Ultrafast All-optical Switching Research Articles

Bistable soliton switching dynamics in a PT -symmetric coupler with saturable nonlinearity

We investigate the switching dynamics in a $\mathcal{PT}$-symmetric fiber coupler composed of a saturable nonlinear material as the core. In such a saturable nonlinear medium, bistable solitons may evolve due to the balance between dispersion and saturable nonlinearity, which we extend in the context of the $\mathcal{PT}$-symmetric coupler. Our investigations of power-controlled and phase-sensitive switching show richer soliton switching dynamics than the currently existing conventional counterparts, which may lead to ultrafast and efficient all-optical switching dynamics at very low power owing to the combined effects of $\mathcal{PT}$ symmetry and saturable nonlinearity. In addition to the input power, the relative phase of the input solitons and saturable coefficient are additional controlling parameters that efficiently tailor the switching dynamics. Also, we provide a suitable range of system and pulse parameters that would be helpful for the practical realization of the coupler to use in all-optical switching devices and photonic circuits. Finally, we develop a variational approach to analytically investigate the switching dynamics in such $\mathcal{PT}$-symmetric couplers that excellently predicts the numerical findings.

Investigating the influence of ITO thin film thickness on the optical Kerr nonlinearity using ultrashort laser pulses

In this paper, we present results from the measurements of the optical Kerr nonlinearity of indium tin oxide (ITO) thin films of different thicknesses using the femtosecond (fs) Z-scan technique. ITO thin films were prepared by radio frequency magnetron sputtering on a glass substrate at room temperature. The coated ITO thin films were subsequently characterized by UV–visible absorption spectroscopy, x-ray diffraction, and scanning electron microscopy. Using ∼ 100 f s pulses at 80 MHz repetition rate, the optical Kerr nonlinearity in ITO with different thicknesses was investigated at different excitation wavelengths and incident pulse energies. The optical Kerr nonlinearity was found to be dependent on excitation wavelength, incident power, and ITO thickness, with a maximum value of ∼ 9 × 1 0 − 12 c m 2 / W at a wavelength of 820 nm, power of 1 W, and 170 nm ITO thickness. These results suggest that Kerr nonlinearity in ITO can be tailored by varying the film thickness, which would be ideal for ultrafast all-optical switching in future optoelectronic devices.

Ultrafast all-optical phase switching enabled by epsilon-near-zero materials in silicon.

Transparent conducting oxides (TCOs) have emerged as both particularly appealing epsilon-near-zero (ENZ) materials and remarkable candidates for the design and fabrication of active silicon nanophotonic devices. However, the leverage of TCO's ultrafast nonlinearities requires precise control of the intricate physical mechanisms that take place upon excitation. Here we investigate such behavior for ultrafast all-optical phase switching in hybrid TCO-silicon waveguides through numerical simulation. The model is driven from the framework of intraband-transition-induced optical nonlinearity. Transient evolution is studied with a phenomenological two-temperature model. Our results reveal the best compromise between energy consumption, insertion losses and phase change per unit length for enabling ultrafast switching times below 100 fs and compact active lengths in the order of several micrometers.

Interacting plexcitons for designed ultrafast optical nonlinearity in a monolayer semiconductor

Searching for ideal materials with strong effective optical nonlinear responses is a long-term task enabling remarkable breakthroughs in contemporary quantum and nonlinear optics. Polaritons, hybridized light-matter quasiparticles, are an appealing candidate to realize such nonlinearities. Here, we explore a class of peculiar polaritons, named plasmon–exciton polaritons (plexcitons), in a hybrid system composed of silver nanodisk arrays and monolayer tungsten-disulfide (WS2), which shows giant room-temperature nonlinearity due to their deep-subwavelength localized nature. Specifically, comprehensive ultrafast pump–probe measurements reveal that plexciton nonlinearity is dominated by the saturation and higher-order excitation-induced dephasing interactions, rather than the well-known exchange interaction in traditional microcavity polaritons. Furthermore, we demonstrate this giant nonlinearity can be exploited to manipulate the ultrafast nonlinear absorption properties of the solid-state system. Our findings suggest that plexcitons are intrinsically strongly interacting, thereby pioneering new horizons for practical implementations such as energy-efficient ultrafast all-optical switching and information processing.

Realistic micromagnetic description of all-optical ultrafast switching processes in ferrimagnetic alloys

Both helicity-independent and helicity-dependent all-optical switching processes driven by single ultrashort laser pulse have been experimentally demonstrated in ferrimagnetic alloys as GdFeCo. Although the switching has been previously reproduced by atomistic simulations, the lack of a robust micromagnetic framework for ferrimagnets limits the predictions to small nanosystems, whereas the experiments are usually performed with lasers and samples of tens of micrometers. Here we develop a micromagnetic model based on the extended Landau-Lifshitz-Bloch equation, which is firstly validated by directly reproducing atomistic results for small samples and uniform laser heating. After that, the model is used to study ultrafast single shot all-optical switching in ferrimagnetic alloys under realistic conditions. We find that the helicity-independent switching under a linearly polarized laser pulse is a pure thermal phenomenon, in which the size of inverted area directly correlates with the maximum electron temperature in the sample. On the other hand, the analysis of the helicity-dependent processes under circular polarized pulses in ferrimagnetic alloys with different composition indicates qualitative differences between the results predicted by the magnetic circular dichroism and the ones from inverse Faraday effect. Based on these predictions, we propose experiments that would allow one to resolve the controversy over the physical phenomenon that underlies these helicity-dependent all optical processes.

Ultrafast metamaterial all-optical switching based on coherent modulation.

We report a demonstration of an ultrafast all-optical switching with unique light control effects. The all-optical switching consists of a gold film with asymmetric split rings and a silica substrate. The device effectively controls the transmission and absorption of continuous pulses in the communication band (1200-1800 nm) and short pulses with a pulse duration of 80 fs by using the interaction of two coherent beams on nano-metamaterials with a thickness of only 50 nm. The metamaterial can achieve more than 90 % output control under continuous light irradiation. When the pulse duration is 80 fs, the switching contrast ratio is greater than 3 : 1 and the modulation bandwidth is greater than 12.5 THz. Switching time can be on the order of femtosecond. This paper provides a new structure for ultra-high speed optical data processing components in coherent networks.

Thermal impact on ultrafast helicity independent all-optical switching of GdxCo100-x

Ultrafast all-optical switching by single femtosecond laser pulse has recently been considered as one of the most potential technologies for the application on high-speed and energy efficient memory device. Here, to ensure such a sophisticated technology to be applicable to more complex environments, we report the dependence of all-optical switching of the ferrimagnet, GdCo, on the ambient temperature. Through heating the GdCo film up to 380 K, we demonstrate the distinct magnetic behaviours and the critical laser fluences of all-optical switching under different composition of Gd. It is found that the thermal stability for AOS will be significantly improved when there are more rare-earth elements, Gd in GdCo. This work provides a deep insight into the thermal effect on helicity independent all-optical switching and will optimize the application of ultrafast technology on the spintronic devices and magnetic memories.

All-optical switching based on interacting exciton polaritons in self-assembled perovskite microwires.

Ultrafast all-optical switches and integrated circuits call for giant optical nonlinearity to minimize energy consumption and footprint. Exciton polaritons underpin intrinsic strong nonlinear interactions and high-speed propagation in solids, thus affording an intriguing platform for all-optical devices. However, semiconductors sustaining stable exciton polaritons at room temperature usually exhibit restricted nonlinearity and/or propagation properties. Delocalized and strongly interacting Wannier-Mott excitons in metal halide perovskites highlight their advantages in integrated nonlinear optical devices. Here, we report all-optical switching by using propagating and strongly interacting exciton-polariton fluids in self-assembled CsPbBr3 microwires. Strong polariton-polariton interactions and extended polariton fluids with a propagation length of around 25 μm have been reached. All-optical switching on/off of polariton propagation can be realized in picosecond time scale by locally blue-shifting the dispersion with interacting polaritons. The all-optical switching, together with the scalable self-assembly method, highlights promising applications of solution-processed perovskites toward integrated photonics operating in strong coupling regime.

Ultrafast all-optical metamaterial terahertz switch by exploiting nanoparticle-enriched substrate nonlinearity and graphene resonators

The emergence of graphene as a new material with several extraordinary optical properties paves a way to introduce new electromagnetic devices in the terahertz (THz) band. In this paper, we have exploited the unique qualities of graphene in combination with the optical Kerr effect in order to propose a new THz switch. In the proposed structure, two graphene layers with circular and notch ring patterns are stacked to obtain two narrowband absorption peaks at the frequencies of 3.65 and 6.5 THz with high absorption values of 92% and 97%, respectively. The imposed pump power in interaction with the nanocomposite substrate provides a controlling mechanism that makes this structure an ideal candidate for all-optical devices in the THz regime. The obtained switching time of the proposed structure is 2 ps, and the maximum extinction ratio of 83% is achieved for the pump power of 273 mW. Moreover, the absorption spectrum of the switch can be manipulated by the chemical potential of graphene layers, which is useful for electro-optical modulation application. The application of the THz switch as an all-optical tunable filter is also studied in this paper.

Ultrafast All-Optical Switching in the Visible Spectrum with 6H Silicon Carbide

Ultrafast All-Optical Switching in the Visible Spectrum with 6H Silicon Carbide

Broadband and ultrafast all-optical switching based on transition metal carbide

Abstract Ultrafast all-optical switches have attracted considerable attention for breaking through the speed limitation of electric devices. However, ultrafast and high-efficiency all-optical switches based on two-dimensional (2D) materials can be achieved due to their strong nonlinear optical response and ultrafast carrier dynamic. For this reason, we propose the pump-probe method to achieve an ultrafast optical switcher with a response time of 192 fs and a switching energy of 800 nJ by using transition metal carbide (Nb2C). The response time and switching energy are far smaller than that of the all-optical device based on the saturable absorption effect of 2D materials. It is believed that the Nb2C-based all-optical switch provides a novel idea to achieve a high-performance all-optical device and has the potential for application in high-speed photonics processing.

High Q Chalcogenide Photonic Crystal Nanobeam Cavities

We report high quality (Q) photonic crystal (PhC) nanobeam cavities in the chalcogenide glass Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> near 1550 nm wavelength. By optimizing the structure of the PhC cavity, an ultra-high Q factor as large as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times 10^{6}$ </tex-math></inline-formula> is predicted by simulation, and up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.2\times 10^{4}$ </tex-math></inline-formula> is measured in the experiment. Such high-Q cavities in this strongly nonlinear material create an attractive platform for compact, ultrafast all-optical switches.

Ultrafast all-optical diffraction switching using semiconductor metasurfaces

Ultrafast all-optical switching using Mie resonant metasurfaces requires both on-demand tunability of the wavefront of the light and ultrafast time response. However, devising a switching mechanism that has a high contrast between its “on” and “off” states without compromising speed is challenging. Here, we report the design of a tunable Mie resonant metasurface that achieves this behavior. Our approach utilizes a diffractive array of semiconductor resonators that support both dipolar and quadrupolar Mie resonances. By balancing the strengths of the dipole and quadrupole resonances, we can suppress radiation into the first diffraction order, thus creating a clearly delineated “off”-state at the operating wavelength. Then, we use optical injection of free- carriers to spectrally shift the multipoles and rebalance the multipole strengths, thereby enabling radiation into the diffraction order—all on an ultrafast timescale. We demonstrate ultrafast off-to-on switching with Ion/Ioff ≈ 5 modulation of the diffracted intensity and ultrafast on-to-off switching with Ion/Ioff ≈ 9 modulation. Both switches exhibit a fast τtr ≈ 2.7 ps relaxation time at 215 μJ cm−2 pump fluence. Further, we show that for higher fluences, the temporal response of the metasurface is governed by thermo-optic effects. This combination of multipole engineering with lattice diffraction opens design pathways for tunable metasurface-based integrated devices.

Ultrafast and low power all-optical switching in the mid-infrared region based on nonlinear highly doped semiconductor hyperbolic metamaterials

Guided wave modes in the uniaxial anisotropic hyperbolic metamaterials (HMMs) based on highly doped semiconductor instead of metal in the mid-infrared region are investigated theoretically. The heavily doped semiconductor is used to overcome the restrictions of the conventional metal-based structures caused by the lake of tunability and high metal loss at mid-infrared wavelengths. The unit cells of our proposed metamaterial are composed of alternating layers of undoped InAs as a dielectric layer and highly doped InAs as a metal layer. We numerically study the linear and nonlinear behavior of such multilayer metamaterials, for different arrangements of layers in the parallel (vertical HMM) and perpendicular (horizontal HMM) to the input wave vector. The effect of doping concentration, metal to dielectric thickness ratio in the unit cell (fill-fraction), and the total thickness of structure on the guided modes and transmission/reflection spectra of the metamaterials are studied. Moreover, the charge redistribution due to band-bending in the alternating doped and undoped layers of InAs is considered in our simulations. We demonstrate that the guided modes of the proposed hyperbolic metamaterial can change by increasing the intensity of the incident lightwave and entering the nonlinear regime. Therefore, the transition from linear to the nonlinear region leads to high-performance optical bistability. Furthermore, the switching performance in the vertical and horizontal HMMs are inspected and an ultrafast, low power, and high extinction ratio all-optical switch is presented based on a vertical structure of nonlinear highly doped semiconductor hyperbolic metamaterials.

All-optical switching of ultrafast solitons at 1560 nm in dual-core fibers with high contrast of refractive index

We present a study of ultrafast solitonic all-optical switching in the C-band using dual-core optical fibers fabricated in-house. The two soft glasses used for the fiber express high contrast of refractive index at the level of 0.4. The core was made of a highly nonlinear lead-silicate glass, which, combined with the anomalous dispersion, allowed the solitonic propagation at low pulse energies. A convincing switching performance was demonstrated using the optimized fiber at hundreds of picojoule level of the in-coupled energy. The dedicated numerical simulations unveiled the solitonic propagation character of 75 fs excitation pulses influenced by dual-core asymmetry. All the key parameters of the switching, such as the optimal fiber length (38 mm), the switching contrast (4.5 dB) and the switching energy levels (200–700 pJ), were confirmed by the numerical results.

Nonlinear optical properties of 6H-SiC and 4H-SiC in an extensive spectral range

Silicon carbide (SiC), which is the leading representative of the third-generation of semiconductors, possesses many excellent physical properties. However, its advantages also incur difficulties in processing, which calls for special processing techniques, such as femtosecond laser machining. In addition, SiC has shown unprecedented potential for optoelectronic applications. Knowledge of the nonlinear absorption coefficient and the nonlinear refractive index coefficient of SiC is required in both of the fields. In this work, the nonlinear absorption coefficient and the nonlinear refractive index coefficient of semi-insulating (SI) 6H-SiC and SI 4H-SiC, the most pervasive SiC polytypes, are measured in an extensive spectral range from 400 nm to 1000 nm with the Z-scan technique. Besides, the spectral dependence of the nonlinear optical properties is analyzed, facilitated by linear absorption spectrum. Especially, two-photon absorption (2PA) and three-photon absorption (3PA) coefficients of SI 6H-SiC and SI 4H-SiC are characterized in the respective spectral ranges. From the characterization of SiC, we can observe self-focusing phenomenon for nonlinear refraction. In the end, we unravel the potential of SiC for ultrafast all-optical switching based on the measured nonlinear optical properties.

Transmitting Surface Plasmon Polaritons across Nanometer-Sized Gaps by Optical near-Field Coupling

Metallic nanostructures can transport electromagnetic fields in the form of surface plasmon polariton (SPP) excitations, focus them into nanometric spots, and transfer them to nearby nanostructures by near-field coupling. This provides a basic functionality for designing new plasmonic devices that can greatly enhance light–matter coupling and facilitate ultrafast and efficient all-optical switching on the nanoscale. Here, we study a prototypical device geometry, a bow-tie antenna equipped with curved line gratings, for the efficient coupling of light into and across the antenna nanogap. We experimentally demonstrate the spectrally broadband launching and propagation of SPP waves over more than 10 μm on one arm of the antenna, their focusing into and transmission across the gap being studied by the plasmon outcoupling on the other arm. A substantial increase in the coupling efficiency for antennas with gap widths below 20 nm proves that the optical near-field coupling between the two antenna arms dominates the gap transmission. We find overall transmission efficiencies for nanofocusing, gap transmission, and plasmon outcoupling of up to 4%. A finite-difference time-domain simulation supports our experimental findings. This makes such bow-tie couplers an interesting platform for sensitively probing near-field coupling to single quantum emitters and for the ultrafast switching of light by light on the nanoscale.

Spatiotemporal Metasurfaces: Spatiotemporal Terahertz Metasurfaces for Ultrafast All‐Optical Switching with Electric‐Triggered Bistability (Laser Photonics Rev. 15(3)/2021)

In article number 2000456, Tian Jiang and co-workers propose a novel spatiotemporal terahertz meta-device, which equips temporally ultrafast all-optical terahertz switching with spatially selective nonvolatile resonances. By leveraging both degrees of freedom in space and time domains, this work directly promotes the emergence of highly functional spatiotemporal memory metadevices.

Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films

Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal (\textit{a}-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic \textit{a}-Gd$_{22-x}$Tb$_x$Co$_{78}$ thin films, from x = 0 to x = 18, and elucidate the role of Gd in HI-AOS in \textit{a}-RE-TM alloys and multilayers. Increasing Tb content results in increasing perpendicular magnetic anisotropy and coercivity, without modifying magnetization density, and slower remagnetization rates and higher critical fluences for switching but still shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on the two-temperature model reproduce these results qualitatively and predict that the lower damping on the RE sublattice arising from the small spin-orbit coupling of Gd (with $L = 0$) is instrumental for the faster dynamics and lower critical fluences of the Gd-rich alloys. Annealing \textit{a}-Gd$_{10}$Tb$_{12}$Co$_{78}$ leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that acounting for element-specific damping is crucial in understanding HI-AOS phenomena. The results suggest that engineering the element specific damping of materials can open up new classes of materials that exhibit low-energy, ultrafast HI-AOS.

Femtosecond-scale all-optical switching in oxyfluorogallate glass induced by nonlinear multiphoton absorption.

Herein, all-optical switching based on a new type of oxyfluorogallate glass with high switching efficiency and ultrafast response time was reported in the near-infrared wavelength range, which is of great importance for applications in optical telecommunication. The structural and optical properties, as well as the nonlinear optical effects, of the oxyfluorogallate glass were investigated, demonstrating a good figure of merit applicable to nonlinear optical devices. Using pump–probe experiments, we found that the switching time in the oxyfluorogallate glass due to nonlinear multiphoton absorption was approximately 350 fs, which was limited by the pulse duration of the near-infrared probe pulse. Additionally, the largest on–off amplitude of this optical switching device could reach ∼12%, which is in sharp contrast to that of quartz glass. Thus, this study provides a suitable material for the manufacture of integrated photonic devices, which are crucial for promoting the application of glass on-chip photonics.