Xiaoqin Wu

Wavelength Tunable CdSe Nanowire Lasers Based on the Absorption‐Emission‐Absorption Process

Wavelength tunability of lasers is one of the most important parameters for practical applications such as optical communication, environmental monitoring, and spectroscopy analysis. [ 14–18 ] Recently, tunable semiconductor NW and nanoribbon lasers have been realized, using tunable bandgap nanostructures as the composition tunable gain media. [ 19 –26 ] In this approach, alloyed semiconductor NWs with different bandgaps are necessary components for wavelength tuning; a careful composition control is required in order to obtain high crystal quality stoichiometric NWs. [ 27–29 ] Alternatively, the peak wavelength of NW lasers can also be tuned by changing the geometry of the cavity structures, but the tunable range is limited to about 10 nm. [ 7 , 13 ]

Hybrid Photon-Plasmon Nanowire Lasers

By near-field coupling a high-gain CdSe nanowire (NW) and a 100-nm-diameter Ag NW, we demonstrate a hybrid photon-plasmon laser operating at 723-nm wavelength at room temperature, with a plasmon mode area of 0.008λ2. This device simultaneously provides spatially separated photonic far-field output and highly localized coherent plasmon modes with sub-diffraction-limited beam size.

Semiconductor nanowire lasers

Semiconductor nanowires (or other wire-like nanostructures, including nanoribbons and nanobelts) synthesized by bottom-up chemical growth show single-crystalline structures, excellent geometric uniformities, subwavelength transverse dimensions, and relatively high refractive indices, making these one-dimensional structures ideal optical nanowaveguides with tight optical confinement and low scattering loss. When properly pumped by optical or electrical means, lasing oscillation can be readily established inside these high-gain active nanowires with feedback from endface reflection or near-field coupling effects, making it possible to realize nanowire lasers with miniature sizes and high flexibilities. Also, the wide-range material availability bestows the semiconductor nanowire with lasing wavelength selectable within a wide spectral range from ultraviolet (UV) to near infrared (IR). As nanoscale coherent light sources, in recent years, nanowire lasers have been attracting intensive attention for both fundamental research and technological applications ranging from optical sensing, signal processing, and on-chip communications to quantum optics. Here, we present a review of the status and perspectives of semiconductor nanowire lasers, with a particular emphasis on their optical characteristics categorized in two groups: (1)xa0waveguiding related properties in Sectionxa03, which includes waveguide modes, near-field coupling, endface reflection, substrate-induced effects, and nanowire microcavities, and (2)xa0optically pumped semiconductor nanowire lasers in Sectionxa04, starting from principles and basic types of UV, visible, and near-IR nanowire lasers relying on Fabry–Perot cavities, to advanced configurations including wavelength-tunable, single-mode operated, fiber-coupled, and metal-incorporated nanowire lasing structures for more possibilities. In addition, the material aspects of semiconductor nanowires, including nanowire synthesis and electrically driven nanowire lasers, are briefly reviewed in Sectionsxa02 and 5, respectively. Finally, in Sectionxa06 we present a brief summary of semiconductor nanowire lasers regarding their current challenges and future opportunities.

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 material-based optical modulation in three categories is reviewed: free-space, fiber-based, 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.

Optical microfibers and nanofibers

Abstract As a combination of fiber optics and nanotechnology, optical microfibers and nanofibers (MNFs) have been emerging as a novel platform for exploring fiber-optic technology on the micro/nanoscale. Typically, MNFs taper drawn from glass optical fibers or bulk glasses show excellent surface smoothness, high homogeneity in diameter and integrity, which bestows these tiny optical fibers with low waveguiding losses and outstanding mechanical properties. Benefitting from their wavelength- or sub-wavelength-scale transverse dimensions, waveguiding MNFs exhibit a number of interesting properties, including tight optical confinement, strong evanescent fields, evident surface field enhancement and large and abnormal waveguide dispersion, which makes them ideal nanowaveguides for coherently manipulating light, and connecting fiber optics with near-field optics, nonlinear optics, plasmonics, quantum optics and optomechanics on the wavelength- or sub-wavelength scale. Based on optical MNFs, a variety of technological applications, ranging from passive micro-couplers and resonators, to active devices such as lasers and optical sensors, have been reported in recent years. This review is intended to provide an up-to-date introduction to the fabrication, characterization and applications of optical MNFs, with emphasis on recent progress in our research group. Starting from a brief introduction of fabrication techniques for physical drawing glass MNFs in Section 2, we summarize MNF optics including waveguiding modes, evanescent coupling, and bending loss of MNFs in Section 3. In Section 4, starting from a “MNF tree” that summarizes the applications of MNFs into 5 categories (waveguide & near field optics, nonlinear optics, plasmonics, quantum & atom optics, optomechanics), we go to details of typical technological applications of MNFs, including optical couplers, interferometers, gratings, resonators, lasers and sensors. Finally in Section 5 we present a brief summary of optical MNFs regarding their current challenges and future opportunities.

Single mode lasing in coupled nanowires

We demonstrate single mode lasing in coupled CdSe nanowires. By coupling two 420 nm diameter CdSe nanowires to form an X-structure cavity, single-mode lasing emission around 734.3 nm is obtained with line width of 0.11 nm and lasing threshold of about 120 μJ/cm2. Mode selection in the lasing nanowire is realized via Vernier effect in the coupled cavities. Our results suggest a simple approach to single-mode nanowire lasers.

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 on-chip ultrafast optical technology.

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.

Graphene decorated microfiber for ultrafast optical modulation.

With a convenient and controllable evanescent-field-induced transfer method, graphene flakes were deposited on the surface of a 1-μm-diameter microfiber, which can be used for ultrafast optical modulation based on its distinct saturable absorption.

Short-range structure of Zr41Ti14Cu12.5Ni10Be22.5 glass prepared by shock wave

Short-range structure of a Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass prepared by shock-wave treatment was investigated by x-ray diffraction using synchrotron radiation with a wavelength of 0.112u200a71 A. The radial distribution function was obtained from S(Q) with a large Q value up to 20 A−1. The oscillation in S(Q) of the glass definitely persists up to Q∼14u2002A−1. The shoulder on the high Q side of the second peak is observed in the bulk glass. It is found that the glass has higher coordination numbers in the range of r∼2.4–5.6 A and lower numbers in the range of r∼5.6–9.5 A than those for a water-quenched glass while the shell distances are similar in both glasses prepared by shock-wave and water-quenching methods. In the shock-waved glass, atomic configurations in the first, fourth, or fifth coordination shells are modified, i.e., atoms are packed denser in the first two coordination shells and less (or more free volume) in the third and fourth coordination shells as compared to those for the water-que...