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As a new class of 2D materials, MXenes have attracted a lot of interest because of their prominent performance in versatile applications, such as batteries, supercapacitors, catalysts, electronics, and optics. In this work, an all‐optical modulator using MXene Ti3C2Tx deposited on a microfiber is proposed. By inserting an MXene‐deposited phase shifter into one arm of a Mach–Zehnder interferometer, the MXene Ti3C2Tx absorbs the control light and generates heat, which induces significant refractive index changes through strong light–matter interactions and thermo‐optic effects. In this study, a maximum phase shift of 16π is obtained, and an efficient all‐optical switch with an extinction ratio of more than 18.53 dB and a rise time constant of 4.10 ms are demonstrated. The advantages of this modulator include its all‐fiber content, low cost, ease of integration, and compactness. All‐optical modulators based on thermo‐optical effects will play an active role in the future of optical communications and optical information processing.

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An electrorheological and magnetorheological fluid has small spherical particles in a space where a uniform electric field is perpendicular to a uniform magnetic field. When the ratio between the electric field and magnetic field varies, structural transitions occur. If the electric ͑or magnetic͒ field is dominant, the ideal structure of the system is a body-centered tetragonal lattice with its fourfold rotational axis in the electric ͑or magnetic͒ field direction. When the electric field and magnetic field are compatible, under the dipolar approximation, the system may have a hexagonal close-packed ͑hcp͒ structure. However, because the energy difference between the hcp structure and face-centered cubic ͑fcc͒ lattice is small and compatible with the thermal energy at room temperature, the system may develop into a hcp-fcc mixed structure when the electric field and magnetic field are compatible. ͓S1063-651X͑98͒04305-0͔PACS number͑s͒: 83.80.Gv, 83.20.Di, 61.90.ϩd Transitions between different crystallographic states are an important phenomenon in nature, because they involve intriguing physics and have many applications. Since the basic degrees of freedom in a solid are electronic and vibrational, we usually classify these transitions by whether they are driven primarily by electronic or vibrational instabilities. For example, the cooperative John-Teller transitions ͓1͔ are structural transitions of electronic states and the displacive transition ͓2͔ is caused by phonons.In this paper, we study a system that will have structural transitions by a change of applied electric field and magnetic field. The system consists of fine spherical particles of radius a in micrometers, which are placed in a capacitor whose spacing Lӷa. In addition, there is a uniform magnetic field acting on the spheres. An N pole and an S pole separated by a distance Wӷa produce a magnetic field perpendicular to the electric field. The particles are either in microgravity or in a liquid which provides a buoyancy to neutralize the gravity. Therefore, they are randomly distributed in space before the application of electric and magnetic fields. The media have both dielectric constants ⑀ f and magnetic permeability f close to unity, while the particles have ⑀ p Ͼ⑀ f and p f . Such a system acts as both electrorheological ͑ER͒ fluid ͓3,4͔ and magnetorheological ͑MR͒ fluid ͓5͔. We take the electric field direction as the z direction and the magnetic field direction as the x direction.One candidate for our system is a suspension of titaniumcoated iron particles in silicon oil ͓6͔. Other interesting candidates include a slurry of superconducting spheres coated with insulating film in a liquified gas. In a static electric field, superconducting spheres, though having insulating surface, have a high effective ⑀ p . Meanwhile the Meissner effect makes the superconducting spheres a diamagnetic substance with p ϭ0. The research on ER fluids and MR fluids has found a variety of particles that can be polarized by both electric field and magnetic field ͓3...