Near-field Optics Research Articles

Dynamic control and manipulation of near-fields using direct feedback

Shaping and controlling electromagnetic fields at the nanoscale is vital for advancing efficient and compact devices used in optical communications, sensing and metrology, as well as for the exploration of fundamental properties of light-matter interaction and optical nonlinearity. Real-time feedback for active control over light can provide a significant advantage in these endeavors, compensating for ever-changing experimental conditions and inherent or accumulated device flaws. Scanning nearfield microscopy, being slow in essence, cannot provide such a real-time feedback that was thus far possible only by scattering-based microscopy. Here, we present active control over nanophotonic near-fields with direct feedback facilitated by real-time near-field imaging. We use far-field wavefront shaping to control nanophotonic patterns in surface waves, demonstrating translation and splitting of near-field focal spots at nanometer-scale precision, active toggling of different near-field angular momenta and correction of patterns damaged by structural defects using feedback enabled by the real-time operation. The ability to simultaneously shape and observe nanophotonic fields can significantly impact various applications such as nanoscale optical manipulation, optical addressing of integrated quantum emitters and near-field adaptive optics.

(Invited) Near-Field Optics and Its Applications in Nanoscale Materials: A Review

In this presentation, we first offer an overview of aperture-type scanning near field optical microscopy –a family of super-resolution imaging techniques based on evanescent waves, which can be combined with atomic force microscopy and are capable of subwavelength resolution nano-optical imaging. In the second part of this review, we will discuss a few applications in which our group capitalized on the super-resolution resolving power of SNOM to design specific nano-optical and nano-photonic systems for light harvesting, resistive memory device applications and nanoscale thermo-optical management, as well as to characterize nanoscale luminescent materials. Specific case studies that will be presented include the characterization of weakly photoluminescent and curved carbon dots for memory device applications, the three-dimensional characterization of plasmon-enhanced nanophotonic devices, as well as the development of near-field thermoreflectance imaging for nanophotonic-based thermal management applications. Collectively, our study well represents the versability of SNOM as a unique super-resolution nanophotonic tool for the investigation of light-matter interaction at the nanoscale.

Near-field induced local excitation dynamics of Na10 and Na10-N2 from real-time TDDFT.

Electron dynamics of the Na10 chain and the Na10-N2 complex locally excited by an atomistic optical near-field are investigated using real-time time-dependent density functional theory calculations on real-space grids. Ultrafast laser pulses were used to simulate the near-field excitation under on- and off-resonance conditions. Off-resonance excitation did not lead to the propagation of the excitation through the Na10 chain. In contrast, under the resonance conditions, the excited state is delocalized over the entire Na chain. Analysis of the local dipole moment of each atom in Na10 indicates that this behavior is consistent with the transition density. Adding an N2 molecule to the opposite end of the local excitation region results in energy transfer via the Na10 chain. The energy transfer efficiency of the N2 molecule is well correlated with the absorption spectrum of Na10. The present study paves the way for realizing remote excitation and photonic devices at the atomic scale.

(Invited) Near-Field Optics and Its Applications in Nanoscale Materials: A Review

In this paper, we first offer an overview of aperture-type scanning near field optical microscopy –a family of super-resolution imaging techniques based on evanescent waves, which can be combined with atomic force microscopy and are capable of subwavelength resolution nano-optical imaging. In the second part of this review, we will discuss a few applications in which our group capitalized on the super-resolution resolving power of SNOM to design specific nano-optical and nano-photonic systems for light harvesting, resistive memory device applications and nanoscale thermo-optical management. Specific case studies that will be presented include the characterization of weakly photoluminescent and curved carbon dots for memory device applications, the three-dimensional characterization of plasmon-enhanced nanophotonic devices, as well as the development of near-field thermoreflectance imaging for nanophotonic-based thermal management applications. Collectively, our study well represents the versability of SNOM as a unique super-resolution nanophotonic tool for the investigation of light-matter interaction at the nanoscale.

Temporal Dynamics of Collective Resonances in Periodic Metasurfaces.

Temporal dynamics of confined optical fields can provide valuable insights into light-matter interactions in complex optical systems, going beyond their frequency-domain description. Here, we present a new experimental approach based on interferometric autocorrelation (IAC) that reveals the dynamics of optical near-fields enhanced by collective resonances in periodic metasurfaces. We focus on probing the resonances known as waveguide-plasmon polaritons, which are supported by plasmonic nanoparticle arrays coupled to a slab waveguide. To probe the resonant near-field enhancement, our IAC measurements make use of enhanced two-photon excited luminescence (TPEL) from semiconductor quantum dots deposited on the nanoparticle arrays. Thanks to the incoherent character of TPEL, the measurements are only sensitive to the fundamental optical fields and therefore can reveal clear signatures of their coherent temporal dynamics. In particular, we show that the excitation of a high-Q collective resonance gives rise to interference fringes at time delays as large as 500 fs, much greater than the incident pulse duration (150 fs). Based on these signatures, the basic characteristics of the resonances can be determined, including their Q factors, which are found to exceed 200. Furthermore, the measurements also reveal temporal beating between two different resonances, providing information on their frequencies and their relative contribution to the field enhancement. Finally, we present an approach to enhance the visibility of the resonances hidden in the IAC curves by converting them into spectrograms, which greatly facilitates the analysis and interpretation of the results. Our findings open up new perspectives on time-resolved studies of collective resonances in metasurfaces and other multiresonant systems.

Subwavelength resolution using the near field of quantum emitters.

We propose a novel, to the best of our knowledge, approach to superresolution optical imaging by combining quantum optics and near-field optics. Our concept involves the utilization of single-photon quantum emitters to generate a standalone evanescent wave. We demonstrate that the quantum interference effects of single-photon emitters, in conjunction with their near-field, result in a higher resolution of subwavelength structures than systems that are only quantum enhanced or only near-field enhanced. We believe that nano-sized emitters could be employed to accomplish the goals of this research, taking into account the current progress in nanophotonics and quantum optics technology.

History-dependent nano-photoisomerization by optical near-field in photochromic single crystals

The history dependence of physical systems is promising for solving complex problems. To exploit spontaneous nanometre-scale optical memory for computation, the history dependence must be elucidated. Here, we demonstrate history-dependent or dynamic nano-photoisomerization through the sequential formation of multiple memory pathways in photochromic crystals via optical near-field interactions. Incident photons passing through photoisomerization memory pathways are observed using a double-probe optical near-field microscope, with one probe located on the front surface for local excitation and the other on the rear surface for near-field observations. By performing localised near-field excitation twice but at spatially different positions, negatively correlated near-field output patterns are observed between the first and second memory pathways. We also confirm that the first memory pathway is preserved after the formation of the second memory pathway. This result indicates that photoisomerization using an optical near field in diarylethene crystals has a history dependence, leading to brain-like dynamic information memorisation.

A historical overview of Nano-optics: From Near-Field Optics to Plasmonics

Abstract Nano-optics is an emergent research field in physics that appeared in the 1980s, which deals with light-matter optical interactions at the nanometer scale. In early studies of nano-optics, the main concern focus was to obtain higher optical resolution over the diffraction limit. The researches of near-field imaging and spectroscopy based on Scanning Near-Field Optical Microscopy (SNOM) were developed. The exploration of improving SNOM probe for near-field detection led to the emergence of surface plasmons. In the sense of resolution and wider application, there has been a significant transition from seeking higher resolution microscopy to plasmonic near-field modulations in the nano-optics community during the nano-optic development. Nowadays, studies of nano-optics prefer the investigation of plasmonics in different material systems. In this article, the history of the development of near-field optics is briefly reviewed. The difficulties of conventional SNOM to achieve higher resolution are discussed. As an alternative solution, surface plasmons have shown the advantages of higher resolution, wider application, and flexible nano-optical modulation for new devices. The typical studies in different periods are introduced and analyzed characteristics of nano-optics in each stage. In this way, the evolution progress from near-field optics to plasmonics of nano-optics research is presented. The future development of nano-optics is discussed then.

Advances in fundamentals and application of plasmon-assisted CO2 photoreduction

Abstract Artificial photosynthesis of hydrocarbons from carbon dioxide (CO2) has the potential to provide renewable fuels at the scale needed to meet global decarbonization targets. However, CO2 is a notoriously inert molecule and converting it to energy dense hydrocarbons is a complex, multistep process, which can proceed through several intermediates. Recently, the ability of plasmonic nanoparticles to steer the reaction down specific pathways and enhance both reaction rate and selectivity has garnered significant attention due to its potential for sustainable energy production and environmental mitigation. The plasmonic excitation of strong and confined optical near-fields, energetic hot carriers and localized heating can be harnessed to control or enhance chemical reaction pathways. However, despite many seminal contributions, the anticipated transformative impact of plasmonics in selective CO2 photocatalysis has yet to materialize in practical applications. This is due to the lack of a complete theoretical framework on the plasmonic action mechanisms, as well as the challenge of finding efficient materials with high scalability potential. In this review, we aim to provide a comprehensive and critical discussion on recent advancements in plasmon-enhanced CO2 photoreduction, highlighting emerging trends and challenges in this field. We delve into the fundamental principles of plasmonics, discussing the seminal works that led to ongoing debates on the reaction mechanism, and we introduce the most recent ab initio advances, which could help disentangle these effects. We then synthesize experimental advances and in situ measurements on plasmon CO2 photoreduction before concluding with our perspective and outlook on the field of plasmon-enhanced photocatalysis.

Studying near-field propagation of generalized parabolic beams by virtual source method

Generalized parabolic beams have various optical morphologies. They can be used in different research fields, such as component design, aero-optics, and microwave wireless power transmission. Studying the near-field transmission characteristics of these beams is important for improving utilization efficiency. We develop a more accurate theoretical framework to precisely understand the propagation behaviors of complex light fields in the near-field range, especially to break through the limitations of conventional near-axis approximation. This framework fully reveals the propagation mechanism of parabolic beams and their energy transmission modes. Here, based on the principle of independent propagation and the virtual source method, a group of virtual sources are introduced to analyze generalized parabolic beams. These beams can be expanded into the superposition of infinite continuous integer Bessel beams. Then, by combining the Weber integral formula and the Fourier Bessel transform, we rigorously derive an integral expression for generalized parabolic beams during near-field propagation. This expression breaks through the limitation of the traditional paraxial approximation and contains all the key propagation parameters of the family of beams. Based on this integral expression, the intensity distribution and phase characteristics of the generalized parabolic beam along the optical axis are further calculated and analyzed to reveal its energy transfer mode and phase characteristics. By comparing the paraxial approximate solution with the nonparaxial corrected solution for generalized parabolic beams, the far-field propagation of generalized parabolic beams is found to be the same when the propagation distance is sufficiently long. Such simulation results indirectly confirm the correctness of the obtained theoretical solution. The simple paraxial approximation theory can be used conveniently to calculate the far-field propagation of generalized parabolic beams. However, large errors exist when paraxial theory is used to calculate the near-field distribution of generalized parabolic beams. Although calculating nonparaxial propagation is especially complex, the nonparaxial correction solution is necessary when generalized parabolic beams are used in near-field research. Such research results not only deepen the understanding of the propagation mechanism of generalized parabolic beams but also lay a theoretical foundation for studying the precise propagation behaviors of other complex light fields in near-field optics.

Far-near-field coupling calculation method for a 110 m radio telescope based on the parabolic equation and time domain near-field physical optics

Far-near-field coupling calculation method for a 110 m radio telescope based on the parabolic equation and time domain near-field physical optics

(Invited) Near-Field Optics and Its Applications in Nanophotonic Devices: A Review

In our presentation, we will offer an overview of aperture-type scanning near field optical microscopy (SNOM) – a family of nano-optical imaging techniques derived from scanning probe microscopy which are capable of subwavelength resolution, and the development of three dimensional (3D) SNOM methods undertaken by our group to locally image the distribution of the electromagnetic radiation in the proximity of nanoparticles and nano-objects.[1] We will discuss a few applications in which we took advantage of 3D-SNOM to design specific optical nanosystems for light harvesting device applications. Specific case studies that will be presented include the design of plasmonic thin-film solar cells enhanced by random arrays of copper nanoparticles,[2] and the use of 3D-SNOM for characterizing evanescent waveguides self-assembled from of copper nanoparticles assembled on thin films of graphene.[3] In the final part of our talk, we will we present near-field scanning thermoreflectance imaging (NeSTRI), a new pump-probe technique invented in our group (see Figure 1) in which an aperture-type SNOM is used to contactlessly determine the thermal conductivity of inhomogeneous thin films and low-dimensional systems at the nanoscale for heat-spreading and thermoelectric applications.[4,5] These examples well represent the versatility of SNOM imaging and its potential for designing an even wider family of nano-optical devices.[1] P Bazylewski, S Ezugwu, G Fanchini, Applied Sciences 7 (2017) 973[2] S Ezugwu, H Ye, G Fanchini, Nanoscale 7 (2016) 252-260[3] T Ouyang, A Akbari-Sharbaf, J Park, R Bauld, MG Cottam, G Fanchini, RSC Advances 5 (2015) 98814-98821[4] S Ezugwu, S Kazemian, DYW Choi, G Fanchini, Nanoscale 9 (2017) 4097-4106[5] S Kazemian, S Ezugwu, G Fanchini, Proc. SPIE 10926, Quantum Sensing and Nano Electronics and Photonics XVI, 109260L (1 February 2019); doi: 10.1117/12.2509828 Figure 1

Analysis of the electromagnetic scattering characteristics of large-aperture parabolic surface based on symmetrical time-domain near-field physical optics method

Analysis of the electromagnetic scattering characteristics of large-aperture parabolic surface based on symmetrical time-domain near-field physical optics method

Fano Resonance-Assisted All-Dielectric Array for Enhanced Near-Field Optical Trapping of Nanoparticles.

Near-field optics can overcome the diffraction limit by creating strong optical gradients to enable the trapping of nanoparticles. However, it remains challenging to achieve efficient, stable trapping without heating and thermal effects. Dielectric structures have been used to address this issue but usually offer weak trap stiffness. In this work, we exploit the Fano resonance effect in an all-dielectric quadrupole nanostructure to realize a 20-fold enhancement of trap stiffness, compared to the off-resonance case. This enables a high effective trap stiffness of 1.19 fN/nm for 100 nm diameter polystyrene nanoparticles with 4.2 mW/μm2 illumination. Furthermore, we demonstrate the capability of the structure to simultaneously trap two particles at distinct locations within the nanostructure array.

Numerical investigation of sequential phase-locked optical gating of free electrons

Recent progress in coherent quantum interactions between free-electron pulses and laser-induced near-field light have revolutionized electron wavepacket shaping. Building on these advancements, we numerically explore the potential of sequential interactions between slow electrons and localized dipolar plasmons in a sequential phase-locked interaction scheme. Taking advantage of the prolonged interaction time between slow electrons and optical near-fields, we aim to explore the effect of plasmon dynamics on the free-electron wavepacket modulation. Our results demonstrate that the initial optical phase of the localized dipolar plasmon at the starting point of the interaction, along with the phase offset between the interaction zones, can serve as control parameters in manipulating the transverse and longitudinal recoil of the electron wavefunction. Moreover, it is shown that the incident angle of the laser light is an additional control knop for tailoring the longitudinal and transverse recoils. We show that a sequential phase-locking method can be employed to precisely manipulate the longitudinal and transverse recoil of the electron wavepacket, leading to selective acceleration or deceleration of the electron energy along specific diffraction angles. These findings have important implications for developing novel techniques for ultrafast electron-light interferometry, shaping the electron wavepacket, and quantum information processing.

Photoluminescence upconversion in monolayer WSe2 activated by plasmonic cavities through resonant excitation of dark excitons

Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe2 monolayers through resonant excitation of a dark exciton at room temperature. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.

Three-dimensional polarization effects in optical tunneling.

We consider the three-dimensional (3D) polarimetric properties of an evanescent optical field excited in the gap of a double-prism system by a random plane wave. The analysis covers the case of frustrated total internal reflection (FTIR), i.e.,optical tunneling, and relies on the characteristic decomposition of the 3×3 polarization matrix. We find in particular that, for any incident partially polarized plane wave, the evanescent field inside the gap is necessarily in a nonregular, genuine 3D polarization state. We also show that the 3D polarimetric properties of the field at the second boundary are sensitive to the changes of the gap width and that the relevant effects occur for the smaller widths when the angle of incidence of the plane wave becomes larger. The results of this work uncover new aspects of the polarimetric structure of genuine 3D evanescent fields and may find applications in near-field optics and surface nanophotonics.

(Invited) Ultrafast Charge Transfer Dynamics in WS2–Au Nanohybrid System Fabricated by Pulsed Laser Ablation in Liquid

Nanosheet semiconductor materials have attracted much attention in recent years due to their unique optical, electronic, and chemical properties for various applications such as photodetectors, light emitters, photocatalysts, and biosensors. We have fabricated tungsten disulfide (WS2) nanostructures by pulsed laser ablation in liquid. We applied nanosecond and femtosecond lasers and found the latter to be suitable for nanosheet fabrication [1]. Visible light response was enhanced by attaching gold nanoparticles (AuNPs) onto WS2 nanosheets [2]. The charge transfer dynamics in the WS2-Au nanohybrid system was characterized using femtosecond diffuse reflectance spectroscopy. The nanosheet structure was found to reduce the bulk carrier property. Furthermore, modification with AuNPs was found to enhance the formation of carriers trapped deeply in the WS2 nanosheets. These findings will be useful for the design of future metal-nanosheet semiconductor hybrid systems.Reference Pankaj Koinkar, Kohei Sasaki, Tetsuro Katayama and Akihiro Furube, International Journal of Modern Physics B, 35, 2140007 (2021)Tatsuki Kokufu, Daichi Nakayama, Tetsuro Katayama, Koinkar Pankaj, and Akihiro Furube, The 13th Asia-Pacific Conference on Near-Field Optics, OL30-A13 (2022)

Advanced active polymer probe for near-field optics.

We report on a novel, to the best of our knowledge, active probe for scanning near-field optical microscopy (SNOM). A fluorescent nanosphere, acting as the secondary source, is grafted in an electrostatic manner at the apex of a polymer tip integrated into the extremity of an optical fiber. Thanks to the high photostability and sensitivity of the secondary source, the near-field interaction with a gold nanocube is investigated. It is shown that the spatial resolution is well defined by the size of the fluorescent nanosphere. The polarization-dependent near-field images, which are consistent with the simulation, are ascribed to the local excitation rate enhancement. Meanwhile, measurement of the distance-dependent fluorescence lifetime of the nanosphere provides strong evidence that the local density of states is modified so that extra information on nano-emitters can be extracted during near-field scanning. This advanced active probe can thus potentially broaden the range of applications to include nanoscale thermal imaging, biochemical sensors, and the manipulation of nanoparticles.

Imaging the field inside nanophotonic accelerators

Controlling optical fields on the subwavelength scale is at the core of nanophotonics. Laser-driven nanophotonic particle accelerators promise a compact alternative to conventional radiofrequency-based accelerators. Efficient electron acceleration in nanophotonic devices critically depends on achieving nanometer control of the internal optical nearfield. However, these nearfields have so far been inaccessible due to the complexity of the devices and their geometrical constraints, hampering the design of future nanophotonic accelerators. Here we image the field distribution inside a nanophotonic accelerator, for which we developed a technique for frequency-tunable deep-subwavelength resolution of nearfields based on photon-induced nearfield electron-microscopy. Our experiments, complemented by 3D simulations, unveil surprising deviations in two leading nanophotonic accelerator designs, showing complex field distributions related to intricate 3D features in the device and its fabrication tolerances. We envision an extension of our method for full 3D field tomography, which is key for the future design of highly efficient nanophotonic devices.