Tailoring the Lasing Modes in Semiconductor Nanowire Cavities Using Intrinsic Self-Absorption

Understanding the optical gain and mode-selection mechanisms in semiconductor nanowire (NW) lasers is key to the development of high-performance nanoscale oscillators, amplified semiconductor/plasmon lasers and single photon emitters, and so forth. Modification of semiconductor band structure/bandgap through electric field modulation, elemental doping, or alloying semiconductors has so far gained limited success in achieving output mode tunability of the NW laser. One stifling issue is the considerable optical losses induced in the NW cavities by these extrinsic methods that limit their applicability. Herein we demonstrate a new optical self-feedback mechanism based on the intrinsic self-absorption of the gain media to achieve low-loss, room-temperature NW lasing with a high degree of mode selectivity (over 30 nm). The cadmium sulfide (CdS) NW lasing wavelength is continuously tunable from 489 to 520 nm as the length of the NWs increases from 4 to 25 μm. Our straightforward approach is widely applicable in most semiconductor or semiconductor/plasmonic NW cavities

Total Syntheses of β‑Carboline Alkaloids Manzamine C, Orthoscuticelline C, and Quassidine S

A regioselective olefin hydrofunctionalization reaction of pavettine (4) with various nucleophiles was developed and used as the key step in the total syntheses of β-carboline natural products manzamine C (3), orthoscuticelline C (5), and quassidine S (6). In the 6-step total synthesis of manzamine C (3), an efficient two-step procedure, comprising a Wittig olefination reaction and a Fukuyama–Mitsunobu reaction, was devised for the synthesis of the N-macrocycle with a Z-olefin

Inflection Point of the Localized Surface Plasmon Resonance Peak: A General Method to Improve the Sensitivity

The shift of the localized surface plasmon resonance (LSPR) spectrum is widely used in bio- and chemical sensing. Traditionally, the shift is monitored at the peak maximum of the extinction spectrum. We demonstrate that the inflection point at the long wavelength side of the peak maximum shows better refractive index sensitivity than the peak maximum. A consistent improvement in bulk refractive index sensitivity of 18–55% is observed for six different nanoparticles such as spherical particles of different sizes, nanostar and nanorods with different aspect ratios. Local refractive index changes induced by molecular adsorption confirm the superior performance of the method. We contribute this improvement in sensitivity to the change in shape of the LSPR peak in response to an increase of the local refractive index. We further illustrate the advantage of using the inflection point method for analyzing DNA adsorption on U-shaped metamaterials, and for using 17 nm spherical gold nanoparticles for detection of matrix metalloprotease 7 (MMP-7), a biomarker that is heavily up-regulated during certain cancers. With the inflection point, the limit of detection (LOD) for MMP-7 is improved to 0.094 μg/mL from 0.22 μg/mL. This improvement may facilitate early diagnosis of salivary and colorectal cancers. We also envision that this generic method can be employed to track minute optical responses in other analytical areas

Plasmonic Hot Carriers-Controlled Second Harmonic Generation in WSe₂ Bilayers

Modulating second harmonic generation (SHG) by a static electric field through either electric-field-induced SHG or charge-induced SHG has been well documented. Nonetheless, it is essential to develop the ability to dynamically control and manipulate the nonlinear properties, preferably at high speed. Plasmonic hot carriers can be resonantly excited in metal nanoparticles and then injected into semiconductors within 10–100 fs, where they eventually decay on a comparable time scale. This allows one to rapidly manipulate all kinds of characteristics of semiconductors, including their nonlinear properties. Here we demonstrate that plasmonically generated hot electrons can be injected from plasmonic nanostructure into bilayer (2L) tungsten diselenide (WSe₂), breaking the material inversion symmetry and thus inducing an SHG. With a set of pump–probe experiments we confirm that it is the dynamic separation electric field resulting from the hot carrier injection (rather than a simple optical field enhancement) that is the cause of SHG. Transient absorption measurement further substantiate the plasmonic hot electrons injection and allow us to measure a rise time of ∼120 fs and a fall time of 1.9 ps. Our study creates opportunity for the ultrafast all-optical control of SHG in an all-optical manner that may enable a variety of applications

Minority Carrier Blocking to Enhance the Thermoelectric Performance of Solution-Processed Bi_<i>x</i>Sb_2–<i>x</i>Te₃ Nanocomposites via a Liquid-Phase Sintering Process

Minority carrier blocking through heterointerface barriers has been theoretically proposed to enhance the thermoelectric figure of merit (<i>ZT</i>) of bismuth telluride based nanocomposites at elevated temperatures recently (<i>Phys. Rev. B</i> <b>2016</b>, <i>93</i>, 165209). Here, to experimentally realize the minority carrier blocking, a liquid-phase sintering process enabled by excess Te is applied to the solution-processed Bi_<i>x</i>Sb_2–<i>x</i>Te₃ nanocomposites to introduce interfacial energy barriers. The controlling parameters in the liquid-phase sintering process such as the amount of excess Te, sintering temperature and holding time, and the Bi composition (<i>x</i>) are systemically tuned and investigated to fully understand the minority carrier blocking mechanism. These interface-engineering parameters are optimized for introducing maximum lattice imperfections and band-bending interfaces that are responsible for blocking the minority carrier and wide-range scattering of the phonons toward enhanced thermoelectric performance. High <i>ZT</i> > 1.4 at 375 K is realized in the Bi_0.5Sb_1.5Te₃ sample, which is much higher than those of the state-of-the-art commercial ingots (<i>ZT</i> ∼ 1) and other solution-processed nanocomposites. The enhanced <i>ZT</i> at elevated temperatures is mostly due to the suppression of bipolar thermal conductivity by minority carrier blocking as well as the reduction of lattice thermal conductivity. Adapting this solution synthesis process to design favorable heterointerfaces for minority carrier blocking in the liquid-phase sintering process holds promise to further enhance the <i>ZT</i> values

Size-Dependent Exciton Recombination Dynamics in Single CdS Nanowires beyond the Quantum Confinement Regime

A deep understanding of the size, surface trapping, and scattering effects on the recombination dynamics of CdS nanowires (NWs) is a key step for the design of on-demand CdS-based nanodevices. However, it is often very difficult to differentiate these intertwined effects in the NW system. In this article, we present a comprehensive study on the size-dependent exciton recombination dynamics of high-quality CdS NWs (with diameters from 80 to 315 nm) using temperature-dependent and time-resolved photoluminescence (TRPL) spectroscopy in a bid to distinguish the contributions of size and surface effects. TRPL measurements revealed two distinct processes that dominate the band edge recombination dynamicsa fast decay process (τ₁) originating from the near-surface recombination and a slower decay process (τ₂) arising from the intrinsic free exciton A decay. With increasing NW diameters, τ₁ increases from ∼0.10 to ∼0.42 ns due to the decreasing surface-to-volume ratio of the NWs, whereas τ₂ increases from ∼0.36 to ∼1.21 ns due to decreased surface scattering in the thicker NWsas validated by the surface passivation and TRPL studies. Our findings have discerned the interplay between size and surface effects and advanced the understanding of size-dependent optoelectronic properties of one-dimensional semiconductor nanostructures for applications in surface- and size-related nanoscale devices

Wavelength Tunable Single Nanowire Lasers Based on Surface Plasmon Polariton Enhanced Burstein–Moss Effect

Wavelength tunable semiconductor nanowire (NW) lasers are promising for multifunctional applications ranging from optical communication to spectroscopy analysis. Here, we present a demonstration of utilizing the surface plasmon polariton (SPP) enhanced Burstein–Moss (BM) effect to tune the lasing wavelength of a single semiconductor NW. The photonic lasing mode of the CdS NW (with length ∼10 μm and diameter ∼220 nm) significantly blue shifts from 504 to 483 nm at room temperature when the NW is in close proximity to the Au film. Systematic steady state power dependent photoluminescence (PL) and time-resolved PL studies validate that the BM effect in the hybrid CdS NW devices is greatly enhanced as a consequence of the strong coupling between the SPP and CdS excitons. With decreasing dielectric layer thickness <i>h</i> from 100 to 5 nm, the enhancement of the BM effect becomes stronger, leading to a larger blue shift of the lasing wavelength. Measurements of enhanced exciton emission intensities and recombination rates in the presence of Au film further support the strong interaction between SPP and excitons, which is consistent with the simulation results

Room-Temperature Near-Infrared High‑Q Perovskite Whispering-Gallery Planar Nanolasers

Near-infrared (NIR) solid-state micro/nanolasers are important building blocks for true integration of optoelectronic circuitry. Although significant progress has been made in III–V nanowire lasers with achieving NIR lasing at room temperature,− challenges remain including low quantum efficiencies and high Auger losses. Importantly, the obstacles toward integrating one-dimensional nanowires on the planar ubiquitous Si platform need to be effectively tackled. Here we demonstrate a new family of planar room-temperature NIR nanolasers based on organic–inorganic perovskite CH₃NH₃PbI_3-aX_a (X = I, Br, Cl) nanoplatelets. Their large exciton binding energies, long diffusion lengths, and naturally formed high-quality planar whispering-gallery mode cavities ensure adequate gain and efficient optical feedback for low-threshold optically pumped in-plane lasing. We show that these remarkable wavelength tunable whispering-gallery nanolasers can be easily integrated onto conductive platforms (Si, Au, indium tin oxide, and so forth). Our findings open up a new class of wavelength tunable planar nanomaterials potentially suitable for on-chip integration

Multiple Magnetic Mode-Based Fano Resonance in Split-Ring Resonator/Disk Nanocavities

Plasmonic Fano resonance, enabled by the weak interaction between a bright super-radiant and a subradiant resonance mode, not only is fundamentally interesting, but also exhibits potential applications ranging from extraordinary optical transmission to biosensing. Here, we demonstrate strong Fano resonances in split-ring resonators/disk (SRR/D) nanocavities. The high-order magnetic modes are observed in SRRs by polarization-resolved transmission spectroscopy. When a disk is centered within the SRRs, multiple high-order magnetic modes are coupled to a broad electric dipole mode of SRR/D, leading to significant Fano resonance spectral features in near-IR regime. The strength and line shape of the Fano resonances are tuned through varying the SRR split-angle and interparticle distance between SRR and disk. Finite-difference-time-domain (FDTD) simulations are conducted to understand the coupling mechanism, and the results show good agreement with experimental data. Furthermore, the coupled structure gives a sensitivity of ∼282 nm/RIU with a figure of merit ∼4

Controllable Fabrication of Two-Dimensional Patterned VO₂ Nanoparticle, Nanodome, and Nanonet Arrays with Tunable Temperature-Dependent Localized Surface Plasmon Resonance

A universal approach to develop various two-dimensional ordered nanostructures, namely nanoparticle, nanonet and nanodome arrays with controllable periodicity, ranging from 100 nm to 1 μm, has been developed in centimeter-scale by nanosphere lithography technique. Hexagonally patterned vanadium dioxide (VO₂) nanoparticle array with average diameter down to sub-100 nm as well as 160 nm of periodicity is fabricated, exhibiting distinct size-, media-, and temperature-dependent localized surface plasmon resonance switching behaviors, which fits well with the predication of simulations. We specifically explore their decent thermochromic performance in an energy saving smart window and develop a proof-of-concept demo which proves the effectiveness of patterned VO₂ film to serve as a smart thermal radiation control. This versatile and facile approach to fabricate various ordered nanostructures integrated with attractive phase change characteristics of VO₂ may inspire the study of temperature-dependent physical responses and the development of smart devices in extensive areas