All-optical Switching Dynamics Research Articles

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.

Metaphotonic Devices: Pump‐Color Selective Control of Ultrafast All‐Optical Switching Dynamics in Metaphotonic Devices (Adv. Sci. 14/2020)

In article number of 2000799, Tian Jiang and co-workers propose a novel functionality of pump-colorselective ultrafast switching speed in terahertz metadevices by hybridizing meta-atoms with judiciously engineered bilayer semiconductors. The proof-of-concept demonstration breaks the long-standing challenge that all-optical switching dynamics is hardly tunable in metaphotonic devices. This strategy could inspire more opportunities for designing ultrafast, multifunctional, all-optical switchable photonic devices.

Theoretical investigation of integratable photonic crystal nanobeam all-optical switching with ultrafast response and ultralow switching energy

High-performance all-optical switching is of paramount importance to realize integrated photonic circuit. Due to the ultra-sharp resonance mode, it is demonstrated that Fano resonance is far superior to Lorentz resonance for the implementation of all-optical switching. However, it is still a difficulty to realize an integratable all-optical switching with faster response and lower switching energy simultaneously. In this work, we propose double Fano resonances based all-optical switching composed of a silicon-polymer compound photonic crystal nanobeam (PCN) side-coupled with a photonic crystal nanobeam cavity (PCNC). The pump and probe wavelengths locate at the position of two Fano resonant modes. Introducing the excellent Kerr nonlinearity of polymer materials, all-optical switching dynamics are investigated explicitly by numerical pump-probe technique based on the finite-difference time-domain method. Associated with the sharp transmission profile of Fano resonance modes and excellent nonlinear optical property of polymer, sub-picosecond switching time and sub-picojoule switching energy can be realized simultaneously with in-plane pumping scheme. Such PCN-PCNC structures are compact and can be fabricated based on the silicon-on-insulator material, which are compatible with the complementary-metal-oxide-semiconductor technology. Our results eliminate the obstacles for the realization of high-performance optical switching, and unlock the potential for the construction of integrated photonic circuits.

Microscopic three-temperature model for all-optical switching in GdFeCo

Compared with magnetic switching by an external magnetic field or by a heat-assisted manner, all-optical switching (AOS) can complete the switching process within 100 ps, which has attracted extensive attention from researchers. Among the magneto-optical materials which can realize AOS, the ferrimagnetic GdFeCo has the ability to realize single-shot AOS and possesses great potential in all-optical magnetic storage. In this paper, a microscopic three-temperature model (M3TM) is utilized to simulate the AOS process of GdFeCo, which is also demonstrated experimentally, under the excitation of a single laser pulse based on the heating effect. By using this M3TM, the AOS dynamics and the final magnetization states of GdFeCo induced by single laser pulses with different energy and pulse widths are calculated and analyzed concretely. Compared with the atomic spin model and the Landau- Lifshitz-Bloch (LLB) model, M3TM provides a more concise time-varying expression of the magnetization of GdFeCo and explicitly addresses the dissipation of angular momentum after the laser-pulse excitation, which enables faster calculations of the heat-induced magnetization dynamics in magneto-optical materials with large areas.

All-optical switches using solitons within nonlinear fibers

A type of all-optical switches using solitons within nonlinear fibers is studied in this paper. Analytic two-soliton solutions for the coupled nonlinear Schrödinger equations are derived with symbolic computation and Hirota method. Inelastic interactions between solitons are displayed, and the all-optical switching dynamics is analyzed. The high-intensity light is controlled with the low-intense light in the anomalous dispersion regime of fibers. Besides, the bright solitons for the all-optical switches are obtained in the normal dispersion regime of fibers. Influences of the fiber nonlinearity, group-velocity dispersion, and reciprocal of the group velocity are discussed. Nonlinear effects in the normal dispersion regime of fibers has less impact than them in the anomalous dispersion regime, and the switching threshold power is adapted through changing the group velocity. All-optical switches can be achieved with a short fiber length through increasing the group-velocity dispersion, and the size of optical circuits can be reduced without the dispersion limit. Results of this paper may drive potential applications of all-optical switches for computing, information processing, and networking.

Dynamics of all-optical switching in <inline-formula><math display="inline" overflow="scroll"><msub><mrow><mrow><mstyle mathvariant="normal"><mtext>C</mtext></mstyle></mrow></mrow><mrow><mn>6</mn><mn>0</mn></mrow></msub></math></inline-formula> and its application to optical logic gates

The dynamics of all-optical switching in C60 has been theoretically analyzed based on the principle of nonlinear intensity-induced excited-state absorption to achieve high contrast and fast switching. The transmission of a cw probe laser beam at 885 nm corresponding to the peak absorption of the S1 state through C60 in toluene is switched by a pulsed pump laser beam at 532 nm that excites molecules from the ground state. Switching characteristics are shown to be sensitive to the variation of pump pulse width, peak pumping intensity, thickness of the medium, intersystem crossing time between the S1 and T1 states and the absorption cross section of the S1 state at a probe wavelength, and their effect are analyzed in detail. It is shown that the transmission of the probe beam can be completely switched off (100% modulation) by the pump beam at a pump intensity of 100 MW/cm2 and a pulse width 1.5 ns, with switch on and off times of 2.5 and 7.5 ns, respectively. The results are used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses, which are the basic building blocks of computing circuits.