Optical antennas convert propagating far-field radiation into localized near-fields. Perhaps the simplest optical antenna is a sharp metal tip, which is commonly utilized in near-field microscopy and spectroscopy. Usually, the coupling of far-field light to the near-field requires either a field polarized along the tip axis (longitudinal field) or a surface grating on the tip. Here, we demonstrate that efficient transformation of incident far-field radiation to localized energy at the cone apex can be achieved using nanocones with wings oriented perpendicular to the tip axis.
Using three-dimensional (3D) second-harmonic generation (SHG) scanning microscopy, we unravel the formation and distribution of distinct and highly localized persistent luminescent (PeL) microparticles of varied hierarchical levels in glasses prepared using the direct doping method. The PeL microparticles were added in the glasses at different doping temperatures and the glasses were quenched after different dwell time. The SHG maps of the PeL microparticles in the glass, prepared with a doping temperature of 975°C and a dwell time of 3 min, reveal grating-like microscopic domains. This suggests that a large arrangement of PeL crystals spanning several micrometers in three dimensions is manifested by the imbued PeL microparticle. In contrast, the SHG maps of the PeL microparticles inside the glass prepared at doping temperature of 1025°C and dwell time of 10 min, show the existence of single, highly localized and most importantly, submicrometer-sized PeL crystals. These findings substantiate well with the expected behavior of the PeL microparticles in glasses and their physical disintegration in the form of nanoparticles at high doping temperatures and dwell times. The SHG microscopy technique is shown to circumvent the fundamental challenges of traditional and usually destructive imaging methods to detect and visualize PeL nanoparticles in a glass matrix and expected to open a new avenue to evidence the presence of crystals in glasses.
Efficient optical excitation of hybridized plasmon modes in nanoantennas is vital to achieve many promising functionalities, but it can be challenging due to a field-profile mismatch between the incident light and the hybrid mode. We present a general approach for efficient hybrid-mode excitation by focusing the incident light field in the basis of cylindrically polarized vector beams of various higher-order spiral phases. Such basis vector beams are described in the higher-order polarization states and Stokes parameters (both defined locally in polar coordinates), and visualized correspondingly on the higher-order Poincar\'e spheres. The focal field is formulated exclusively in cylindrical coordinates as a series sum of all focused beams of the associated high-order paraxial beams. Our focal field decomposition enables an analysis of hybrid-mode excitation via higher-order vector beams, and thus yields a straightforward design of effective mode-matching field profile in the tightly focused region.
Nonlinear nanophotonics is continuously shaped by advances in nanofabrication, creating new nano-objects that come in unconventional architectures. An intriguing class of such nano-objects includes semiconductor nanowires exhibiting high crystallinity, polarization anisotropy, and high optical nonlinearities. Vertically-aligned nanowires provide the best route to device integration due to maximization of the surface-volume ratio and ease of fabrication [1], [2]. To understand and exploit the nonlinear optical effects in such a nanowire, it is crucial that light can be coupled well into it. Previously, we have shown that the second-harmonic generation (SHG) from a single pristine semiconductor nanowire could be driven well by the longitudinal electric fields and can be even used to reliably map the longitudinal electric fields of focused vector beams [3]. However, little is still known about the higher-order harmonic emissions of strongly absorbing materials like GaAs that could occur in the UV regime. It is believed that such high-harmonic emissions are difficult to probe due to wide absorption resonances obscuring the possible richness of nonlinear phenomena in that spectral regime. Also, the generation of UV light that is compatible for any device engineering effort is becoming essential. New approaches to detect and harness these emissions from advanced nano-objects are thus needed. Here, we show the possibility of probing and manipulating the THG from a single vertically-aligned semiconductor nanowire using polarized vector beams.
Collective effects in assemblies of plasmonic nanostructures are attracting widespread interest. Such effects are governed by the interactions among the constituents of the overall structure, providing an alternative path to modify plasmon resonances [1]. A particularly interesting collective effect in these structures is the so-called dark plasmon mode, which has a net zero dipole moment and cannot be accessed using plane waves or linearly polarized light under normal incidence. An emerging way to excite collective dark modes in such structures is through the use of cylindrical vector beams (CVB) that exhibit inhomogeneous states of polarization such as azimuthal or radial polarizations [2,3]. The use of CVBs to excite collective nonlinear optical effects such as second-harmonic generation (SHG) in oligomers has started only recently [4]. Here, we demonstrate a similar possibility by tailoring SHG in plasmonic radial trimers using CVBs.
We introduce 3D optical antennas based on winged nanocones. The antennas support particle plasmon oscillations with current distributions that facilitate transformation of transverse far-field radiation to strong longitudinal local fields near the cone apices. We characterize the optical responses of the antennas by their extinction spectra and by second-harmonic generation microscopy with cylindrical vector beams. The results demonstrate a new 3D polarization-controllable optical antenna for applications in apertureless near-field microscopy, spectroscopy, and plasmonic sensing.
We employ structured light to study resonantly-enhanced second- and third-harmonic emission from AlGaAs nanoantennas. We demonstrate correlation between nonlinear emissions with the pump polarization state and Mie-resonant excitation.
We use cylindrical vector beams to investigate second-harmonic generation from rotationally symmetric arrangements of plasmonic nanoholes. The second-harmonic efficiency is shown to depend strongly on collective interactions between the nanoholes.
We demonstrate two-photon optical beam-induced current (2P-OBIC) microscopy of light-emitting diodes (LEDs). We utilized a Ti:Sapphire femtosecond laser source operating at 800 nm to derive the 2P-OBIC signal from a 605 nm band-gap LED. The spatial confinement of free carrier generation only at the focus and the quadratic dependence of the 2P-OBIC signal on excitation power are the key principles in two-photon excitation. As a consequence, superior image quality evident in the 2P-OBIC images of LEDs are obtained. These features decrease the linear absorption and wide-angle scattering effects plaguing single-photon optical beam-induced current (1P-OBIC) technique, thereby increasing the resolution of the imaging system in the axial and lateral directions. Thus, the attainment of good axial discrimination in the LED samples is obtained even without a confocal pinhole. In addition, 2P-OBIC images reveal local variations in free carrier densities which are not evident in the single-photon excitation.
To date, majority of the all-optical techniques for the characterization of metal nano-objects rely on the use of fundamental light scattering in conjunction with conventional polarizations such as linear and circular. In this work, we show the opportunities arising from the use of nonlinear optical (NLO) effects combined with cylindrical vector beams (CVB) for scanning microscopy of individual metal nano-objects.
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