Adaptive subwavelength control of nano-optical fields

Electronic structure orientation as a map of in-plane antiferroelectricity in β'-In2Se3

Antiferroelectric (AFE) materials are excellent candidates for sensors, capacitors, and data storage due to their electrical switchability and high-energy storage capacity. However, imaging the nanoscale landscape of AFE domains is notoriously inaccessible, which has hindered development and intentional tuning of AFE materials. Here, we demonstrate that polarization-dependent photoemission electron microscopy can resolve the arrangement and orientation of in-plane AFE domains on the nanoscale, despite the absence of a net lattice polarization. Through direct determination of electronic transition orientations and analysis of domain boundary constraints, we establish that antiferroelectricity in β'-In2Se3 is a robust property from the scale of tens of nanometers to tens of micrometers. Ultimately, the method for imaging AFE domain organization presented here opens the door to investigations of the influence of domain formation and orientation on charge transport and dynamics.

Polarization enhanced two-photon excited fluorescence contrast by shaped laser pulses using a deformable phase plate

We utilize spatially and temporally tailored laser pulses for polarization enhanced two-photon excited fluorescence contrasts of dyes. The shaped laser pulses are produced by first passing through a temporal pulse shaper and then through a two-dimensional spatial pulse shaper with deformable phase plates. Different spatial beam profiles are presented that demonstrate the potential of the spatial pulse shaper. Particularly, a polarization enhanced fluorescence contrast between two dyes is reported by utilizing specific phase shaping in perpendicular polarization directions. The tailored laser pulses are further modified by the deformable phase plate, and a polarization increased depth-dependent contrast is achieved. This spatial shaping for all polarization directions demonstrates the advantage of deformable phase plate spatial shapers compared to liquid crystals, where only one polarization direction can spatially be modified. The described polarization contrast method allows for three-dimensional scanning of probes and provides perspectives for biophotonic applications.

Near- and far-field study of polarization-dependent surface plasmon resonance in bowtie nano-aperture arrays

Bowtie nano-apertures can confine light into deep subwavelength volumes with extreme field enhancement, making them a useful tool for various applications such as optical trapping, deep subwavelength imaging, nanolithography, and sensors. However, the correlation between the near- and far-field properties of bowtie nano-aperture arrays has yet to be fully explored. In this study, we experimentally investigated the polarization-dependent surface plasmon resonance in bowtie nano-aperture arrays using both optical transmission spectroscopy and photoemission electron microscopy. The experimental results reveal a nonlinear redshift in the transmission spectra as the gap size of the bowtie nanoaperture decreases for vertically polarized light, while the transmission spectra remain unchanged with different gap sizes for horizontally polarized light. To elucidate the underlying mechanisms, we present simulated charge and current distributions, revealing how the electrons respond to light and generate the plasmonic fields. These near-field distributions were verified by photoemission electron microscopy. This study provides a comprehensive understanding of the plasmonic properties of bowtie nano-aperture, enabling their further applications, one of which is the optical switching of the resonance wavelength in the widely used visible spectral region without changing the geometry of the nanostructure.

Light-Induced Subnanometric Modulation of a Single-Molecule Electron Source

Single-molecule electron sources of fullerenes driven via constant electric fields, approximately 1 nm in size, produce peculiar emission patterns, such as a cross or a two-leaf pattern. By illuminating the electron sources with femtosecond light pulses, we discovered that largely modulated emission patterns appeared from single molecules. Our simulations revealed that emission patterns, which have been an intractable question for over seven decades, represent single-molecule molecular orbitals. Furthermore, the observed modulations originated from variations of single-molecule molecular orbitals, practically achieving the subnanometric optical modulation of an electron source.

Temporally shaped vortex phase laser pulses for two-photon excited fluorescence

We report temporally shaped vortex phase laser pulses for two-photon excited fluorescence of dyes. The particularly tailored pulses are generated by first utilizing a temporal pulse shaper and subsequently a two-dimensional spatial pulse shaper. Various vortex phase shaped structures are demonstrated by combining different two-dimensional phase patterns. Moreover, perpendicular polarization components are used to achieve an enhanced radial two-photon excited fluorescence contrast by applying third order phase functions on the temporal pulse shaper. Particularly, the spatial fluorescence structure is modulated with a combination of Gaussian and vortex phase shaped pulses by modifying only the phase on the temporal modulator. Thereby, interference structures with high spatial resolution arise. The introduced method to generate temporally shaped vortex phase tailored pulses will provide new perspectives for biophotonic applications.

Simple single-shot complete spatiotemporal intensity and phase measurement of an arbitrary ultrashort pulse using coherent modulation imaging

We propose a simple single-shot spatiotemporal measurement technique called coherent modulation imaging for the spatio-spectrum (CMISS), which reconstructs the full three-dimensional high-resolution characteristics of ultrashort pulses based on frequency-space division and coherent modulation imaging. We demonstrated it experimentally by measuring the spatiotemporal amplitude and phase of a single pulse with a spatial resolution of 44 µm and a phase accuracy of 0.04 rad. CMISS has good potential for high-power ultrashort-pulse laser facilities and can measure even spatiotemporally complicated pulses with important applications.

Wide-Field Super-Resolution Optical Fluctuation Imaging through Dynamic Near-Field Speckle Illumination

Stochastic optical fluctuation imaging (SOFI) generates super-resolution fluorescence images by emphasizing the positions of fluorescent emitters via statistical analysis of their on-and-off blinking dynamics. In SOFI with speckle illumination (S-SOFI), the diffraction-limited grain size of the far-field speckles prevents independent blinking of closely located emitters, becoming a hurdle to realize the full super-resolution granted by SOFI processing. Here, we present a surface-sensitive super-resolution technique exploiting dynamic near-field speckle illumination to bring forth the full super-resolving power of SOFI without blinking fluorophores. With our near-field S-SOFI technique, up to 2.8- and 2.3-fold enhancements in lateral spatial resolution are demonstrated with computational and experimental fluorescent test targets labeled with conventional fluorophores, respectively. Fluorescent beads separated by 175 nm are also super-resolved by near-field speckles of 150 nm grain size, promising sub-100 nm resolution with speckle patterns of much smaller grain size.

Resolving plasmonic hotspots by label-free super-resolution microscopy

The plasmonic hotspot of metal nanostructures has small dimension far beyond the optical diffraction limit. When trying to locate the hotspot using fluorescent probes, the localization is significantly distorted due to the coupling of emission and surface plasmon. A label-free technique can solve the problem, which uses hotspot emission as the native probe. We demonstrate a super-resolution microscopy investigation based on this idea. By modulating hotspot emission of crossed silver nanowires, which have a pair of plasmonic hotspots approximately 100 nm apart at the intersection, we precisely locate and separate them with nanometer precision. This label-free technique could be applied for analyzing hotspot distribution with high efficiency and precision.

Imaging through a scattering medium: the Fisher information and the generalized Abbe limit

Enhanced-resolution imaging in complex scattering media is revisited from a parameter estimation perspective. A suitably defined Fisher information is shown to offer useful insights into the limiting precision of parameter estimation in a scattering environment and, hence, into the limiting spatial resolution that can be achieved in imaging-through-scattering settings. The Fisher information that defines this resolution limit via the Cramér-Rao lower bound is shown to scale with the number of adaptively controlled space-time modes of the probe field, suggesting a physically intuitive generalization of the Abbe limit to the spatial resolution attainable for complex scattering systems. In a conventional, direct-imaging microscopy setting, this bound is shown to converge to the canonical Abbe limit.

Coherent Interference Fringes of Two-Photon Photoluminescence in Individual Au Nanoparticles: The Critical Role of the Intermediate State

The interaction between light and metal nanoparticles enables investigations of microscopic phenomena on nanometer length and ultrashort timescales, benefiting from strong confinement and enhancement of the optical field. However, the ultrafast dynamics of these nanoparticles are primarily investigated by multiphoton photoluminescence on picoseconds or photoemission on femtoseconds independently. Here, we presented two-photon photoluminescence (TPPL) measurements on individual Au nanobipyramids (AuNP) to reveal their ultrafast dynamics by double-pulse excitation on a global timescale ranging from subfemtosecond to tens of picoseconds. Two orders of magnitude photoluminescence enhancement, namely, coherent interference fringes, has been demonstrated. Power-dependent measurements uncovered the transform of the nonlinearity from 1 to 2 when the interpulse delay varied from tens of femtoseconds to tens of picoseconds. We proved that the real intermediate state plays a critical role in the observed phenomena, supported by numerical simulations with a three-state model. Our results provide insight into the role of intermediate states in the ultrafast dynamics of noble metal nanoparticles. The presence of the intermediate states in AuNP and the coherent control of state populations offer interesting perspectives for imaging, sensing, nanophotonics, and in particular, for preparing macroscopic superposition states at room temperature and low-power superresolution stimulated emission depletion microscopy.

Orientation and Alignment dynamics of polar molecule driven by shaped laser pulses

We review the theoretical status of intense laser induced orientation and alignment-a field of study which lies at the interface of intense laser physics and chemical dynamics and having potential applications such as high harmonic generation, nano-scale processing and control of chemical reactions. The evolution of the rotational wave packet and its dynamics leading to orientation and alignment is the topic of the present discussion. The major part of this article primarily presents an overview of recent theoretical progress in controlling the orientation and alignment dynamics of a molecule by means of shaped laser pulses. The various theoretical approaches that lead to orientation and alignment such as static electrostatic field in combination with laser field(s), combination of orienting and aligning field, combination of aligning fields, combination of orienting fields, application of train of pulses etc. are discussed. It is observed that the train of pulses is quite an efficient tool for increasing the orientation or alignment of a molecule without causing the molecule to ionize. The orientation and alignment both can occur in adiabatic and non-adiabatic conditions with the rotational period of the molecule taken under consideration. The discussion is mostly limited to non-adiabatic rotational excitation (NAREX) i.e. cases in which the pulse duration is shorter than the rotational period of the molecule. We have emphasised on the so called half-cycle pulse (HCP) and square pulse (SQP). The effect of ramped pulses and of collision on the various laser parameters is also studied. We summarize the current discussion by presenting a consistent theoretical approach for describing the action of such pulses on movement of molecules. The impact of a particular pulse shape on the post-pulse dynamics is also calculated and analysed. In addition to this, the roles played by various laser parameters including the laser frequency, the pulse duration and the system temperature etc. are illustrated and discussed. The concept of alignment is extended from one-dimensional alignment to three-dimensional alignment with the proper choice of molecule and the polarised light. We conclude the article by discussing the potential applications of intense laser orientation and alignment.

Modulation of Cathodoluminescence Emission by Interference with External Light

Spontaneous processes triggered in a sample by free electrons, such as cathodoluminescence, are commonly regarded and detected as stochastic events. Here, we supplement this picture by showing through first-principles theory that light and free-electron pulses can interfere when interacting with a nanostructure, giving rise to a modulation in the spectral distribution of the cathodoluminescence light emission that is strongly dependent on the electron wave function. Specifically, for a temporally focused electron, cathodoluminescence can be canceled upon illumination with a spectrally modulated dimmed laser that is phase-locked relative to the electron density profile. We illustrate this idea with realistic simulations under attainable conditions in currently available ultrafast electron microscopes. We further argue that the interference between excitations produced by light and free electrons enables the manipulation of the ultrafast materials response by combining the spectral and temporal selectivity of the light with the atomic resolution of electron beams.

Information transfer from the optical phase to a plasmonic digital encoder composed of a gold nanorod pair

Small all-optical devices are central to the optical computing. Plasmonic digital encoders (PDEs) with a featured dimension of ∼1µm hold the key for transferring information from far field to photonic processing systems. Here we propose a PDE design composed of two gold nanorods (AuNRs), whose pattern represents 2-bit digital information. We implanted information into the spectral phase of a femtosecond pulse by pulse shaping and controlled the two-photon photoluminescence pattern of an AuNR pair. The high contrast ratios were achieved with 13.01 and 6.02 dB for binary codes "1-0" and "0-1", respectively.

Turning a hot spot into a cold spot: polarization-controlled Fano-shaped local-field responses probed by a quantum dot

Optical nanoantennas can convert propagating light to local fields. The local-field responses can be engineered to exhibit nontrivial features in spatial, spectral and temporal domains, where local-field interferences play a key role. Here, we design nearly fully controllable local-field interferences in the nanogap of a nanoantenna, and experimentally demonstrate that in the nanogap, the spectral dispersion of the local-field response can exhibit tuneable Fano lineshapes with nearly vanishing Fano dips. A single quantum dot is precisely positioned in the nanogap to probe the spectral dispersions of the local-field responses. By controlling the excitation polarization, the asymmetry parameter q of the probed Fano lineshapes can be tuned from negative to positive values, and correspondingly, the Fano dips can be tuned across a broad spectral range. Notably, at the Fano dips, the local-field intensity is strongly suppressed by up to ~50-fold, implying that the hot spot in the nanogap can be turned into a cold spot. The results may inspire diverse designs of local-field responses with novel spatial distributions, spectral dispersions and temporal dynamics, and expand the available toolbox for nanoscopy, spectroscopy, nano-optical quantum control and nanolithography.

Plasmonic nanoantennas on VO2 films for active switching of near-field intensity and radiation from nanoemitters

In this paper, we propose novel plasmonic switches based on plasmonic nanoantennas lying on top of a thin film of a phase change material such as vanadium dioxide (VO₂), such that the near-field properties of these nanoantennas can be actively switched by varying the phase of the VO₂ film. We employ finite difference time domain (FDTD) simulations to first demonstrate that the near-field intensity in the vicinity of the plasmonic nanoantennas can be substantially switched by changing the phase of the vanadium dioxide film from the semiconductor state to the metallic state. We demonstrate that a ring-bowtie nanoantenna (RBN) switch can switch the near-field intensity by ∼ 59.5 times and ring-rhombus nanoantenna (RRN) switch can switch the near-field intensity by a factor of ∼ 80.8. These values of the maximum intensity switching ratios are substantially higher than those previously reported in the literature. In addition, we optimize the various geometrical parameters of the plasmonic switches to maximize the intensity switching ratio and to understand how the different parameters affect the performance of the plasmonic switches. In this paper, we also show that the intensity of emission from a nanoemitter placed in the gap between the two arms of a plasmonic nanoantenna can be significantly switched by changing the phase of the VO₂ film between its semiconductor state and the metallic state. To quantify the switching of emission from the nanoemitters placed in the near-field of the nanoantennas, we define and calculate a parameter, called FESR, the ratio of fluorescent enhancement factors in the on-state and off-state of the plasmonic switch. The maximum fluorescence enhancement switching ratio (FESR) of ∼ 163 is obtained for the RBN switch and FESR of ∼ 200 is obtained for RRN switch. The plasmonic switches being proposed by us can be easily fabricated by employing the conventional nanofabrication and thin film deposition processes.

Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

Plasmonics is a rapidly growing field spanning research and applications across chemistry, physics, optics, energy harvesting, and medicine. Ultrafast photoemission electron microscopy (PEEM) has demonstrated unprecedented power in the characterization of surface plasmons and other electronic excitations, as it uniquely combines the requisite spatial and temporal resolution, making it ideally suited for 3D space and time coherent imaging of the dynamical plasmonic phenomena on the nanofemto scale. The ability to visualize plasmonic fields evolving at the local speed of light on subwavelength scale with optical phase resolution illuminates old phenomena and opens new directions for growth of plasmonics research. In this review, we guide the reader thorough experimental description of PEEM as a characterization tool for both surface plasmon polaritons and localized plasmons and summarize the exciting progress it has opened by the ultrafast imaging of plasmonic phenomena on the nanofemto scale.

Distinct spatiotemporal imaging of femtosecond surface plasmon polaritons assisted with the opening of the two-color quantum pathway effect

Accurately capturing the spatiotemporal information of surface plasmon polaritons (SPPs) is the basis for expanding SPP applications. Here, we report spatio-temporal evolution imaging of femtosecond SPPs launched from a rectangular trench in silver film with a 400-nm light pulse assisted femtosecond laser interferometric time-resolved (ITR) photoemission electron microscopy. It is found that introducing the 400nm light pulse in the spatially separated near-infrared (NIR) laser pump-probe ITR scheme enables distinct spatiotemporal imaging of the femtosecond SPPs with a weak probe pulse in the ITR scheme, which is free from the risk of sample damage due to the required high monochromatic field for a clear photoelectron image as well as the entangled interference fringe (between the SPPs and probe pulse) in the usual spatially overlapped pump-probe ITR scheme. The demonstrated great improvement of the visibility of the SPPs spatiotemporal image with an additional 400nm light pulse scheme facilitates further analysis of the femtosecond SPPs, and carrier wavelength (785nm), group velocity (0.94C) and phase velocity (0.98C) of SPPs are extracted from the distinct spatio-temporal evolution images of SPPs. Furthermore, the modulation of photoemission induced by the quantum pathway interference effect in the 400nm-assisted scheme is proposed to play a major role in the distinct visualization for SPPs. The probabilities of electrons in different quantum pathways are obtained quantitatively through fitting the experimental results with the quantum pathway interference model. The probability that electrons emit through the quantum pathway allows us to quantitatively analyze the contribution to electron emission from the different quantum pathways. These findings pave a way for the spatiotemporal imaging of the near-infrared light-induced SPPs, such as the communication wave band using PEEM.

Modulation of spin-dependent diffraction based on dielectric metasurfaces

We propose theoretically and realize experimentally a tunable single-slit diffraction based on dielectric metasurfaces. Our dielectric metasurfaces can be regarded as polarization converters to generate inhomogeneous polarized light periodically variant in x direction. Different from the well-known single-slit diffraction of the scalar light field, our diffraction patterns exhibit two columns of diffraction fringes, which conceals spin-dependent splitting phenomenon. The underlying mechanism is attributed to the inherent nature of the Pancharatnam-Berry phase in the inhomogeneous polarized light. Interestingly, the spin-dependent splitting can be enhanced by increasing the polarization rotation rate of the inhomogeneous polarized beam or the transmission distance. Further, tunable diffraction phenomenon is observed with different slit widths or variant rotation angles of the dielectric metasurface and the slit. Our results may offer potential applications in spin-controlled nanophotonics.

Far-Field Wavefront Control of Nonlinear Luminescence in Disordered Gold Metasurfaces

We demonstrate the local optimization of nonlinear luminescence from disordered gold metasurfaces by shaping the phase of femtosecond excitation. This process is enabled by the far-field wavefront control of plasmonic modes delocalized over the sample surface, leading to a coherent enhancement of subwavelength electric fields. In practice, the increase in nonlinear luminescence is strongly sensitive to both the nanometer-scale morphology and the level of structural complexity of the gold metasurface. We typically observe a 2 orders of magnitude enhancement of the luminescence signal for an optimized excitation wavefront compared to a random one. These results demonstrate how disordered metasurfaces made of randomly coupled plasmonic resonators, together with wavefront shaping, provide numerous degrees of freedom to program locally optimized nonlinear responses and optical hotspots.

Azimuthal multiplexing 3D diffractive optics

Diffractive optics have increasingly caught the attention of the scientific community. Classical diffractive optics are 2D diffractive optical elements (DOEs) and computer-generated holograms (CGHs), which modulate optical waves on a solitary transverse plane. However, potential capabilities are missed by the inherent two-dimensional nature of these devices. Previous work has demonstrated that extending the modulation from planar (2D) to volumetric (3D) enables new functionalities, such as generating space-variant functions, multiplexing in the spatial or spectral domain, or enhancing information capacity. Unfortunately, despite significant progress fueled by recent interest in metasurface diffraction, 3D diffractive optics still remains relatively unexplored. Here, we introduce the concept of azimuthal multiplexing. We propose, design, and demonstrate 3D diffractive optics showing this multiplexing effect. According to this new phenomenon, multiple pages of information are encoded and can be read out across independent channels by rotating one or more diffractive layers with respect to the others. We implement the concept with multilayer diffractive optical elements. An iterative projection optimization algorithm helps solve the inverse design problem. The experimental realization using photolithographically fabricated multilevel phase layers demonstrates the predicted performance. We discuss the limitations and potential of azimuthal multiplexing 3D diffractive optics.

Superlensing plano-convex-microsphere (PCM) lens for direct laser nano-marking and beyond

A high-performance all-dielectric lens, formed by integrating a conventional plano-convex lens with a high-index microsphere lens (PCM), was developed for far-field super-resolution applications. The PCM lens features a theoretical resolution of $\sim\lambda /{2.5}$∼λ/2.5 in air with a WD $\sim 2\;{\unicode{x00B5} \rm m}$∼2µm away from the lens. When combined with a femtosecond laser, the actual patterning resolution can reach $\sim\lambda /{3.5}$∼λ/3.5. The unusual focusing properties were theoretically and experimentally verified, and direct laser nano-writing of arbitrary patterns and nanostructures on various substrates was demonstrated. This Letter can be naturally extended to other super-resolution applications, including imaging, sensing, and trapping, with the potential of developing next-generation low-cost direct laser nano-marking machine and super-resolution imaging nanoscope.

Interferometric time- and energy-resolved photoemission electron microscopy for few-femtosecond nanoplasmonic dynamics

We report a time-resolved normal-incidence photoemission electron microscope with an imaging time-of-flight detector using ∼7-fs near-infrared laser pulses and a phase-stabilized interferometer for studying ultrafast nanoplasmonic dynamics via nonlinear photoemission from metallic nanostructures. The interferometer's stability (35 ± 6 as root-mean-square from 0.2 Hz to 40 kHz) as well as on-line characterization of the driving laser field, which is a requirement for nanoplasmonic near-field reconstruction, is discussed in detail. We observed strong field enhancement and few-femtosecond localized surface plasmon lifetimes at a monolayer of self-assembled gold nanospheres with ∼40 nm diameter and ∼2 nm interparticle distance. A wide range of plasmon resonance frequencies could be simultaneously detected in the time domain at different nanospheres, which are distinguishable already within the first optical cycle or as close as about ±1 fs around time-zero. Energy-resolved imaging (microspectroscopy) additionally revealed spectral broadening due to strong-field or space charge effects. These results provide a clear path toward visualizing optically excited nanoplasmonic near-fields at ultimate spatiotemporal resolution.

Measurement of propagation of ultrafast surface plasmon polariton pulses using dual-probe scanning near-field optical microscopy

We report the construction of diagnostics for ultrafast surface plasmon polariton (SPP) pulses that evolve spatiotemporally in femtosecond and nanometer scales. We constructed two types of scanning near-field optical microscopes (SNOMs) and verified that the temporal waveform of ultrafast SPP pulses can be measured by combining spectral interferometry (SI). In the illumination-collection (I-C) mode SNOM, which uses a single fiber probe to excite samples and collect optical responses, a lock-in detection scheme using a lock-in camera detects SI fringes even for extremely weak signal light pulses. With this I-C SI-SNOM scheme, we measured the complex plasmon response functions of gold (Au) nanorods on Ge₂Sb₂Te₅ thin film, both in the crystal and amorphous phases. For a dual-probe SNOM (DSNOM), a dual-band modulation technique was introduced to independently control the probe-sample and probe-probe distances. With the DSNOM and by employing femtosecond SPP pulse excitation, we successfully measured the temporal waveform of an ultrafast SPP pulse that is propagating on an Au thin layer.

Femtosecond time-resolved photoemission electron microscopy operated at sample illumination from the rear side

We present an advanced experimental setup for time-resolved photoemission electron microscopy (PEEM) with sub-20 fs resolution, which allows for normal incidence and highly local sample excitation with ultrashort laser pulses. The scheme makes use of a sample rear side illumination geometry that enables us to confine the sample illumination spot to a diameter as small as 6 µm. We demonstrate an operation mode in which the spatiotemporal dynamics following a highly local excitation of the sample is globally probed with a laser pulse illuminating the sample from the front side. Furthermore, we show that the scheme can also be operated in a time-resolved normal incidence two-photon PEEM mode with interferometric resolution, a technique providing a direct and intuitive real-time view onto the propagation of surface plasmon polaritons.

Optimizing the Nonlinear Optical Response of Plasmonic Metasurfaces

Controlling the nonlinear optical response of nanoscale metamaterials opens new exciting applications such as frequency conversion or flat metal optical elements. To utilize the already well-developed fabrication methods, a systematic design methodology for obtaining high nonlinearities is required. In this paper we consider an optimization-based approach, combining a multiparameter genetic algorithm with three-dimensional finite-difference time domain (FDTD) simulations. We investigate two choices of the optimization function: one which looks for plasmonic resonance enhancements at the frequencies of the process using linear FDTD, and another one, based on nonlinear FDTD, which directly computes the predicted nonlinear response. We optimize a four-wave-mixing process with specific predefined input frequencies in an array of rectangular nanocavities milled in a thin free-standing gold film. Both approaches yield a significant enhancement of the nonlinear signal. Although the direct calculation gives rise to the maximum possible signal, the linear optimization provides the expected triply resonant configuration with almost the same enhancement, while being much easier to implement in practice.

Symmetry Protection of Photonic Entanglement in the Interaction with a Single Nanoaperture

In this work, we experimentally show that quantum entanglement can be symmetry protected in the interaction with a single subwavelength plasmonic nanoaperture, with a total volume of V∼0.2λ^{3}. In particular, we experimentally demonstrate that two-photon entanglement can be either completely preserved or completely lost after the interaction with the nanoaperture, solely depending on the relative phase between the quantum states. We achieve this effect by using specially engineered two-photon states to match the properties of the nanoaperture. In this way we can access a symmetry protected state, i.e., a state constrained by the geometry of the interaction to retain its entanglement. In spite of the small volume of interaction, we show that the symmetry protected entangled state retains its main properties. This connection between nanophotonics and quantum optics probes the fundamental limits of the phenomenon of quantum interference.

Biomedical Applications for Gold Nanoclusters: Recent Developments and Future Perspectives

Gold nanoclusters (AuNCs) have been extensively applied as a fluorescent probe for biomedical applications in imaging, detection, and therapy due to their unique chemical and physical properties. Fluorescent probes of AuNCs have exhibited high compatibility, superior photostablility, and excellent water solubility which resulted in remarkable biomedical applications for long-term imaging, high-sensitivity detection, and target-specific treatment. Recently, great efforts have been made in the developments of AuNCs as the fluorescent probes for various biomedical applications. In this review, we have collected fluorescent AuNCs prepared by different ligands, including small molecules, polymers, and biomacromolecules, and highlighted current achievements of AuNCs in biomedical applications for imaging, detection, and therapy. According to these advances, we further provided conclusions of present challenges and future perspectives of AuNCs for fundamental investigations and practical biomedical applications.

Ultrashort pulse synthesis for energy concentration control in nanostructures

A waveform synthesis technique is introduced and applied to the femtosecond pulse excitation of plasmonic nanoantennas for temporal and spatial energy concentration control. The waveform synthesis process is based on phase and polarization shaping and an understanding of the electromagnetic response of the nanostructure. Linear and radial nano-dipole arrays are analyzed before the log-periodic toothed nanoantenna is investigated as a nanostructure capable of combining the benefits of the nano-dipole arrays. The consistent superiority of the log-periodic toothed nanoantenna is established by comparing its electromagnetic response to that of the radial nano-dipole array using a variety of synthesized excitation waveforms.

Selective far-field addressing of coupled quantum dots in a plasmonic nanocavity

Plasmon–emitter hybrid nanocavity systems exhibit strong plasmon–exciton interactions at the single-emitter level, showing great potential as testbeds and building blocks for quantum optics and informatics. However, reported experiments involve only one addressable emitting site, which limits their relevance for many fundamental questions and devices involving interactions among emitters. Here we open up this critical degree of freedom by demonstrating selective far-field excitation and detection of two coupled quantum dot emitters in a U-shaped gold nanostructure. The gold nanostructure functions as a nanocavity to enhance emitter interactions and a nanoantenna to make the emitters selectively excitable and detectable. When we selectively excite or detect either emitter, we observe photon emission predominantly from the target emitter with up to 132-fold Purcell-enhanced emission rate, indicating individual addressability and strong plasmon–exciton interactions. Our work represents a step towards a broad class of plasmonic devices that will enable faster, more compact optics, communication and computation.

Plasmonic nanostructures can tailor excitation and emission for quantum emitters, but generally only for a single emitter. In this work, the authors selectively excite and detect one out of two quantum dots coupled to a deep-subwavelength cavity composed of three gold nanorods assembled into a U-shape.

Spatial Control of Multiphoton Electron Excitations in InAs Nanowires by Varying Crystal Phase and Light Polarization

We demonstrate the control of multiphoton electron excitations in InAs nanowires (NWs) by altering the crystal structure and the light polarization. Using few-cycle, near-infrared laser pulses from an optical parametric chirped-pulse amplification system, we induce multiphoton electron excitations in InAs nanowires with controlled wurtzite (WZ) and zincblende (ZB) segments. With a photoemission electron microscope, we show that we can selectively induce multiphoton electron emission from WZ or ZB segments of the same wire by varying the light polarization. Developing ab initio GW calculations of first to third order multiphoton excitations and using finite-difference time-domain simulations, we explain the experimental findings: While the electric-field enhancement due to the semiconductor/vacuum interface has a similar effect for all NW segments, the second and third order multiphoton transitions in the band structure of WZ InAs are highly anisotropic in contrast to ZB InAs. As the crystal phase of NWs can be precisely and reliably tailored, our findings open up for new semiconductor optoelectronics with controllable nanoscale emission of electrons through vacuum or dielectric barriers.

Observing charge separation in nanoantennas via ultrafast point-projection electron microscopy

Observing the motion of electrons on their natural nanometer length and femtosecond time scales is a fundamental goal of and an open challenge for contemporary ultrafast science^1-5. At present, optical techniques and electron microscopy mostly provide either ultrahigh temporal or spatial resolution, and microscopy techniques with combined space-time resolution require further development^6-11. In this study, we create an ultrafast electron source via plasmon nanofocusing on a sharp gold taper and implement this source in an ultrafast point-projection electron microscope. This source is used in an optical pump-electron probe experiment to study ultrafast photoemissions from a nanometer-sized plasmonic antenna^12-15. We probe the real space motion of the photoemitted electrons with a 20-nm spatial resolution and a 25-fs time resolution and reveal the deflection of probe electrons by residual holes in the metal. This is a step toward time-resolved microscopy of electronic motion in nanostructures.

Holographic Plasmonic Nanotweezers for Dynamic Trapping and Manipulation

We demonstrate dynamic trapping and manipulation of nanoparticles with plasmonic holograms. By tailoring the illumination pattern of an incident light beam with a computer-controlled spatial light modulator, constructive and destructive interference of plasmon waves create a focused hotspot that can be moved across a surface. Specifically, a computer-generated hologram illuminating the perimeter of a silver Bull's Eye nanostructure generates surface plasmons that propagate toward the center. Shifting the phase of the plasmon waves as a function of space gives complete control over the location of the focus. We show that 200 nm diameter nanoparticles trapped in this focus can be moved in arbitrary patterns. This allows, for example, circular motion with linearly polarized light. These results show the versatility of holographically generated surface plasmon waves for advanced trapping and manipulation of nanoparticles.

Optics of exciton-plasmon nanomaterials

This review provides a brief introduction to the physics of coupled exciton-plasmon systems, the theoretical description and experimental manifestation of such phenomena, followed by an account of the state-of-the-art methodology for the numerical simulations of such phenomena and supplemented by a number of FORTRAN codes, by which the interested reader can introduce himself/herself to the practice of such simulations. Applications to CW light scattering as well as transient response and relaxation are described. Particular attention is given to so-called strong coupling limit, where the hybrid exciton-plasmon nature of the system response is strongly expressed. While traditional descriptions of such phenomena usually rely on analysis of the electromagnetic response of inhomogeneous dielectric environments that individually support plasmon and exciton excitations, here we explore also the consequences of a more detailed description of the molecular environment in terms of its quantum density matrix (applied in a mean field approximation level). Such a description makes it possible to account for characteristics that cannot be described by the dielectric response model: the effects of dephasing on the molecular response on one hand, and nonlinear response on the other. It also highlights the still missing important ingredients in the numerical approach, in particular its limitation to a classical description of the radiation field and its reliance on a mean field description of the many-body molecular system. We end our review with an outlook to the near future, where these limitations will be addressed and new novel applications of the numerical approach will be pursued.

Minimization of 1/fn phase noise in liquid crystal masks for reliable femtosecond pulse shaping

Liquid-crystal spatial light modulators (LCM) are a common tool to tailor femtosecond laser pulses. The phase stability of 1 kHz, sub-20 fs visible shaped and unshaped pulses are investigated. Our results show that the spectral phase after the LCM varies from pulse to pulse leading to strong deviations from the predicted pulse shapes. This phase instability is generated only by LCM and is strongly temperature dependent. Based on the experimental data, a numerical model for the phase was developed that takes the temperature-dependent phase instability as well as pixel coupling across the LCM into account. Phase stability after the LCM can be improved by an order of magnitude by combining the control the temperature of the LCM and by using rapid-scan averaging. Reliable pulse shapes on a pulse-to-pulse basis are crucial, especially in coherent control experiments, where small differences between pulse shape are important.

Generating laser-pulse enantiomers

We present an optical setup capable of mirroring an arbitrary, potentially time-varying, polarization state of an ultrashort laser pulse. The incident beam is split up in two and the polarization of one beam is mirrored by reflection off a mirror in normal incidence. Afterwards, both beams are recombined in time and space such that two collinear ultrashort laser pulses with mutually mirrored polarization, i.e., laser-pulse enantiomers, leave the setup. We employ the Jones formalism to describe the function of the setup and analyze the influence of alignment errors before describing the experimental implementation and alignment protocol. Since no wave plates are utilized, broadband pulses in a large wavelength range can be processed. In particular, we show that the setup outperforms broadband achromatic wave plates. Furthermore, since the two beams travel separately through the optical system they can be blocked independently. This opens the possibility for circular dichroism, ellipsometry, and anisotropy spectroscopy with shot-to-shot chopping and detection schemes as well as chiral coherent control applications.

Magnetic Switching in Granular FePt Layers Promoted by Near-Field Laser Enhancement

Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle X-ray scattering at an X-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, 1 order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer.

Secondary Electron Imaging of Light at the Nanoscale

The interaction of fast electrons with metal atoms may lead to optical excitations. This exciting phenomenon forms the basis for the most powerful inspection methods in nanotechnology, such as cathodoluminescence and electron-energy loss spectroscopy. However, direct nanoimaging of light based on electrons is yet to be introduced. Here, we experimentally demonstrate simultaneous excitation and nanoimaging of optical signals using unmodified scanning electron microscope. We use high-energy electron beam for plasmon excitation and rapidly image the optical near fields using the emitted secondary electrons. We analyze dipole nanoantennas coupled with channel nanoplasmonic waveguides and observe both surface plasmons and surface plasmon polaritons with spatial resolution of 25 nm. Our experimental results are confirmed by rigorous numerical calculations based on full-wave solution of Maxwell's equations, showing high correlation between optical near fields and secondary electrons images. This demonstration of optical near-field mapping using direct electron imaging provides essential insights to the exciting relations between electrons plasmons and photons, paving the way toward secondary electron-based plasmon analysis at the nanoscale.

Analytic Optimization of Near-Field Optical Chirality Enhancement

We present an analytic derivation for the enhancement of local optical chirality in the near field of plasmonic nanostructures by tuning the far-field polarization of external light. We illustrate the results by means of simulations with an achiral and a chiral nanostructure assembly and demonstrate that local optical chirality is significantly enhanced with respect to circular polarization in free space. The optimal external far-field polarizations are different from both circular and linear. Symmetry properties of the nanostructure can be exploited to determine whether the optimal far-field polarization is circular. Furthermore, the optimal far-field polarization depends on the frequency, which results in complex-shaped laser pulses for broadband optimization.

Use of polarization freedom beyond polarization-division multiplexing to support high-speed and spectral-efficient data transmission

Increasing the system capacity and spectral efficiency (SE) per unit bandwidth is one of the ultimate goals for data network designers, especially when using technologies compatible with current embedded fiber infrastructures. Among these, the polarization-division-multiplexing (PDM) scheme, which supports two independent data channels on a single wavelength with orthogonal polarization states, has become a standard one in most state-of-art telecommunication systems. Currently, however, only two polarization states (that is, PDM) can be used, setting a barrier for further SE improvement. Assisted by coherent detection and digital signal processing, we propose and experimentally demonstrate a scheme for pseudo-PDM of four states (PPDM-4) by manipulation of four linearly polarized data channels with the same wavelength. Without any modification of the fiber link, we successfully transmit a 100-Gb s^-1 PPDM-4 differential-phase-shift-keying signal over a 150-km single-mode fiber link. Such a method is expected to open new possibilities to fully explore the use of polarization freedom for capacity and SE improvement over existing fiber systems.

Fluorescent probes and materials for detecting formaldehyde: from laboratory to indoor for environmental and health monitoring

Formaldehyde (FA), as a vital industrial chemical, is widely used in building materials and numerous living products.

Normal-Incidence PEEM Imaging of Propagating Modes in a Plasmonic Nanocircuit

The design of noble-metal plasmonic devices and nanocircuitry requires a fundamental understanding and control of the interference of plasmonic modes. Here we report the first visualization of the propagation and interference of guided modes in a showcase plasmonic nanocircuit using normal-incidence nonlinear two-photon photoemission electron microscopy (PEEM). We demonstrate that in contrast to the commonly used grazing-incidence illumination scheme, normal-incidence PEEM provides a direct image of the structure's near-field intensity distribution due to the absence of beating patterns and despite the transverse character of the plasmonic modes. Based on a simple heuristic numerical model for the photoemission yield, we are able to model all experimental findings if global plane wave illumination and coupling to multiple input/output ports, and the resulting interference effects are accounted for.

High resolution scanning near field mapping of enhancement on SERS substrates: comparison with photoemission electron microscopy

The need for a dedicated spectroscopic technique with nanoscale resolution to characterize SERS substrates pushed us to develop a proof of concept of a functionalized tip-surface enhanced Raman scattering (FTERS) technique. We have been able to map hot spots on semi-continuous gold films; in order to validate our approach we compare our results with photoemission electron microscopy (PEEM) data, the complementary electron microscopy tool to map hot spots on random metallic surfaces. Enhanced Raman intensity maps at high spatial resolution reveal the localisation of hotspots at gaps for many neighboring nanostructures. Finally, we compare our findings with theoretical simulations of the enhancement factor distribution, which confirms a dimer effect as the dominant origin of hot spots.

Imaging and controlling plasmonic interference fields at buried interfaces

Capturing and controlling plasmons at buried interfaces with nanometre and femtosecond resolution has yet to be achieved and is critical for next generation plasmonic devices. Here we use light to excite plasmonic interference patterns at a buried metal–dielectric interface in a nanostructured thin film. Plasmons are launched from a photoexcited array of nanocavities and their propagation is followed via photon-induced near-field electron microscopy (PINEM). The resulting movie directly captures the plasmon dynamics, allowing quantification of their group velocity at ∼0.3 times the speed of light, consistent with our theoretical predictions. Furthermore, we show that the light polarization and nanocavity design can be tailored to shape transient plasmonic gratings at the nanoscale. This work, demonstrating dynamical imaging with PINEM, paves the way for the femtosecond and nanometre visualization and control of plasmonic fields in advanced heterostructures based on novel two-dimensional materials such as graphene, MoS₂, and ultrathin metal films.

Visualizing surface plasmon polaritons at buried interfaces has remained elusive. Here, the authors develop a methodology to study the spatiotemporal evolution of buried near-fields within complex heterostructures, enabling the characterization of the next generation of plasmonic devices.

Ultrashort Pulses for Far-Field Nanoscopy

We show that ultrashort pulses can be focused, in a particular instant, to a spot size given by the wavelength associated with its spectral width. For attosecond pulses this spot size is within the nanometer scale. Then we show that a two-level system can be left excited after interacting with an ultrashort pulse whose spectral width is larger than the transition frequency, and that the excitation probability depends not on the field amplitude but on the field intensity. The latter makes the excitation profile have the same spot size as the ultrashort pulse. This unusual phenomenon is caused by quantum electrodynamics in the ultrafast light-matter interaction regime since the usually neglected counterrotating terms describing the interaction with the free electromagnetic modes are crucial for making the excitation probability nonzero and depend on the field intensity. These results suggest that a train of coherent attosecond pulses could be used to excite fluorescent markers with nanoscale resolution. The detection of the light emitted after fluorescence-or any other method used to detect the excitation-could then lead to a new scheme for far-field light nanoscopy.

Ultrafast control of third-order optical nonlinearities in fishnet metamaterials

Nonlinear photonic nanostructures that allow efficient all-optical switching are considered to be a prospective platform for novel building blocks in photonics. We performed time-resolved measurements of the photoinduced transient third-order nonlinear optical response of a fishnet metamaterial. The mutual influence of two non-collinear pulses exciting the magnetic resonance of the metamaterial was probed by detecting the third-harmonic radiation as a function of the time delay between pulses. Subpicosecond-scale dynamics of the metamaterial’s χ⁽³⁾ was observed; the all-optical χ⁽³⁾ modulation depth was found to be approximately 70% at a pump fluence of only 20 μJ/cm².

Nanophotonics gets twisted

Simple structures process complex light beams that carry angular momentum

Two-Phonon Quantum Coherences in Indium Antimonide Studied by Nonlinear Two-Dimensional Terahertz Spectroscopy

We report the first observation of two-phonon quantum coherences in a semiconductor. Two-dimensional terahertz (THz) spectra recorded with a sequence of three THz pulses display strong two-phonon signals, clearly distinguished from signals due to interband two-photon absorption and electron tunneling. The two-phonon coherences originate from impulsive off-resonant excitation in the nonperturbative regime of light-matter interaction. A theoretical analysis provides the relevant Liouville pathways, showing that nonlinear interactions using the large interband dipole moment generate stronger two-phonon excitations than linear interactions.