator. By utilizing ultrafast tuning of the pump frequency through single-sideband modulation, in combination with an optimized wavelength scan and pump power-ramp patterns, we can deterministically elongate a ∼400  ns short-lived soliton to a time span as long as we wish to hold it.

High-fidelity cavity soliton generation in crystalline AlN micro-ring resonators.

Kerr soliton microcombs have recently emerged as a prominent topic in integrated photonics and have enabled new horizons for optical frequency metrology. Kerr soliton microcombs, as the name suggests, are based on high-order cubic optical nonlinearity. It is desirable to exploit quadratic photonic materials, namely Pockels materials, for soliton generation and on-chip implementation of 1f–2f comb self-referencing. Such quadratically driven solitons have been proposed theoretically, but have not yet been observed in a nanophotonic platform, despite recent progress in quadratic comb generation in free-space and crystalline resonators. Here, we report photonic-chip-based Pockels microcomb solitons driven by three-wave mixing in an aluminium nitride microring resonator. In contrast to typical Kerr solitons, the Pockels soliton features unity soliton generation fidelity, two-by-two evolution of multi-soliton states, favourable tuning dynamics and high pump-to-soliton conversion efficiency. Photonic-chip-based microcomb solitons driven by Pockels nonlinearity—the quadratic χ(2) effect—instead of the Kerr soliton are demonstrated in an aluminium nitride microring resonator with a conversion efficiency of 17%.

Pockels soliton microcomb

Lithium niobate (LN), dubbed by many as the silicon of photonics, has recently risen to the forefront of chip-scale nonlinear optics research since its demonstration as an ultralow-loss integrated photonics platform. Due to its significant quadratic nonlinearity ($\chi^{(2)}$), LN inspires many important applications such as second-harmonic generation (SHG), spontaneous parametric down-conversion, and optical parametric oscillation. Here, we demonstrate high-efficiency SHG in dual-resonant, periodically poled z-cut LN microrings, where quasi-phase matching is realized by field-assisted domain engineering. Meanwhile, dual-band operation is accessed by optimizing the coupling conditions in fundamental and second-harmonic bands via a single pulley waveguide. As a result, when pumping a periodically poled LN microring in the low power regime at around 1617nm, an on-chip SHG efficiency of 250,000%/W is achieved, a state-of-the-art value reported among current integrated photonics platforms. An absolute conversion efficiency of 15% is recorded with a low pump power of 115$\mu$W in the waveguide. Such periodically poled LN microrings also present a versatile platform for other cavity-enhanced quasi-phase matched $\chi^{(2)}$ nonlinear optical processes.

Xianwen Liu

Papers

Periodically poled thin-film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/W

Structure and optical properties of periodically twinned ZnS nanowires

High-fidelity cavity soliton generation in crystalline AlN micro-ring resonators.

Pockels soliton microcomb

Periodically poled thin film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/W