Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.

Light strongly interacts with structures that are of a similar scale to its wavelength, typically nanoscale features for light in the visible spectrum. However, the optical response of these nanostructures is usually fixed during the fabrication. Phase change materials offer a way to tune the properties of these structures in nanoseconds. Until now, phase change active photonics has used materials that strongly absorb visible light, which limits their application in the visible spectrum. In contrast, Sb2S3 is an underexplored phase change material with a bandgap that can be tuned in the visible spectrum from 2.0 to 1.7 eV. This tuneable bandgap is deliberately coupled to an optical resonator such that it responds dramatically in the visible spectrum to Sb2S3 reversible structural phase transitions. It is shown that this optical response can be triggered both optically and electrically. High‐speed reprogrammable Sb2S3 based photonic devices, such as those reported here, are likely to have wide applications in future intelligent photonic systems, holographic displays, and microspectrometers.

We report a broadband polarization-independent perfect absorber with wide-angle near unity absorbance in the visible regime. Our structure is composed of an array of thin Au squares separated from a continuous Au film by a phase change material (Ge2Sb2Te5) layer. It shows that the near perfect absorbance is flat and broad over a wide-angle incidence up to 80° for either transverse electric or magnetic polarization due to a high imaginary part of the dielectric permittivity of Ge2Sb2Te5. The electric field, magnetic field and current distributions in the absorber are investigated to explain the physical origin of the absorbance. Moreover, we carried out numerical simulations to investigate the temporal variation of temperature in the Ge2Sb2Te5 layer and to show that the temperature of amorphous Ge2Sb2Te5 can be raised from room temperature to > 433 K (amorphous-to-crystalline phase transition temperature) in just 0.37 ns with a low light intensity of 95 nW/μm2, owing to the enhanced broadband light absorbance through strong plasmonic resonances in the absorber. The proposed phase-change metamaterial provides a simple way to realize a broadband perfect absorber in the visible and near-infrared (NIR) regions and is important for a number of applications including thermally controlled photonic devices, solar energy conversion and optical data storage.

The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.

The limit to which the phase change memory material Ge(2)Sb(2)Te(5) can be scaled toward the smallest possible memory cell is investigated using structural and optical methodologies. The encapsulation material surrounding the Ge(2)Sb(2)Te(5) has an increasingly dominant effect on the material's ability to change phase, and a profound increase in the crystallization temperature is observed when the Ge(2)Sb(2)Te(5) layer is less than 6 nm thick. We have found that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress, we have maintained the bulk crystallization temperature in Ge(2)Sb(2)Te(5) films just 2 nm thick.

A novel optical switch operating at a wavelength of 1.55 µm and showing a 12 dB modulation depth is introduced. The device is implemented in a silicon microring resonator using an overcladding layer of the phase change data storage material Ge 2 Sb 2 Te 5 (GST), which exhibits high contrast in its optical properties upon transitions between its crystalline and amorphous structural phases. These transitions are triggered using a pulsed laser diode at λ = 975 nm and used to tune the resonant frequency of the microring resonator and the resultant modulation depth of the 1.55 µm transmitted light.The ever-increasing demand for high speed optical communication networks is driving the development of new photonic devices that can process optical signals in a reliable, low-cost manner. Among competing technologies, Si-based devices have emerged as one of the main candidates for such applications, and several devices, including modulators 1-5 , add-drop filters 6 and wavelength division multiplexers (WDM) 7 have already been demonstrated. An important branch of this technology is the ability to program reconfigurable optical circuits. Indeed, a reprogrammable optical circuit that can hold its configuration without an external continuous source is extremely desirable for a multitude of applications ranging from photonic routers to optical cognitive networks. Recently, new solutions for non-volatile photonic memories have been proposed, involving the use of phase-change materials (PCM) and microring resonators 8,9 .Herein, a non-volatile Si microring resonator optical switch is demonstrated. A thin film of the phasechange material 10 (PCM) Ge 2 Sb 2 Te 5 (GST), which is commonly encountered in optical and electrical data storage applications [11][12][13][14] , is used to switch the resonant frequency and Q-factor of the microring resonator. GST shows high optical contrast between its amorphous, covalently bonded, and crystalline, resonantly bonded, structural phases 15-18 (n cryst − n amorph = 2.5 ; k cryst − k amorph = 1 at 1.55 µm) 19 . Moreover, transitions between the two phases can take place on a sub-ns timescale 20,22 while the resulting final state is stable for several years. These characteristics deem this material appropriate for application in reconfigurable optical circuits.The device, shown in Fig. 1, consists of a Si microring resonator with a bend radius of 5 µm and a coupling region of 3 µm, on top of which a GST thin film with an area of 3×1.5 µm 2 has been deposited. A second Si a) miquel.rude@icfo.es microring with identical dimensions but free of GST is used as a reference during the measurements. A 200 nm gap separates both microrings from a Si strip waveguide (220×440 nm 2 ) with grating couplers 23 at both ends, which are used to deliver light into the device and monitor the transmitted spectrum using single-mode fibers (SMF).

Van der Waals heterostructure superlattices of Sb2 Te1 and GeTe are strain-engineered to promote switchable atomic disordering, which is confined to the GeTe layer. Careful control of the strain in the structures presents a new degree of freedom to design the properties of functional superlattice structures for data storage and photonics applications.

Strain engineering is an emerging route for tuning the bandgap, carrier mobility, chemical reactivity and diffusivity of materials. Here we show how strain can be used to control atomic diffusion in van der Waals heterostructures of two-dimensional (2D) crystals. We use strain to increase the diffusivity of Ge and Te atoms that are confined to 5 Å thick 2D planes within an Sb2Te3–GeTe van der Waals superlattice. The number of quintuple Sb2Te3 2D crystal layers dictates the strain in the GeTe layers and consequently its diffusive atomic disordering. By identifying four critical rules for the superlattice configuration we lay the foundation for a generalizable approach to the design of switchable van der Waals heterostructures. As Sb2Te3–GeTe is a topological insulator, we envision these rules enabling methods to control spin and topological properties of materials in reversible and energy efficient ways.