Excitation Polarization Research Articles

Enhanced high-harmonic generation from an all-dielectric metasurface

The recent observation of high-harmonic generation from solids creates a new possibility for engineering fundamental strong-field processes by patterning the solid target with subwavelength nanostructures. All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field, making them attractive platforms to control high-harmonics and other high-field processes at nanoscales. Here we report enhanced non-perturbative high-harmonic emission from a Si metasurface that possesses a sharp Fano resonance resulting from a classical analogue of electromagnetically induced transparency. Harmonic emission is enhanced by more than two orders of magnitude compared to unpatterned samples. The enhanced high harmonics are highly anisotropic with excitation polarization and are selective to excitation wavelength due to its resonant feature. By combining nanofabrication technology and ultrafast strong-field physics, our work paves the way for designing new compact ultrafast photonic devices that operate under high intensities and short wavelengths.

Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons.

Using two-photon photoemission electron microscopy (2P-PEEM) we image the polarization dependence of coupling and propagation of surface plasmon polaritons (SPPs) launched from edges of a triangular, micrometer size, single-crystalline Ag crystal by linearly or circularly polarized light. 2P-PEEM records interferences between the optical excitation field and SPPs it creates with nanofemto space-time resolution. Both the linearly and circularly polarized femtosecond light pulses excite spatially asymmetric 2PP yield distributions, which are imaged. We attribute the asymmetry for linearly polarized light to the relative alignments of the laser polarization and triangle edges, which affect the efficiency of excitation of the longitudinal component of the SPP field. For circular polarization, the asymmetry is caused by matching of the spin angular momenta (SAM) of light and the transverse SAM of SPPs. Moreover, we show that the interference patterns recorded in the 2P-PEEM images are cast by phase shifts and amplitudes for coupling of light into the longitudinal and transverse components of SPP fields. While the interference patterns depend on the excitation polarization, nanofemto movies show that the phase and group velocities of SPPs are independent of SAM of light in time-reversal invariant media. Simulations of the wave interference reproduce the polarization and spin-dependent coupling of optical pulses into SPPs.

Understanding photoluminescence of metal nanostructures based on an oscillator model

Scattering and absorption properties of metal nanostructures have been well understood based on the classic oscillator theory. Here, we demonstrate that photoluminescence of metal nanostructures can also be explained based on a classic model. The model shows that inelastic radiation of an oscillator resembles its resonance band after external excitation, and is related to the photoluminescence from metallic nanostructures. The understanding based on the classic oscillator model is in agreement with that predicted by a quantum electromagnetic cavity model. Moreover, by correlating a two-temperature model and the electron distributions, we demonstrate that both one-photon and two-photon luminescence of the metal nanostructures undergo the same mechanism. Furthermore, the model explains most of the emission characteristics of the metallic nanostructures, such as quantum yield, spectral shape, excitation polarization and power dependence. The model based on an oscillator provides an intuitive description of the photoluminescence process and may enable rapid optimization and exploration of the plasmonic properties.

Linearly polarized photoluminescence of InGaN quantum disks embedded in GaN nanorods

We have investigated the emission from InGaN/GaN quantum disks grown on the tip of GaN nanorods. The emission at 3.21 eV from the InGaN quantum disk doesn’t show a Stark shift, and it is linearly polarized when excited perpendicular to the growth direction. The degree of linear polarization is about 39.3% due to the anisotropy of the nanostructures. In order to characterize a single nanostructure, the quantum disks were dispersed on a SiO2 substrate patterned with a metal reference grid. By rotating the excitation polarization angle from parallel to perpendicular relative to the nanorods, the variation of overall PL for the 3.21 eV peak was recorded and it clearly showed the degree of linear polarization (DLP) of 51.5%.

Investigation and Control of Single Molecular Structures of Meso- Meso Linked Long Porphyrin Arrays.

We have investigated conformational structures of meso- meso linked porphyrin arrays (Z n) by single molecule fluorescence spectroscopy. Modulation depths ( M values) were measured by excitation polarization fluorescence spectroscopy. The M value decreases from 0.85 to 0.46 as the number of porphyrin units increases from 3 to 128, indicating that longer arrays exhibit coiled structures. Such conformational changes depending on the length have been confirmed by coarse-grained simulation. The histograms of M values and traces of centroid position of emitting sites by localization microscopy showed that the structures of longer arrays changed to more stretched after solvent vapor annealing with tetrahydrofuran.

Color Centers in Hexagonal Boron Nitride Monolayers: A Group Theory and Ab Initio Analysis

We theoretically study physical properties of the most promising color center candidates for the recently observed single-photon emissions in hexagonal boron nitride (h-BN) monolayers. Through our group theory analysis combined with density functional theory (DFT) calculations we provide several pieces of evidence that the electronic properties of the color centers match the characters of the experimentally observed emitters. We calculate the symmetry-adapted multi-electron wavefunctions of the defects using group theory methods and analyze the spin-orbit and spin-spin interactions in detail. We also identify the radiative and non-radiative transition channels for each color center. An advanced ab-initio DFT method is then used to compute energy levels of the color centers and their zero-phonon-line (ZPL) emissions. The computed ZPLs, the profile of excitation and emission dipole polarizations, and the competing relaxation processes are discussed and matched with the observed emission lines. By providing evidence for the relation between single-photon emitters and local defects in h-BN, this work provides the first steps towards harnessing quantum dynamics of these color centers.

Anisotropic visible photoluminescence from thermally annealed few-layer black phosphorus

Black phosphorus, a two-dimensional material, with high carrier mobility, tunable direct bandgap and anisotropic electronic properties has attracted enormous research interest towards potential application in electronic, optoelectronic and optomechanical devices. The bandgap of BP is thickness dependent, ranging from 0.3 eV for bulk to 1.3 eV for monolayer, while lacking in the visible region, a widely used optical regime for practical optoelectronic applications. In this work, photoluminescence (PL) centered at 605 nm is observed from the thermally annealed BP with thickness ≤20 nm. This higher energy PL is most likely the consequence of the formation of higher bandgap phosphorene oxides and suboxides on the surface BP layers as a result of the enhanced rate of oxidation. Moreover, the polarization-resolved PL measurements show that the emitted light is anisotropic when the excitation polarization is along the armchair direction. However, if excited along zigzag direction, the PL is nearly isotropic. Our findings suggest that the thermal annealing of BP can be used as a convenient route to fill the visible gap of the BP-based optoelectronic and optomechanical devices.

Quantitative retardance imaging by means of quadri-wave lateral shearing interferometry for label-free fiber imaging in tissues

We describe the use of quantitative retardance imaging, previously introduced in Aknoun et al. (2015), as a label-free method for in situ tissue imaging and extra-cellular matrix fiber organization study. A technical improvement of this quantitative retardance imaging technique, based on the use of a high-resolution quadri-wave lateral shearing interferometer, is implemented to perform fast quantitative linear retardance and retardance measurements on biological tissues. The technique combines a stack of quantitative phase images with varying polarization excitation to create retardance and orientation images at 1 Hz. This technique gives information about local retardance and structure of anisotropic components. In addition to bringing morphological information on unstained biopsies, it can reveal tumors development grades thanks to specific fiber organization imaging. In this present study, we demonstrate how this information can be used as tumor associated signature. We show the capability of our approach on mouse skin and human breast tissues.

Surface enhanced fluorescence effect of SiO2 coated silver nanoparticles and clusters

Composite Ag@SiO2 nanoparticles are prepared for investigating a metal-enhanced fluorescence effect. Due to the competition between enhanced electromagnetic field intensity and fluorescence emission decay, the distance between the molecules and metal surface is an important surface-enhanced fluorescence factor for enhancing or quenching. This distance can be controlled by the size of the separation-layer, and thus the surface-enhanced fluorescence enhancement can be controlled. To avoid fluorescence quenching and obtain a considerable fluorescence enhancement, SiO2 shell is used to separate the molecules from the metal surface. The surface modification of metallic nanoparticles by SiO2 can not only improve the dispersibility and stability of metal particles, but also expanding its application effectively in the biological field. For fluorophores located around metal nanostructures, many factors would affect a fluorescence enhancement effect. Investigation of the enhancement mechanism usually becomes difficult and complicated due to too many variables and factors involved in the enhancement process. In order to systematically study the influence of these factors on fluorescence intensity, single polymer nanoparticles are chosen to be an enhanced substrate. The particle distribution is labeled through electron beam lithography and the surface-enhanced fluorescence spectroscopy of single nanoparticle or cluster is achieved by in - situ measurement method. The influence of metal particle size, cluster property and excitation polarization on the surface enhanced fluorescence is studied by using Rh6G as the probe molecule. Obvious fluorescence enhancement effect of Rh6G molecules at the surface of Ag@SiO2 is observed with nanocomposite, and the enhancement effect is controlled by the metal core particle size, aggregation property, and the polarization direction of the excitation light.

Microscale two-dimensional (2D) temperature mapping by ratiometric fluorescence imaging under orthogonal excitations

Microscale temperature mapping in liquids is of great importance in many areas of research such as microfluidics and biology. Among the current thermometric approaches, optical probing using fluorescence is particularly desirable because of its high spatial resolution and non-invasive nature. Here we report a new microscale two-dimensional (2D) fluorescence thermometry. This method exploits the temperature dependence of rotational molecular motion and its influence on the depolarization of fluorescence light, by measuring the difference in fluorescence intensities excited by orthogonal polarizations. With one charge coupled device (CCD) camera, we get 2D ratiometric mappings of temperature from successively recorded fluorescence images under alternatively polarized excitations. We demonstrate reliable temperature mapping in liquids at sub-1 °C temperature accuracy and sub-10 µm spatial resolution. We also show that the proposed thermometry approach is robust against fluorescence intensity variations, suitable for 2D mapping and of fast readout that is comparable to CCD framerate. Moreover, it is easy to be integrated into microscope systems since only rotation of excitation polarization is needed.

Polarization control of high transmission/reflection switching by all-dielectric metasurfaces

Metasurfaces built of high refractive dielectric nanostructures could play a key role in controlling the electromagnetic wave propagation, due to their low energy losses and their ability to excite not only electric but also magnetic resonances. In this study, we theoretically and experimentally demonstrate that an array of high-index dielectric nanodimers can perform as tuneable metasurfaces that can be switched from a high transmitter to a high reflector, by just changing the linear polarization of excitation. The incident polarization alters the hybridization mode of the excited electric and magnetic dipoles in the dimer, and this leads to either spectral overlap or separation of the two dipoles. The hybridization of the electric and magnetic modes modifies the effective permittivity and permeability of the tuneable dielectric metasurface, exhibiting the high transmission and reflection that can be easily switched by simply changing the linear polarization.

Piezo‐Phototronic Effect Modulated Deep UV Photodetector Based on ZnO‐Ga2O3 Heterojuction Microwire

AbstractA strain modulated solar‐blinded photodetector (PD) based on ZnO‐Ga2O3 core–shell heterojuction microwire is developed. This PD is highly sensitive to deep UV light centered at 261 nm. It performs ultrahigh sensitivity and spectral selectivity, which can response to rare weak deep UV light (≈1.3 µw cm−2) and almost no response to visible light wavelength ranges. Moreover, by using the piezo‐phototronic effect, the deep UV current response is enhanced to about three times under −0.042% static strain. This is a three way coupling effect among pizoelectric polarization, simiconductor properties, and optical excitation, which exists in noncentral symmetric wurtzite semiconductors such as ZnO, GaN, and CdS. By modulating the energy band diagrams and charge carriers in the junction area upon straining, the optoelectronic processes are regulated. The strain induced piezopotential modulates carrier transport in the heterostructure, which improves the response of the PD, with potential applications for health monitoring, smart systems, deep space exploration, and security communication.

Polarization-resolved optical response of plasmonic particle-on-film nanocavities

Placing a metal nanoparticle atop a metal film forms a plasmonic particle-on-film nanocavity. Such a nanocavity supports strong plasmonic coupling that results in rich hybridized plasmon modes, rendering the cavity a versatile platform for exploiting a wide range of plasmon-enhanced spectroscopy applications. In this paper, we fully address the polarization-resolved, orientation-dependent far-field optical responses of plasmonic monomer- and dimer-on-film nanocavities by numerical simulations and experiments. With polarization-resolved dark-field spectroscopy, the distinct plasmon resonances of these nanocavities are clearly determined from their scattering spectra. Moreover, the radiation patterns of respective plasmon modes, which are often mixed together in common dark-field imaging, can be unambiguously resolved with our proposed quasi-multispectral imaging method. Explicitly, the radiation pattern of the monomer-on-film nanocavity gradually transitions from a solid spot in the green imaging channel to a doughnut ring in the red channel when tuning the excitation polarization from parallel to perpendicular to the sample surface. This observation holds true for the plasmonic dimer-on-film nanocavity with the dimer axis aligned in the incidence plane; when the dimer axis is normal to the incidence plane, the pattern transitions from a solid spot to a doughnut ring both in the red channel. These studies not only demonstrate a flexible polarization control over the optical responses of plasmonic particle-on-film nanostructures but also enrich the optical tool kit for far-field imaging and spectroscopy characterization of various plasmonic nanostructures.

Recent Progress on Piezotronic and Piezo‐Phototronic Effects in III‐Group Nitride Devices and Applications

Wurtzite‐structured III‐group nitrides, like GaN, InN, AlN, and their alloys, present both piezoelectric and semiconducting properties under straining owing to the polarization of ions in a crystal with non‐central symmetry. The piezoelectric polarization charges are created at the interface when a strain is applied. As a result, a piezoelectric potential (piezopotential) is produced, which is used as a “gate” to tune/control the charge transport behavior across a metal/semiconductor interface or a p‐n junction. This is called as piezotronic effect. A series of piezotronic devices and applications have been developed, such as piezotronic nanogenerators (NGs), piezotronic transistors, piezotronic logic devices, piezotronic electromechanical memories, piezotronic enhanced biochemical, and gas sensors and so on. With the flourished development of piezotronic effect, the piezo‐phototronic effect, as the three‐way coupling of piezoelectric polarization, semiconductor properties, and optical excitation, utilizes the piezopotential to modulate the energy band profile and control the carrier generation, transportation, separation, and/or recombination for improving performances of optoelectronic devices. This paper intends to provide an overview of the rapid progress in the emerging fields of piezotronics and piezo‐phototronics, covering from the fundamental principles to devices and applications. This study will provide important insight into the potential applications of GaN based electronic/optoelectronic devices in sensing, active flexible/stretchable electronics/optoelectronics, energy harvesting, human‐machine interfacing, biomedical diagnosis/therapy, and prosthetics.

Identifying light impurities in transition metal dichalcogenides: the local vibrational modes of S and O in ReSe2 and MoSe2

The transition metal dichalcogenides provide a rich field for the study of two-dimensional materials, with metals, semiconductors, superconductors and charge density wave materials being known. Members of this family are typically hexagonal, but those based on rhenium (ReSe2 and ReS2) and their ternary alloys are attracting attention due to their triclinic structure and their resulting, strong in-plane anisotropy. Here, Raman spectra of dilute ReSe2 - xSx alloys containing low levels of sulfur (x ≤ 0.25) were obtained in order to investigate the distribution of substitutional sulfur atoms over the non-equivalent chalcogen sites of the ReSe2 unit cell. Four different Raman bands arising from the local vibrational modes of sulfur atoms were observed, corresponding to these four sites. One local vibrational mode has a substantially in-plane displacement of the sulfur atom, two are partially out-of-plane and one is completely out-of-plane. The interpretation of the experimental data is based on calculations of the lattice dynamics and non-resonant Raman tensors of a model alloy via density functional theory. For comparison, polarization-dependent Raman spectra of pure ReS2 are also presented; a dramatic increase in the Raman cross-section is found for the out-of-plane modes when the excitation polarization is normal to the layers and the light propagates in the layer plane. A similar increase in cross-section is found experimentally for the local vibrational modes of sulfur in dilute ReSe2 -x Sx alloys and is predicted for dilute sulfur-containing alloys based on MoSe2. The analogous local vibrational modes of substitutional oxygen impurities in ReSe2 were also investigated computationally.

Nodeless vibrational amplitudes and quantum nonadiabatic dynamics in the nested funnel for a pseudo Jahn-Teller molecule or homodimer.

The nonadiabatic states and dynamics are investigated for a linear vibronic coupling Hamiltonian with a static electronic splitting and weak off-diagonal Jahn-Teller coupling through a single vibration with a vibrational-electronic resonance. With a transformation of the electronic basis, this Hamiltonian is also applicable to the anti-correlated vibration in a symmetric homodimer with marginally strong constant off-diagonal coupling, where the non-adiabatic states and dynamics model electronic excitation energy transfer or self-exchange electron transfer. For parameters modeling a free-base naphthalocyanine, the nonadiabatic couplings are deeply quantum mechanical and depend on wavepacket width; scalar couplings are as important as the derivative couplings that are usually interpreted to depend on vibrational velocity in semiclassical curve crossing or surface hopping theories. A colored visualization scheme that fully characterizes the non-adiabatic states using the exact factorization is developed. The nonadiabatic states in this nested funnel have nodeless vibrational factors with strongly avoided zeroes in their vibrational probability densities. Vibronic dynamics are visualized through the vibrational coordinate dependent density of the time-dependent dipole moment in free induction decay. Vibrational motion is amplified by the nonadiabatic couplings, with asymmetric and anisotropic motions that depend upon the excitation polarization in the molecular frame and can be reversed by a change in polarization. This generates a vibrational quantum beat anisotropy in excess of 2/5. The amplitude of vibrational motion can be larger than that on the uncoupled potentials, and the electronic population transfer is maximized within one vibrational period. Most of these dynamics are missed by the adiabatic approximation, and some electronic and vibrational motions are completely suppressed by the Condon approximation of a coordinate-independent transition dipole between adiabatic states. For all initial conditions investigated, the initial nonadiabatic electronic motion is driven towards the lower adiabatic state, and criteria for this directed motion are discussed.

Switching between H- and J-type electronic coupling in single conjugated polymer aggregates

The aggregation of conjugated polymers and electronic coupling of chromophores play a central role in the fundamental understanding of light and charge generation processes. Here we report that the predominant coupling in isolated aggregates of conjugated polymers can be switched reversibly between H-type and J-type coupling by partially swelling and drying the aggregates. Aggregation is identified by shifts in photoluminescence energy, changes in vibronic peak ratio, and photoluminescence lifetime. This experiment unravels the internal electronic structure of the aggregate and highlights the importance of the drying process in the final spectroscopic properties. The electronic coupling after drying is tuned between H-type and J-type by changing the side chains of the conjugated polymer, but can also be entirely suppressed. The types of electronic coupling correlate with chain morphology, which is quantified by excitation polarization spectroscopy and the efficiency of interchromophoric energy transfer that is revealed by the degree of single-photon emission.

Determining the Liquid Light Scattering Cross Section and Depolarization Spectra Using Polarized Resonance Synchronous Spectroscopy.

Rayleigh scattering is a universal material property because all materials have nonzero polarizability. Reliable quantification of the material light scattering cross section in the liquid phase and its depolarization spectra is, however, challenging due to a host of sample and instrument issues. Using the recently developed polarized resonance synchronous spectroscopic method, we reported the light scattering cross section and depolarization spectra measured for a total of 29 liquids including water, methanol, ethanol, 1-propanol, 1-butanol, dimethylformamide, carbon disulfide, dimethyl sulfoxide, hexane and two hexane isomers (3-methylpentane and 2,3-dimethylbutane), tetrahydrofuran, cyclohexane, acetonitrile, pyridine, chloromethanes including di-, tri, tetrachloromethane, acetone, benzene and eight benzene derivatives (toluene, fluorobenzene, 1,2-, 1,3-, and 1,4-difluorobenzene, chlorobenzene, 1,2- and 1,3-dichlorobenzene, and nitrobenzene). The solvent light scattering depolarization is wavelength-independent for the model solvents, and it varies from 0.023 ± 0.011 for CCl4 to 0.619 ± 0.022 for nitrobenzene. The light scattering cross-section spectra can be approximated with the function of σ(λ) = αλ-4 with the α value varying from 7.2 ± 0.2 × 10-45 cm6 for water to a maximum of 8.5 ± 0.6 × 10-43 cm6 for nitrobenzene. Structural isomerization has no significant effect on either the depolarization or the scattering cross sections for both hexanes and difluorobenzene isomers. This work represents the most comprehensive experimental study on liquid light scattering features. The insight from this work should be important for understanding the correlation between the material structure and optical properties. The described method can be readily implemented by researchers with access to conventional spectrofluorometers equipped with excitation and detection polarizers.

Two-dimensional coherent random laser in photonic crystal fiber with dye-doped nematic liquid crystal.

A random laser of a photonic crystal fiber (PCF) with holes filled with laser dye-doped nematic liquid crystal (NLC) is reported. When the excitation polarization was along the PCF axis, the measured laser threshold was 80 μJ/mm2 per pulse, which is much lower than the previously reported random laser of PCF filled with laser dye-doped organic solvent. This low threshold is due to the high refractive index of the NLC, which produces a greater scattering efficiency. In contrast, when the excitation polarization is perpendicular to the PCF axis, the threshold was much higher or the laser oscillation was absent. This is because of the lower refractive index of the NLC for the perpendicular polarization. The laser oscillation was absent in the isotropic phase because of a low fluorescence efficiency at high temperatures.

Azimuthally and radially excited charge transfer plasmon and Fano lineshapes in conductive sublayer-mediated nanoassemblies.

Here, the plasmon responses of both symmetric and antisymmetric oligomers on a conductive substrate under linear, azimuthal, and radial polarization excitations are analyzed numerically. By observing charge transfer plasmons under cylindrical vector beam (CVB) illumination for what we believe is the first time, we show that our studies open new horizons to induce significant charge transfer plasmons and antisymmetric Fano resonance lineshapes in metallic substrate-mediated plasmonic nanoclusters under both azimuthal and radial excitation as CVBs.