Scientific research
Generating a nanoscale blade-like optical field in a coupled nanofiber pair

An optical field with sub-nm confinement is essential for exploring atomic- or molecular-level light-matter interaction.While such fields demonstrated so far have typically point-like cross-sections, an optical field having a higher-dimensional cross-section may offer higher flexibility and/or efficiency in applications. Here, we propose generating a nanoscale blade-like optical field in a coupled nanofiber pair (CNP) with a 1-nm-width central slit. Based on a strong mode coupling-enabled slit waveguide mode, a sub-nm-thickness blade-like optical field can be generated with a cross-section down to ∼0.28 nm × 38 nm at 1550 nm wavelength (i.e., a thickness of ∼λ0∕5000) and a peak-to-background intensity ratio higher than 20 dB. The slit waveguide mode of the CNP can be launched from one of the two nanofibers that are connected to a standard optical fiber via an adiabatical fiber taper, in which a fundamental waveguide mode of the fiber can be converted into a high-purity slit mode with high efficiency (>98%) within a CNP length of less than 10 μm at 1550 nm wavelength. The wavelength-dependent behaviors and group velocity dispersion in mode converting processes are also investigated, showing that such a CNP-based design is also suitable for broadband and ultrafast pulsed operation. Our results may open up new opportunities for studying light-matter interaction down to the sub-nm scale, as well as for exploring ultra-high-resolution optical technology ranging from super-resolution nanoscopy to chemical bond manipulation.
— Y. X. Yang et al., Photonics Res. 12, 154-162 (2024).

Generating a sub-nanometer-confined optical field in a nanoslit waveguiding mode

We propose to generate a sub-nanometer-confined optical field in a nanoslit waveguiding mode in a coupled nanowire pair (CNP). We show that, when a conventional waveguide mode with a proper polarization is evanescently coupled into a properly designed CNP with a central nanoslit, it can be efficiently channeled into a high-purity nanoslit mode within a waveguiding length <10 μm. The CNP can be either freestanding or on-chip by using a tapered fiber or planar waveguide for input-coupling, with a coupling efficiency up to 95%. Within the slit region, the output diffraction-limited nanoslit mode offers an extremely confined optical field (∼0.3 nm × 3.3 nm) with a peak-to-background ratio higher than 25 dB and can be operated within a 200-nm bandwidth. The group velocity dispersion of the nanoslit mode for ultrafast pulsed operation is also briefly investigated. Compared with the previous lasing configuration, the waveguiding scheme demonstrated here is not only simple and straightforward in structural design but is also much flexible and versatile in operation. Therefore, the waveguiding scheme we show here may offer an efficient and flexible platform for exploring light–matter interactions beyond the nanometer scale, and developing optical technologies ranging from superresolution nanoscopy and atom/molecule manipulation to ultra-sensitivity detection.
— L. Yang et al., Adv. Photonics 5, 046003 (2023).

Efficient fiber-to-chip interface via an intermediated CdS nanowire

An efficient fiber-to-chip interface via an intermediated CdS nanowire is demonstrated. The fiber mode is firstly squeezed through a fiber taper drawn at the end of a single-mode fiber, then evanescently coupled into an intermediated CdS nanowire with a longitudinally tapering profile, and finally coupled into an on-chip silicon waveguide (SiW) via a waveguide taper fabricated on it. Since the fiber-nanowire-SiW cascade structure is designed to match effective indices in each coupling area, such a fiber-to-chip interface ensures a bidirectional coupling efficiency up to 90% and a 3-dB bandwidth over 100 nm in experiments. The difference of coupling efficiencies between the TM or TE modes is less than 0.5 dB in the spectral range of 1545–1635 nm. The footprint of the on-chip coupling structure is about 10 µm in size. The results may provide a compact, efficient, and versatile fiber-to-chip interface in applications including optical interconnects, coherent communication, and quantum optical circuitry.
— Y. Y. Jin et al., Laser Photon. Rev. 17, 2200919 (2023).

Waveguide-integrated light-emitting metal-insulator-graphene tunnel junctions

Ultrafast interfacing of electrical and optical signals at the nanoscale is highly desired for on-chip applications including optical interconnects and data processing devices. Here, we break through the limitation of low-efficiency waveguided output of inelastic tunneling-based light sources by developing waveguide-integrated metal−insulator−graphene tunnel junctions (MIG-TJs) based on an organic layer-coated silver nanowire (AgNW) interfaced with graphene. The use of graphene instead of one of the metal electrodes of conventional tunnel junctions not only eliminates the highly lossy MIM modes and therefore ensures efficient delivery of the IET-excited optical signal to the edge of the tunnel junction where it can be extracted but also offers a small mode mismatch between the tunnel junction region and the plasmonic nanowire waveguide, which enables efficient coupling (∼70%) of the optical signal from the edge of the junction to the output AgNW waveguide. Furthermore, we demonstrate the coupling of the AgNW-integrated tunnel junction with a low-loss semiconductor nanowire waveguide, thus realizing efficient outcoupling of the IET-excited optical signal to both highly integrated plasmonic and low-loss photonic channels. Finally, as a proof of principle, a direct electrical modulation of the outputted optical signal has been also demonstrated.
— L. F. Liu et al., Nano Lett. 23, 3731−­3738 (2023).

Effect of Mirror Quality on Optical Response of Nanoparticle-on-Mirror Plasmonic Nanocavities

As an essential part of nanoparticle-on-mirror (NPoM) plasmonic nanocavities, metal mirrors play an important role not only in determining the optical response of the nanocavities but also their performance in applications. In this work, we experimentally study the effect of mirror quality on the optical response of NPoM nanocavities including nanosphere-on-mirror (NSoM) and nanocube-on-mirror (NCoM) designs. Polycrystalline sputtered gold films (SGFs), template-stripped gold films (TSGFs) and single-crystalline gold microflakes (GMFs) were investigated and compared. Due to the great improvement in the surface roughness that can minimize fluctuations in the gap morphology, NSoM and NCoM nanocavities formed on smooth TSGFs and GMFs have a better cavity-to-cavity homogeneity in the scattering spectrum than those formed on comparably rougher SGFs. In addition, there is an obvious change in the spectral positions of the resonance modes of NSoM and NCoM nanocavities formed on different gold films due to the variation in dielectric functions and surface quality of the gold films, which is reproduced very well by theoretical calculations based on measured dielectric functions of the gold films. Moreover, due to the reduction in electron scattering losses from SGF to TSGF and GMF, an increase in the quality factors and scattering intensities of the resonance modes was observed for nanocavities formed on the corresponding films.
— Z. X. Wang et al., Adv. Opt. Mater. 11, 2201914 (2023).

High-power continuous-wave optical waveguiding in a silica micro/nanofibre

As miniature fibre-optic platforms, micro/nanofibres (MNFs) taper-drawn from silica fibres have been widely studied for applications from optical sensing, nonlinear optics to optomechanics and atom optics. While continuous-wave (CW) optical waveguiding is frequently adopted, so far almost all MNFs are operated in low-power region (e.g., <0.1 W). Here, we demonstrate high-power low-loss CW optical waveguiding in MNFs around 1550-nm wavelength. We show that a pristine MNF, even with a diameter down to 410 nm, can waveguide an optical power higher than 10 W, which is about 30 times higher than demonstrated previously. Also, we predict an optical damage threshold of 70 W. In high-power CW waveguiding MNFs, we demonstrate high-speed optomechanical driving of microparticles in air, and second harmonic generation efficiency higher than those pumped by short pulses. Our results may pave a way towards high-power MNF optics, for both scientific research and technological applications.
— J. B. Zhang et al., Light-Sci. Appl. 12, 89 (2023).

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes. As a versatile platform that can enhance and manipulate optical phenomena on the nanoscale, plasmonic metamaterials will continue to advance molecular plasmonics for the development of future photonic applications including ultrahigh-sensitivity optical sensing, nanolasers, active functionalities, and photoactuated nanochemistry.
— P. Wang et al., Chem. Rev. 122, 15031–15081 (2022)

Strong Mode Coupling Enabled Hybrid Photon-plasmon Laser with a Microfiber-coupled Nanorod

Laser based on single plasmonic nanoparticle can provide optical frequency radiation far beyond the diffraction limit and is one of the ultimate goals of nanolasers, yet it remains a challenge to be realized because of the inherently high Ohmic loss. Here, we report the direct observation of lasing in microfiber-coupled single plasmonic nanoparticles enabled by strong mode coupling. We show that, by strongly coupling a gold nanorod (GNR) with the whispering gallery cavity of a dye-doped polymer microfiber (with diameter down to 2.0 μm), the substantially enhanced optical coherence of the hybrid photon-plasmon mode and effective gain accumulated from the active microfiber cavity enable single-mode laser emission from the GNR at room temperature with a threshold as low as 2.71 MW/cm2 and a linewidth narrower than 2 nm. The results demonstrated here provide a feasible approach for the realization of lasers based on single plasmonic nanoparticles and may find applications in areas such as ultraconfined field manipulation, ultrasensitive sensing, and on-chip optical interconnects.
— N. Zhou et al., Sci. Adv. 8, eabn2026 (2022).

Photonic Nanolaser with Extreme Optical Field Confinement

We proposed a photonic approach to a lasing mode supported by low-loss oscillation of polarized bound electrons in an active nano-slit-waveguide cavity, which circumvents the confinement-loss trade-off of nanoplasmonics, and offers an optical confinement down to sub-1-nm level with a peak-to-background ratio of ~30 dB. Experimentally, the extremely confined lasing field is realized as the dominant peak of a TE0-like lasing mode around 720-nm wavelength, in 1-nm-level width slit-waveguide cavities in coupled CdSe nanowire pairs. The measured lasing characteristics agree well with the theoretical calculations. Our results may pave a way towards new regions for nanolasers and light-matter interaction.
— H. Wu et al., Phys. Rev. Lett. 129, 013902 (2022).

Atomically Smooth Single-Crystalline Platform for Low-Loss Plasmonic Nanocavities

As a typical type of plasmonic nanostructures, nanoparticle-on-mirror nanocavity (NPoM), with the capability of extreme optical confinement and enhancement, has triggered a series of breakthrough in nanophotonic researches and applications. Here, we study the optical response of nanorod on mirror structure both experimentally and theoretically. The strong coupling of a gold nanorod with an ultraflat single-crystalline gold microflake produces rich and high-quality plasmonic modes related to complex interaction of film-coupled nanorod modes and waveguide modes in the gap. Their optical responses are very sensitive to the thickness (down to 15 nm) and refractive index of the dielectric spacer. Compared with the traditional nanocavity based on thermally evaporated gold film, our nanocavity not only has stronger scattering intensity, but also has a significant improvement in the quality factor of each mode. This low-loss NPoM nanocavity structure can also integrate with fiber system efficiently and compactly. High-quality NPoM nanocavities are attractive for studying light-matter interaction at the nanoscale, and promising for the construction of nanophotonic devices such as nanolasers and plasmon-enhanced sensors.
— L. F. Liu et al., Nano Lett. 22, 1786-1794 (2022).

Strong Coupling of a Plasmonic Nanoparticle to a Semiconductor Nanowire

Strong coupling of a localized plasmonic mode to a low-loss photonic mode in a hybrid plasmonic-photonic cavity provides a promising solution to elongate the LSPR lifetime via coherent energy exchange and recirculation between the two otherwise uncoupled modes. Here, by placing a single Au nanoparticle on the surface of a cadmium sulfide (CdS) nanowire, strong coupling of localized surface plasmon resonance (LSPR) modes in the nanoparticle and whispering gallery modes (WGMs) in the nanowire is demonstrated. For a 50-nm-diameter Au-nanosphere particle, strong coupling occurs when the nanowire diameter is between 300 nm and 600 nm, with a mode splitting up to 80 meV. Using a temperature-induced spectral shift of the resonance wavelength, the anti-crossing behavior in the strongly coupled system is observed. In addition, since the Au nanosphere has spherical symmetry, the supported LSPR mode can be selectively coupled with TE and TM WGMs in the nanowire. The ultra-compact strong-coupling system shown here may provide a versatile platform for studying hybrid “photon-plasmon” nanolasers, nonlinear optical devices and nanosensors.
— Y. Y. Jin et al., Nanophotonics 10, 2875-2881 (2021).

Elastic Ice Microfibers

Ice is known to be a rigid and brittle crystal that fractures when deformed. We demonstrate that ice grown as single-crystal ice microfibers (IMFs) with diameters ranging from 10 micrometers to less than 800 nanometers is highly elastic. Under cryotemperature, we could reversibly bend the IMFs up to a maximum strain of 10.9%, which approaches the theoretical elastic limit. We also observed a pressure-induced phase transition of ice from Ih to II on the compressive side of sharply bent IMFs. The high optical quality allows for low-loss optical waveguiding and whispering-gallery-mode resonance in our IMFs. The discovery of these flexible ice fibers opens opportunities for exploring ice physics and ice-related technology on micro- and nanometer scales.
— P. Z. Xu et al., Science 373, 187-192 (2021).
See also news and views in
                                  1、E. M. Schulson, “Ice physics: A flexible and springy form of ice”, Science 373, 158 (2021).
                                  5、The New York Times —— A New Kind of Ice That Bends Like a Noodle Without Breaking
                                  6、New Scientist —— New kind of ice is so bendy it can curl and uncurl without breaking
                                  7、ScienceNews —— These weird, thin ice crystals are springy and bendy
                                  12、DeepTech深科技——冰也有弹性!浙大团队成功生长高质量冰单晶微纳光纤,实现10.9%的弹性                                             应变 | 专访
                                  13、Wissenschaft aktuell —— Elastische Fasern aus Eis
                                  14、NRC —— Perfect ijs breekt niet, maar buigt
                                  15、GIZMODO —— This Bendable Ice Is Freaking Us Out

Batch Fabrication of High-Quality Infrared Chalcogenide Microsphere Resonators

Optical microsphere resonators working in the near- to mid-infrared regions are highly required for a variety of applications, such as optical sensors, filters, modulators and microlasers. Here, a simple and low-cost approach for batch fabrication of high-quality chalcogenide glass (ChG) microspheres is demonstrated. By melting high-purity ChG powders in a heated oil environment, As2S3 and As2Se3 microspheres with excellent surface smoothness and extremely low eccentricity (~0.13%) can be readily fabricated. They exhibit excellent resonant responses both in near- and mid-infrared spectral ranges (Q factor of ~106 around 1550 nm, and ~104 around 4.5 µm). The high-quality ChG microspheres demonstrated here are highly attractive for near- and mid-infrared optics, including optical sensing, optical nonlinearity, cavity quantum electrodynamics, microlasers, nanofocusing and microscopic imaging.
— Y. Xie et al., Small 17, 2100140 (2021).

Optical Micro/Nanofiber-Enabled Compact Tactile Sensor for Hardness Discrimination

Optical micro/nanofibers (MNFs) can be applied for ultrasensitive tactile sensing with fast response and compact size, which are attractive for restoring tactile information in minimally invasive robotic surgery and tissue palpation. Herein, we present a compact tactile sensor (CTS) with a diameter of 1.5 mm enabled by an optical MNF. The CTS provides continuous readouts for high-fidelity transduction of touch and pressure stimuli into interpretable optical signals, which permits instantaneous sensing of contact and pressure with pressure-sensing sensitivity as high as 0.108 mN−1 and a resolution of 0.031 mN. Working in pressing mode, the CTS can discriminate the difference between samples with shore a hardness of 36 and 40, a hardness resolving ability even beyond the human hands. Benefitting from the fast response feature, the CTS can also be operated in either scanning or tapping mode, making it feasible for hardness identification by analyzing the shape of the response curve. Such MNF-enabled compact tactile sensors may pave the way for hardness sensing in tissue palpation, surgical robotics, and object identification.
— Y. Tang et al., ACS Appl. Mater. Interfaces 13, 4560-4566 (2021).

Wearable Optical Sensors Based on Glass Micro/nanofibers Nonuniformity

Wearable sensor is of vital importance for health monitoring, human-machine interaction and smart robot. In order to monitor pressure or displacement with high sensitivity and fast response, we demonstrate a skin-like wearable optical sensor (SLWOS) based on glass optical micro/nanofibers (MNFs) in thin layers of polydimethylsiloxane (PDMS). Enabled by the transition from guided modes into radiation modes of the waveguiding MNFs upon external stimuli, the skin-like optical sensors show ultrahigh sensitivity (1870 kPa-1), low detection limit (7 mPa) and fast response (10 μs) for pressure sensing, significantly exceeding the performance metrics of state-of-the-art electronic skins. Electromagnetic interference (EMI)-free detection of high-frequency vibrations, wrist pulse and human voice are realized. Moreover, a five-sensor optical data glove and a 2×2-MNF tactile sensor are demonstrated. These initial results pave the way toward a new category of optical devices ranging from ultrasensitive wearable sensors to optical skins.
— L. Zhang et al., Opto-Electron Adv. 3, 190022 (2020).
See also highlight in Opto-Electronic Advances

On-Chip Single-Mode Nanowire Lasers

By integrating a free-standing cadmium sulfide (CdS) nanowire onto a silicon nitride (SiN) photonic chip, we demonstrate a highly compact on-chip single-mode CdS nanowire laser. The mode selection is realized using a Mach-Zehnder interferometer (MZI) structure. When the pumping intensity exceeds the lasing threshold of 4.9 kW/cm2, on-chip single-mode lasing at ~518.9 nm is achieved with a linewidth of 0.1 nm and a side-mode suppression ratio of up to a factor of 20 (13 dB). The output of the nanowire laser is channelled into an on-chip SiN waveguide with high efficiency (up to 58%) by evanescent coupling, and the directional coupling ratio between the two output ports can be varied from 90 to 10% by predesigning the coupling length of the SiN waveguide. Our results open new opportunities for both nanowire photonic devices and on-chip light sources and may pave the way towards a new category of hybrid nanolasers for chip-integrated applications.
— Q. Y. Bao et al., Light-Sci. Appl. 9, 42 (2020).
See also highlight in Sensorexpert

Mid-Infrared Chalcogenide Glass Microfiber and Its Applications

Owing to their special merits including broadband transparency, high optical nonlinearity, and hospitality to rare-earth dopants, chalcogenide glasses (ChGs) have been considered as highly promising mid-infrared (mid-IR) photonic materials. Here, we demonstrate ChG microfiber knot resonators (MKRs) operating in the mid-IR for the first time, and investigate the wavelength tunability, the structure of a PMMA-embedded on-chip ChG MKR. ChG microfibers with typical diameters around 3 μm are taper-drawn from As2S3 glass fibers and assembled into MKRs in liquid without surface damage. The measured Q factor of a typical 824 μm diameter ChG MKR is about 2.84 × 104 at the wavelength of 4469.14 nm. The free spectral range (FSR) of the MKR can be tuned from 2.0 nm (28.4 GHz) to 9.6 nm (135.9 GHz) by tightening the knot structure in liquid. Benefitting from the high thermal expansion coefficient of As2S3 glass, the MKR exhibits a thermal tuning rate of 110 pm • °C-1 at the resonance peak. When embedded in polymethyl methacrylate (PMMA) film, a 551 μm diameter MKR retains a Q factor of 1.1 × 104. The ChG MKRs demonstrated here are highly promising for resonator-based optical technologies and applications in the mid-IR spectral range.
— Y. Xie et al., Photonics Res. 8, 616-621 (2020).

A New Route for Fabricating Polymer Optical Microcavities

In this study, SU-8 WGM microcavities are fabricated via a simple self-assembling method, with smooth surface (σ < 0.6 nm) and high Q factors (~104). The obtained microcavities show hemispherical shape, and can be transferred onto arbitrary substrates with flat surfaces for characterizations and applications. As an application of the microcavities, a sensitive temperature sensor is demonstrated by evanescently coupling a SU-8 hemisphere with a silica microfiber. Owing to the package of polydimethylsiloxane (PDMS), the sensor shows excellent long-term stability over one year with a sensitivity as high as 120 pm/°C in a range of 25-125°C. These results illustrate broad application potential of these simple fabricated polymer microcavities in ultrasensitive sensors and microlasers.
— Z. Zhang et al., Nanoscale 11, 5203-5208 (2019).

Broadband Quasi-Phase-Matched Harmonic Generation from Lithium Niobate Microdisk

For light waves in the TE mode propagating along the circumference of a lithium niobate (LN) microdisk, the effective nonlinear optical coefficients naturally oscillate periodically to change both the sign and magnitude, facilitating natural quasi-phase matching (QPM) without the necessity of domain engineering in the micrometer-scale LN disk. The second harmonic and cascaded third-harmonic waves are simultaneously generated with normalized conversion efficiencies as high as 9.8% mW-1 and 1.05% mW-2, respectively, thanks to the utilization of the highest nonlinear coefficient d33 of LN. The high efficiency achieved with the microdisk of a radius of ~30 μm is beneficial for realizing high-density integration of nonlinear photonic devices such as wavelength convertors and entangled photon sources.
— J. T. Lin, N. Yao et al., Phys. Rev. Lett. 122, 173903 (2019).

High-purity Electrically-driven Single-photon Source at Room Temperature

Photonic quantum information requires high-purity, easily-accessible, and scalable single-photon sources. Here we report an electrically driven single-photon source based on colloidal quantum dots. Our solution-processed devices consist of isolated CdSe/CdS core/shell quantum dots sparsely buried in an insulating layer that is sandwiched between electron-transport and hole-transport layers. The devices generate single photons with near-optimal antibunching at room temperature without any spectral filtering. Such highly suppressed multi-photon-emission probability is attributed to both novel device design and carrier injection/recombination dynamics.
— X. Lin et al., Nat. Commun. 8, 1132 (2017).

Integrating Nanowires into Silicon Photonics

Silicon photonics has been developed successfully with a top-down fabrication technique to enable large-scale photonic integrated circuits with high reproducibility, but is limited intrinsically by the material capability for active or nonlinear applications. On the other hand, free-standing nanowires synthesized via a bottom-up growth present great material diversity and structural uniformity, but precisely assembling free-standing nanowires for on-demand photonic functionality remains a great challenge. Here we report hybrid integration of free-standing nanowires into silicon photonics with high flexibility by coupling free-standing nanowires onto target silicon waveguides that are simultaneously used for precise positioning. Coupling efficiency between a free-standing nanowire and a silicon waveguide is up to ~97% in the telecommunication band. A hybrid nonlinear-free-standing nanowires–silicon waveguides Mach–Zehnder interferometer and a racetrack resonator for significantly enhanced optical modulation are experimentally demonstrated, as well as hybrid active-free-standing nanowires–silicon waveguides circuits for light generation. These results suggest an alternative approach to flexible multifunctional on-chip nanophotonic devices.
— B. G. Chen et al., Nat. Commun. 8, 20 (2017).

2D Materials for Optical Modulation: Challenges and Opportunities

Owing to their atomic layer thickness, strong light-material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D materialbased optical modulation in three categories is reviewed: free-space, fiberbased, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.
— S. L. Yu et al., Adv. Mater. 29, 1606128 (2017).

Single CdTe Nanowire Optical Correlator for Femtojoule Pulses

Relying on the transverse second harmonic (TSH) generation in a highly nonlinear CdTe nanowire, we demonstrate a single nanowire optical correlator. Pulses to be measured were equally split and coupled into two ends of a suspending nanowire via tapered optical fibers. By transferring the spatial intensity profile of the TSH image into the time-domain temporal profile of the input pulses, we operate the nanowire as a miniaturized optical correlator with input energy goes down to 2 fJ/pulse for 1064 nm 200 fs pulses. The miniature fJ-pulse correlator may find applications from low power on-chip optical communication, biophotonics to ultracompact laser spectroscopy.
— C. G. Xin et al., Nano Lett. 16, 4807-4810 (2016).

All-optical graphene modulator based on optical Kerr phase shift

Graphene-based optical modulators have recently attracted much attention because of their characteristic ultrafast and broadband response. Their modulation depth (MD) and overall transmittance (OT), however, are often limited by optical loss arising from interband transitions. We report here an all-optical, all-fiber optical modulator with a Mach-Zehnder interferometer structure that has significantly higher MD and OT than graphene-based loss modulators. It is based on the idea of converting optically induced phase modulation in the graphene-cladded arm of the interferometer to intensity modulation at the output of the interferometer. The device has the potential to be integrable into a photonic system in real applications.
— S. L. Yu et al., Optica 3, 541-544 (2016).

Single-band 2-nm-linewidth plasmon resonance in a strongly coupled Au nanorod

This paper reports a dramatic reduction in plasmon resonance line width of a single Au nanorod by coupling it to a whispering gallery cavity of a silica microfiber. With fiber diameter below 6 μm, strong coupling between the nanorod and the cavity occurs, leading to evident mode splitting and spectral narrowing. Using a 1.46-μm-diameter microfiber, we obtained single-band 2-nm-line-width plasmon resonance in an Au nanorod around a 655-nm-wavelength, with a quality factor up to 330 and extinction ratio of 30 dB. Compared to an uncoupled Au nanorod, the strongly coupled nanorod offers a 30-fold enhancement in the peak intensity of plasmonic resonant scattering.
— P. Wang et al., Nano Lett. 15, 7581–7586 (2015).

Graphene-doped polymer optical nanofibers

Graphene-doped polymer nanofibers are fabricated by taper drawing solvated polyvinyl alcohol doped with liquid-phase exfoliated graphene flakes. As-drawn nanofibers, with typically diameter of hundreds of nanometers and length up to tens of millimeters, show excellent uniformity and surface smoothness for optical waveguiding. Owing to their tightly confined waveguiding behavior, light-matter interaction in these subwavelength-diameter nanofibers is significantly enhanced. Using femto-second pulses around 1350-nm wavelength, we demonstrate saturable absorption behaviors of such nanofibers with a saturation threshold down to 0.25 pJ/pulse (peak power ~1.3 W). Additionally, using 1064-nm-wavelength naosecond pulses as switching light, we show all-optical modulation of a 1550-nm-wavelength signal light guided along a single nanofiber with switching peak power of ~3.2 W.
— C. Meng et al., Light: Science & Applications 4, e348 (2015).

Single nanowire optical correlator

Integration of miniaturized elements has been a major driving force behind modern photonics. Nanowires have emerged as potential building blocks for compact photonic circuits and devices in nanophotonics. We demonstrate here a single nanowire optical correlator (SNOC) for ultrafast pulse characterization based on imaging of the second harmonic (SH) generated from a cadmium sulfide (CdS) nanowire by counterpropagating guided pulses. The SH spatial image can be readily converted to the temporal profile of the pulses, and only an overall pulse energy of 8 μJ is needed to acquire a clear image of 200 fs pulses. Such a correlator should be easily incorporated into a photonic circuit for future use of onchip ultrafast optical technology.
— H. K. Yu et al., Nano Lett. 14, 3487-3490 (2014).

Ultrafast (200-GHz) graphene all-optical modulator

Graphene offers broadband light-matter interactions with ultrafast responses. The bandwidth of previous graphene-based optical modulators were usually limited to ~1 GHz by electric parasite response. Now by using an all-optical scheme, Li et al. show that a graphene-clad microfiber all-optical modulator can achieve a modulation depth of 38% and a response time of ∼2.2 ps, corresponding to a bandwidth of ~200 GHz. This modulator is compatible with current high-speed fiber-optic communication networks and may open the door to meet future demand of ultrafast optical signal processing.
— W. Li et al., Nano Lett. 14, 955-959 (2014). ACS Editor's Choice

Subtle balance in nanowaveguides: loss v.s. confinement

In deep-subwavelength optical nanowires, the waveguiding loss increases with increasing optical confinement, which may also lead to long-wavelength cutoff. Guo et al. pictured subtle balance between loss, confinement and more in a recent article.
— X. Guo et al., Acc. Chem. Res. 47, 656-666 (2014).

Photon-plasmon hybrid nanowire (NW) laser

By near-field coupling a CdSe and a Ag nanowires, Wu et al. demonstrated a hybrid photon-plasmon laser operating at 723 nm wavelength at room temperature, which offers subdiffraction-limited beam size and pure plasmon modes with mode area of 0.008λ2.
— X. Q. Wu et al., Nano Lett. 13, 5654-5659 (2013).
See also highlight in Phys.ORG
"Photon-plasmon nanowire laser offers new opportunities in light manipulation" by Lisa Zyga

70% photon-to-plasmon conversion efficiency in a single Au nanorod

When a Au nanorod is placed inside an optical nanofiber, it can be efficiently excited by the waveguiding mode with photon-to-plasmon conversion efficiency as high as 70%, and is highly potential for realizing ultra-low power nanoparticle plasmonic devices. Check out more details of our Au-nanorod-doped polymer nanofibers.
— P. Wang et al., Nano Lett. 12, 3145-3150 (2012).