Photon-Pair Generation with a 100 nm Thick Carbon Nanotube Film

Engineering Quantum Light Sources with Flat Optics

Quantum light sources are essential building blocks for many quantum technologies, enabling secure communication, powerful computing, and precise sensing and imaging. Recent advancements have witnessed a significant shift toward the utilization of "flat" optics with thickness at subwavelength scales for the development of quantum light sources. This approach offers notable advantages over conventional bulky counterparts, including compactness, scalability, and improved efficiency, along with added functionalities. This review focuses on the recent advances in leveraging flat optics to generate quantum light sources. Specifically, the generation of entangled photon pairs through spontaneous parametric down-conversion in nonlinear metasurfaces, and single photon emission from quantum emitters including quantum dots and color centers in 3D and 2D materials are explored. The review covers theoretical principles, fabrication techniques, and properties of these sources, with particular emphasis on the enhanced generation and engineering of quantum light sources using optical resonances supported by nanostructures. The diverse application range of these sources is discussed and the current challenges and perspectives in the field are highlighted.

High Quantum Yield Amino Acid Carbon Quantum Dots with Unparalleled Refractive Index

Carbon quantum dots (CQDs) are one of the most promising types of fluorescent nanomaterials due to their exceptional water solubility, excellent optical properties, biocompatibility, chemical inertness, excellent refractive index, and photostability. Nitrogen-containing CQDs, which include amino acid based CQDs, are especially attractive due to their high quantum yield, thermal stability, and potential biomedical applications. Recent studies have attempted to improve the preparation of amino acid based CQDs. However, the highest quantum yield obtained for these dots was only 44%. Furthermore, the refractive indices of amino acid derived CQDs were not determined. Here, we systematically explored the performance of CQDs prepared from all 20 coded amino acids using modified hydrothermal techniques allowing more passivation layers on the surface of the dots to optimize their performance. Intriguingly, we obtained the highest refractive indices ever reported for any CQDs. The values differed among the amino acids, with the highest refractive indices found for positively charged amino acids including arginine-CQDs (∼2.1), histidine-CQDs (∼2.0), and lysine-CQDs (∼1.8). Furthermore, the arginine-CQDs reported here showed a nearly 2-fold increase in the quantum yield (∼86%) and a longer decay time (∼8.0 ns) compared to previous reports. In addition, we also demonstrated that all amino acid based CQD materials displayed excitation-dependent emission profiles (from UV to visible) and were photostable, water-soluble, noncytotoxic, and excellent for high contrast live cell imaging or bioimaging. These results indicate that amino acid based CQD materials are high-refractive-index materials applicable for optoelectronic devices, bioimaging, biosensing, and studying cellular organelles in vivo. This extraordinary RI may be highly useful for exploring cellular elements with different densities.

Photon pairs bi-directionally emitted from a resonant metasurface

Metasurfaces are artificially structured surfaces able to control the properties of light at subwavelength scales. While, initially, they have been proposed as means to control classical optical fields, they are now emerging as nanoscale sources of quantum light, in particular of entangled photons with versatile properties. Geometric resonances in metasurfaces have been recently used to engineer the frequency spectrum of entangled photons, but the emission directivity was so far less studied. Here, we generate photon pairs via spontaneous parametric down conversion from a metasurface supporting a quasi-bound state in the continuum (BIC) leading to remarkable emission directivities. The pair generation rate is enhanced 67 times compared to the case of an unpatterned film of the same thickness and material. At the wavelength of the quasi-BIC resonance, photons are mostly emitted backwards, while their partners, spectrally detuned by only 8 nm, are emitted forwards. This behavior demonstrates fine spectral splitting of entangled photons and their bi-directional emission, never before observed in nanoscale sources. We expect this work to be a starting point for the efficient demultiplexing of photons in nanoscale quantum optics.

Flat-optics generation of broadband photon pairs with tunable polarization entanglement

The concept of "flat optics" is quickly conquering different fields of photonics, but its implementation in quantum optics is still in its infancy. In particular, polarization entanglement, strongly required in quantum photonics, is so far not realized on "flat" platforms. Meanwhile, relaxed phase matching of "flat" nonlinear optical sources enables enormous freedom in tailoring their polarization properties. Here we use this freedom to generate photon pairs with tunable polarization entanglement via spontaneous parametric downconversion (SPDC) in a 400-nm GaP film. By changing the pump polarization, we tune the polarization state of photon pairs from maximally entangled to almost disentangled, which is impossible in a single bulk SPDC source. Polarization entanglement, together with the broadband frequency spectrum, results in an ultranarrow (12 fs) Hong-Ou-Mandel effect and promises extensions to hyperentanglement.

Carbon Nanotube Devices for Quantum Technology

Carbon nanotubes, quintessentially one-dimensional quantum objects, possess a variety of electrical, optical, and mechanical properties that are suited for developing devices that operate on quantum mechanical principles. The states of one-dimensional electrons, excitons, and phonons in carbon nanotubes with exceptionally large quantization energies are promising for high-operating-temperature quantum devices. Here, we discuss recent progress in the development of carbon-nanotube-based devices for quantum technology, i.e., quantum mechanical strategies for revolutionizing computation, sensing, and communication. We cover fundamental properties of carbon nanotubes, their growth and purification methods, and methodologies for assembling them into architectures of ordered nanotubes that manifest macroscopic quantum properties. Most importantly, recent developments and proposals for quantum information processing devices based on individual and assembled nanotubes are reviewed.

Photon Pairs from Resonant Metasurfaces

All-dielectric optical metasurfaces are a workhorse in nano-optics, because of both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to 2 orders of magnitude, compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons-a new promising platform for quantum optics experiments.

Entangled photons from subwavelength nonlinear films

Miniaturized entangled photon sources, in particular based on subwavelength metasurfaces, are highly demanded for the development of integrated quantum photonics. Here, as a first step towards the development of quantum optical metasurfaces (QOMs), we demonstrate generation of entangled photons via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieve photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. By implementing the fiber spectroscopy of SPDC in nanofilms, we measure a spectrum with a bandwidth of 500 nm, limited only by the overall detection efficiency. The spectrum reveals vacuum field enhancement due to a Fabry-Perot resonance inside the nonlinear films. It also suggests a strategy for observing SPDC from QOM. Our experiments lay the groundwork for future development of flat SPDC sources, including QOM.

Evaluation of graphene optical nonlinearity with photon-pair generation in graphene-on-silicon waveguides

We evaluate the nonlinear coefficient of graphene-on-silicon waveguides through the coincidence measurement of photon-pairs generated via spontaneous four-wave mixing. We observed the temporal correlation of the photon-pairs from the waveguides over various transfer layouts of graphene sheets. A simple analysis of the experimental results using coupled-wave equations revealed that the atomically-thin graphene sheets enhanced the nonlinearity of silicon waveguides up to ten-fold. The results indicate that the purely χ ⁽³⁾-based effective nonlinear refractive index of graphene is on the order of 10^-13 m ²/W, and provide important insights for applications of graphene-based nonlinear optics in on-chip nanophotonics.

Ionic Liquid Gated Carbon Nanotube Saturable Absorber for Switchable Pulse Generation

Materials with electrically tunable optical properties offer a wide range of opportunities for photonic applications. The optical properties of the single-walled carbon nanotubes (SWCNTs) can be significantly altered in the near-infrared region by means of electrochemical doping. The states' filling, which is responsible for the optical absorption suppression under doping, also alters the nonlinear optical response of the material. Here, for the first time we report that the electrochemical doping can tailor the nonlinear optical absorption of SWCNT films and demonstrate its application to control pulsed fiber laser generation. With a pump-probe technique, we show that under an applied voltage below 2 V the photobleaching of the material can be gradually reduced and even turned to photoinduced absorption. Furthermore, we integrated a carbon nanotube electrochemical cell on a side-polished fiber to tune the absorption saturation and implemented it into the fully polarization-maintaining fiber laser. We show that the pulse generation regime can be reversibly switched between femtosecond mode-locking and microsecond Q-switching using different gate voltages. This approach paves the road toward carbon nanotube optical devices with tunable nonlinearity.

Rayleigh-Instability-Induced Bismuth Nanorod@Nitrogen-Doped Carbon Nanotubes as A Long Cycling and High Rate Anode for Sodium-Ion Batteries

Sodium-ion battery (SIB) as one of the most promising large-scale energy storage devices has drawn great attention in recent years. However, the development of SIBs is limited by the lacking of proper anodes with long cycling lifespans and large reversible capacities. Here we present rational synthesis of Rayleigh-instability-induced bismuth nanorods encapsulated in N-doped carbon nanotubes (Bi@N-C) using Bi₂S₃ nanobelts as the template for high-performance SIB. The Bi@N-C electrode delivers superior sodium storage performance in half cells, including a high specific capacity (410 mA h g^-1 at 50 mA g^-1), long cycling lifespan (1000 cycles), and superior rate capability (368 mA h g^-1 at 2 A g^-1). When coupled with homemade Na₃V₂(PO₄)₃/C in full cells, this electrode also exhibits excellent performances with high power density of 1190 W kg^-1 and energy density of 119 Wh kg^-1_total. The exceptional performance of Bi@N-C is ascribed to the unique nanorod@nanotube structure, which can accommodate volume expansion of Bi during cycling and stabilize the solid electrolyte interphase layer and improve the electronic conductivity.

Nonlinear Optics with 2D Layered Materials

2D layered materials (2DLMs) are a subject of intense research for a wide variety of applications (e.g., electronics, photonics, and optoelectronics) due to their unique physical properties. Most recently, increasing research efforts on 2DLMs are projected toward the nonlinear optical properties of 2DLMs, which are not only fascinating from the fundamental science point of view but also intriguing for various potential applications. Here, the current state of the art in the field of nonlinear optics based on 2DLMs and their hybrid structures (e.g., mixed-dimensional heterostructures, plasmonic structures, and silicon/fiber integrated structures) is reviewed. Several potential perspectives and possible future research directions of these promising nanomaterials for nonlinear optics are also presented.

Nanomaterial-Based Plasmon-Enhanced Infrared Spectroscopy

Surface-enhanced infrared absorption (SEIRA) has attracted increasing attention due to the potential of infrared spectroscopy in applications such as molecular trace sensing of solids, polymers, and proteins, specifically fueled by recent substantial developments in infrared plasmonic materials and engineered nanostructures. Here, the significant progress achieved in the past decades is reviewed, along with the current state of the art of SEIRA. In particular, the plasmonic properties of a variety of nanomaterials are discussed (e.g., metals, semiconductors, and graphene) along with their use in the design of efficient SEIRA configurations. To conclude, perspectives on potential applications, including single-molecule detection and in vivo bioassays, are presented.

Towards spontaneous parametric down conversion from monolayer MoS2

We present a detailed study of the second order nonlinearity of 2D (mono-atomic layer) dichalcogenide MoS₂, both in the visible and in the IR regime, and test its potential for spontaneous parametric down-conversion (SPDC), the amplification of vacuum fluctuations mediated by optical nonlinearity. We develop a model of SPDC from a deeply subwavelength nonlinear medium, where phase matching conditions are completely relaxed, and make predictions about the rate of emitted photons, their momentum, polarisation and spectrum. We show that detection in the visible spectral region is hindered by the strong photoluminescence background. Moving to the IR regime we observe indications of SPDC by performing polarization, power dependence and lifetime measurements around 1560 nm. We show that the signal from a single monolayer is qualitatively different from that generated by multi-layer MoS₂. Finally, we characterize the latter as a new kind of photo-luminescence emission which is enhanced at the edges of multi-layer MoS₂.

Wavelength and pulse duration tunable ultrafast fiber laser mode-locked with carbon nanotubes

Ultrafast lasers with tunable parameters in wavelength and time domains are the choice of light source for various applications such as spectroscopy and communication. Here, we report a wavelength and pulse-duration tunable mode-locked Erbium doped fiber laser with single wall carbon nanotube-based saturable absorber. An intra-cavity tunable filter is employed to continuously tune the output wavelength for 34 nm (from 1525 nm to 1559 nm) and pulse duration from 545 fs to 6.1 ps, respectively. Our results provide a novel light source for various applications requiring variable wavelength or pulse duration.