Optical Property Regulation Of Chalcogenide Phase Change Materials And Their Applications In Color And Absorption Modulators

In the past five years,chalcogenide phase change materials（PCMs）have attracted much attention due to the fast and reversible phase transition,optical contrast between amorphous and crystalline phases,as well as their applications in color and absorption modulators.At present,Ge₂Sb₂Te₅ is one of the most studied and widely used PCMs,but there are some problems:（1）The reflectivity contrast of pure Ge₂Sb₂Te₅ and doped Ge₂Sb₂Te₅ is not high enough,which limits the improvement of the signal-to-noise ratio and modulation capability of the photonic devices;（2）Since the optical contrast of cubic-hexagonal phase transition of Ge₂Sb₂Te₅（only 2.8%）is much smaller than that of amorphous-cubic phase transition（11.4%）,the existing non-volatile photonics are on the basis of the amorphous-cubic phase transition and limit the switching states to two,which greatly limits the miniaturization and integration of photonic devices;（3）The dielectric-metallic transition（DMT）range of Ge₂Sb₂Te₅ is only limited to 400-615 nm,and the dielectric contrast ratio is low（16.7）,which cannot meet the urgent need for arbitrary tuning of optical performance in the whole visible range.In this paper,we address the problem that the performance of the conventional material Ge₂Sb₂Te₅and related devices is not satisfactory,carry out research on the design of new materials,and their applications to color and absorption modulators by experiment,first-principles calculation and electromagnetic field finite element simulation.First,we elucidate the influence mechanism of atomic composition on the optical properties of PCMs（Chapter 3）.Then based on this,we design and fabricate three new types of films that exhibit high reflectivity contrast（Chapter 4）,tertiary optical switching states（Chapter 5）,and dielectric-metallic transition across the whole visible range（Chapter 6）,respectively.Finally,the three new types of films are applied to non-volatile color modulators and absorption modulators,resolving the problems of Ge₂Sb₂Te₅-based photonic devices including low signal-to-noise ratio and poor switching performance.The main research contents and conclusions are as follows:1.By analyzing the reflectivity of（Ge Te）_x（Sb₂Te₃）_1-x with different atomic compositions,we elucidate the new mechanism of atomic composition on the optical properties of PCMs.Compared with Sb₂Te₃,Ge Sb₄Te₇,Ge₂Sb₂Te₅ and Ge₃Sb₂Te₆,Ge Te has higher reflectivity contrast and higher crystalline extinction coefficient in the visible range.This is because crystalline Ge Te is of the structure with non-stoichiometric vacancy-free,leading to high structural order and delocalization of electrons.This study reveals the intrinsic relationship between atomic composition and reflectivity in the crystalline phase,and provides another idea for regulating the reflectivity of PCMs.2.By introducing a small amount of Sb in the fifth main group into Ge Te,we obtain Ge_0.9Sb_0.1Te with high reflectivity contrast（50%）far higher than conventional materials Ge₂Sb₂Te₅（22%）.The high reflectivity contrast of Ge_0.9Sb_0.1Te originates from the unique combination of high structural order which stems from its crystalline structure with non-stoichiometric vacancy-free,and narrow optical bandgap which arises from the formation of weak Sb-Te bonds.Furthermore,we find that Ge_0.9Sb_0.1Te-based color modulators and absorption modulators have much higher resonance tuning capabilities than conventional modulators.This study provides a new material design strategy for PCMs with high optical contrast.3.By introducing small interstitial atoms N into Ge₂Sb₂Te₅,we achieve the tertiary optical switching states and develop the color and absorption modulators based on amorphous-cubic-hexagonal phase,which improves the switching performance of Ge₂Sb₂Te₅-based modulators by 50%.This is attributed to the fact that interstitial atomic N significantly reduces the structural order of amorphous and cubic phases,while it has little effect on the structural order of hexagonal phase,thereby widening the structural order difference and optical contrast of cubic-hexagonal phase transtion close to that of the amorphous-cubic phase transition.This study provides material design guidelines and candidate materials for the development of multi-state nonvolatile photonic devices.4.PCMs with dielectric-metallic transition（DMT）are developed through the substitution of heavy atoms in the fourth main group in the Ge Te parent.We demonstrate that the DMT wavelength range and dielectric contrast of Ge_0.9Sn_0.1Te are 400-760 nm and 24.3,respectively,which are much higher than those of Ge₂Sb₂Te₅（400-615 nm and 16.7）.Through ultrashort-pulse laser induction,we show that the crystalline phase of Ge_0.9Sn_0.1Te can be arbitrarily written,erased,and modified.We obtain grating metasurfaces with alternating dielectric amorphous and metallic crystalline phases,whose resonance peaks and colors can be continuously modulated in the whole visible range.This study provides a new material platform and laser processing method for programmable metasurfaces.