Near-field Spectra Research Articles

Tunable Optical Bistability and Tristability in Nonlinear Graphene-Wrapped Nanospheres

We develop the nonlinear electromagnetic theory (NET) including the self-consistent mean-field approach to investigate the optical multistability of graphene-wrapped dielectric nanoparticles. We demonstrate the optical bistability (OB) of the graphene-wrapped nanoparticle in both near-field and far-field spectra due to electric dipolar modes for small sizes, as predicted in the quasistatic limit (QL). For small sizes, two OB regions can be observed when the magnetic dipolar modes arise under the strong field. On the other hand, for large sizes, one observes the optical tristability (OT) and even optical multistability arising from the contributions of higher-order magnetic modes. Furthermore, both the optically stable region and the switching threshold values can be tuned by changing either the Fermi level or the size of the nanoparticles. Our results show promise for the graphene-wrapped dielectric nanoparticle as a candidate for multistate optical switching, optical memories, and relevant optoelectronic...

Wigner molecules and charged excitons in near-field magnetophotoluminescence spectra of self-organized InP/GaInP quantum dots

We used high-spatial-resolution, low-temperature near-field scanning optical microscopy (NSOM) operating at magnetic fields $B=0--10\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ to study the effects of Wigner localization (WL) on emission spectra of single self-organized InP/GaInP quantum dots (QDs) and investigate the stability of singly (trion) and doubly (tetron) charged exciton complexes in the weak quantum confinement regime. Using NSOM measurements together with configuration interaction calculations, we identify the dots having different electron population $\mathrm{N}(\mathrm{N}=1--12)$, quantum confinement $(\ensuremath{\hbar}{\ensuremath{\omega}}_{0}=0.6--8\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ and size $D(D=70--170\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$. For $\mathrm{N}=2$, we observed a magnetic-field-induced molecular-droplet transition, accompanied by the decomposition of the tetron into a Wigner molecule complex (WMC), and the activation of rotovibronic structure. For $\mathrm{N}=1$, unusually strong vibronic structure resulting from a trion-type WMC was observed. We have shown that magnetic-field-induced shifts of this structure allow measurement of single particle Fock-Darwin levels and angular momentum transitions of the WMC. In addition, we demonstrated the use of NSOM imaging to probe the charge density distribution and observed anomalous dependence of the image size on the quantum confinement, implying a pairing of electrons or formation of whispering gallery modes in the QD. We demonstrated that InP/GaInP QDs, provide a Wigner-Seitz radius $({r}_{s})$ up to 13, and that the measurements of NSOM magneto-optical spectroscopy using these dots makes it possible to study effects arising from strong Coulomb interaction of a few confined electrons (holes).

Near-field spectroscopic properties of complementary gold nanostructures: applicability of Babinet's principle in the optical region.

We examine the far-field and near-field properties of complementary screens made of nanostructured gold thin films, a rectangular nanowire and a nanovoid, using an aperture-type scanning near-field optical microscope and electromagnetic field calculations, and discuss the applicability of Babinet's principle in the optical region. The far-field transmission spectra of the complementary screens are considerably different from each other. On the other hand, genuine near-field extinction spectra exhibit nearly complementary characteristics. The spatial features of the observed near-field images for the complementary screens show little correlation. We have found from the Fourier analysis of the simulated images that high spatial-frequency components of the electromagnetic fields show mutual spatial correlation. These results suggest that Babinet's principle is applicable to the high spatial-frequency components of electromagnetic fields for the complementary screens.

Finite dipole model for extreme near-field thermal radiation between a tip and planar SiC substrate

Recent experimental studies have measured the infrared (IR) spectrum of tip-scattered near-field thermal radiation for a SiC substrate and observed up to a 50cm−1 redshift of the surface phonon polariton (SPhP) resonance peak [1,2]. However, the observed spectral redshift cannot be explained by the conventional near-field thermal radiation model based on the point dipole approximation. In the present work, a heated tip is modeled as randomly fluctuating point charges (or fluctuating finite dipoles) aligned along the primary axis of a prolate spheroid, and quasistatic tip-substrate charge interactions are considered to formulate the effective polarizability and self-interaction Green's function. The finite dipole model (FDM), combined with fluctuational electrodynamics, allows the computation of tip-plane thermal radiation in the extreme near-field (i.e., H/R≲1, where H is the tip-substrate gap distance and R is the tip radius), which cannot be calculated with the point dipole approximation. The FDM provides the underlying physics on the spectral redshift of tip-scattered near-field thermal radiation as observed in experiments. In addition, the SPhP peak in the near-field thermal radiation spectrum may split into two peaks as the gap distance decreases into the extreme near-field regime. This observation suggests that scattering-type spectroscopic measurements may not convey the full spectral features of tip-plane extreme near-field thermal radiation.

Laser heating of scanning probe tips for thermal near-field spectroscopy and imaging

Spectroscopy and microscopy of the thermal near-field yield valuable insight into the mechanisms of resonant near-field heat transfer and Casimir and Casimir-Polder forces, as well as providing nanoscale spatial resolution for infrared vibrational spectroscopy. A heated scanning probe tip brought close to a sample surface can excite and probe the thermal near-field. Typically, tip temperature control is provided by resistive heating of the tip cantilever. However, this requires specialized tips with limited temperature range and temporal response. By focusing laser radiation onto AFM cantilevers, we achieve heating up to ∼1800 K, with millisecond thermal response time. We demonstrate application to thermal infrared near-field spectroscopy (TINS) by acquiring near-field spectra of the vibrational resonances of silicon carbide, hexagonal boron nitride, and polytetrafluoroethylene. We discuss the thermal response as a function of the incident excitation laser power and model the dominant cooling contributions. Our results provide a basis for laser heating as a viable approach for TINS, nanoscale thermal transport measurements, and thermal desorption nano-spectroscopy.

Photobleaching-Assisted Near-Field Absorption Spectroscopy: Its Application to Single Tubular J-Aggregates

We propose photobleaching-assisted near-field optical spectroscopy to obtain the absorption properties of nanomaterials. We used bis(3-sulfopropyl)-5,5′,6,6′-tetrachloro-11′-dioctylbenzimidacarbocyanine (C8S3) double-wall tubular J-aggregates for demonstrating this method. We photobleached the J-aggregate by photoirradiation. We obtained the absorption spectrum of the J-aggregates by subtracting the transmission spectra observed before and after the photobleaching. The spectrum showed an intense asymmetric peak near 595 nm, which was assigned to the J-bands. These features were well reproduced by theoretical model calculations. We estimated the oscillator strength per molecule from the observed absorbance, and we found that the determined oscillator strength is comparable to that of the monomer. These results indicate that the proposed method is applicable for obtaining near-field absorption spectra of nanomaterials in a semi-quantitative manner.

Ga–In intermixing, intrinsic doping, and Wigner localization in the emission spectra of self-organized InP/GaInP quantum dots

We present study of structural and optical properties of InP/GaInP quantum (QDs) providing a weak quantum confinement and creating a platform to study Wigner localization (WL) effects using high spatial resolution optical spectroscopy. Self-organized QD structures were grown using metal-organic chemical phase epitaxy by using different substrate misorientations and cap layer deposition temperatures. Using transmission electron microscopy and energy dispersive x-ray spectroscopy, we demonstrated a bimodal height distribution with peaks at ~5 and ~20 nm and a control of both the lateral size distribution, peaked from ~100 to ~160 nm, and the amount of Ga–In intermixing in the QDs (up to 20%). Using photoluminescence (PL) spectroscopy in combination with circular polarization degree and time resolved micro-PL measurements, we demonstrated control of the emission energy, the intrinsic doping, and the emission decay of these In(Ga)P QDs. Using high-spatial-resolution near-field PL spectra and imaging of single dots, we demonstrated WL effects in dots having a population of up to nine electrons and a parabolic confinement down to ħω0 ~ 1 meV. We performed a self-consistent calculation of exciton transitions using an effective mass, mean field theory with an isotropic elasticity model to describe the effect of Ga–In intermixing on the emission properties of these dots; and we used calculations of shell splitting, using mean field Hartree–Fock approach and calculations of electron density distribution using configuration interaction approach, to described effects of enhancement of WL in non-circular dots with hard-wall potentials.

Exploring Coupled Plasmonic Nanostructures in the Near Field by Photoemission Electron Microscopy.

The extraordinary optical properties of coupled plasmonic nanostructures make these materials potentially useful in many applications; thus, they have received enormous attention in basic and applied research. Coupled plasmon modes have been characterized predominantly using far-field spectroscopy. In near-field spectroscopy, the spectral response of local field enhancement in coupled plasmonic nanostructures remains largely unexplored, especially experimentally. Here, we investigate the coupled gold dolmen nanostructures in the near field using photoemission electron microscopy, with wavelength-tunable femtosecond laser pulses as an excitation source. The spatial evolution of near-field mapping of an individual dolmen structure with the excitation wavelength was successfully obtained. In the near field, we spatially resolved an anti-bonding mode and a bonding mode as the result of plasmon hybridization. Additionally, the quadrupole plasmon mode that could be involved in the formation of a Fano resonance was also revealed by spatially resolved near-field spectra, but it only contributed little to the total near-field enhancement. On the basis of these findings, we obtained a better understanding of the near-field properties of coupled plasmonic nanostructures, where the plasmon hybridization and the plasmonic Fano resonance were mixed.

Tunable Optical Performances on a Periodic Array of Plasmonic Bowtie Nanoantennas with Hollow Cavities.

We propose a design method to tune the near-field intensities and absorption spectra of a periodic array of plasmonic bowtie nanoantennas (PBNAs) by introducing the hollow cavities inside the metal nanostructures. The numerical method is performed by finite element method that demonstrates the engineered hollow PBNAs can tune the optical spectrum in the range of 400–3000 nm. Simulation results show the hollow number is a key factor for enhancing the cavity plasmon resonance with respect to the hotspot region in PBNAs. The design efforts primarily concentrate on shifting the operation wavelength and enhancing the local fields by manipulating the filling dielectric medium, outline film thickness, and hollow number in PBNAs. Such characteristics indicate that the proposed hollow PBNAs can be a potential candidate for plasmonic enhancers and absorbers in multifunctional opto-electronic biosensors.

SERS-Active Ag Nanoparticles on Porous Silicon and PDMS Substrates: A Comparative Study of Uniformity and Raman Efficiency

Silver-coated porous silicon and polydimethylsiloxane (PDMS) are systematically analyzed as substrates for surface-enhanced Raman scattering (SERS). They were selected as representative metal–dielectric nanostructures characterized by different morphology and substrate dielectric constant which is reflected in the electromagnetic near-field intensity spectra. The study is conducted using 4-mercaptobenzoic acid as probe molecule with the aim to compare the scattering efficiency and the homogeneity of the Raman signal on the selected substrates. A larger SERS enhancement is evidenced for the silicon-based plasmonic nanostructures (>106, despite the electronic off-resonant excitation of the analyte). On the other hand, the silvered elastomeric substrates, characterized by a good Raman efficiency, show good repeatability featured by a low inter- and intrasubstrate standard deviation of the SERS signal intensity, which make them suitable for quantitative analysis. The influence of the excitation wavelength on such properties is deeply discussed taking into account experimental results and 3D modeling.

'Squeezing' near-field thermal emission for ultra-efficient high-power thermophotovoltaic conversion.

We numerically demonstrate near-field planar ThermoPhotoVoltaic systems with very high efficiency and output power, at large vacuum gaps. Example performances include: at 1200 °K emitter temperature, output power density 2 W/cm2 with ~47% efficiency at 300 nm vacuum gap; at 2100 °K, 24 W/cm2 with ~57% efficiency at 200 nm gap; and, at 3000 °K, 115 W/cm2 with ~61% efficiency at 140 nm gap. Key to this striking performance is a novel photonic design forcing the emitter and cell single modes to cros resonantly couple and impedance-match just above the semiconductor bandgap, creating there a ‘squeezed’ narrowband near-field emission spectrum. Specifically, we employ surface-plasmon-polariton thermal emitters and silver-backed semiconductor-thin-film photovoltaic cells. The emitter planar plasmonic nature allows for high-power and stable high-temperature operation. Our simulations include modeling of free-carrier absorption in both cell electrodes and temperature dependence of the emitter properties. At high temperatures, the efficiency enhancement via resonant mode cross-coupling and matching can be extended to even higher power, by appropriately patterning the silver back electrode to enforce also an absorber effective surface-plasmon-polariton mode. Our proposed designs can therefore lead the way for mass-producible and low-cost ThermoPhotoVoltaic micro-generators and solar cells.

Localized spoof surface plasmons in textured open metal surfaces

We experimentally demonstrate that textured open metal surfaces, i.e., the ultrathin fan-shaped metallic strips, are able to support spoof localized surface plasmons (spoof-LSPs) in the microwave frequencies. Unlike conventional spoof-LSPs supported on textured closed metal surfaces, which originate from the interference of clockwise and counterclockwise propagating surface modes, spoof-LSPs on textured open metal surfaces arise from the Fabry-Perot-like resonances due to the terminations of the open surfaces. We show that both the number of modes and the resonance frequencies of spoof-LSPs on textured open metal surfaces can be engineered through tuning the grating numbers (or total length) of the structured fan-shaped metallic strip. This enables the tuning of the spoof-plasmonic resonator by simply changing its length, rather than the complete geometry, simplifying the design to just one degree of freedom. Experimental evidence of the spoof-LSP Fabry-Perot resonators in the microwave regimes is presented with near-field response spectra and mode profiles imaged directly.

Continuous distributed phase-plate advances for high-energy laser systems

The distributed phase plate (DPP) design code Zhizhoo’ has been used to design full- aperture, continuous near-field transmission optics for a wide variety of high-fidelity focal-spot shapes for high-energy laser systems: OMEGA EP, Dynamic Compression Sector (DCS), and the National Ignition Facility (NIF). The envelope shape, or profile, of the focal spot affects the hydrodynamics of directly driven targets in these laser systems. Controlling the envelope shape to a high degree of fidelity impacts the quality of the ablatively driven implosions. The code Zhizhoo’ not only produces DPP's with great control of the envelope shape, but also spectral and gradient control as well as robustness from near-field phase aberrations. The focal-spot shapes can take on almost any profile from symmetric to irregular patterns and with high fidelity relative to the objective function over many decades of intensity. The control over the near-field phase spectrum and phase gradients offer greater manufacturability of the full- aperture continuous surface-relief pattern. The flexibility and speed of the DPP design code Zhizhoo’ will be demonstrated by showing the wide variety of successful designs that have been made and those that are in progress.

Large near-to-far field spectral shifts for terahertz resonances

We have performed far-field extinction measurements and near electric field measurements on gold bowtie antennas resonant at THz frequencies. These measurements show a very large shift between the resonant frequencies of the near-field and the far-field spectra. We use the established damped-driven harmonic oscillator model for resonators to model the far-field response of the antennas from the near-field spectrum and show that there is a large discrepancy between the predicted and measured far-field response. We were able to explain this discrepancy by improving the oscillator model with a Fano model. This large shift makes the prediction of the near-field response of resonant structures at THz frequencies very imprecise, provided that only information of the far-field response is available and establishes the necessity of measuring near fields for a correct and accurate characterization of these structures.

Metal-dielectric-metal resonators with deep subwavelength dielectric layers increase the near-field SEIRA enhancement.

Plasmonic nanostructures presenting either structural asymmetry or metal-dielectric-metal (M-D-M) architecture are commonly used structures to increase the quality factor and the near-field confinement in plasmonic materials. This characteristic can be leveraged for example to increase the sensitivity of IR spectroscopy, via the surface enhanced IR absorption (SEIRA) effect. In this work, we combine structural asymmetry with the M-D-M architecture to realize Ag-Ag(2)O-Ag asymmetric ring resonators where two Ag layers sandwich a native silver oxide (Ag(2)O) layer. Their IR response is compared with the one of fully metallic (Ag) resonators of the same size and shape. The photothermal induced resonance technique (PTIR) is used to obtain near-field SEIRA absorption maps and spectra with nanoscale resolution. Although the native Ag(2)O layer is only 1 nm to 2 nm thick, it increases the quality factor of the resonators' dark-mode by ≈27% and the SEIRA enhancement by ≈44% with respect to entirely Ag structures.

Experimental demonstration of high-order magnetic localized spoof surface plasmons

We experimentally demonstrate that an ultrathin metallic spiral structure is able to support multiple high-order magnetic localized spoof surface plasmons (LSSPs), which were absent in previously reported magnetic LSSPs. Near-field response spectra and near-field mapping are performed in the microwave regime to confirm this phenomenon. We also show that the high-order magnetic LSSPs are more sensitive to the surrounding refractive index change than the previously reported magnetic dipole mode. Our study may be useful in electromagnetic near-field sensing from microwave to infrared frequencies.

Improvement of infrared near-field spectrum by asymmetric interferometer configuration

Infrared synchrotron radiation (IR-SR) is a highly brilliant white light source. We are developing an infrared near-field spectroscopy system with an IR-SR light source. The near-field spectroscopy system previously reported comprised an atomic force microscope (AFM) and a commercial Fourier transform infrared (FTIR) spectrometer. In the present study, the configuration of the FTIR interferometer has been modified to an asymmetric one. In the asymmetric interferometer, one beam split by a beamsplitter is focused onto the tip of an AFM probe, and the other beam goes to a movable mirror. The scattered light from the probe and the light reflected by the movable mirror interfere with each other. The near-field signal is extracted by a modulation method with an AFM oscillation frequency. The signal-to-noise ratio has been improved 6-fold and the signal-to-background ratio is improved 8-fold compared with those observed in the previous system.

Magnitude and phase-resolved infrared vibrational nanospectroscopy with a swept quantum cascade laser.

We demonstrate a method of rapidly acquiring background-free infrared near-field spectra by combining magnitude and phase resolved scattering-type scanning near-field optical microscopy (s-SNOM) with a wavelength-swept quantum cascade laser (QCL). Background-free measurement of both near-field magnitude and phase allows for direct comparison with far-field absorption spectra, making the technique particularly useful for rapid and straightforward nanoscale material identification. Our experimental setup is based on the commonly used pseudo-heterodyne detection scheme, which we modify by operating the interferometer in the white light position; we show this adjustment to be critical for measurement repeatability. As a proof-of-principle experiment we measure the near-field spectrum between 1690 and 1750 cm(-1) of a PMMA disc with a spectral resolution of 1.5 cm(-1). We finish by chemically identifying two fibers on a sample surface by gathering their spectra between 1570 and 1750 cm(-1), each with a measurement time of less than 2.5 minutes. Our method offers the possibility of performing both nanoscale-resolved point spectroscopy and monochromatic imaging with a single laser that is capable of wavelength-sweeping.

Wide operation range in-phase coherently coupled vertical cavity surface emitting laser array based on proton implantation.

In-phase coherently coupled vertical cavity surface emitting laser (VCSEL) hexagonal arrays were fabricated using proton implantation. The near-field profiles, far-field profiles, and emission spectra under different injection currents were tested and analyzed. The arrays can maintain in-phase single mode in a considerably wide current range from 10 mA (I(th)) to 35 mA (3.5×I(th)), exhibiting excellent beam quality. The far-field divergence angle of the in-phase coupled array is 2.5 degrees. Approximately 29% of total power is localized in the central lobe. Compared with square structure arrays, hexagonal arrays can maintain a more stable in-phase mode because of stronger coupling among the elements. The maximum output power of 4.9 mW was obtained under pulse wave condition. The simulation of far-field was carried out to match the in-phase operation test results. The performance enhancement of the array is attainable if the condition of heat dissipation is better. The process procedure of proton implantation is relatively simple and of low cost. It can be used as an alternative to coherently coupled array implementations.

Nanoscale-resolved chemical identification of thin organic films using infrared near-field spectroscopy and standard Fourier transform infrared references

We establish a solid basis for the interpretation of infrared near-field spectra of thin organic films on highly reflective substrates and provide guidelines for their straightforward comparison to standard far-field Fourier transform infrared (FTIR) spectra. Particularly, we study the spectral behavior of near-field absorption and near-field phase, both quantities signifying the presence of a molecular resonance. We demonstrate that the near-field phase spectra only weakly depend on the film thickness and can be used for an approximate comparison with grazing incidence FTIR (GI-FTIR) spectra. In contrast, the near-field absorption spectra can be compared more precisely with far-field spectra: for ultrathin films they match well GI-FTIR spectra, while for thick films a good agreement with standard transmission FTIR spectra is found. Our results are based on experimental data obtained by nanoscale FTIR (nano-FTIR) spectroscopy and supported by a comprehensive theoretical analysis.