Ultrafast Switching Devices Research Articles

Dynamical aspects of excitonic Floquet states generated by a phase-locked mid-infrared pulse in a one-dimensional Mott insulator

A periodic electric field of light applied on a solid is predicted to generate coupled states of the light electric fields and electronic system called photon-dressed Floquet states. Previous studies of those Floquet states have focused on time-averaged energy-level structures. Here, we report time-dependent responses of Floquet states of excitons generated by a mid-infrared (MIR) pulse excitation in a prototypical one-dimensional (1D) Mott insulator, a chlorine-bridged nickel-chain compound, [Ni(chxn)2Cl](NO3)2 (chxn = cyclohexanediamine). Sub-cycle reflection spectroscopy on this compound using a phase-locked MIR pump pulse and an ultrashort visible probe pulse with the temporal width of ∼7 fs revealed that large and ultrafast reflectivity changes occur along the electric field of the MIR pulse; the reflectivity change reached approximately 50% of the original value around the exciton absorption peak. It comprised a high-frequency oscillation at twice the frequency of the MIR pulse and a low-frequency component following the intensity envelope of the MIR pulse, which showed different probe-energy dependences. Simulations considering one-photon-allowed and one-photon-forbidden excitons reproduced the temporal and spectral characteristics of both the high-frequency oscillation and low-frequency component. These simulations demonstrated that all responses originated from the quantum interferences of the linear reflection process and nonlinear light-scattering processes owing to the excitonic Floquet states characteristic of 1D Mott insulators. The present results lead to the developments of Floquet engineering, and demonstrate the possibility of rapidly controlling the intensity of visible or near-IR pulse by varying the phase of MIR electric fields, which will be utilized for ultrafast optical switching devices.

Observation of negative terahertz photoconductivity in strongly correlated electron-doped CaMnO3 thin film

Electron-doped Ca0.96Ce0.04MnO3 (CCMO) possesses a unique band structure and exhibits a giant topological Hall effect contrary to other correlation-driven manganites known for insulator-to-metal transition, magnetoresistance, complex magnetic order, etc. The interaction mechanisms among the fundamental entities and their dynamical evolutions responsible for this unusual topological phase are yet to be understood. Here, we employ time-averaged and sub-picosecond time-resolved terahertz (THz) spectroscopy to explore the low-energy steady-state and ultrafast carrier dynamics, respectively, to unravel the complexity of charge carriers during their transition from a non-equilibrium state to the ground state in CCMO thin film. The THz optical conductivity confirms the presence of dichotomic charge carriers, i.e. heavy and light carriers throughout the temperature range of 15–300 K. A rare observation of both positive and negative photoconductivities along with a sharp crossover between the two resolved to a few picoseconds of illumination confirms the formation of polaron with a lifetime of a few nanoseconds. These optical evidences of dichotomic charge carriers, along with manipulation of the sign of photoconductivity induced by dynamics of related quasiparticles could facilitate a new mechanism for ultrafast optoelectronic switching devices.

Si nanopillar arrays as possible electronic device platforms

This paper explores the possible use of silicon nanopillars as electronic devices. The silicon nanopillars studied in this work were fabricated by electron beam lithography and by plasma ion etching of silicon wafers. The electrical and physical properties of these pillars with full width at half maximum ranging from few nanometers to 100 nm and length of few hundreds of nanometer have been investigated by time-resolwed photoluminescence, infrared spectroscopy and current-voltage characteristics. An ultrafast blue luminescence component competing with non-radiative recombination at surface defects was quantified as originating from the no-phonon recombination. This component involved two decay processes with a peak energy at around 500 nm, which have the fast component close to femtosecond time scale. The emission exhibits also a slow component in the red spectral region with a time constant in the nanosecond regime. The nanopillars have been smoothened in an attempt to passivate surface defects. The results are indicative of an increase in the lifetimes-of carriers and an enhancement in the red component of the emission with much slower sates. The presence of ultrafast decay at blue-green spectral region is suggestive of the possibility of using the silicon nanopillars as ultrafast switching devices. Silicon nanopillar arrays can also be an ideal platform in trapping and sensing chemical or biological species using diamond NV centers.

Observation of giant persistent photoconductivity on vanadium dioxide thin film device

We report a giant persistent photoconductivity (GPPC) phenomenon in VO2 film device prepared on corning glass substrate. With a single pulse laser irradiation onto the device at bias voltages of 7 V and 8 V, the VO2 film makes an insulator-to-metal transition with a sharp increase of photo current. The photo current remained at a highly conductive state for long period, e.g. the photo current for VO2 device was decreased only ~1% in one day. The GPPC was found within the limit of critical voltages of the hysteresis loop in voltage vs current curve of the device. High speed time-current measurements on the device with a 20 ms single pulse laser irradiation revealed that the onset of abrupt photo current occurs in less than 1 ms time scale. The GPPC in VO2 device can contribute in realizing ultrafast optical remote control of advanced electronic devices i.e. optical memories, displays, and remote ultrafast switching devices.

Optical Polarization Sensitive Ultra-Fast Switching and Photo-Electrical Device.

Ultra-fast electrical switches activated with an optical-polarized light trigger, also called photo-polarized activated electrical switches, are presented. A set of new transistor circuits is switched by light from above, illuminating deep V-grooves, whose angle is sensitive to the polarization of the incident. Thus, this application may serve for encryption/decryption devices since the strongest electrical responsivity is only obtained for very specific spatial polarization directions of the illumination beam. When this V-groove is sufficiently narrow, the device mainly responds to one polarization and not to the other. In such a way, electrons are generated only for one specific polarization. While the nature of the data remains electronic, the modulation control is optic, creating a photo-induced current depending on the polarization direction. This coupled device acts as a polarization modulator as well as an intensity modulator. The article focuses on the integration of several devices in different configurations of circuitry: dual, triple, and multi-element. Case studies of several adjacent devices are presented with varying critical variables, such as the V-groove aperture dimensions. Analytical models and complementary numerical analyses are presented for the future smooth integration into Complementary Metal-Oxide-Semiconductor (CMOS) technology.

Ultrafast Plasmonic Optical Switching Structures and Devices

[This submission is based on the contacts with Prof. Pavesi by code JMDC(100%.2019.FPHY.832).] Plasmonic structures possess rich physics related to the sensitivity of plasmon resonance to the change in the environmental dielectric constant, the enhanced light scattering and optical extinction, and the local field enhancement enabled strong light-matter interactions, which have been applied in refractive-index sensors, optical feedback in various micro- or nano-cavity lasers, surface enhanced Raman scattering spectroscopy, and high-sensitivity molecular detection. However, ultrafast optical response is another important aspect of plasmons, which can be utilized to achieve switching of optical signals in different spectral bands. These optical switching designs are very important for applications in optical logic circuits and optical communication system. In this review, we summarize a series of reports on ultrafast plasmonic optical switches, where we focus our discussions on the structural and device designs, instead of on their physics. By categorizing the designs of optical switches into different groups by their featured performances, we intend to propose the development trend and the commonly interested mechanisms of such ultrafast optical switches. We hope this review will supply helpful concepts and technical information for further development and new applications of ultrafast optical switching devices.

Tailoring the properties of zinc oxide films by incorporating gold nanoparticles using RF magnetron sputtering

Nanoscale noble metal-incorporated ZnO nanostructures can have potential applications in developing chemical and biological sensors, optical filters, ultrafast switching devices, etc. The structural, morphological and optical properties of the as-deposited pure and Au-incorporated RF-sputtered ZnO films were investigated. XRD analysis suggests that the presence of Au nanoparticles strongly promotes the growth of well-crystalline grains of ZnO along $$\left\langle {00{\text{1}}} \right\rangle$$ direction. Surface morphology examined using AFM and FESEM micrographs and compositional analysis using EDX spectra reveal that Au acts as a nucleating center for the growth of high-crystalline grains, leading to the formation of specific structures. The observed reduction in transmittance with Au incorporation concentration can be attributed to LSPR of gold nanoparticles, scattering, increased surface roughness and increased thicknesses of the films. Systematic increase in the intensity of the plasmonic peak along with a shift in its position towards longer wavelength region is observed with increase in Au incorporation concentration. The PL spectra of the films show a NBE emission due to excitonic transition in ZnO and blue emission originating from deep-level defects in ZnO. The influence of Au nanoparticles on the crystallization of ZnO grains, evolution of surface morphology of ZnO nanostructured films and modification in the UV emission efficiency of ZnO films due to SPR of Au nanoparticles are studied in detail.

Ultrafast photoinduced insulator-metal transition in epitaxial samarium nickelate thin films investigated by time-resolved terahertz spectroscopy

We investigate the ultrafast dynamic behavior of photoinduced insulator-metal phase transition in an epitaxially grown $\mathrm{SmNi}{\mathrm{O}}_{3}$ thin film by using time-resolved terahertz (THz) spectroscopy at different excitation fluences. It is found that the relaxation of the photoexcited carriers is characterized by two components with different time scales: (i) an ultrafast one with time constant varying between 0.3 ps (at 4.4 $\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$) and 0.35 ps (at 0.88 $\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$) and (ii) a slow one with a time constant varying between 1.7 ps (at 2 $\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$) and 4.7 ps (at 0.88 $\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$). These relaxation times could be primarily attributed to the coexistence of conducting and insulating charge-ordering domains. The observed behavior of the transient THz conductivity of $\mathrm{SmNi}{\mathrm{O}}_{3}$ thin film is well fitted by the Drude-Smith model, revealing that insulating domains emerge between the metallic domains, which in turn increase the carrier confinement during the relaxation process. Furthermore, the ultrafast dynamic behavior at different temperatures suggests that the insulating gap shrinks gradually between the Ni $3d$ and O $2p$ states as the temperature applied to the film increases. Our experimental findings pave the way to scientific opportunities and to technological applications of epitaxially grown $\mathrm{SmNi}{\mathrm{O}}_{3}$ thin film such as ultrafast optical switching and modulation devices.

A new coupling mechanism between two graphene electron waveguides for ultrafast switching

In this paper, we report a novel coupling between two graphene electron waveguides, in analogy the optical waveguides. The design is based on the coherent quantum mechanical tunneling of Rabi oscillation between the two graphene electron waveguides. Based on this coupling mechanism, we propose that it can be used as an ultrafast electronic switching device. Based on a modified coupled mode theory, we construct a theoretical model to analyze the device characteristics, and predict that the switching speed is faster than 1 ps and the on–off ratio exceeds 106. Due to the long mean free path of electrons in graphene at room temperature, the proposed design avoids the limitation of low temperature operation required in the traditional design by using semiconductor quantum-well structure. The layout of our design is similar to that of a standard complementary metal-oxide-semiconductor transistor that should be readily fabricated with current state-of-art nanotechnology.

Impact of tungsten doping on the dynamics of the photo-induced insulator-metal phase transition in VO2 thin film investigated by optical pump-terahertz probe spectroscopy

The influence of tungsten (W) doping on the ultrafast dynamics of the photo-induced insulator-metal phase transition (IMT) is investigated at room temperature in epitaxially grown vanadium dioxide (VO2) thin films by means of optical pump-terahertz (THz) probe spectroscopy. It is observed that the THz transmission variation of the films across the IMT follows a bi-exponential decrease characterized by two time constants, one corresponding to a fast process and the other to a slower process. W-doping (i) reduces the photo-excitation fluence threshold required for triggering the IMT, (ii) accelerates the slow process, and (iii) increases the THz transient transmission variation for corresponding fluences. From the Drude-Smith model, it is deduced that a strong carrier confinement and an enhancement of the transient conductivity occur across the IMT. The IMT is also accompanied by an increase in the carrier concentration in the films, which is enhanced by W-doping. Our results suggest that W-doped VO2 could be advantageously exploited in applications such as ultrafast THz optical switching and modulation devices.

Ultrafast Multipolar Plasmon for Unidirectional Optical Switching in a Hemisphere‐Nanoshell Array

This study reports ultrafast multipolar plasmons in a large‐area periodic array of hemisphere gold nanoshells. A dipolar plasmon is observed as a resonance mode at longer wavelengths, where the free electrons are driven to oscillate through the nanoshells and the corresponding resonance spectrum is determined by the size of the hemisphere nanoshells. However, a hexapolar plasmon is observed to resonate at shorter wavelengths, where the free electrons are driven to oscillate in the inner shell and in the surrounding gold film and this inner‐shell “bright” dipole induces charge redistribution in the surrounding gold film, leading to the formation of a quadrupolar “dark” plasmon. Different physical mechanisms of these two resonance modes lead to different interactions between plasmonic electrons and photons, resulting in different lifetimes of the plasmonic dynamics. Both resonance modes can be exploited to achieve ultrafast optical switching devices. Large‐area structures with simple fabrication techniques enable potentially practical applications.

Quantum Design of π-Electron Ring Currents in Polycyclic Aromatic Hydrocarbons: Parallel and Antiparallel Ring Currents in Naphthalene.

Control of π-electrons in polycyclic aromatic hydrocarbons (PAHs) is one of the fundamental issues in optoelectronics for ultrafast optical switching devices. We have proposed an effective scenario for design of the generation of coherent ring currents in naphthalene (D2h), which is the smallest unit of planar PAHs. It has been demonstrated by using quantum chemical calculations and quantum optimal control (QOC) simulations that two types of ring currents, parallel and antiparallel, can be generated by resonance excitations by two linearly polarized lasers. A parallel (antiparallel) ring current means that the currents of two benzene rings run in the same (opposite) directions. The two types of ring currents may be experimentally identified by magnetic force microscopy. The QOC simulations indicate that a parallel ring current can be generated by using continuous wave and Gaussian pulse lasers with their time delay without relying on a sophisticated experimental apparatus. The present results provide a guiding principle of coherent π-electronics in PAHs for next-generation organic optical switching devices.

Focus ion beam-induced mechanical stress switching in an ultra-fast resistive switching device

The Mo/Si3N4:Pt/Pt nanometallic resistive switching devices with ultra-fast write/erase speed (<50 ns) were fabricated. Other than conventional electrical switching, a mechanical stress-induced switching was demonstrated. Such mechanical stress was provided by momentum transfer of 30 keV Ga+ ions in a focus ion beam system, enabling a one-way high resistance state (HRS) to low resistance state (LRS) transition. The capability of mechanical stress switching provides evidence that electron trapping/detrapping mechanism is responsible for nanometallic resistive switching. It was further demonstrated that HRS (trapping state) is a meta-stable state, while LRS (detrapping state) is a stable state. Strong mechanical stress facilitates local bond distortion in dielectric films and thus lowers the energy barrier between HRS and LRS, eventually leading to a barrier-less state transition. A quantitative model based on stress-mediated parallel conduction paths were established to provide a more accurate description of the resistive switching devices.

Plasmon induced transparency in loop–stub resonator-coupled waveguide systems

We firstly investigate plasmon induced transparency (PIT) effect in a metal–dielectric–metal (MDM) waveguide coupled to a single loop stub resonator by finite difference time domain method (FDTD). Compared with previous PIT sup based on MDM waveguide, PIT phenomena can be realized in a single plasmonic composite nanocavity without employment of additional optical elements. Plasmon induced transparency windows can be controlled by adjusting the geometrical parameters of the vertical branches or the horizontal branch in the plasmonic structure. The red-shift of PIT peak is almost linearly proportional to the refractive index of the horizontal branch. This plasmonic system takes the advantages of easy fabrication and compactness. The results may pave a way for the dynamic control of light in highly integrated optical circuits, which can realize ultrafast switching, light storage and nanosensor devices.

Nanoscale manipulation of the Mott insulating state coupled to charge order in 1T-TaS2

The controllability over strongly correlated electronic states promises unique electronic devices. A recent example is an optically induced ultrafast switching device based on the transition between the correlated Mott insulating state and a metallic state of a transition metal dichalcogenide 1T-TaS2. However, the electronic switching has been challenging and the nature of the transition has been veiled. Here we demonstrate the nanoscale electronic manipulation of the Mott state of 1T-TaS2. The voltage pulse from a scanning tunnelling microscope switches the insulating phase locally into a metallic phase with irregularly textured domain walls in the charge density wave order inherent to this Mott state. The metallic state is revealed as a correlated phase, which is induced by the moderate reduction of electron correlation due to the charge density wave decoherence.

Control of the Magnetic Configuration of Ferromagnetic Nanostructures Across the Structural Phase Transition of Vanadium Dioxide

We investigate the effect of the structural phase transition in VO 2 in magnetoelastically coupled heterostructures of VO 2 and Ni. Continuous and nano-patterned Ni layers were used, and we found reversible and reproducible magnetic domain switching induced by the VO 2 structural phase transition. The magnetic states of the nano-patterned ferromagnetic elements were dominated by topographic features which generated strong pinning but still allowed for a reversible switching between the states. Our measurements constitute a key step for the use of the VO 2 phase transition for ultrafast dynamical studies of the inverse mangetostrictive effect, and eventually employing the effect for ultrafast low-power switching devices.

Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons.

Multicomponent chalcogenides, such as quasi-binary GeTe–Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge2Sb2Te5 (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 106 write–erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase change in GST remain controversial, and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photoexcited GST superlattice, where GeTe and Sb2Te3 are spatially separated, using coherent phonon spectroscopy with pump–pump–probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in the GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest the possible applicability of GST superlattices for ultrafast switching devices.

Quantum Control of Coherent ρρ‐Electron Dynamics in Chiral Aromatic Molecules

AbstractControl of mobile π‐electrons is one of the fundamental issues in the organic optoelectronics for designing the next generation ultrafast switching devices. The optimal control simulations of coherent π‐electron rotations in (P)‐2,2’‐biphenol, which is the typical nonplanar aromatic molecule with axial chirality, were performed by taking into account two types of the control targets: one is generation of the maximum π‐angular momentum, and the other is the maintaining of the generated unidirectional angular momentum during a setting time duration. The optimal control pulse for each target is designed. The analysis of the simulation results shows that the effective maintaining of the unidirectional angular momentum can be realized by applying 2π pulse to one of the electronic excited states forming the coherent electronic state. The 2π pulse prevents the reverse rotation of the π‐electrons by dumping the excited state population to the ground state and subsequently by pumping the population back to the excited state. The present results provide a theoretical basis for the designing next generation ultrafast switching devices made by organic aromatic molecules.

Ultrafast optical switching in quantum dot-metallic nanoparticle hybrid systems.

We study ultrafast excitonic population inversion resulting from the interaction of a semiconductor quantum dot (SQD) with localized surface plasmons. The plasmonic enhanced fields are generated when a metallic nanoparticle (MNP) is subject to a nonlinear chirped few-cycle pulse train. By numerically solving the time-dependent Bloch equations beyond the rotating-wave approximation, we show that the complete population inversion can be achieved for small interparticle distance and the dynamic in population inversion exhibits a steplike transition between absorption and amplifying. This phenomenon can be exploited as an all-optical ultrafast switching device. Moreover, the final state of population inversion is shown to be modified significantly with the interparticle distances, which is not only robust against the variation of probe pulse parameters but also suggests a straightforward method for measuring the interparticle distances via probing the final populations.

Multi-Dimensional Switching of Ring Currents in Nonplanar Chiral Aromatic Molecules by Ultrashort Linearly Polarized UV Laser Pulses

Nonplanar chiral aromatic ring molecules are candidates for use as ultrafast multi-dimensional optical switching devices. Nonplanar (P)-2, 2’-biphenol exhibits four patterns of π-electron rotations along the two phenol rings. It can induce two-dimensional coherent ring currents. Each rotational pattern can be driven by a coherent excitation of two quasi-degenerate electronic states using linearly polarized ultrashort UV pulses. We present the quantum dynamical simulation results of a four-step switching of the two-dimensional ring currents within 300 fs.