Polarization-dependent Excitation Research Articles

Theory of photonic crystal polaritons in periodically patterned multilayer waveguides

We present a formalism for studying the radiation-matter interaction in multilayered dielectric structures with active semiconductor quantum wells patterned with an in-plane periodic lattice. The theory is based on the diagonalization of the generalized Hopfield matrix, and it includes loss channels in a non-Hermitian formulation. Hybrid elementary excitations named photonic crystal polaritons arise in these systems, whose detailed dispersion and loss characteristics are shown to depend on material composition as well as on symmetry properties of the lattice. We show the generality of the approach by calculating polariton dispersions in very diverse material platforms, such as multilayered perovskite-based lattices or inorganic semiconductor heterostructures. As an application of the method, we show how to engineer lossless polariton modes through excitonic coupling to bound states in the continuum at either zero or finite in-plane wave vector, and discuss their topological properties. Detailed comparison with a semiclassical approach based on the scattering matrix method is provided, which allows to interpret the optical spectra in terms of polarization-dependent excitation of the different polariton branches. This paper introduces an efficient and invaluably versatile numerical approach to engineer photonic crystal polaritons, with potential applications ranging from low-threshold lasers to symmetry-protected propagating modes of hybrid radiation-matter states.

A library of thermotropic liquid crystals of inorganic nanoparticles and extraordinary performances based on their collective ordering

Inorganic nanoparticles are widely investigated due to their unique and fascinating physical and chemical properties. However, due to the strong interparticle interactions, they generally form irreversible random aggregates with elevated temperatures. This limits their technological applications considerably as the latter requires structural ordering and tunability. To address this, a versatile method of preparing solvent-free thermotropic liquid crystals of inorganic nanoparticles is presented. By chemically modifying the nanoparticles with a flexible organic surface layer, the nanoparticles self-assemble into ordered microstructures, possessing both ordering and fluidity in a solvent-free condition. A library of thermotropic liquid crystals, including 6 different shapes, 8 different compositions, and 18 different dimensions, is constructed. The flexible tuning of the particle properties and the intrinsic ordering and fluidity provide a powerful platform for the design and development of functional materials and devices with unique properties, such as polarization-dependent excitation, polarized photoluminescence, polarized photothermal response, and designable magnetic robot. • A versatile method of preparing solvent-free thermotropic liquid crystals of inorganic nanoparticles is presented. • A library of solvent-free TLCs of inorganic NPs, including 6 shapes, 8 compositions, and 18 dimensions, is constructed. • The intrinsic ordering and fluidity provide a platform for the design and development of functional materials and devices.

Oxygen annealing induced changes in defects within β-Ga2O3 epitaxial films measured using photoluminescence

In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy induced by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in the formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O2 annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find activation energy for the generation of non-radiative defects of 1.34 eV in the bulk but 2.53 eV near the surface. We also deduce activation energies for the green emission-related defects of 1.20 eV near the surface and 2.21 eV at low temperatures and 0.74 eV at high temperatures through the films, whereas the blue-related defects have activation energy in the range 0.72–0.77 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 °C. These observations are consistent with the formation and diffusion of V Ga and its complexes as a dominant process during O2 annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.

Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO.

The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3–4 µm. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

2D Square Lattice Photonic Crystal Phosphor Films for Efficient and Excitation Polarization Insensitive Color Conversion

AbstractAs a continuing effort to improve the performance of photonic crystal (PhC) phosphors—an engineered nanophotonic phosphor structure developed by the authors' group—hereby 2D PhC phosphor films are introduced. In particular, the excitation polarization dependence inherent to and suffered by the precedent 1D PhC phosphors can be eliminated using a square lattice PhC structure due to its fourfold rotational symmetry. In addition, the thickness of the high index PhC backbone layer is intentionally increased so that dual excitation resonances occur for both the transverse‐electric (TE) and transverse‐magnetic (TM) waveguide modes, thereby effectively doubling the phosphor efficiency. 2D PhC phosphor structures are implemented and examined using colloidal quantum dots (CQDs) as phosphor material. Unlike conventional impurity‐doped phosphor materials composed of micron‐sized grains, nanometer‐sized CQDs are structurally compatible with PhCs whose feature sizes are ≈100 nm. Excitation polarization–insensitive CQD fluorescence is observed approx. four times enhanced at dual resonance wavelengths for the TE‐ and TM‐guided modes. The validity of the observed results is confirmed through simulations.

Light Coupling between Plasmonic Nanowire and Nanoparticle

In this work, we investigate polarization-dependent excitation of the propagating surface plasmon polariton (SPP) modes in gold nanowires (Au NWs) combined with gold nanoparticles (Au NPs). The light coupling from focused light to SPPs on Au NWs is investigated for different structural combinations of Au NWs with Au NPs, using full-wave finite-element numerical simulations. The results show that the excitation of SPPs changes remarkably on varying the orientation of the NP on NW or the polarization angle of the incident light. Metallic NWs combined with NPs can be applied to the polarization-resolved SPP coupling in various optical and optoelectronic devices including photonic circuits and optical sensors.

Long-range surface plasmon polariton detection with a graphene photodetector.

We present an integration of a single Ag nanowire (NW) with a graphene photodetector and demonstrate an efficient and compact detection of long-range surface plasmon polaritons (SPPs). Atomically thin graphene provides an ideal platform to detect the evanescent electric field of SPPs extremely bound at the interface of the Ag NW and glass substrate. Scanning photocurrent microscopy directly visualizes a polarization-dependent excitation and detects the SPPs. The SPP detection responsivity is readily controlled up to ∼17 mA/W by the drain-source voltage. We believe that the graphene SPP detector will be a promising building block for highly integrated photonic and optoelectronic circuits.

Excitation polarization dependence of optical coupled exciton states in a single coupled quantum dot molecule

The optically coupled exciton states have been investigated in terms of power dependent photoluminescence at the coupled GaAs quantum dots, which were laterally coupled along the [10] direction. The power dependent photoluminescence results are compared with two different excitation directions which are parallel to the direction of [10] and [110]. In coupled direction along the [10] direction, the power dependent redshift and coupled biexciton are observed, a fact that may be attributed to optical couplings via dipole-dipole interactions. Also, the redshift of excitons was manifested based on theoretical model.

Ultrafast Zero-Bias Photocurrent in GeS Nanosheets: Promise for Photovoltaics

Ferroelectric semiconductors have been predicted to exhibit strong zero-bias shift current, spurring the search for ferroelectric semiconductors with band gaps in the visible range as candidates for so-called shift current photovoltaics with efficiencies not constrained by the Schockley–Queisser limit. Recent theoretical works have predicted that two-dimensional IV–VI monochalcogenides are multiferroic and capable of generating significant shift currents. Here we present experimental validation of this prediction, observing ultrafast shift currents by detecting terahertz electromagnetic pulses emitted by the photoexcited GeS nanosheets without external bias. We explore excitation fluence, orientation, and excitation polarization dependence of the terahertz emission to confirm that shift currents are indeed responsible for the observed emission. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third-generation photovoltaics ...

Single Particle Studies on Two-Photon Photoluminescence of Gold Nanorod–Nanosphere Heterodimers

The localized surface plasmon resonance (SPR) property of metal nanoparticles has numerous potential applications. Coupling between different SPR modes of adjacent metal nanoparticles can result in significantly enhanced optical responses such as surface-enhanced Raman scattering (SERS) and two-photon photoluminescence (2PPL). In this work, gold (Au) nanorod (NR)–nanosphere (NS) heterodimers with side-linked and end-linked spatial arrangements have been systematically studied on the single-particle level to investigate the plasmon coupling effects on their 2PPL properties. Compared to a single Au NR, both end-linked and side-linked Au NR-NS heterodimers displayed red-shifted scattering spectra with similar cos2θ excitation polarization dependence, which can be understood in terms of plasmon hybridization theory. However, the 2PPL intensities of end-linked and side-linked Au NR-NS heterodimers displayed strikingly different excitation polarization dependence. The 2PPL signals of the end-linked Au NR-NS heterodimer displayed cos4θ excitation polarization dependence, similar to that of Au NR monomer. The 2PPL intensity of the end-linked heterodimer is the largest with polarization along the long axis of Au NR, which is ∼300 times that of the Au NR monomer component and even ∼5 times that of a longer Au NR with a similar overall size to the heterodimer. In a striking contrast, 2PPL intensity of the side-linked heterodimer is largest for excitation along the transverse direction of Au NR, which is ∼25 times that under excitation along the longitudinal direction. Despite these differences, an interesting feature is that the largest 2PPL signals occur along the coupling directions for both end-linked and side-linked heterodimers. Plasmon-coupling-enhanced 2PPL is generally believed to arise from enhanced two-photon excitation efficiency through two effects: improved resonance effects due to the plasmon-coupling-induced red-shifted SPR mode and plasmon-coupling-induced giant local electric field amplification. Stronger 2PPL signals have been observed for coupled nanostructures with excitation along the coupling direction but less favorable overlap between the SPR scattering spectra and excitation wavelength, which unambiguously supports that plasmon-coupling-induced local electric field amplification is the dominant effect responsible for the big difference in 2PPL intensities of different nanostructures. This conclusion has been further confirmed by the excellent agreement between the numerically calculated integrated |E/E0|4 and the experimentally obtained 2PPL signal intensities for various nanostructures. These studies offer some fundamental understanding of plasmon coupling effects on optical responses and excitation mechanisms of 2PPL of metal nanostructures, which provide insight on designing nanostructures with tailored optical properties for various potential photonic and optoelectronic applications.

Upconversion photoluminescence imaging and spectroscopy of individual single-walled carbon nanotubes

We demonstrate microscopic observations of recently discovered efficient upconversion (anti-Stokes) photoluminescence (UCPL) [Nat. Commun. 6, 8920 (2015)] of single-walled carbon nanotubes (SWNTs). UCPL images and spectra of the identical isolated SWNTs were obtained. Unlike previous observations on the SWNT ensemble, some individual SWNTs exhibit UCPL spectra coincident with Stokes PL spectra. The excitation polarization dependence of UCPL intensity indicates that the intermediate states in the upconversion process are extrinsic. These observations are likely to facilitate a comprehensive understanding of the UCPL mechanism towards the realization of highly efficient UCPL of SWNTs.

Plasmonically enhanced electron escape from gold nanoparticles and their polarization-dependent excitation transfer along DNA nanowires.

Here we show plasmon mediated excitation transfer along DNA nanowires over up to one micrometer. Apparently, an electron excitation is initiated by a femtosecond laser pulse that illuminates gold nanoparticles (AuNP) on double stranded DNA (dsDNA). The dependency of this excitation on laser wavelength and polarization are investigated. Excitation of the plasmon resonance of the AuNPs via one- and two-photon absorption at 520 and 1030 nm, respectively, was explored. We demonstrate an excitation transfer along dsDNA molecules at plasmon supported four-photon excitation of AuNP cluster or at laser field driven nanoparticle electron tunneling for an alignment of the attached dsDNA to the polarization of the electric field of the laser light. These results extend the previously observed plasmonically induced three-photon excitation transfer along DNA nanowires to another nanoparticle material (gold) and the adapted irradiation wavelengths.

Two-photon excited fluorescence from a pseudoisocyanine-attached gold-coated tip via a thin tapered fiber under a weak continuous wave excitation

A simple tapered fiber based photonic-plasmonic hybrid nanostructure composed of a thin tapered fiber and a pseudoisocyanine (PIC)-attached Au-coated tip was demonstrated. Using this simple hybrid nanostructure, we succeeded in observing two-photon excited fluorescence from the PIC dye molecules under a weak continuous wave excitation condition. From the results of the tip-fiber distance dependence and excitation polarization dependence, we found that using a thin tapered fiber and an Au-coated tip realized efficient coupling of the incident light (~95%) and LSP excitation at the Au-coated tip, suggesting the possibility of efficiently inducing two-photon excited fluorescence from the PIC dye molecules attached on the Au-coated tip. This simple photonic-plasmonic hybrid system is one of the promising tools for single photon sources, highly efficient plasmonic sensors, and integrated nonlinear plasmonic devices.

Beam deflection measurement of time and polarization resolved ultrafast nonlinear refraction

We modify the well-known photothermal beam deflection technique to study ultrafast nonlinearities. Using phase-sensitive detection we directly measure the temporal and polarization dynamics of nonlinear refraction (NLR) with sensitivity to optically induced phase changes of approximately λ/20,000. We use the relative polarization dependence of excitation and probe to separate the isotropic and reorientational components of the NLR.

Experimental evidence for cm propagation lengths of long-range guided terahertz radiation by thin layers of water

Terahertz waves can be guided with low losses along thin layers of a strongly absorbing medium embedded in a dielectric. By means of end-fire coupling experiments, we demonstrate the polarization dependent excitation, propagation, and detection of this loss-guided mode, demonstrating long propagation lengths for terahertz radiation along thin water films. Broadband frequency and time resolved analysis using transmission terahertz time domain spectroscopy are performed. A theoretical model is used to isolate the coupled mode from co-propagating spurious modes. Hereby, experimental propagation lengths in the cm range are demonstrated, 2–3 orders of magnitude above the intrinsic propagation length in bulk water.

Spectrally-Resolved Polarization Anisotropy of Single Plasmonic Nanoparticles Excited by Total Internal Reflection

Polarization-resolved optical studies of individual plasmonic nanoparticles are valuable for providing orientation information about anisotropic plasmonic nanostructures, such as nanorods and nanoparticle dimers, but are less useful for structures such as nanoprisms. In this study, the anisotropic polarization-dependent excitation field generated by through-the-objective total internal reflection (TIR) is used to probe the orientation of single gold nanorods and single silver triangular nanoprisms with spectral and polarization resolution. By correlating localized surface plasmon resonance (LSPR) and polarization-resolved spectra with scanning electron microscopy (SEM) imaging, we show that both nanorods and nanoprisms show anisotropic polarization-sensitive scattering. While the polarization response of the nanorods is consistent with previous reports, the nanoprisms show an unexpected anisotropy that is related to their orientation in the x–y plane. Exploiting the anisotropic polarization-dependent excitation field offered by TIR allows us to access orientational information about a variety of nanostructures through a nonperturbative optical technique.

Quantitative myelin imaging with coherent anti-Stokes Raman scattering microscopy: alleviating the excitation polarization dependence with circularly polarized laser beams

The use of coherent anti-Stokes Raman scattering microscopy tuned to the lipid vibration for quantitative myelin imaging suffers from the excitation polarization dependence of this third-order nonlinear optical effect. The contrast obtained depends on the orientation of the myelin membrane, which in turn affects the morphometric parameters that can be extracted with image analysis. We show how circularly polarized laser beams can be used to avoid this complication, leading to images free of excitation polarization dependence. The technique promises to be optimal for in vivo imaging and the resulting images can be used for coherent anti-Stokes Raman scattering optical histology on native state tissue.

Ex vivo and in vivo imaging of myelin fibers in mouse brain by coherent anti-Stokes Raman scattering microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy was applied to image myelinated fibers in different regions of a mouse brain. The CARS signal from the CH2 symmetric stretching vibration allows label-free imaging of myelin sheath with 3D sub-micron resolution. Compared with two-photon excited fluorescence imaging with lipophilic dye labeling, CARS microscopy provides sharper contrast and avoids photobleaching. The CARS signal exhibits excitation polarization dependence which can be eliminated by reconstruction of two complementary images with perpendicular excitation polarizations. The capability of imaging myelinated fibers without exogenous labeling was used to map the whole brain white matter in brain slices and to analyze the microstructural anatomy of brain axons. Quantitative information about fiber volume%, myelin density, and fiber orientations was derived. Combining CARS with two-photon excited fluorescence allowed multimodal imaging of myelinated axons and other cells. Furthermore, in vivo CARS imaging on an upright microscope clearly identified fiber bundles in brain subcortex white matter. These advances open up new opportunities for the study of brain connectivity and neurological disorders.

Confocal white light reflection imaging for characterization of metal nanostructures

A simple far-field confocal white light reflection imaging system was proposed using a xenon (Xe) lamp as the incident light source. The best spatial resolution was determined to be about 410nm. Localized surface plasmons (LSPs) of isolated single and dimer gold nanospheres were studied and the resonance energy difference between their LSPs was extracted. Polarization dependent reflection imaging and contrast spectrum of single silver nanowires were also investigated, which correlate closely with the polarization dependent excitation of their LSP.

Strong Near-infrared Avalanche Photoluminescence from Ag Nanowire Arrays

Ag nanowire (NW) arrays with NW diameter d NW = 12–120 nm were electrodeposited in anodic aluminum oxide templates. Strong avalanche photoluminescence (PL) from Ag NW arrays with small d NW were observed near 914 nm by using picosecond laser at the excitation wavelength 808 nm, which is originated from the plasmon-enhanced radiative intraband transitions. The peak PL intensity of the avalanche PL from the sample with small diameter d NW = 12 nm is about 102 times stronger than that of the linear PL from the sample with large diameter d NW = 120 nm. The opposite excitation polarization dependence and emission polarization distribution of the PL from Ag NW array with d NW = 12 nm and d NW = 120 nm were also observed.