Metamaterials With Phase-change Materials For Optical Manipulation Of Absorption And Chiroptical Responses

Metamaterials?MMs?are artificial,engineered materials with rationally designed compositions and arrangements of nanostructured building blocks.These materials can be tailored for almost any applications because of their extraordinary response to force,heat,light,acoustics,electricity and magnetism,and thus become one of the most exciting topics in the science of light.Although MMs have been widely studied for more than 20 years,they flourish rapidly in recent years thanks to combined advances in nanofabrication,numerical modeling,and characterization tools.Phase-change materials?PCMs?,such as VO₂ and Ge₂Sb₂Te₅?GST?,representing one of the best candidates for future electronic FLASH memory,have been widely used in commercial optical disks and rewritable optical data storage media.PCM is a prototypical and smart material,which undergoes a thermally driven phase transition that is accompanied by changes in light,electricity and magnetic properties.Benefiting from these special properties,PCMs have found a wide range of applications in many fields,such as optoelectronic devices.However,there are still some problems for PCMs in the integration of optoelectronic devices,nano-processing technology and performance-characterizations.Therefore,more studies are still needed to promote the further development of active optoelectronic devices based on PCMs.In this thesis,we demonstrate our research works on 3D functional metamaterials based on PCMs from three aspects,including ultra-broadband metamaterial absorbers,ultrafast controllable photothermal devices and 3D novel chiral metamaterials.First,wenumericallydemonstrateanovelultra-broadband polarization-independent metamaterial perfect absorber in the visible and near-infrared region involving the phase-change material Ge₂Sb₂Te₅?GST?.The novel perfect absorber scheme consists of an array of high-index strong-absorbance GST square resonators separated from a continuous Au substrate by a low-index lossless dielectric layer?silica?and a high-index GST planar cavity.Three absorption peaks with the maximal absorbance up to 99.94%and the integrated absorbance up to92.92%in the whole wavelength range are achieved,owing to the excitation of plasmon-like dipolar or quadrupole resonances from the high-index GST resonators and cavity resonances generated by the GST planar cavity.The intensities and positions of the absorption peaks show strong dependence on the structural parameters.A heat transfer model is used to investigate the temporal variation of temperature within the GST region.The results show that the temperature of amorphous GST can reach up to 433 K of the phase transition temperature from room temperature in just 0.37 ns with a relatively low incident light intensity of 1.11×10⁸W?m²,due to the enhanced ultra-broadband light absorbance through strong plasmon resonances and cavity resonance in the absorber.Additionally,when switching the GST thin film to crystalline phase,the integrated absorbance for the design over the whole wavelength range can maintain 86.2%.Hence,the study suggests a feasible means to lower the power requirements for photonic devices based on a thermal phase change via engineering ultra-broadband light absorbers and verifies the feasibility of the proposed design in the applications of solar cells and thermal photovoltaics.Second,switchable nanoscale devices can be implemented in heterostructures that integrate plasmonic nanostructures with functional active materials and hence hold great potential for nanoscale-integrated circuits.The phase-change material of vanadium dioxide?VO₂?has reversibly switchable optical/electrical properties and huge contrast in its refractive index within the infrared spectral range between insulator and metallic states.In this work,we numerically demonstrate all-optical manipulation of switchable absorption effect using the heterostructure incorporating the plasmonic resonance of Au nanoantennas with vanadium dioxide.Compared with the planar control device?without Au nanoantennas?,the proposed design exhibits a pronounced resonant field enhancement as well as polarization-insensitive and omnidirectional absorption response.Meanwhile,the proposed device shows a large switching contrast?from⁹9.9 to¹0%in absorption efficiency?at the mid-infrared wavelength of 3609 nm.Interestingly,the resonance of the proposed device can be continuously tuned by varying the side length of the antennas or governing the metallization level of vanadium dioxide layer.The photothermal mechanism is further investigated by numerical model calculations,indicating that the resonant,antenna-mediated local heating occurs on a sub-nanosecond time scale of 0.26 ns under a quite low incident intensity of 1.9×10⁶ W/m²,which is about 12.5 times reduced with respect to that of the reported control devices.Therefore,the hybrid strategy of plasmonic antennas and vanadium dioxide provides a conceptual framework of switchable metamaterials for actively steering in ultrafast,energy-efficient electronic and photonic devices.Third,chiral metamaterials with versatile designs can exhibit orders of magnitude enhancement in chiroptical response compared with that of the natural chiral media.Here,we propose an ease-of-fabrication three-dimensional?3D?chiral metamaterial consisting of vertical asymmetric plate-shape resonators along a planar air hole array with extraordinary optical transmission.It is theoretically predicted and experimentally proved that the chiral metamaterials simultaneously support five Fano resonance states and exhibit giant circular dichroism?CD?with a contrast value as large as 1.6 due to the distinctive local electric field distributions.More interestingly,a“bridge”in the proposed double-plate-based architectures can act as a flipped ruler that is able to continuously manipulate optical chirality including the handedness-selective enhancement and the switching of CD signals.These features are promising for the improvement of chiral optical systems,such as ultrasensitive biosensing,polarimetric imaging,quantum information processing,and so forth.Fourth,we propose two novel schemes that utilize phase-change material of GST to achieve efficient circular conversion dichroism?CCD?and to manipulate circular dichroism?CD?.The unit cells of the two schemes are both composed of two misaligned rectangular holes supporting extraordinary optical transmission and vertical GST plate-shaped resonators standing on the two edges of two misaligned rectangular holes.By means of focused-ion-beam irradiation-induced folding,the vertical GST plate-shaped resonators could be fabricated.The difference between the two schemes is that the two dislocated rectangular air holes do not overlap in the first scheme and we call it NICMM,while overlap partially in the second scheme and we call it ICMM.For the first scheme,prior to phase transition of GST,NICMM can achieve robust Fano resonances excited by the linear polarized light.Meanwhile,for NICMM,left-and right-circularly polarized light transmit quite differently,indicating an enormous CD,about 0.8.However,when amorphous GST is transformed into crystalline state,the circular dichroism of NICMM was significantly reduced to only 0.3,which did not meet actual needs.The analysis of circular polarization states of the transmitted light reveals that under the right-handed circularly polarized light?RCP?,NICMM can effectively convert the right-handed circularly polarized light into the left-handed circularly polarized light?LCP?and CCD can reach up to 93.6%.Furthermore,the right-handed circular polarization conversion efficiency T_rl in the wavelength range of 2405 to 2530 nm far exceeds the theoretical limit of 0.25 for that of single-layer planar chiral metamaterials and T_rll obtains its maximum value of 0.53 at the wavelength of 2435 nm.All proves that the proposed NICMM can effectively improve CCD,and thus can be applied to many fields,such as circular polarization converters.For the second scheme,ICMM with amorphous GST can also achieve robust Fano resonance under the linearly polarized light and obtain huge CD under circular polarized light.Specially,when amorphous GST is transformed into crystalline state,ICMM still maintains huge CD although the CD spectrum redshifts.Notably,the full width at half maximum?FWHM?of the CD spectrum broadens obviously.Therefore,the proposed ICMM has much potential in several fields,such as chiral manipulations and detections.