Thin Film Nanophotonics For Next Generation Energy And Information Technology Applications

By tailoring optical properties of materials at the sub wavelength scale,new fields of metamaterial and metasurface have emerged that demonstrate remarkable control over electromagnetic fields.The control and manipulation of electromagnetic waves,however,commonly require structuring materials at the nanometer scale,something that is only possible via advanced nanofabrication processes with high fidelity.The stacking of ultra-thin dielectric films,semiconductors,and metal layers provided an alternative platform with similar or unique characteristics.The new era of thin films in photonics offers many new possibilities other than interference optical coatings.These applications include perfect and selective light absorption,structural coloring,gas sensing,and enhanced spontaneous emission,biosensing,energy,imaging applications and as superlenses.Additionally,the discovery of the intriguing optical properties of phase-change materials based on chalcogenides has created opportunities for photonic data storage again since the optical constants of these materials can be rapidly activated by thermal,optical,or electronic means.As these materials can be modulated rapidly and with versatility,they are ideally suited to reconfigurable photonic devices.Also,use of graphene as a potential active photonics material in thin film configuration.This thesis discusses the design strategy of smart,functional and tunable thin film-based metal-dielectric nano cavity for the next generation of thin film based nanophotonics devices.We present potential applications of these thin-film structures,including,spontaneous emission,optoelectronics,reconfigurable thin film nano-photonic devices,optical computing and thermal design management.Energy and optoelectronics are the two key sectors where thin films nanophotonics are crucial.Spontaneous emission is a fundamental phenomenon associated with the creation of light.It is essential for diverse applications ranging from miniature lasers and light-emitting diodes,to single photon sources for quantum information,and to solar energy harvesting.We numerically investigate the dynamic control over the spontaneous emission rate of quantum emitters using tunable HMMs.Tunability is due to PCMs being able to change phase optically or electrically,and to graphene’s ability to change conductivity with a chemical potential when externally biased.We investigate several materials as possible building blocks of tunable Hyperbolic Metamaterials(HMMs)to modulate the SE rate in the visible,NIR,and mid-IR wavelength ranges.The dispersion of a metal-dielectric thin film stack at a given frequency can undergo a topological transition from an elliptical to a hyperbolic dispersion by incorporating a tunable metal or dielectric film in the HMM.This transition modifies the local density of optical states of the emitter,hence,its emission rate.Due to PCMs’ tunable properties,PCMs are becoming increasingly important in optoelectronic applications.Exploiting reversible and sub-ns fast switching in PCM with huge contrast in optical constant,we proposed nano-cavity to provide near-perfect optical modulation.The modulator exhibits tunable,perfect,and multi-band absorption from visible to the near-infrared region(NIR).Interestingly,the designed cavity supports critical resonance in an ultrathin(～λ/15)Sb2S3 film with perfect,broadband,and tunable absorption due to high refractive index of Sb2S3.Also,as Sb2S3 shows significant contrast in the bandgap(Eg)upon phase transition from Crystalline(Cry)(Eg=2.01 eV)to Amorphous(Amp)(Eg=1.72 eV)phase and forms the lowest Schottky barrier with Au in its Amp phase compare to conventional semiconductors such as Si,MoS2 and TiO2.We demonstrate PCM based design as a hot electron photodetector.Proposed HEPD is tunable for the absorption and responsively in the range of 720 nm<λ<1250 nm for the Cry phase and 604 nm<λ<3542 nm for the Amp phase.We also introduced a novel scheme to switch between the single and double cavity by exploiting semiconductor to metal phase transition in PCM called VO2.The integration of VO2 as a coupling medium in the double cavity has increased the responsively up-to 50%upon phase transition to the metal phase.Further,we present a few thin film-based designs with spectrally and angularly selective responses.Such multilayer designs find application in radiative cooling(RC),thermal camouflage and thermal management.RC is a passive cooling approach that,in principle,requires no energy consumption and can significantly reduce the carbon footprint of the cooling sector and be a major thrust towards a global net zero carbon emission.Although it is an ancient cooling technique,the advent of nanophotonics modelling,tools,and fabrication led to major breakthroughs in radiative cooling technologies that could expand the domain of applications for radiative cooling after overcoming a few bottlenecks.For this purpose,we analyze the impact of angular selectivity on the radiative cooling performance of thermal emitters.We proposed a design strategy for a practical emitter,the emitter has a thickness of 9 μm,which is more than 2 orders of magnitude thinner and has 3 orders of magnitude less layers than previously proposed angularly selective thin films.We show that combining angular and spectral selectivity is necessary to reach deep subfreezing temperatures.While spectrally selective thermal emitters have poor cooling performance in humid environments,angular selective emitters can reach subfreezing temperatures even in high humidity levels.We show that angular selectivity is not robust to parasitic heating and requires stringent thermal management.Following that,we design a multilayer thin film-based design that combines both thermal camouflage and efficient thermal management via radiative cooling.Various thermal applications have attracted growing attention to camouflage technology.In particular,infrared(IR)camouflage is essential to effectively concealing high-temperature objects,however,it’s challenging because object’s thermal radiation is depending on the fourth power of the object’s temperature.IR camouflage with thermal management at high temperatures is demonstrated with efficient heat management.We design a smart and functional angular and spectrally selective coating made of multilayer thin films for thermal camouflage and efficient thermal management.This coating is combination of HMM,gradient epsilon near zero material(ENZ)and polymer called PEI on the top.HMM makes the coating visibly transparent and fully reflective in IR region.ENZ layers provide BZ mode(at higher angles～50 deg-80 deg)in atmospheric window 8-12μm and PEI provides absorption band in 5-8 μm non-atmospheric window.So,with this combination,it is possible to achieve efficient thermal management due to radiative cooling(5-8 μm nonatmospheric)due to polytherimide absorption band and due to gradient ENZ layer in atmospheric window(at higher angles～50 deg-80 deg).At the same time the coating can provide IR camouflage(8-14 μm atmospheric at lower angles until 50 deg).Hyperbolic metamaterial makes the coating visibly transparent(i.e.,like a dielectric)and reflective(i.e.,like a metal)in the IR region.Therefore,it is possible to achieve simultaneous radiative cooling in atmosphere in conjunction with non-atmospheric windows and IR camouflage,thus providing a more effective solution to manage heat in devices.As a result of our coating with silica aerogel layer,the surface temperature decreased from 800 K to 400 K in simulations.We conclude by discussing thin film’s potential in optical computing.There has recently been a surge of interest in performing wave based analogue computations that avoid analogue-to-digital conversion and allow massively parallel operation.In particular,novel schemes for wave based analogue computing have been proposed based on artificially engineered photonic structures,nanostructure based design such as metasurface.We proposed a lithographic free thin film-based metal-dielectric cavity design for bright and dark mode imaging by tuning wavelength.Such a design could be directly applied to microscopy and serve as a very useful tool in biological imaging.The concept of spatial differentiation plays an important role in applications for example edge-based segmentation and image sharpening.Gradient(1st order)and Laplacian(2nd order)are the simplest derivative operators in two dimensions,and thus plays an essential role in these applications.It is possible to implement spatial differentiation electronically.However,digital computations prove challenging to perform in real-time applications requiring high-throughput image differentiation.It may be possible to overcome these challenges using optical analog computing,which is likely to offer high-throughput along with low-energy operations.Prior spatial differentiation research has focused on either one-dimensional or reflection mode nanophotonics structures,while transmission mode operation is more preferable due to its direct implementation with traditional image processing and recognition systems.In addition,transmission mode-based image differentiators are often made of nanostructures,metasurface,or gratings,or thin film-based image differentiators require proper optimization due to the multiple layers involved in their design(>23),resulting in a high overall thickness.We demonstrate and discuss,theoretically and experimentally an ultrathin(～600 nm)image differentiator with a numerical aperture of near one working in transmission as well as in reflection mode.With this ultrathin metal-and-dielectric coating,bright and dark field imaging can be achieved using both reflection and transmission modes with different operating wavelengths.