Combining Switchable Phase‐Change Materials and Phase‐Transition Materials for Thermally Regulated Smart Mid‐Infrared Modulators

Abstract

enables applications in optical components such as anti-reflective coatings, [1] optical filters, [2] and optical absorbers. [3] Generally, their optical properties can be tuned by designing the structural geometries and materials involved, which remain fixed after fabrication. Using phase-change materials (PCMs) and phase-transition materials (PTMs) as active optical layers provides the optical components with new features of switchable optical properties. PCMs have at least two (meta-)stable phases, that is, one amorphous phase and one or more crystalline phases with high contrast in their electrical and optical properties. [4] A PCM can be crystallized by a thermal, optical, or electrical stimulus which heats it above its glass transition temperature T g . The phase change is non-volatile, which means the switched phase is maintained even after the switching stimulus ends. [4,5] Common PCMs like GeTe and GeSbTe-derivatives are, for example, applied in rewritable data storage devices. [6][7][8] In contrast, PTMs also have a temperature-stimulated, but volatile phase transition, where the switched phase is only maintained at temperatures above its transition temperature (T c ). [9] These materials automatically change back to the original phases when cooled down below T c . Therefore, their optical and electrical properties change reversibly with the temperature. A commonly used example of PTMs is vanadium dioxide (VO 2 ), which undergoes a semiconductor-metal Phase-change materials (PCMs) and phase-transition materials (PTMs) both show a large contrast in their respective optical properties upon switching, enabling compact optical components with diverse functionalities like sensing, thermal imaging, and data recording. However, their switching properties differ significantly, that is, the switching is non-volatile for PCMs while volatile for PTMs. Here, new-generation smart mid-infrared modulators with switchable transmission, reflection, and absorption are demonstrated conceptually and experimentally, which combine one PCM (Ge 3 Sb 2 Te 6 or In 3 SbTe 2 ) with one PTM (VO 2 ) as two active layers. The bottom VO 2 layer is employed as a thermally regulated (modulated) dynamic mirror, facilitating the switching of transmission between "on" state (using VO 2 in its semiconducting state at temperatures below its phase transition temperature T c ) and "off" state (metallic VO 2 at temperatures above T c ). The PCM layer on top of the metallic VO 2 layer is used either for continuously adjusting the absorption peak spectrally (by up to 1.8 µm using different phases of Ge 3 Sb 2 Te 6 ) or for switching between absorption mode (A = 0.99 with amorphous In 3 SbTe 2 ) and reflection mode (R = 0.85 with crystalline In 3 SbTe 2 ). The presented concept of merging static, non-volatile thermal switching (via PCMs) with dynamic, volatile thermal modulation (via PTMs) empowers a new generation of optical devices for smart optical switching, for example in spectrally tunable safety optical switches.

Nonvolatile High-Contrast Whole Long-Wave Infrared Emissivity Switching Based on In3SbTe2

“…While other PCMs change from dielectrics ( n a > k a ) to dielectrics ( n c > k c ) with higher refractive indices upon crystallization, IST changes from a lossless dielectric to a lossy metal (for IST at 10 μm wavelength, n a ≈ 3.8, k a ≈ 0; n c ≈ 12.6, k c ≈ 18.1), providing a tremendous contrast in refractive index (Figure b and Figure S1). , The two emitters possess the following advantages: (i) realizing negative and positive differential thermal emissivity over the whole LWIR range; (ii) exhibiting higher emissivity contrasts ( Δε N,8–14 μm ≈ −0.75, Δε P,8–14 μm ≈ 0.83) compared to previous works (Figure a); (iii) being nonvolatile and thereby energy efficient; (iv) being film structured and thus possible for large-scale fabrication.…”

Broadband hyperbolic thermal metasurfaces based on the plasmonic phase-change material In3SbTe2

“…[19][20][21][22] In recent years, phase change materials (PCMs) have gained particular interest for the dynamic modulation of thermal radiation because of their distinct optical properties in amorphous and crystalline phases and their switchability, either thermally or optically, between these two states. [23][24][25][26][27][28][29] Furthermore, a mixture of amorphous and crystalline phases of PCM can be utilized to engineer the multi-level control of thermal radiation by changing the mixing ratio. [30][31][32] However, complications persist in the PCM-aided modulation of thermal radiation to realize a balance between different characteristics.…”

Reconfiguring Magnetic Infrared Resonances with the Plasmonic Phase-Change Material In3SbTe2

“…30 While the atoms are covalently bound in the amorphous phase, there is a unique bonding mechanism, called metavalent bonding, 30−34 in the crystalline phase. This change in bonding is the reason for the pronounced change of the refractive index of PCMs, which is key to many designs 35 like reconfigurable waveguides, 29 polariton optics, 36−39 tunable lenses, 40,41 polarization filters, 42 beam steerers, 41,43,44 light absorbers, 45,46 switches. 46,47 Furthermore, PCM thin films can be locally switched with a pulsed laser.…”

“…While the atoms are covalently bound in the amorphous phase, there is a unique bonding mechanism, called metavalent bonding, − in the crystalline phase. This change in bonding is the reason for the pronounced change of the refractive index of PCMs, which is key to many designs like reconfigurable waveguides, polariton optics, − tunable lenses, , polarization filters, beam steerers, ,, light absorbers, , and optical switches. , Furthermore, PCM thin films can be locally switched with a pulsed laser. This has been previously employed for reconfigurable diffractive MSs, , sub-diffraction polaritonic resonators, and, more recently, for programmable metallic MSs and dielectric Huygens’ MSs. , …”