Search

Aluminum has attracted a great deal of attention as an alternative plasmonic material to silver and gold because of its natural abundance on Earth, material stability, unique spectral capability in the ultraviolet spectral region, and complementary metal-oxide-semiconductor compatibility. Surprisingly, in some recent studies, aluminum has been reported to outperform silver in the visible range due to its superior surface and interface properties. Here, we demonstrate excellent structural and optical properties measured for aluminum epitaxial films grown on sapphire substrates by molecular-beam epitaxy under ultrahigh vacuum growth conditions. Using the epitaxial growth technique, distinct advantages can be achieved for plasmonic applications, including high-fidelity nanofabrication and wafer-scale system integration. Moreover, the aluminum film thickness is controllable down to a few atomic monolayers, allowing for plasmonic ultrathin layer devices. Two kinds of aluminum plasmonic applications are reported here, including precisely engineered plasmonic substrates for surface-enhanced Raman spectroscopy and high-quality-factor plasmonic surface lattices based on standing localized surface plasmons and propagating surface plasmon polaritons, respectively, in the entire visible spectrum (400–700 nm).

Excitons in monolayer transition metal dichalcogenides are formed at K and K′ points at the boundary of the Brillouin zone. They acquire a valley degree of freedom, which has been explored as an alternative information carrier, analogous to charge or spin. Two opposite valleys in transition metal dichalcogenides can be optically addressed using light with different helicity. Here, we demonstrate that valley-polarized excitons can be sorted and spatially separated at room temperature by coupling a MoS2 monolayer to a subwavelength asymmetric groove array. In addition to separation of valley excitons in real space, emission from valley excitons is also separated in photon momentum-space; that is, the helicity of photons determines a preferential emission direction. Our work demonstrates that metasurfaces can facilitate valley transport and establish an interface between valleytronic and photonic devices, thus addressing outstanding challenges in the field of valleytronics. Helicity of photons is exploited for preferential emission direction and sorting of valley-polarized excitons is achieved at room temperature.

Single crystalline Ag films on dielectric substrates have received tremendous attention recently due to their technological potentials as low loss plasmonic materials. Two different growth approaches have been used to produce single crystalline Ag films previously. One approach is based on repetitive cycles of a two-step process (low temperature deposition followed by RT annealing) using molecular beam epitaxy (MBE), which is extremely time-consuming due to the need for repeat growth cycles. Another approach is based on rapid e-beam deposition which is capable of growing thick single crystalline Ag films (300 nm) but lacks the precision in thickness control of thin epitaxial films. Here, we report a universal approach to grow atomically smooth epitaxial Ag films by eliminating the repetitive cycles used in the previous two-step MBE method while maintaining the precise thickness control from a few monolayers to the optically thick regime, thus overcoming the limitations of the two aforementioned methods. In addition, we develop an in situ growth of aluminum oxide as the capping layer to protect the epitaxial Ag films. The quality of the epitaxial Ag films was evaluated using a variety of techniques, and the superior optical performance of the films is demonstrated by measuring the propagation length of surface plasmon polaritons (∼80 μm at 632 nm) as well as their capability to support a plasmonic nanolaser in infrared incorporating an InGaAsP quantum well as the gain media.

Non-Hermitian photonic systems with gains and/or losses have recently emerged as a powerful approach for topology-protected optical transport and novel device applications. To date, most of these systems employ coupled optical systems of diffraction-limited dielectric waveguides or microcavities, which exchange energy spatially or temporally. Here, we introduce a diffraction-unlimited approach using a plasmon-exciton coupling (polariton) system with tunable plasmonic resonance (energy and line width) and coupling strength. By designing a chirped silver nanogroove cavity array and coupling a single tungsten disulfide monolayer with a large contrast in resonance line width, we show the tuning capability through energy level anticrossing and plasmon-exciton hybridization (line width crossover), as well as spontaneous symmetry breaking across the exceptional point at zero detuning. This two-dimensional hybrid material system can be applied as a scalable and integratable platform for non-Hermitian photonics, featuring seamless integration of two-dimensional materials, broadband tuning, and operation at room temperature.

The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals respectively. Time-resolved experiments indicate that the critical process for photo-excited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the photoconductivity is stronger in CVD samples than exfoliated monolayers with a lower defect density. Our work reveals the intrinsic time and length scales of electrical response to photo-excitation in van der Waals materials, which is essential for their applications in novel optoelectronic devices., 21 pages, 4 figures

Quantum dots (QDs) are steadily being implemented as down-conversion phosphors in market-ready display products to enhance color rendering, brightness, and energy efficiency. However, for adequate longevity, QDs must be encased in a protective barrier that separates them from ambient oxygen and humidity, and device architectures are designed to avoid significant heating of the QDs as well as direct contact between the QDs and the excitation source. In order to increase the utility of QDs in display technologies and to extend their usefulness to more demanding applications as, for example, alternative phosphors for solid-state lighting (SSL), QDs must retain their photoluminescence emission properties over extended periods of time under conditions of high temperature and high light flux. Doing so would simplify the fabrication costs for QD display technologies and enable QDs to be used as down-conversion materials in light-emitting diodes for SSL, where direct-on-chip configurations expose the emitters to temperatures approaching 100 °C and to photon fluxes from 0.1 W/mm

We have investigated how the photoluminescence (PL) of WSe2 is modified when coupled to Ag plasmonic structures at low temperature. Chemical vapor deposition (CVD) grown monolayer WSe2 flakes were transferred onto a Ag film and a Ag nanotriangle array that had a 1.5 nm Al2O3 capping layer. Using low-temperature (7.5 K) micro-PL mapping, we simultaneously observed enhancement of the defect-bound exciton emission and quenching of the band edge exciton emission when the WSe2 was on a plasmonic structure. The enhancement of the defect-bound exciton emission was significant with enhancement factors of up to ∼200 for WSe2 on the nanotriangle array when compared to WSe2 on a 1.5 nm Al2O3 capped Si substrate with a 300 nm SiO2 layer. The giant enhancement of the luminescence from the defect-bound excitons is understood in terms of the Purcell effect and increased light absorption. In contrast, the surprising result of luminescence quenching of the bright exciton state on the same plasmonic nanostructure is due to...

We report on the first demonstration of metal–insulator–semiconductor-type plasmonic lasers at the telecom wavelength (∼1.3 μm) using top-down fabricated semiconductor waveguides on single-crystalline metallic platforms formed using epitaxially grown Ag films. The critical role of the Ag film thickness in sustaining plasmonic lasing at the telecom wavelength is investigated systematically. Low-threshold (0.2 MW/cm2) and continuous-wave operation of plasmonic lasing at cryogenic temperatures can be achieved on a 150 nm Ag platform with minimum radiation leakage into the substrate. Plasmonic lasing occurs preferentially through higher-order surface-plasmon-polariton modes, which exhibit a higher mode confinement factor, lower propagation loss, and better field–gain coupling. We observed plasmonic lasing up to ∼200 K under pulsed excitations. The plasmonic lasers on large-area epitaxial Ag films open up a scalable platform for on-chip integrations of plasmonics and optoelectronics at the telecom wavelength.

Optical dielectric constants are critical to modeling the electronic and optical properties of materials. Silver, as a noble metal with low loss, has been extensively investigated. The recently developed epitaxial growths of single crystalline Ag on dielectric substrates have prompted efforts to characterize their intrinsic optical dielectric function. In this paper, we report spectral ellipsometry measurements and analysis of a thick, epitaxially-grown, single-crystalline Ag film. We focus on the range of 0.18 – 1.0 eV or 1.24 – 7 µm, an energy and wavelength range that has not been examined previously using epitaxial films. We compare the extracted dielectric constants and the predicted optical performances with previous measurements. The loss is appreciably lower than the widely quoted Palik’s optical constants (i.e., up to a factor of 2) in the infrared frequency range. The improved knowledge of fundamental optical properties of the high-quality epitaxial Ag film will have a broad impact on simulations and practical applications based on Ag in the long wavelength range.

Valley degree of freedom in monolayer transition metal dichalcogenides have been explored as an information carrier. In a different context, metasurfaces consisting of engineered components have enabled the manipulation of light in unprecedented ways. Here, we demonstrate that by placing a MoS2 monolayer onto a metasurface consisting of asymmetric grooves, valley polarized excitons can be spatially separated even at room temperature.

We demonstrated a low-threshold lasing from a monolayer gain medium which is made possible due to 2D feedback mechanism via strong exciton-SPP coupling in both real space and a wide range of momentum space.

We report on scanning spreading resistance microscopy on cross sections of thin-film CdTe devices. The results show the capability of identifying the multiple layers, the depletion region, and the nonuniform doping. We observe carrier injection and depletion region movement by laser illumination or by electrically biasing the device, directly revealing the underlying physics of the solar cell junction in real space with resolutions of nanometer scale.

Atomically thin lateral heterostructures based on transition metal dichalcogenides have recently been demonstrated. In monolayer transition metal dichalcogenides, exciton energy transfer is typically limited to a short range (~1 μm), and additional losses may be incurred at the interfacial regions of a lateral heterostructure. To overcome these challenges, here we experimentally implement a planar metal-oxide-semiconductor structure by placing a WS2/MoS2 monolayer heterostructure on top of an Al2O3-capped Ag single-crystalline plate. We find that the exciton energy transfer range can be extended to tens of microns in the hybrid structure mediated by an exciton-surface plasmon polariton–exciton conversion mechanism, allowing cascaded exciton energy transfer from one transition metal dichalcogenides region supporting high-energy exciton resonance to a different transition metal dichalcogenides region in the lateral heterostructure with low-energy exciton resonance. The realized planar hybrid structure combines two-dimensional light-emitting materials with planar plasmonic waveguides and offers great potential for developing integrated photonic and plasmonic devices., Exciton energy transfer in monolayer transition metal dichalcogenides is limited to short distances. Here, Shi et al. fabricate a planar metal-oxide-semiconductor structure and show that exciton energy transfer can be extended to tens of microns, mediated by an exciton-surface-plasmon-polariton–exciton conversion mechanism.

We report longer surface plasmon polariton propagation distance based on crystalline crystal silver at low temperature. Although enhanced plasmonic performance at low temperature has been predicted for a long time, it has not been directly observed on polycrystalline silver films which suffer from significant plasmonic losses due to grain boundaries and rough silver surface. Here we show that longer propagation distance can be achieved with epitaxial silver at low temperature. Importantly, the enhancement at low temperature are consistent across silver films grown with different methods.

Using atomically smooth epitaxial silver films, new optical permittivity highlighting significant loss reduction in the visible frequency range is extracted. Largely enhanced propagation distances of surface plasmon polaritons are measured, confirming the low intrinsic loss in silver. The new permittivity is free of extrinsic spectral features associated with grain boundaries and localized plasmons inevitably present in thermally deposited films.

Aluminum (Al) provides an excellent material platform for plasmonic applications in the ultraviolet (UV) regime due to its low loss coefficient at UV wavelengths. To fully realize the potential of this material, it is imperative to create nanostructures with minimal defects in order to prevent light scattering and better support plasmonic resonances. In this work, we report the successful development of atomically smooth epitaxial Al films on silicon. These epitaxial Al thin films facilitate the creation of fine plasmonic nanostructures and demonstrate considerable loss reduction in the UV frequency range, in comparison to the polycrystalline Al films based on spectroscopic ellipsometry measurements. Remarkably, our measurements on the epitaxial Al film grown using the two-step method suggest that the intrinsic loss in Al is significantly lower, by up to a factor of 2 in the UV range, with respect to current widely quoted Palik's values extracted from polycrystalline films. These high-quality epitaxial Al films provide an ideal platform for UV plasmonics. In addition, the availability of intrinsic optical constants will enable more accurate theoretical predictions to guide the device design.

Going Green with Nanophotonics Plasmons are optically induced collective electronic excitations tightly confined to the surface of a metal, with silver being the metal of choice. The subwavelength confinement offers the opportunity to shrink optoelectronic circuits to the nanometer scale. However, scattering processes within the metal lead to losses. Lu et al. (p. 450 ) developed a process to produce atomically smooth layers of silver, epitaxially grown on silicon substrates. A cavity in the silver layer is capped with a SiO insulating layer and an AlGaN nanorod was used to produce a low-threshold emission at green wavelengths.

Two experiments observe the so-called Mollow triplet in the emission spectrum of a quantum dot—originating from resonantly driving a dot transition—and demonstrate the potential of these systems to act as single-photon sources, and as a readout modality for electron-spin states. Single-quantum emitters emit only one photon at a time1,2, but the properties of the photon depend on how the emitter is excited3. Incoherent excitation is simple and broadly used with solid-state emitters such as quantum dots, but does not allow direct manipulation of the quantum state. Coherent, resonant excitation on the other hand is used in pump–probe techniques to examine the quantum state of the emitter4, but does not permit collection of the single-photon emission. Coherent control with simultaneous generation of photons has been an elusive goal in solid-state approaches, where, because of strong laser scattering at the detection wavelength, measurement of resonant emission has been limited to cross-polarized detection5 or Stokes-shift techniques6,7. Here we demonstrate that a semiconductor quantum dot in a microcavity can be resonantly driven and its single-photon emission extracted background free. Under strong continuous-wave excitation, the dot undergoes several Rabi oscillations before emitting, which are visible as oscillations in the second-order correlation function. The quantum-dot states are therefore ‘dressed’, resulting in a Mollow-triplet emission spectrum. Such coherent control will be necessary for future high-efficiency sources of indistinguishable single photons3,8, which can be used for quantum key distribution9 or through post-selection10 to generate entangled photon pairs11,12.

Linear CdTe|CdSe|CdTe heterostructure nanorods are synthesized by using a colloidal sequential reactant injection technique [Shieh et al., J. Phys. Chem. B 2005, 109, 8538-8542]. The composition profiles of the individual nanorods are verified by using nanobeam elemental mapping by energy dispersive X-ray spectroscopy (EDS) and the photoluminescence emission spectra of the linear CdTe|CdSe|CdTe heterostructure nanorods are measured as a function of the temperature (down to 5 K). Photoluminescence is observed to occur from electron-hole recombination in both the CdSe core and across the heterojunction. Thermally activated trapping is found to influence both luminescence processes, thereby being more significant for the type II recombination across the CdSe|CdTe interface.

We report on the design of a compact low temperature scanning tunneling microscope (STM) having in-situ sample preparation capability. The in-situ sample preparation chamber was designed to be compact allowing quick transfer of samples to the STM stage, which is ideal for preparing temperature sensitive samples such as ultra-thin metal films on semiconductor substrates. Conventional spring suspensions on the STM head often cause mechanical issues. To address this problem, we developed a simple vibration damper consisting of welded metal bellows and rubber pads. In addition, we developed a novel technique to ensure an ultra-high-vacuum (UHV) seal between the copper and stainless steel, which provides excellent reliability for cryostats operating in UHV. The performance of the STM was tested from 2 K to 77 K by using epitaxial thin Pb films on Si. Very high mechanical stability was achieved with clear atomic resolution even when using cryostats operating at 77 K. At 2 K, a clean superconducting gap was observed, and the spectrum was easily fit using the BCS density of states with negligible broadening.

Semiconductor heterostructures are fundamental building blocks for many important device applications. The emergence of two-dimensional semiconductors opens up a new realm for creating heterostructures. As the bandgaps of transition metal dichalcogenides thin films have sensitive layer dependence, it is natural to create lateral heterojunctions using the same materials with different thicknesses. Using scanning tunneling microscopy and spectroscopy, here we show the real space image of electronic structures across the bilayer-monolayer interface in MoSe2 and WSe2. Most bilayer-monolayer heterojunctions are found to have a zigzag-orientated interface, and the band alignment of such atomically sharp heterojunctions is of type-I with a well-defined interface mode which acts as a narrower-gap quantum wire. The ability to utilize such commonly existing thickness terrace as lateral heterojunctions is a crucial addition to the tool set for device applications based on atomically thin transition metal dichalcogenides, with the advantage of easy and flexible implementation., 19 pages in total, 4 figures and 1 table

Many remarkable properties of graphene are derived from its large energy window for Dirac-like electronic states and have been explored for applications in electronics and photonics. In addition, strong electron-phonon interaction in graphene has led to efficient photo-thermo energy conversions, which has been harnessed for energy applications. By combining the wavelength independent absorption property and the efficient photo-thermo energy conversion, here we report a new type of applications in sound wave generation underlined by a photo-thermo-acoustic energy conversion mechanism. Most significantly, by utilizing ultrafast optical pulses, we demonstrate the ability to control the phase of sound waves generated by the photo-thermal-acoustic process. Our finding paves the way for new types of applications for graphene, such as remote non-contact speakers, optical-switching acoustic devices, etc.

By using silver nanoplatelets with a widely tunable localized surface plasmon resonance (LSPR) and their corresponding local field enhancement, here we show large manipulation of plasmonic enhanced upconversion in NaYF4:Yb3+/Er3+ nanocrystals at the single particle level. In particular, we show that when the plasmonic resonance of silver nanolplatelets is tuned to 656 nm, matching the emission wavelength, an upconversion enhancement factor ~5 is obtained. However, when the plasmonic resonance is tuned to 980 nm, matching the nanocrystal absorption wavelength, we achieve an enhancement factor of ~22 folds. The precise geometric arrangement between fluorescent nanoparticles and silver nanoplatelets allows us to make, for the first time, a comparative analysis between experimental results and numerical simulations, yielding a quantitative agreement at the single particle level. Such a comparison lays the foundations for a rational design of hybrid metal-fluorescent nanocrystals to harness the upconversion enhancement for biosensing and light harvesting applications.

Self-organized Co nanoplatelets with a singular height, quantized lateral sizes, and unique shape and orientation have been fabricated on a template consisting of ordered Al nanocluster arrays on Si(111)-7 x 7 surfaces. Despite their small volume (a few nm(3)), these nanomagnets exhibit an unusually high blocking temperature (>100 K). The perpendicular direction for easy magnetization, the high blocking temperature, the size tunability, and the epitaxial growth on Si substrates make these nanomagnets important for applications in information technology.

This study conducts experiments on a planetary three-roll experimental apparatus and uses plasticine as the working material. Certain rolling characteristics were observed and compared with the data from finite element simulation, which include diameter of workpieces after deformation, the pitch length and threaded angle of spiral marks, and the cavity at the leading end of rolled plasticine. The comparison confirms the correctness of the simulation model for PSW.

In this paper, the finite element method and the dual-stream functions upper bound method are selected to analyze the tube rolling process with a planetary rolling mill. Three-dimensional elastic–plastic finite element simulations with special emphasis on the determination of the roller profile and realistic contact conditions between the rollers, workpiece and mandrel are used to reveal the deformation characteristics of seamless tube during the rolling process. In addition, the kinematically admissible velocity field of the deforming tube is described in terms of two stream functions so that the velocity components of the deforming tube can be obtained through the upper bound analysis. In both analysis methods, the equation of meshing is used to determine the contact condition between the rollers and the workpiece. The results from both methods with different variables are observed and compared.

Up to now, significantly reducing the size of semiconductor lasers in all three dimensions is the ultimate challenge for the development of nanolasers, which is a key component for long-waited on-chip optical communications and computing systems. However, the minimum size of conventional semiconductor lasers utilizing dielectric resonators is governed by the optical diffraction limit (λ/2n)3. Recently, we have published the world’s smallest semiconductor laser [1] based on a new concept in laser feedback mechanism. We report on the low-threshold, continuous-wave operation of a sub-diffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation (spaser ) [2]. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium.

We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spas- ers) emitting in the full visible spectrum. These nanolasers are based on a single metal−oxide−semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al2O3-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresh- olds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of "thresholdless" lasing for the blue plasmonic nanolaser.

This proposal is aimed at investigating the influence of substrate steps on molecular beam epitaxial growth of Bi2Se3 and Bi2Te3 topological insulators on Si (111). It uses a novel combinatorial approach by creating micro-lens structures on Si(111) substrate. The microlens contains a combination of numerous micro-surfaces with different miscut angles and miscut orientation. By using scanning probe microscopy, we can investigate how different miscut angles and orientations influence the growth process.

Using Scanning Tunneling Microscopy and Spectroscopy, we probe the electronic structures of single layer ${\small MoS_2}$ on graphite. We show that the quasiparticle energy gap of single layer ${\small MoS_2}$ is 2.15 $\pm$ 0.07 eV at 77 K. Combining with temperature dependent photoluminescence studies, we deduce an exciton binding energy of 0.22 $\pm$ 0.1 eV, a value that is much lower than current theoretical predictions. Consistent with theoretical predictions we directly observed metallic edge states of single layer ${\small MoS_2}$. In the bulk region of ${\small MoS_2}$, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/${\small MoS_2}$ heterojunction. At the edge, however, we observe an upward band bending of 0.6 eV within a short depletion length of about 5 nm, analogous to the phenomena of Fermi level pinning of a 3D semiconductor by metallic surface states., 17 pages, 4 figures

The emergence of transition metal dichalcogenides (TMDs) as 2D electronic materials has stimulated proposals of novel electronic and photonic devices based on TMD heterostructures. Here we report the determination of band offsets in TMD heterostructures by using microbeam X-ray photoelectron spectroscopy ({\mu}-XPS) and scanning tunneling microscopy/spectroscopy (STM/S). We determine a type-II alignment between $\textrm{MoS}_2$ and $\textrm{WSe}_2$ with a valence band offset (VBO) value of 0.83 eV and a conduction band offset (CBO) of 0.76 eV. First-principles calculations show that in this heterostructure with dissimilar chalcogen atoms, the electronic structures of $\textrm{WSe}_2$ and $\textrm{MoS}_2$ are well retained in their respective layers due to a weak interlayer coupling. Moreover, a VBO of 0.94 eV is obtained from density functional theory (DFT), consistent with the experimental determination., Comment: ^ These authors contributed equally. *Corresponding author E-mail: lanceli@gate.sinica.edu.tw, shih@physics.utexas.edu 20 pages, 4 figures in main text

Three kinds of 12-fold quasi-periodic subwavelength hole arrays have been designed using the same dodecahedral supercell arranged with different local rotational symmetries. Fano resonances associated with spoof surface plasmons in these structures have been studied by far-infrared transmission measurements. The resonant transmission channels of the lowest-order Fano resonance mode have been compared directly between these structures, benefitting from constant non-resonant transmission channel. It is found that the higher is the local rotational symmetry of the supercell array, the higher the transmission intensity and the narrower the linewidth of the Fano resonance.

This paper is focused on using the finite element method to simulate and analyze the planetary rolling process. First, a basic geometric model of the planetary rolling mill that considers roll profiles and offset angle of the rolls was constructed. Then, a three-dimensional elastic–plastic finite element simulation was used to analyze both the deformation characteristics of this process and the distributions of stress and strain in the workpieces. During simulations, an algorithm called Equation of Meshing was proposed by which the initial contact conditions between the rollers and the workpiece were successfully derived. Finally, an optimum design method was integrated into this analysis for seeking the best design variables in order to reduce the cavity conditions in the leading end of milled steel rods.

We demonstrate that the limited coherence length of a one-dimensional quasiperiodically ordered superstructure of a thin Ag film on GaAs(1 1 0) surfaces is due to structural defects. We identify dislocations, phason defects, and domain walls. The domain walls end at dislocations, whose stress field is found to induce a wide distribution of oriented phason defects.

We present a novel {open_quotes}electronic growth{close_quotes} model for metallic thin films on semiconductor substrates. Depending on the competition between the effects of quantum confinement, charge spilling, and interface-induced Friedel oscillations, different types of film stability are defined, as characterized by the existence of critical/magic thicknesses for smooth growth. In particular, smooth growth can be achieved only above a few monolayers for noble metals, and only for the first layer for alkali metals. {copyright} {ital 1998} {ital The American Physical Society}

Self-assembled InGaAs quantum dots (QDs) were fabricated inside a planar microcavity with two vertical cavity modes. This allowed us to excite the QDs coupled to one of the vertical cavity modes through two propagating cavity modes to study their down- and up-converted photoluminescence (PL). The up-converted PL increased continuously with the increasing temperature, reaching an intensity level comparable to that of the down-converted PL at ~120 K. This giant efficiency in the up-converted PL of InGaAs QDs was enhanced by about 2 orders of magnitude with respect to a similar structure without cavity. We tentatively explain the enhanced up-converted signal as a direct consequence of the modified spontaneous emission properties of the QDs in the microcavity, combined with the phonon absorption and emission effects.

Bi2Se3 initially emerged as a particularly promising host of topological physics. However, in actual materials, several issues have been uncovered including strong surface band bending and potential fluctuations. To investigate these concerns, we study nominally stoichiometric Bi2Se3 using scanning tunnelling microscopy. Here we identify two distinct distributions of Bi-Se antisites that act as nanometer-scale sensors for the surface band-bending field. To confirm this, we examine bulk Cu-doped Bi2Se3 and demonstrate a significantly reduced surface band-bending field. In addition, we find that in the case of unintentionally doped Bi2Se3, lateral fluctuations of the Dirac point can be directly correlated with specific near-surface point defects, namely Se vacancies.

Diffraction-unlimited semiconductor nanolasers are demonstrated by using single shape-controlled InGaN/GaN core-shell nanorods as laser gain media on Ag films epitaxially grown on Si substrates. The use of atomically smooth Ag films allows us to fabricate low-loss plasmonic cavities for ultralow-threshold, continuous-wave (CW) nanolaser operation above liquid nitrogen temperature. Furthermore, the tunable band-gap energy of the InxGa1-xN alloy makes it possible to realize laser emission in the full visible spectrum.

Miniaturization of semiconductor lasers holds the key to the development of compact, low-threshold, and fast coherent on-chip light sources/amplifiers, which are critically important for emerging applications in nanophotonics, integrated optics, and information technology (1–3). However, on-chip integration of nanoscale electronic components with conventional semiconductor lasers utilizing dielectric optical cavities is impeded by the diffraction limit-i.e., ∼(λ/2n)3 for three-dimensional (3D) cavities, where λ is the free-space wavelength and n is the refractive index of the dielectric (4–7). The recent advent of nanoplasmonics based on metallodielectric structures has led to the design of optical components and optoelectronic devices in the deep subwavelength regime (8–13). In particular, a new class of lasers based on surface plasmon amplification by stimulated emission of radiation (SPASER) has recently been proposed (14, 15) and experimentally demonstrated (16–19). In the SPASER operation, surface plasmons excited in noble-metal structures adjacent to gain media dramatically increase the optical mode density, shrink the optical mode volume, and provide the necessary feedback mechanism.

We report the measurements of one-photon and two-photon interference of light scattered by two quantum dots, separated by 40 μm. We show how fringe contrasts are strongly affected by spectral diffusion.

We have probed the local thermoelectric power of semiconductor nanostructures with the use of ultrahigh-vacuum scanning thermoelectric microscopy. When applied to a p-n junction, this method reveals that the thermoelectric power changes its sign abruptly within 2 nanometers across the junction. Because thermoelectric power correlates with electronic structure, we can profile with nanometer spatial resolution the thermoelectric power, band structures, and carrier concentrations of semiconductor junctions that constitute the building blocks of thermoelectric, electronic, and optoelectronic devices.

Scaling down semiconductor lasers in all three dimensions holds the key to the development of compact, low-threshold, and ultrafast coherent light sources, as well as integrated optoelectronic and plasmonic circuits. However, the minimum size of conventional semiconductor lasers utilizing dielectric cavity resonators (photonic cavities) is limited by the diffraction limit. To date, surface plasmon amplification by stimulated emission of radiation (spaser)-based plasmonic nanolaser is the only photon and plasmon-emitting device capable of this remarkable feat. Specifically, it has been experimentally demonstrated that the use of plasmonic cavities based on metal-insulator-semiconductor (MIS) nanostructures can indeed break the diffraction limit in all three dimensions. In this review, we present an updated overview of the current status for plasmonic nanolasers using the MIS configuration and other related metal-cladded semiconductor microlasers. In particular, by using composition-varied indium gallium nitride/gallium nitride core-shell nanorods, it is possible to realize all-color, single-mode nanolasers in the full visible wavelength range with ultralow continuous-wave (CW) lasing thresholds. The lasing action in these subdiffraction plasmonic cavities is achieved via a unique auto-tuning mechanism based on the property of weak size dependence inherent in plasmonic nanolasers. As for the choice of metals in the plasmonic structures, epitaxial silver films and giant colloidal silver crystals have been shown to be the superior constituent materials for plasmonic cavities due to their low plasmonic losses in the visible and near-infrared (NIR) spectral regions. In this review, we also provide some perspectives on the challenges and opportunities in this exciting new research frontier.

In this Letter we report a comparative study, in the infrared regime, of surface plasmon polariton (SPP) propagation in epitaxially grown Ag films and in polycrystalline Ag films, all grown on Si substrates. Plasmonic resonance features are analyzed using extraordinary optical transmission (EOT) measurements, and SPP band structures for the two dielectric/metal interfaces are investigated for both types of film. At the Si/Ag interface, EOT spectra show almost identical features for epitaxial and polycrystalline Ag films and are characterized by sharp Fano resonances. On the contrary, at the air/Ag interface, dramatic differences are observed: while the epitaxial film continues to exhibit sharp Fano resonances, the polycrystalline film shows only broad spectral features and much lower transmission intensities. In corroboration with theoretical simulations, we find that surface roughness plays a critical role in SPP propagation for this wavelength range.

We report the measurement of field-field and photon-photon correlations of light scattered by two InAs quantum dots separated by ≈40 μm. Near 4 K a large fraction of photons can be scattered coherently by each quantum dot leading to one-photon interference at a beam splitter (visibility ≈20%). Simultaneously, two-photon interference is also observed (visibility ≈40%) due to the indistinguishability of photons scattered by the two different quantum emitters. We show how spectral diffusion accounts for the reduction in interference visibility through variations in photon flux.

Successful development of epitaxial Ag as a new plasmonic platform has enabled low-threshold, continuous-wave (CW) operation of a SPASER-enabled nanolaser comprised of a subdiffraction plasmonic cavity and a single InGaN@GaN core-shell nanorod gain medium.

Semiconductor heterojunctions (HJs) have played a critical role in many modern electronic and photonic devices. The emergence of transition metal dichalcogenides (TMDs) as a new class of two-dimensional semiconducting materials creates exciting new opportunities to push semiconductor heterostructures toward a new frontier. Vertically stacked van der Waals heterostructures have been quickly recognized as a powerful platform to create atomically thin heterostructures.1 Recently, attention has been directed to the creation of monolayer ‘lateral heterojunctions’ with a line interface between two different TMDs, pushing semiconductor HJs to a new dimension. In late 2014, three papers published simultaneously, demonstrating direct growth of lateral heterojunctions using chemical vapor deposition.2, 3, 4 The papers demonstrated growth including both lattice matched and mismatched HJs; however, the interfaces were not chemically abrupt (that is, diffuse interface). In particular, the lattice mismatched system (for example, WS2-WSe2) displayed a compositional transition width of 40 nm. Presumably, the gradual composition alleviates the abrupt change of the lattice constant, similar to the epitaxial growth of conventional heterostructures with a large lattice mismatch. Conventional wisdom suggests a chemically abrupt lateral interface in a lattice mismatched system would be difficult to achieve. In contrast, in 2015 by using a two-step process, Li et al.5 reported the successful growth of lattice-mismatched lateral HJs of WSe2–MoS2 with an atomically abrupt compositional interface as shown schematically in Figure 1a. This system is coherently strained judging by the high Photoluminescence efficiency of individual TMDs. This scenario is similar to the growth of coherently strained InAs quantum dots in GaAs, although in this case, compositional mixing at the interface has been difficult to eliminate. Thus, an atomically abrupt compositional interface in a system with such a large lattice mismatch comes as a pleasant surprise.

We report on the development of a new two‐dimensional micropositioning device, or walker, which is capable of moving across very large distances (in principle unlimited) and with a very small step size (as small as 100 A/step) in both directions. Based on a unique tracking design, the motion is extremely orthogonal with very little crosstalk between the two directions. Additionally, there is no detectable backlash in either direction. The walker performance has been extensively tested by using a position‐sensitive proximitor probe. Tests have been done between 77 and 300 K. However, we project that the walker will be able to operate at temperatures as low as 4 K. This walker system has shown extremely reliable performance in a UHV environment for use with scanning tunneling microscopy and has been especially useful for cross‐sectional scanning tunneling microscopy and spectroscopy studies of semiconductor hetero‐ and homostructures. We show one example of results on the (AlGa)As/GaAs heterostructure system.

We present a cross-sectional scanning tunneling microscopy (STM) study of heterogeneous-droplet-epitaxy (HDE)-grown InGaAs quantum dots (QDs). We found that the structural properties of HDE-grown QDs such as size, shape, etc., are quite different from that of Stranski–Krastanov (SK)-grown InGaAs QDs. HDE-grown InGaAs QDs exhibit a reverse trapezoidal shape, opposite to the SK-grown QDs. In addition, the In concentration within individual HDE QDs is rather uniform, contrary to the case in SK QDs. These HDE QDs also show large size fluctuation. However, we found that there is a size dependence in the In concentration within the QD—the larger QD has lower In concentration, suggesting a self-compensation effect which gives rise to a sharp photoluminescence linewidth.

In the absence of tip-sample mechanical interaction, graphite images show very different behaviors from those of typical images. Contrary to the current interpretation of site asymmetry, where protrusions in images are considered as B-site atoms, we have found that one can selectively image A- or B-site atoms depending on the bias polarity. Furthermore, the corrugation amplitude is extremely small. If measurements are performed under tip-sample mechanical interaction, one observes larger corrugations and in-phase images at opposite polarities. A new interpretation of site asymmetry is proposed.