Ultrafast Plasmon Research Articles

Ultrafast Plasmonics for All-Optical Switching and Pulsed Lasers

Surface plasmon resonances (SPRs) are often regarded as the collective oscillations of charge carriers localized at the dielectric–metal interface that display an ultrafast response upon light excitation. The recent developments in the fabrication and characterization of plasmonic nanostructures have stimulated continuous effects in the search for their potential applications in the photonic fields. Concentrating on the role of plasmonics in photonics, this review covers recent advances in ultrafast plasmonic materials with a prime focus on all-optical switching. Fundamental phenomena of plasmonic light–matter interaction and plasmon dynamics are discussed by elaborating on the ultrafast processes unraveled by both experimental and theoretical methods, along with a comprehensive illustration of leveraging ultrafast plasmonics for all-optical switching and pulse laser generation with a focus on device design and performance. This review is concluded with a brief highlight of the current progress and the potential future directions in ultrafast plasmonics.

Employing surface plasmons for ultrafast nanoscale applications

Ultrafast plasmonics can lead to novel applications in photochemistry, spectroscopy, and information processing.

Advances in ultrafast plasmonics

In the past 20 years, we have reached a broad understanding of many light-driven phenomena in nanoscale systems. The temporal dynamics of the excited states are instead quite challenging to explore, and, at the same time, crucial to study for understanding the origin of fundamental physical and chemical processes. In this review, we examine the current state and prospects of ultrafast phenomena driven by plasmons both from a fundamental and applied point of view. This research area is referred to as ultrafast plasmonics and represents an outstanding playground to tailor and control fast optical and electronic processes at the nanoscale, such as ultrafast optical switching, single photon emission, and strong coupling interactions to tailor photochemical reactions. Here, we provide an overview of the field and describe the methodologies to monitor and control nanoscale phenomena with plasmons at ultrafast timescales in terms of both modeling and experimental characterization. Various directions are showcased, among others recent advances in ultrafast plasmon-driven chemistry and multi-functional plasmonics, in which charge, spin, and lattice degrees of freedom are exploited to provide active control of the optical and electronic properties of nanoscale materials. As the focus shifts to the development of practical devices, such as all-optical transistors, we also emphasize new materials and applications in ultrafast plasmonics and highlight recent development in the relativistic realm. The latter is a promising research field with potential applications in fusion research or particle and light sources providing properties such as attosecond duration.

All-optical control of ultrafast plasmon resonances in the pulse-driven extraordinary optical transmission

Understanding ultrafast processes in their natural timescale is crucial for controlling and manipulating nanoscale optoelectronic devices under light–matter interaction. Here, we demonstrate that ultrafast plasmon resonances, attributed to the phenomenon of extraordinary optical transmission (EOT), can be significantly modified by tuning the spectral and temporal properties of the ultrashort light pulse. In this scheme, all-optical active tuning governs the spatial and temporal enhancement of plasmon oscillations in the EOT system without device customization. We analyze the spectral and temporal evolution of the system using two approaches. First, we develop a theoretical framework based on the coupled harmonic oscillator model, which analytically describes the dynamics of plasmon modes in the coupled and uncoupled states. Later, we compare the evolution of the system under continuous-wave and pulsed illumination. Further, we discuss the time-resolved spectral and spatial dynamics of plasmon modes using a 3D finite difference time-domain simulation method and wavelet transform. Our results show that optical tuning of the oscillation time, intensity, and spectral properties of propagating and localized plasmon modes yields a three-fold enhancement in the EOT signal. The active tuning of the EOT sensor through ultrashort light pulses paves the way for the development of on-chip photonic devices employing high-resolution imaging and sensing of abundant atomic and molecular systems.

Observation of Charge Separation Enhancement in Plasmonic Photocatalysts under Coupling Conditions.

Surface plasmon resonance-induced charge separation plays key roles in plasmon-related applications, especially in photocatalysis and photovoltaics. Plasmon coupling nanostructures exhibit extraordinary behaviors in hybrid states, phonon scattering, and ultrafast plasmon dephasing, but plasmon-induced charge separation in these materials remains unknown. Here, we design Schottky-free Au nanoparticle (NP)/NiO/Au nanoparticles-on-a-mirror plasmonic photocatalysts to support plasmon-induced interfacial hole transfer, evidenced by surface photovoltage microscopy at the single-particle level. In particular, we observe a nonlinear increase in charge density and photocatalytic performance with an increase in excitation intensity in plasmonic photocatalysts containing hot spots as a result of varying the geometry. Such charge separation increased the internal quantum efficiency by 14 times at 600 nm in catalytic reactions as compared to that of the Au NP/NiO without a coupling effect. These observations provide an improved understanding of charge transfer management and utilization by geometric engineering and interface electronic structure for plasmonic photocatalysis.

Ultrafast Plasmonic Nucleic Acid Amplification and Real-Time Quantification for Decentralized Molecular Diagnostics.

Point-of-care real-time reverse-transcription polymerase chain reaction (RT-PCR) facilitates the widespread use of rapid, accurate, and cost-effective near-patient testing that is available to the public. Here, we report ultrafast plasmonic nucleic acid amplification and real-time quantification for decentralized molecular diagnostics. The plasmonic real-time RT-PCR system features an ultrafast plasmonic thermocycler (PTC), a disposable plastic-on-metal (PoM) cartridge, and an ultrathin microlens array fluorescence (MAF) microscope. The PTC provides ultrafast photothermal cycling under white-light-emitting diode illumination and precise temperature monitoring with an integrated resistance temperature detector. The PoM thin film cartridge allows rapid heat transfer as well as complete light blocking from the photothermal excitation source, resulting in real-time and highly efficient PCR quantification. Besides, the MAF microscope exhibits close-up and high-contrast fluorescence microscopic imaging. All of the systems were fully packaged in a palm size for point-of-care testing. The real-time RT-PCR system demonstrates the rapid diagnosis of coronavirus disease-19 RNA virus within 10 min and yields 95.6% of amplification efficiency, 96.6% of classification accuracy for preoperational test, and 91% of total percent agreement for clinical diagnostic test. The ultrafast and compact PCR system can decentralize point-of-care molecular diagnostic testing in primary care and developing countries.

Chemical Interface Damping in Nonstoichiometric Semiconductor Plasmonic Nanocrystals: An Effect of the Surrounding Environment.

Semiconductor plasmonic nanocrystals (NCs) have been utilized for an enormous number of plasmon-enhanced spectroscopic and energy conversion applications. Plasmonic NCs are extremely high light absorbers, and optical properties can be easily manipulated across the UV-vis-NIR spectrum region by changing mere chemical compositions and the surrounding environment of the NCs. This feature article focuses on reassessing plasmon dynamics by changing the interface composition between NCs and the surrounding medium to ascertain the damping contribution from chemical interface damping (CID). Also, this feature article deciphers a fundamental understanding of hot-carrier relaxation and extraction from plasmonic materials. On the route to determining the different relaxation dynamics of nonstoichiometric Cu2-xS/Se NCs, we have employed a transient ultrafast pump-probe broadband spectrometer. First, we have described the ultrafast plasmon relaxation dynamics of nonstoichiometric Cu2-xS NCs by varying the copper to sulfur ratio, and then we carefully compare how two surface ligands (oleylamine and 3-mercaptopropionic acid) lead to significantly different transient kinetics of the same plasmonic (Cu2-xSe) NCs because of different capping agents. Along with this, we have described the impact of a molecular adsorbate (methylene blue) on ultrafast plasmon relaxation dynamics of the nonstoichiometric Cu2-xSe NCs system. Finally, the chemical interface damping effect has been compared in the Cu2-xS NCs system after capping with two distinct capping ligands: oleylamine and oleic acid. For the proof of concept, plasmonic thin-film devices were fabricated and exhibited higher conductivity/photoconductivity performance in oleic acid-capped NCs because of a deprotonated carboxyl functional group. We have also introduced a model and mechanism of chemical interface damping in a nonstoichiometric plasmonic semiconductor (Cu2-xS/Se) NC system. This feature article highlights the importance of the surface functionalization of nonstoichiometric plasmonic semiconductors to develop new advanced semiconductor-based devices such as infrared photodetectors, plasmonic solar cells, and efficient NIR phototransistors.

Recent advances in ultrafast plasmonics: from strong field physics to ultraprecision spectroscopy

Abstract Surface plasmons, the collective oscillation of electrons, enable the manipulation of optical fields with unprecedented spatial and time resolutions. They are the workhorse of a large set of applications, such as chemical/biological sensors or Raman scattering spectroscopy, to name only a few. In particular, the ultrafast optical response configures one of the most fundamental characteristics of surface plasmons. Thus, the rich physics about photon–electron interactions could be retrieved and studied in detail. The associated plasmon-enhanced electric fields, generated by focusing the surface plasmons far beyond the diffraction limit, allow reaching the strong field regime with relatively low input laser intensities. This is in clear contrast to conventional optical methods, where their intrinsic limitations demand the use of large and costly laser amplifiers, to attain high electric fields, able to manipulate the electron dynamics in the non-linear regime. Moreover, the coherent plasmonic field excited by the optical field inherits an ultrahigh precision that could be properly exploited in, for instance, ultraprecision spectroscopy. In this review, we summarize the research achievements and developments in ultrafast plasmonics over the last decade. We particularly emphasize the strong-field physics aspects and the ultraprecision spectroscopy using optical frequency combs.

Ultrafast infrared plasmon switching in aligned carbon-nanotube optical resonators

Reconfigurable plasmonic materials are crucial components in active photonic devices for optical communication, signal processing, and sensing applications. Certain candidates including transparent conducting oxides, small-bandgap semiconductors (i.e. Ge, InSb), and conducting polymers can exhibit switchable free carrier concentrations triggered by optical, electrical or thermal excitations. Here, we show that aligned films of single-walled carbon nanotubes (SWCNTs) can serve as all-optically tunable plasmonic material in the mid-infrared range, thereby adding them to the library of switchable plasmonic materials. Interband optical pumping with femtosecond laser pulses results in the photoexcitation of free charge carriers, which transiently blueshifts the plasmon resonances of patterned, periodic SWCNT nano-resonators spanning the mid-infrared spectral range from 1700 to 2700 cm−1. The ultrafast plasmon modulation exhibits a nearly single-picosecond decay time, attributed to interband carrier relaxation and inter-tube charge transfer. Given that aligned films of SWCNTs have high thermal stability, excellent photostability, epsilon-near-zero property, and extremely large optical anisotropy, their dynamic tunability represents a promising pathway towards active optical devices in the technologically important mid-infrared range.

Ultrafast Plasmonic Response Ensured by Atomic Scale Confinement

Ultrafast Plasmonic Response Ensured by Atomic Scale Confinement

Ultrafast plasmon dynamics in asymmetric gold nanodimers

We theoretically investigate the effect of symmetry breaking on the ultrafast plasmon responses of Au nanodisk (ND) dimers by varying the diameter of one of the constituent nanodisks. In the case of a single ultrafast laser pulse, we demonstrate that the ultrafast responses of Au ND homodimer can be significantly modified due to the effect of symmetry breaking. The symmetric dimer shows a single broad spectral peak, whereas the size-asymmetric dimer shows three spectral peaks. The first system displays at most one temporal maximum and no beats in ultrafast temporal, whereas the second system may have three temporal maxima and two beats due to a combination of broken symmetry and the coherent superposition between various plasmon modes induced by the ultra-short laser pulse. Moreover, the shape of temporal dynamics of the size-asymmetric dimer is significantly deformed due to the excitation of local plasmon modes with different wavelength components. Furthermore, the decay time of the amplitude of the local field is longer and oscillates with a high frequency due to the narrower linewidth and red-shifted spectral peaks. We show that the ultrafast plasmon responses of both dimers can be controlled by varying the relative phase and time delays between a pair of two pulses. Our results will open new paths to understanding ultrafast plasmon responses in asymmetric heterodimers with suitable properties for different applications.

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

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

Ultrafast Plasmon Thermalization in Epitaxial Graphene Probed by Time‐Resolved THz Spectroscopy

AbstractThe control of carrier transport by electrical, chemical, or optical Fermi level tuning is central to graphene electronics. Here, an optical pump—terahertz (THz) probe spectroscopy—is applied to investigate ultrafast sheet conductivity dynamics in various epitaxially grown graphene layers representing a large variety of carbon allotropes, including H2 intercalated films. The graphene layers display a prominent plasmonic response connected with induced THz transparency spectra on ultrashort timescale. It is generally believed that the plasmonic excitations appear due to wrinkles, and substrate terraces that bring about natural confinement potentials. It is shown that these potentials act within micrometer‐sized domains with essentially isotropic character. The measured ultrafast dynamics are entirely controlled by the quasi‐Fermi level of laser‐excited carriers through their temperature. The photocarriers undergo a disorder‐enabled super‐collision cooling process with an initial picosecond transfer of the optically deposited heat to the lattice followed by a sub‐nanosecond relaxation governed by the lattice cooling. The transient spectra is described by a two‐temperature Drude‐Lorentz model revealing the ultrafast evolution of the carrier temperature and chemical potential and providing crucial material parameters such as Fermi energy, carrier mobility, carrier confinement length, and disorder mean free path.

Ultrafast Plasmon Dynamics in Near-Infrared Active Non-stoichiometric Cu2–xSe Nanocrystals and Effect of Chemical Interface Damping

Ultrafast Plasmon Dynamics in Near-Infrared Active Non-stoichiometric Cu_2–<i>x</i>Se Nanocrystals and Effect of Chemical Interface Damping

Ultrafast Nonradiative Decay of a Dipolar Plasmon-like State in Naphthalene.

Motivated by the uncertainty in our understanding of ultrafast plasmon decay mechanisms, we examine the effect of nuclear vibrations on the dynamical behavior of the strong plasmon-like dipole response of naphthalene, known as the β peak. The real-time time-dependent density functional (RT-TDDFT) method coupled with Ehrenfest molecular dynamics is used to describe the interconnected nuclear and electronic motion. Several vibrational modes promote drastic plasmon decay in naphthalene. The most astonishing finding of this study is that activation of one particular vibrational mode (corresponding to the B1u representation in D2h point group symmetry) leads to a continuous drop of the dipole response corresponding to the β peak into a totally symmetric, dark, quadrupolar electronic state. A second B1u mode provokes the sharp plasmon-like peak to split due to the breaking of structural symmetry. Nonadiabatic coupling between a B2g vibrational mode and the β peak (a B1u electronic state) gives rise to a B3u vibronic state, which can be identified as one of the p-band peaks that reside close in energy to the β peak energy. Overall, strong nonadiabatic coupling initiates plasmon decay into nearby electronic states in acenes, most importantly into dark states. These findings expand our knowledge about possible plasmon decay processes and pave the way for achieving high optical performance in acene-based materials such as graphene.

Parametric dependence of hot electron relaxation timescales on electron-electron and electron-phonon interaction strengths

Understanding how photoexcited electron dynamics depend on electron-electron (e-e) and electron-phonon (e-p) interaction strengths is important for many fields, e.g. ultrafast magnetism, photocatalysis, plasmonics, and others. Here, we report simple expressions that capture the interplay of e-e and e-p interactions on electron distribution relaxation times. We observe a dependence of the dynamics on e-e and e-p interaction strengths that is universal to most metals and is also counterintuitive. While only e-p interactions reduce the total energy stored by excited electrons, the time for energy to leave the electronic subsystem also depends on e-e interaction strengths because e-e interactions increase the number of electrons emitting phonons. The effect of e-e interactions on energy-relaxation is largest in metals with strong e-p interactions. Finally, the time high energy electron states remain occupied depends only on the strength of e-e interactions, even if e-p scattering rates are much greater than e-e scattering rates.

Reduction of graphene oxide by nanofocused ultrafast surface plasmon pulses

We used ultrafast surface plasmon polariton (SPP) pulses that were focused into several tens of nm at the apex of a tapered metal tip to induce photoreduction of graphene oxide (GO), and we successfully fabricated nano graphene stripes with a minimum width of ∼200 nm. GO was reduced using about 1010 shots of SPP pulse irradiation. We evaluated the GO reduction with selective in situ coherent anti-Stokes Raman scattering measurements using spectrally focused SPP pulses.

Strong-field nano-optics

The present status and development of strong-field nano-optics, an emerging field of nonlinear optics, is discussed. A nonperturbative regime of light-matter interactions is reached when the amplitude of the external electromagnetic fields that are driving a material approach or exceed the field strengths that bind the electrons inside the medium. In this strong-field regime, light-matter interactions depend on the amplitude and phase of the field, rather than its intensity, as in more conventional perturbative nonlinear optics. Traditionally such strong-field interactions have been intensely investigated in atomic and molecular systems, and this has resulted in the generation of high-harmonic radiation and laid the foundations for contemporary attosecond science. Over the past decade, however, a new field of research has emerged, the study of strong-field interactions in solid-state nanostructures. By using nanostructures, specifically those made out of metals, external electromagnetic fields can be localized on length scales of just a few nanometers, resulting in signficantly enhanced field amplitudes that can exceed those of the external field by orders of magnitude in the vicinity of the nanostructures. This leads not only to dramatic enhancements of perturbative nonlinear optical effects but also to significantly increased photoelectron yields. It resulted in a wealth of new phenomena in laser-solid interactions that have been discovered in recent years. These include the observation of above-threshold photoemission from single nanostructures, effects of the carrier-envelope phase on the photoelectron emission yield from metallic nanostructures, and strong-field acceleration of electrons in optical near fields on subcycle timescales. The current state of the art of this field is reviewed, and several scientific applications that have already emerged from the fundamental discoveries are discussed. These include, among others, the coherent control of localized electromagnetic fields at the surface of solid-state nanostructures and of free-electron wave packets by such optical near fields, resulting in the creation of attosecond electron bunches, the coherent control of photocurrents on nanometer length and femtosecond timescales by the electric field of a laser pulse, and the development of new types of ultrafast electron microscopes with unprecedented spatial, temporal, and energy resolution. The review concludes by highlighting possible future developments, discussing emerging topics in photoemission and potential strong-field nanophotonic devices, and giving perspectives for coherent ultrafast microscopy techniques. More generally, it is shown that the synergy between ultrafast science, plasmonics, and strong-field physics holds promise for pioneering scientific discoveries in the upcoming years. (Less)

Ultrafast Plasmon Dynamics in Crystalline LiF Triggered by Intense Extreme UV Pulses.

An extreme ultraviolet pump and visible-light probe transmission experiment in crystalline LiF, carried out at the Free Electron Laser facility FERMI, revealed an oscillating time dependence of the plasmon mode excited in the high-density high-temperature electron plasma. The effect is interpreted as a fingerprint of the electron-ion interaction: the ion motion, shaped by the electron dynamic screening, induces, in turn, electron density fluctuations that cause the oscillation of the plasmon frequency at the timescale of the ion dynamics. Fitting the high resolution transmission data with an RPA model for the temperature-dependent dielectric function, which includes electron self-energy and electron-ion coupling, confirms the interpretation of the time modulation of the plasmon mode.

Plasmon charge transfer dynamics in layered Au–ZnO nanocomposites

Understanding the charge transfer dynamics at the interface of metals and semiconductors has received much attention in efficient plasmonic induced photonic devices. Here, we present ultrafast charge transfer dynamics in Au–ZnO nanocomposite systems by exciting them in interband and intraband levels of Au with pump energies higher (2.48 eV) and lower (1.96 eV) than the threshold energy for interband transition (2.4 eV), using the femtosecond time-resolved pump-probe technique. The spectral responses for both the excitations exhibit different behaviors, and these variations are quantitatively interpreted in terms of pump-induced changes in the dielectric constant of Au. It is found from the temporal dynamics that the electron–phonon component in the Au–ZnO system decays relatively faster (∼3 ps in the multilayer Au–ZnO) than that observed in the Au sample (∼7 ps in Au) for both the pump energies. The transfer of highly energetic hot electrons from Au nanoparticles to ZnO across the Schottky barrier results in an accessible optical response for wide bandgap ZnO in the visible to infrared range via plasmon charge collection. The calculated charge transfer rate in the Au–ZnO system is found to be (&amp;gt;1011 s−1). Our results demonstrate the pump excitation dependent ultrafast plasmon charge behavior in an optically active Au–ZnO system that can be attractive for efficient plasmonic-based hybrid photonic devices.