Polarization Of Excitation Light Research Articles

Fluorescence spatial anisotropy of emission dipoles in an orthogonal imaging system

In this study, we explore the relatively unexplored spatially anisotropic fluorescence emission induced by rotationally polarized excitation light in orthogonal imaging systems, a phenomenon that is particularly pronounced in orthogonal setups compared to coaxial configurations. Despite its significance, this aspect has been largely neglected, given the prevalent use of coaxial Fluorescence Polarization Microscope (FPM) setups. Our research endeavors to bridge this gap by formulating a physical model to investigate the polarization-dependent emission of dipoles and the corresponding fluorescence signal within an orthogonal imaging configuration. Additionally, we introduce the Fluorescence Spatial Anisotropy Index (FSAI) to quantify fluorescence spatial anisotropy.

Non-destructive estimation of flesh oil content in avocado (Persea americana Mill.) using fluorescence images from 365-nm UV light excitation.

The flesh oil content (OC) is a crucial commercial indicator of avocado maturity and directly correlates with its nutritional quality. To meet export standards and optimize edible characteristics, avocados must be harvested at the appropriate stage of physiological maturity. The significant variability in OC during maturation, without any external morphological indicators, poses a longstanding challenge. Currently, harvesting maturity is optimized through time-consuming, destructive laboratory methods like freeze-drying and chemical extraction, which use representative samples to estimate the maturity of entire orchards. In this study, for the first time, we employed fluorescence imaging of avocado skin using 365-nm UV polarized light excitation to estimate the OC in the 'Bacon' avocado cultivar. We developed a surface fluorescence index that strongly correlates with OC, achieving correlation coefficients up to -0.91. Our non-destructive and rapid approach achieved a cross-validation accuracy with an R2 value of 0.81, enabling the classification of avocados with low and high OC. This pioneering method shows considerable potential for further improvement and refinement. This study lays the groundwork for developing a portable, cost-effective, and real-time method for non-destructive in situ monitoring of avocado OC in the field and its integration into large-scale post-harvest grading systems.

Anisotropic growth of Ag nanonails induced by plasmon-mediated chemical reactions in cross-shaped nanocavity array

The Ag nanonail rooted in Au cross-channel nanocavity is fabricated by plasmon-mediated chemical reactions (PMCRs). The Au cross-channel nanocavity is prepared based on two-dimensional polystyrene (PS) nanosphere template through wet etching, and the shape of Au cross-channel nanocavity changes from trapezoidal to triangular, depending on the wet etching time. The growth of Ag nanostructures in Au channels by PMCRs indicates the morphology of Ag nanostructures has changed from Ag nanoparticles to nanonails, which is regulated by the shape of Au nanocavities and the exciting light polarization. The hotspots distributions results in the anisotropic growth of Ag nanostructures induced by PMCRs, which leading to the different Ag nanostructures depending on the shape of Au nanocavity and light polarization, as confirmed by FDTD simulations. In comparison with others reports, our nanocavity provide more parameters to control the shapes of Ag nanostructures, the nanocavity shape, the light polarization and the growth time. Our work provides a profound insight into the construction of periodic nanostructures and reconstruction of hotspots.

Giant Photon‐Drag‐Induced Ultrafast Photocurrent in Diamond for Nonlinear Photonics

AbstractDiamond is emerging as an attractive third‐generation wide‐bandgap semiconductor for future on‐chip nonlinear photonics and quantum optics due to its unique thermal, optical, and mechanical properties. However, the light‐driven current under below‐band gap excitation from the second‐order nonlinear optical effect in diamond is still challenging. Herein, a giant second‐order nonlinear photocurrent is observed in the chemical vapor deposition (CVD) diamond by utilizing terahertz (THz) emission spectroscopy. This ultrafast photocurrent originates from the photon drag effect (PDE), during which the momentum transfer from the incident photons to the charge carriers at the rich grain boundaries of the CVD diamond after the exclusive subgap π–π* transition upon femtosecond laser excitation. Especially, the interplay between circular and linear PDE to the THz generation is clarified and distinguished under elliptically polarized light excitation. Furthermore, the picosecond ultrafast dynamics of these charge carriers are also verified by infrared spectroscopy. Owing to the giant photon‐drag‐induced ultrafast photocurrent, the CVD diamond presents the highest THz emission efficiency compared with the reported carbon allotropes, which expands the new functionality of diamond nonlinear photonics into on‐chip THz devices.

Electric field-dependent photoexcited spin-polarized electron transport in Bi2Se3 topological insulator

Three-dimensional (3D) topological insulators (TIs) exhibit spin-polarized surface states in which the spin of electrons is locked to their momentum. The helical surface states can be explored from circularly polarized light-induced spin photocurrent, a phenomenon called circular photogalvanic effect (CPGE). In this work, we fabricate a TI transistor with the Bi2Se3 channel layer synthesized using vapor deposition. The photocurrent response of Bi2Se3 TI is characterized under horizontal and longitudinal electric fields. CPGE and linear photogalvanic effect (LPGE) contribute to the surface state photocurrent at room temperature. The longitudinal electric field provides kinetic energy to the electrons so that the transition of electrons to higher energy bands in momentum space occurs. Under a photon excitation with the energy far above the TI bandgap, we observed a photocurrent difference between left and right circularly polarized light excitation. The photocurrent variation with gate voltage (longitudinal field) is also investigated. Adjusting the Fermi level with the gate bias leads to changes in the population of bulk state carriers and spin electrons in surface states. By shifting the gate bias toward negative, the CPGE current increases because of the reduced scattering with bulk carriers. Our work reveals that longitudinal and horizontal electric fields can manipulate the helical spin-polarized photocurrent of Bi2Se3. From the asymmetry of circularly polarized photoconductive differential current (CPDC) under the horizontal field, we found evidence of spin-polarized electron transition to other conduction band valleys at room temperature under a high-energy photon excitation.

Characterization of Chiral Near Field Within Nanogaps Using Surface‐Enhanced Raman Scattering

AbstractChiral nanogap antennas, which exhibit specific chiroptical responses, hold exceptional potential in various chiral applications, including polarization converters, asymmetric catalysis, label‐free chiral recognition, and chiral sensing. However, characterizing the chiral optical fields within nanogaps presents considerable challenges owing to their complex light field responses and spatial dimensions. In this study, the strong modulation of helicity‐resolved surface‐enhanced Raman spectroscopy (SERS) signals in graphene by chiral plasmonic antennas, offering a promising approach to characterize chiral near fields within nanogaps is demonstrated. The SERS of achiral monolayer graphene is performed within plasmonic nanogap antennas, including single gold nanorods (GNRs) and GNR dimers on gold microflakes. The Raman scattering intensity of graphene within these nanogap antennas is enhanced approximately sixfold. Furthermore, the degree of cross‐circularly polarized Raman intensity reaches up to ≈45%, indicating a robust chiral optical field. Moreover, under opposite circularly polarized light excitation, the SERS of graphene reveals varying emission intensities attributed to the modulation arising from the chiral antennas in both the excitation and emission processes of Raman scattering. Furthermore, the degree of chirality depends on the antenna configuration and the excitation laser wavelength.

Enhancing high harmonic generation in GaAs by elliptically polarized light excitation

Enhancing high harmonic generation in GaAs by elliptically polarized light excitation

Optical chiral gain-tunable metasurface electric field enhancement devices

Nanostructured optical field enhancements have become more attractive in recent years. In this work, we proposed a surface electric field enhancement device based on the combined effect of surface plasmons resonance and surface plasmon polariton. By optimizing the relationship between the morphology of the nanostructures and the Z-direction electric field components, the intensity of the surface polarized plasmons of the meta-atom can be further enhanced. The 633 nm left-circularly polarized light excitation for this device was examined and worked well. It was used as the operating wavelength. A comparison of the results obtained from right-circularly polarized and linearly polarized lights demonstrated only a strong optical field enhancement of the structure with left-handed polarized light. The polarization-selective parameters were also considered. Finally, by designing meta-atoms with different arrangements, we achieved electric field enhancements up to 177.5. It is worth mentioning that this method can control the degree of electric field enhancement by controlling the number of meta-atoms that excite surface plasmon polaritons, which means that obtaining the desired electric field amplitude becomes tunable in the design. With the proper format, the enhancement factor of the surface-enhanced Raman spectroscopy can be increased to approximately 109. In the research of surface-enhanced Raman spectroscopy and optical tweezers, it would be a straightforward and effective design method to create the desired optical field enhancements.

Selective plasmonic trapping of nano-particles by Archimedes metalens.

Optical tweezer is a non-invasive method for optical force tool applied in various fields like biology, physics, and lab on chip manipulation. The Archimedean helix shape is ideal for creating chiral nanostructures, and being able to generate plasmonic focused hotspot field for optical trapping. Here we design a metal disk with the Archimedean shape to own the ability of selective trapping nanoparticles based on the spin-orbit interactions with circularly polarized light. The plasmonic near field on the metalens can be designed by adjusting the geometric parameter flexibly. We numerically analyze the optimal size and screw pitch of the metal disk to realize the switch modulation of hotspot generation, and then demonstrate the novel switchable optical trapping ability in the view of optical force and potential well analysis under the circularly polarized light excitation by a 532 nm laser. The work shows significant potential for on-chip optical trapping in various fields.

Origin of plasmonic Fano resonance in metal-hole/split-ring-resonator metamaterials disclosed by temporal coupled-mode theory.

In plasmonic Fano resonance, the interaction between a discrete plasmonic mode and a continuum of plasmonic mode gives rise to an asymmetric line shape in the scattering or absorption spectrum, enabling a wide range of applications such as sensing, switching, and slow light devices. Here, we establish a theoretical solution in the framework of temporal coupled-mode theory (TCMT) to study the three-dimensional (3D) and two-dimensional (2D) Fano resonances induced by strong coupling between metal hole (MH) and split ring resonator (SRR) array. We first separately analyze the transmission spectra of the MH array and SRR array under different polarized light excitation. We further investigate the electromagnetic field and charge density distribution corresponding to the resonant modes at the peak or valley wavelength of the transmission spectrum and figure out the electric/magnetic dipole feature of these resonance modes. We then establish a theoretical solution by TCMT for Fano resonances arising from the coupling of these modes. The calculated transmission spectrum is closely matching with the numerically simulated transmission spectrum for these Fano resonances in the MH-SRR array, which effectively elucidates that the asymmetry of the Fano resonances is caused by the coupling between bright and dark plasmonic modes involved in the two structures. Our results can help to understand the profound physics in such coupled plasmonic systems.

Electron circular dichroism in hot electron emission from metallic nanohelix arrays.

We investigate the electron emission from 3D chiral silver alloy nanohelices initiated by femtosecond laser pulses with a central photon energy of hν = 1.65eV, well below the work function of the material. We find hot but thermally distributed electron spectra and a strong anisotropy in the electron yield with left- and right-circularly polarized light excitations, which invert in signbetween left- and right-handed helices. We analyze the kinetic energy distribution and discuss the role of effective temperatures. Measurements of the reflectance and simulations of the absorbance of the helices based on retarded field calculations are compared to the anisotropy in photoemission. We find a significant enhancement of the anisotropy in the electron emission in comparison to the optical absorption. Neither simple thermionic nor a multiphoton photoemission can explain the experimentally observed asymmetries. Single photon deep-UV photoemission from these helices together with a change of the work function suggests a contribution of the chirally induced spin selectivity effect to the observed asymmetries.

Brightening up Full-Color and White Circularly Polarized Luminescence through Chiral Induction and Circularly Polarized Light Excitation.

Preparation of chiral materials from achiral helical polymers has aroused great interest among researchers. In this work, chiral small molecules were utilized to accomplish chiral induction toward racemic helical polyacetylene via intermolecular π-π stacking by which chiral films with strong optical activity were fabricated. Furthermore, introducing fluorescent components generated full-color and white circularly polarized luminescence (CPL). A CPL generation mechanism is proposed accordingly, namely circularly polarized light excitation (CP-Ex). CPL emission and amplification of the luminescence dissymmetry factor were achieved under the synergetic effect of CP-Ex and chirality transfer. The CP-Ex mechanism was further verified by the double-layered film consisting of a chiral layer and a fluorescent layer. More noticeably, for double-layered films, the sense of CPL signals can be switched by changing the direction of excitation light. This work opens up new strategies for exploring tunable multiple- and white-color CPL materials.

Light helicity probed through spin dependent recombination in GaAsN alloys

In this paper we briefly review the main experimental and theoretical aspects of the spin-dependent recombination in dilute nitride GaAsN point interstitial defects leading to a large contrast between the photoluminescence intensity under circularly and linearly polarized light excitations as well as to the implementation of a chiral photodetector. We discuss the interplay of the spin-selective capture of free electrons in deep paramagnetic Ga2+ interstitial defects and the hyperfine coupling among the defect nucleus and the bound electron. These factors combined confer GaAsN the capability of discriminating the handedness of an incident light beam.

On the Origin of Photoluminescence Enhancement in Biicosahedral AgxAu25−x Nanoclusters (x = 0–13) and Their Application to Triplet–Triplet Annihilation Photon Upconversion

AbstractNumerous reports hitherto show that the photoluminescence (PL) properties of metal nanoclusters (NCs) can be enhanced by alloying the metal cores. In particular, biicosahedral [AgxAu25−x(PPh3)10(SR)5Cl2]2+ (abbreviated as AgxAu25−x hereafter; with PPh3 = triphenylphosphine; SR = thiolate ligand) NCs attract significant attention because their PL quantum yield is improved by 200 times when 13 Au atoms are replaced with Ag atoms (x = 13). In this contribution, the origin of the PL in the AgxAu25−x system and its remarkable enhancement are investigated on the basis of spectroscopic investigations of the PL behavior and its quenching by an organic fluorophore, finding that (i) the observed PL of AgxAu25−x is phosphorescent; (ii) not only Ag13Au12 but also Ag12Au13 NCs contribute to the PL; and (iii) replacing the central vertex atom of the biicosahedron with an Ag atom causes a blue shift of the triplet states, which suppresses the T1–S0 intersystem crossing and enhances the phosphorescence emission. Additionally, the results of single‐particle PL spectroscopy and defocused imaging with rotation of linearly polarized excitation light reveal that the phosphorescence transition dipole moment of Ag13Au12 exists in the long axis direction of the biicosahedron. Furthermore, the AgxAu25−x NCs can sensitize molecular triplets and efficiently induce red‐to‐blue photon upconversion via triplet–triplet annihilation.

Raman and optical absorption spectra of oriented Se8 and Se12 rings formed in zeolites: Dependence on the Se loading density

We encapsulated selenium (Se) into AFI and LTA zeolites via Se vapour adsorption at ∼450 °C and studied Raman and optical absorption spectra (RS and OAS, respectively) of zeolite single crystals with incorporated Se. At high Se loading density (D), Se8 rings are formed in the AFI channels along with Se chains. The rings are oriented by their 4-fold axis parallel to channels. Se8 is also formed in the LTA large cavities with its 4-fold axis oriented parallel to the 4-fold axis of LTA at any D up to ∼8 Se atoms per cavity (at/c). However, at D > 0.5 at/c, nearly unexplored Se12 rings oriented by their 3-fold axis parallel to the LTA 3-fold axis are formed along with Se8 in neighboring cavities. We discriminate between Se8 and Se12 rings via 1) RS frequencies; 2) RS and OAS band intensities vs. D; 3) RS band intensities vs. excitation wavelength and light polarization configuration. With an increase in D, the number of Se12 rings grows faster than that of Se8. We extract OAS of Se8 and Se12 from the experimental AFI-Se and LTA-Se OAS and show that the 3.0–3.3 eV absorption of Se12 is enhanced compared to that of Se8. This absorption is responsible for the observed resonant Raman effect of Se12 at the 514.5 nm wavelength excitation. Both LTA-confined Se8 and Se12 display rather high thermal stability.

Intelligent infrared sensing enabled by tunable moiré quantum geometry.

Quantum geometric properties of Bloch wave functions in solids, that is, Berry curvature and the quantum metric, are known to significantly influence the ground- and excited-state behaviour of electrons1-5. The bulk photovoltaic effect (BPVE), a nonlinear phenomenon depending on the polarization of excitation light, is largely governed by the quantum geometric properties in optical transitions6-10. Infrared BPVE has yet to be observed in graphene or moiré systems, although exciting strongly correlated phenomena related to quantum geometry have been reported in this emergent platform11-14. Here we report the observation of tunable mid-infrared BPVE at 5 µmand 7.7 µm in twisted double bilayer graphene(TDBG), arising from the moiré-induced strong symmetry breaking and quantum geometric contribution. The photoresponse depends substantially on the polarization state of the excitation light and is highly tunable by external electric fields. This wide tunability in quantum geometric properties enables us to use a convolutional neural network15,16 to achieve full-Stokes polarimetry together with wavelength detection simultaneously, using only one single TDBG device with a subwavelength footprint of merely 3 × 3 µm2. Our work not only reveals the unique role of moiré engineered quantum geometry in tunable nonlinear light-matter interactions but also identifies a pathway for future intelligent sensing technologies in an extremely compact, on-chip manner.

Valence Band of Rutile TiO2(110) Investigated by Polarized-Light-Based Angle-Resolved Photoelectron Spectroscopy.

Band structure dictates optical and electronic properties of solids and eventually the efficiency of the semiconductor based solar conversion. Compared to numerous theoretical calculations, the experimentally measured band structure of rutile TiO2, a prototypical photocatalytic material, is rare. In this work, the valence band structure of rutile TiO2(110) is measured by angle-resolved photoelectron spectroscopy using polarized extreme ultraviolet light. The effective mass of the hole, which has never been measured before, is determined to be 4.66-6.87 m0 (free electron mass) and anisotropic. The dependence of photoemission intensities on excitation light polarization is analyzed by taking into account of the parity symmetry of molecular orbitals in the blocking unit of rutile TiO2. This work reports a direct measurement of valence band structure and hole effective mass of rutile TiO2(110), which will deepen our understanding of the electronic structure and charge carrier properties of the model material and provide reference data for future theoretical calculations.

Metallic nanosphere-assisted coupling ultrafast surface plasmon polaritons background-free tip nanofocusing.

Plasmonic tip nanofocusing has gained much attention owing to its wide application in the field of nanospectroscopy. Here, we present the Au nanosphere (AuNS)-assisted coupling ultrafast surface plasmon polaritons (SPP) background-free tip nanofocusing. The plasmonic tip was prepared by attaching an AuNS on the shaft of an Au conical tip fabricated by electrochemical etching. The AuNS was adopted as an antenna to couple the far-field excitation light to the propagating SPP along the shaft to the tip apex for achieving power compression. Importantly, we experimentally and theoretically demonstrate that such a plasmonic tip can realize background-free ultrafast SPP tip nanofocusing with radially polarized features in a wide spectral range based on the localized SPP resonance effect supported by AuNS. Furthermore, the intensity of the tip nanofocusing light field has strong polarization dependence under linearly polarized light excitation, providing a powerful platform for spatiotemporal light control on the nanoscale. Our technique realizes remote excitation of background-free tip nanofocusing with a structured light feature, and it holds promising potential for tip-enhanced nanospectroscopies, nonlinear nanophotonics, etc.

Resonant light scattering from dielectric ring of rectangular cross section

In this paper we present the results of numerical calculations of electromagnetic properties for cylindrical ring resonators (RRs) with rectangular cross-section and a dielectric permittivity corresponding to silicon ε = 12. The calculation of the scattering spectrum (Radar Cross Section) and the field distribution of the modes were performed at in-plane polarized light excitation. The presence of four side walls in the RRs determines a richer spectrum of eigenmodes in comparison with cylindrical whispering gallery modes of disk resonators and creates more possibilities and diversity for their practical applications.

Linearly polarized excitation enhances signals from fluorescent voltage indicators

Voltage imaging in cells requires high-speed recording of small fluorescent signals, often leading to low signal/noise ratios. Because voltage indicators are membrane bound, their orientations are partially constrained by the plane of the membrane. We explored whether tuning the linear polarization of excitation light could enhance voltage indicator fluorescence. We tested a panel of dye- and protein-based voltage indicators in mammalian cells. The dye BeRST1 showed a 73% increase in brightness between the least and most favorable polarizations. The protein-based reporter ASAP1 showed a 22% increase in brightness, and QuasAr3 showed a 14% increase in brightness. In very thin neurites expressing QuasAr3, improvements were anomalously large, with a 170% increase in brightness between polarization parallel versus perpendicular to the dendrite. Signal/noise ratios of optically recorded action potentials were increased by up to 50% in neurites expressing QuasAr3. These results demonstrate that polarization control can be a facile means to enhance signals from fluorescent voltage indicators, particularly in thin neurites or in high-background environments.