Sharp metallic nanotapers irradiated with few-cycle laser pulses are emerging as a source of highly confined coherent electron wave packets with attosecond duration and strong directivity [1][2][3][4][5][6] . The possibility to steer, control or switch such electron wave packets with light 7 is expected to pave the way towards direct visualization of nanoplasmonic field dynamics [8][9][10] and real-time probing of electron motion 11,12 in solid-state nanostructures 13,14 . Such pulses can be generated by strong-field-induced tunnelling and acceleration of electrons in the near-field of sharp gold tapers within one half-cycle of the driving laser field 1,2,5 . Here, we show the effect of the carrier-envelope phase of the laser field on the generation and motion of strong-field-emitted electrons from such tips. We observe clear variations in the width of plateau-like photoelectron spectra characteristic of the subcycle regime. This is a step towards controlling the coherent electron motion in and around metallic nanostructures over ultrashort lengths and timescales.During the last two decades, strong-field effects in the interaction of light with atomic and molecular systems have led to a wealth of new physical phenomena, including the emission of high-harmonic radiation 15,16 and the generation of attosecond light and X-ray pulses 17 . Key to those phenomena is a field-induced periodic modulation of the tunnelling barrier, acceleration of the photoemitted electrons within an essentially spatially homogeneous laser field, and their recollision with the nuclei 18 . This has opened up the possibility to image molecular wavefunctions 12 and to probe electronic motion in real time. Attosecond science is enabled by the capability to control the carrier-envelope phase (CEP) of the light pulse, giving access to the electric-field waveform rather than the intensity profile.More recently, related strong-field photoemission phenomena have been demonstrated for metallic nanostructures 1,4,5,19,20 . For metallic tapers with modest field enhancement, for example, tungsten tips, multiphoton ionization (MPI) 2,6 and above-threshold ionization (ATI) 4,20 have been observed. CEP effects in this regime are ascribed to changes in the interference pattern created by electrons emitted in subsequent cycles, while the effect of the electric field amplitude is weak 4 . In contrast, for gold tips with large field enhancement, optical field-induced tunnelling becomes important 4,5,21 and, for sharp tips with pronounced spatial field gradients, a new subcycle emission regime has recently been discovered 1 . In this new regime, the electron dynamics are fundamentally different from those known in atomic and molecular systems because the decay length of the optical near-field l F is shorter than the quiver amplitude l q of the electrons (d ¼ l F /l q , 1). Hence, the field-emitted electrons are highly directionally accelerated 5 , escaping the local near-field within less than half an optical cycle (for an overview of the different regimes, s...

We report a strong, laser-field induced modification of the propagation direction of ultrashort electron pulses emitted from nanometer-sized gold tapers. Angle-resolved kinetic energy spectra of electrons emitted from such tips are recorded using ultrafast near-infrared light pulses of variable wavelength and intensity for excitation. For sufficiently long wavelengths, we observe a pronounced strong-field acceleration of electrons within the field gradient at the taper apex. We find a distinct narrowing of the emission cone angle of the fastest electrons. We ascribe this to the field-induced steering of subcycle electrons as opposed to the diverging emission of quiver electrons. Our findings are corroborated by simulations based on a modified Simpleman model incorporating the curved, vectorial field gradient in the vicinity of the tip. Our results indicate new pathways for designing highly directional nanometer-sized ultrafast electron sources.

We investigate the mechanism of light extraction enhancement of a GaN-based light-emitting diode (LED) grown on patterned sapphire substrate (PSS), that has ZnO nanorod arrays (NRAs) fabricated on top of the device using the hydrothermal method. We found that the light output power of the LED with ZnO NRAs increases by approximately 30% compared to the conventional LED without damaging the electrical properties of the device. We argue that the gradual decrease of the effective refractive index, which is caused by the fabrication of ZnO NRAs, is the mechanism of the observed improvement. Our argument is confirmed by cross-sectional confocal scanning electroluminescence microscopy (CSEM) and the theoretical simulations, where we observed a distinct increase of the transmission at the interface between LED and air at the operation wavelength of the LED. In addition, the plane-view CSEM results indicate that ZnO NRAs, which were grown on the bare p-type GaN layer as an electrical safety margin area, also contribute to the enhanced light output power of the LED, which indicate further enhancement is manifested even in the optically ineffective sacrificial area.

Modulation of the carrier concentration and electronic type of monolayer (1L) MoS is highly important for applications in logic circuits, solar cells, and light-emitting diodes. Here, we demonstrate the tuning of the electronic properties of large-area 1L-MoS using graphene oxide (GO). GO sheets are well-known as hole injection layers since they contain electron-withdrawing groups such as carboxyl, hydroxyl, and epoxy. The optical and electronic properties of GO-treated 1L-MoS are dramatically changed. The photoluminescence intensity of GO-treated 1L-MoS is increases by more than 470% compared to the pristine sample because of the increase in neutral exciton contribution. In addition, the A peak in Raman spectra shifts considerably, revealing that GO treatment led to the formation of p-type doped 1L-MoS. Moreover, the current vs voltage (I-V) curves of GO-coated 1L-MoS field effect transistors show that the electron concentration of 1L-MoS is significantly lower in comparison with pristine 1L-MoS. Current rectification is also observed from the I-V curve of the lateral diode structure with 1L-MoS and 1L-MoS/GO, indicating that the electronic structure of MoS is significantly modulated by the electron-withdrawing functional group of GO.

We demonstrate subcycle electron pulse generation in a nanogap of graphene when irradiated by a femtosecond laser pulse in the near-infrared region (800 nm). A strong photoinduced emission was produced when the gap area was irradiated by the ultrashort pulse laser. The graphene, which has atomically sharp edges with a large damage threshold, enables us to achieve a strong tunneling regime for the subcycle field emission. The photoinduced signals exhibited an anomalous increase in nonlinear order as a function of incident pulse energy in the presence of static electric field. A dynamical analysis of tunneling electrons based on the semiclassical model, which considers the contribution from the recoil electrons, reproduced our observation successfully. The large field enhancement near the graphene edge enabled us to reach the deep tunneling regime with the extraordinary Keldysh parameter of 0.2 in the near-infrared region, which has not been accessible by conventional metal nanostructures.

We successfully achieve the tip-enhanced nano Raman scattering images of a tungsten disulfide monolayer with optimizing a fabrication method of gold nanotip by controlling the concentration of etchant in an electrochemical etching process. By applying a square-wave voltage supplied from an arbitrary waveform generator to a gold wire, which is immersed in a hydrochloric acid solution diluted with ethanol at various ratios, we find that both the conical angle and radius of curvature of the tip apex can be varied by changing the ratio of hydrochloric acid and ethanol. We also suggest a model to explain the origin of these variations in the tip shape. From the systematic study, we find an optimal condition for achieving the yield of ~60% with the radius of ~34 nm and the cone angle of ~35°. Using representative tips fabricated under the optimal etching condition, we demonstrate the tip-enhanced Raman scattering experiment of tungsten disulfide monolayer grown by a chemical vapor deposition method with a spatial resolution of ~40 nm and a Raman enhancement factor of ~4,760.

Research on surface plasmon resonance coupling of metallic nanostructures is an important area in the field of plasmonics because distinctive collective optical properties can be realized that are different from the individual constituents. Here we report the localized surface plasmon resonance of hybrid metal-organic nanorods. Colloidal-dispersed Au-PPy nanorods were synthesized as a representative material using a modified electrochemical method, and the collective oscillation properties were systematically investigated by comparing these materials with pure Au nanorods. We observed the extended surface plasmon resonance of a hybrid system. The presence of doped-PPy segments on Au segments induced an enhanced coherent electric field due to the partial contribution of π-electrons on the PPy segment, which led to a red-shifted plasmon feature. Additionally, we demonstrated that surface plasmon resonance extension can be tuned by dopant anions, which demonstrates a way of tuning a dopant-induced plasmonic system.

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