Two dimensional (2D) materials have unique mechanical, optical, electrical and magnetic properties, which are strong and flexible, exhibit large exciton binding energy, cover a broad spectrum of bandgaps, have intensive light matter interactions and are particularly prone to many-body instabilities etc. The diversity provides unlimited possibilities to build various devices achieving different functionalities. To further tailor the properties of 2D materials, many methods have been utilized, including reducing dimensions, intercalation, alloying, gating, strain engineering and heterostructures. Heterostructures of 2D materials have demonstrated rich physics, and wide device applications. For lateral heterostructures with covalent bonding, the formation of misfit dislocations could be a big concern, which may dramatically affect the properties. In vertical heterostructures of 2D/2D and 2D/other dimensions, weak van der Waals interaction enables much wider material choices and functionalities.

This dissertation primarily consists of two projects related to 2D heterostructures. The 1st project is focusing on a large lattice mismatched (~6%) lateral Bi2Se3-Bi2Te3 heterostructure with periodic rippling. Taking advantage of the large lattice mismatch between the constituents, we demonstrate a 3D heterogeneous architecture combining a basal Bi2Se3 nanoplate and wavelike Bi2Te3 edges buckling up and down forming periodic ripples. Unlike 2D heterostructures directly grown on substrates, the solution-based synthesis allows the heterostructures to be free from substrate influence during the formation process. The balance between bending and in-plane strain energies gives rise to controllable rippling of the material. Our experimental results show clear evidence that the wavelengths and amplitudes of the ripples are dependent on both the widths and thicknesses of the rippled material, matching well with continuum mechanics analysis. The rippled Bi2Se3/Bi2Te3 heterojunction demonstrates a unique way to release the strain energy from lattice mismatch through out-of-plane deformation, and also provides a platform to enable nanoscale structure generation and associated photonic/electronic properties manipulation for optoelectronic and electro-mechanic applications.

The 2nd project is related to an ultracompact far-infrared (far-IR) spectrometer based on vertical hybrid heterostructure of InAs, h-BN and graphene. The principle of this ultracompact spectrometer relies on a new surface plasmon polaritons (SPPs) coupling mechanism on the non-structured InAs surface with gradient carrier concentration. When free space light illuminates on the InAs surface with a gradient of negative permittivity, SPPs can be excited and propagates along the planar interface. For every single wavelength, there would be a corresponding location, where ε_m=-ε_d and the surface wave vector reaches its maximum point, and both the group velocity and phase velocity approach zero. As a result, the electric field magnitude is strongly enhanced and localized at the nanoscale. With h-BN and graphene as dielectric spacer and photodetection media, the localized near-field enhancement from the gradient doped surface can be transformed into photocurrent generation, enabling applications such as photodetectors and wavelength sorting spectrometers. We have evaluated the performance of such spectrometer computationally, designed and fabricated the device from scratches, and for measurement, freestanding cross-shaped band pass filters are also designed and made. The ultracompact spectrometer demonstrates a direct application of the unique light coupling theory for rainbow trapping and enables new designs of plasmonic devices. More importantly, the compact spectrometer works within far-IR range without the need for complicated optical components and active cooling, which could be applied in wearable devices and smart phones.

Helicity, a geometric property rendering an object nonsuperimposable on its own mirror image is ubiquitous in Nature. Inorganic crystals can grow into helical form, which is of great interest from the perspective of fundamental material science as well as application. Various inorganic crystals can be grown into helically twisted form on different scales ranging from nanoscale to mesoscale and to macroscale. The natural growth of twisted macroscopic quartz crystals in the bulk form has been documented and studied for centuries. On the nanoscale, quantum dots (QD), nanowires and carbon nanotube into helical structures. Those helical crystals have intriguing optoelectronics properties including rotatory optical activity and circular dichroisms in both absorption and photoluminescence as well as unique stereoselectivity in chemical reactions. These properties render inorganic crystals good potential of applications in polarization optics, chiroptical sensing, enantioselective catalyst and biomedical imaging. The material science responsible for forming these twisted inorganic crystals, nevertheless, remains largely mysterious and elusive.

In recent years, twisted van der Waals materials with rotational stacking of two-dimensional materials have attracted tremendous attention. The twist angle strongly affects the electronic states, excitons and phonons of the twisted structures through interlayer coupling, giving rise to exotic optical, electric, excitonic and spintronic behaviors. In twisted bilayer graphene, at certain twist angles, long-range periodicity associated with moiré patterns introduces flat electronic bands and highly localized electronic states, resulting in Mott insulating behavior and superconductivity. Theoretical studies suggest that these twist-induced phenomena are common to layered materials such as transition-metal dichalcogenides, black phosphorus and germanium monoselenide. In contrast to electronic band structure of the twisted bilayer graphene, unique features such as one-dimensional flat electronic band may emerge in those twisted 2D materials with different structures and symmetries.

The ability to manipulate the twisting topology of van der Waals structures offers a new degree of freedom through which to tailor their electrical and optical properties. Twisted van der Waals materials are usually created using mechanical exfoliation and a transfer-stacking method, but limitations exist to extend this method to a variety of two-dimensional materials. In contrast, bottom-up growth methods could provide an alternative means to create twisted van der Waals structures. This dissertation explores the bottom-up synthesis of twisted van der Waals materials. We demonstrate that the Eshelby twist associated with a screw dislocation (a chiral topological defect), can drive the formation of twisted van der Waals materials.

The Eshelby twist is a continuous crystallographic twist generated by the torsional force of an axial screw dislocation in a one-dimensional structure. It has been shown to result in growth of helically twisted nanowires of various materials. This mechanism potentially provides a means to create twisted van der Waals (vdW) structures. Materials such as germanium sulfide (GeS) can grow into nanowires along the vdW stacking direction (the cross-plane direction), and introducing Eshelby twist into such nanowires naturally leads to twist between the successive layers.

We synthesized twisted GeS crystals at both nanoscale and mesocscale. In the synthesis method, GeS nanowires with axial screw dislocations are first grown along the stacking direction, yielding vdW nanowires with Eshelby twist. These wires possess continuous twists in which the total twist rates are defined by the radii of the nanowires, consistent with Eshelby’s theory. Further radial growth of those twisted nanowires that are attached to the substrate leads to an increase in elastic energy, as the total twist rate is fixed by the substrate. The stored elastic energy can be reduced by accommodating the fixed twist rate in a series of discrete jumps in the twisting profile. This yields mesoscale twisting structures consisting of a helical assembly of nanoplates demarcated by atomically sharp interfaces with a range of twist angles.

The twisting profiles of the structures are tunable. We show that the twisting profile can be tailored by controlling the radial size of the structure. The twisting morphology gradually transitions from initial continuous twisting to intermediate twisting (consisting of both continuous twisting between the twist boundaries and discrete twisting at the boundaries) and eventually to discrete twisting with increasing radial size. This allows us to control the twisting profile and angles at twist interfaces by controlling the radial growth of the structure.

We also demonstrate that the twist rate and period can be tailored by tailoring the radii of the dislocated nanowires first grown in the VLS process. This is achieved by adding GeSe into the growth, which modulates the size of the droplets catalyzing the VLS process, therefore modulating the radii of the nanowires. The chemical modulation demonstrates good potential to tailor the twist rate and period of helical vdW crystals, enabling a new freedom to modulate optoelectronic properties and chiral light-matter interactions.

Last, we explored the anisotropic propagation of surface phonon polaritons in GeS, enabled by the strong in-plane anisotropy of the crystal structure. We demonstrate that GeS thin films at thickness of tens of nanometers support strongly anisotropic surface phonon polaritons in the far-infrared range. The dispersion relations of these phonon polaritons can be tuned by varying the thickness of the GeS film.

Since its inception, the field of valleytronics has always carried the hopes of generating an additional degree of freedom to electronic and spintronic devices, allowing complementary usage of these functions in a single device and hence greatly increasing the processing power. However, several candidates that presumably possess potential in valley manipulation have since fall short of being practical.

Valleytronics is the control of population at “valleys” of certain qualifying materials, and valleys are essentially different conduction band minima or, in general, band gaps that can be distinctively selected by certain selection rules. In diamond-structured materials, this means forcing electrons out of one of the three pairs of degenerate conduction band minima along <100> by magnetic or electric field. For transition metal dichalcogenides, this means using circularly polarized light to selectively excite one of the two band gaps at the ±K points. Both systems require some sort of bias, low temperature, and/ or stringent material quality/ size, making them impractical. Following the hype on layered materials, from graphene to transition metal dichalcogenides, and anisotropic materials, such as black phosphorus, we decided to explore a close analogue in IV-VI systems. Anisotropy is rather common, but IV-VI systems have the unique additional attribute of having two local band gaps that are along high symmetry lines in reciprocal space. Because the global band gap is not at a high symmetry point, the two relevant band gaps becomes significant and the behaviour near these band edges are then well-defined and well-separated by symmetry due to the anisotropic nature of the bandstructure. Using bulk Tin (II) Sulfide (SnS) as a novel system, we found that the two band gaps along ΓX (smaller energy) and ΓY (larger energy) are respectively only excited by light polarized along the x (zigzag) and y (armchair) directions. The implication of this is that we now have a valleytronic system that is inherently polarized and can be activated via light-matter interaction without material or experimental conditional constraints. The non-degeneracy in the band gap values also means that we can decouple the valley degree of freedom with the excitation energy, making the system more robust.

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Theoretical efforts have allowed us to understand this novel form of valleytronics behaviour. We found that the conduction band minima are predominantly made up of 5p orbitals from Sn, while the valence band maxima are mainly composed of 3p orbitals of S atoms. The exact symmetries of the orbitals that make up the bands allow the simplification of transition dipole moment integral analyses, which gives the selection rules. As a proof of concept that such valleytronics can be generalized, we also investigated the effect on SnSxSe1-x and showed both the tunability of both band gaps as well as the retainment of the valleytronics effect across the compositions. The band gap tunability is continuous, a consequence of the hybridization of Se 4p orbitals and S 3p orbitals. Similarly, the retainment of symmetry across all compositions means that the polarization degree, an important figure of merit in valleytronic system, is not compromised.

Finally, we fabricated optoelectronic devices out of SnS with the general configuration of photodetectors, where two pairs of electrodes were placed along the x and y axes respectively. We then subsequently probed the photocurrent of the devices with a combination of excitation wavelengths (above, in between, and below the two band gaps), excitation polarization (along x and y), and current direction (along x and y), obtaining distinctly different photocurrent response. This is the first demonstration of a device that utilizes the valleytronics effect to control current flow, paving the way to a multi-digit switch that can provide additional degrees of freedom per device. Through this multi-digit device, we also identified the key limitations and fundamental mechanisms contributing to the lack of perfect valleytronics selectivity in the photoresponse and propose critical areas of improvements of the system.

In conclusion, we identified a novel form of valleytronics that is more practical than its previous counterpart. Our demonstration includes light-matter interaction and photoresponse of SnS, and also generalization in the SnSxSe1-x system. We acknowledge the limitations of our system but remain optimistic about the potential of such a form of symmetry-driven valleytronics, specifically when key limitations such as growth methods are solved, and/ or the quest of multifunctional materials have been fulfilled.

Light Propagation and Imaging in Indefinite Metamaterials

by

Jie Yao

Doctor of Philosophy in Applied Science & Technology

University of California, Berkeley

Professor Xiang Zhang, Chair

Unlike any traditional materials, indefinite materials possess very unique optical properties. The principal components of its permittivity and/or permeability tensors are not all with the same signs, resulting in a hyperbolic dispersion relation. Such extraordinary dispersion leads to a number of interesting phenomena, such as negative refraction of light at its interface with normal materials, the propagation of electromagnetic waves with very large wave vectors which are evanescent in natural dielectric materials.

Due to the development of metamaterial research and nanofabrication techniques, we have successfully created a type of metamaterial made of parallel silver nanowires embedded in porous alumina matrix. Both effective media theory and finite element simulations showed that such metamaterial possesses indefinite permittivities. We performed optical measurements of visible and infrared light transmitted through such metamaterials. By using an NSOM system capable of mapping the light field distribution with sub-wavelength resolution at both the surface of the nanowire metamaterial and the 3D space near the surface, we have demonstrated all-angle negative refraction and slab lens imaging of visible light in nanowire metamaterials.

The nanowire metamaterials hold the indefinite permittivity over a broad frequency range from visible to infrared, therefore negative refraction phenomena are not restricted in a narrow band any more. By avoiding magnetic resonance, the nanowire metamaterials show substantially reduced energy loss and make negative refraction possible in bulk materials.

Since the indefinite materials support the propagation of very large k waves, deep sub-wavelength 3D nanocavities can be made from them. Theoretically there is no lower limit to the size of the cavity with idea indefinite materials. Unlike traditional dielectric cavities which resonate at higher frequencies when the cavity size reduces, indefinite cavities with sizes several orders different may resonate at the same frequency and same mode order. Furthermore, the size dependence of quality factor due to the radiation loss also shows an inversed behavior compared to traditional dielectric cavities.

Another interesting phenomenon we have discovered is the abnormal diffraction inside the nanowire metamaterials. The hyperbolic iso-frequency contours of such materials are "flatter" for light with longer wavelengths, because of the strong dispersive behavior of the metal in the metamterials. Since the Poynting vector is normal to the iso-frequency contour, the diverging angle of diffracted light inside the nanowire metamterial decreases as the wavelength gets longer, which is the opposite to the diffraction in dielectrics. The flat iso-frequency contour together with the large k wave transmission may lead to the possibility of deep sub-wavelength imaging or lithography with the nanowire metamaterials.

Besides the research on indefinite metamaterials, I also studied the "2D optics" of surface plasmons. Based on the interference of surface plasmons, sub-wavelength patterns can be lithographically created on a metal surface. We also predicted the existence of surface plasmon beats formed by the interference of two different modes excited on the same thin metal film.

Material topology is an exotic degree of freedom in the condensed matter physics. It was initially proposed to explain the electron transportation in ferroelectric materials in 1954, and eventually made its way to the 2016 Nobel Prize in Physics. In recent decades, researchers are dedicated to transplanting the concepts in the condensed matter to the optics and photonics realm. Nowadays, the field of topological photonics is thriving, which allows us to mimic the behaviors of the electrons using photons and discover phenomena exclusive to the Bosonic systems.

In this dissertation, I present my work to show how we can get inspirations from the electronic system, then design the photonic device with new functionalities. Chapter 1 is dedicated to the cornerstone of material topology: geometric phase. The geometric phase determines the topological invariant in a crystal, but we can also use this concept to engineer the phase front in the diffractive optics devices. I show the first-ever metasurface for ultra-violet wavelengths using the geometric phase. In Chapter 2, I discuss the one-dimensional topological insulator and how to realize it in an optical waveguide array. Besides, I reveal the relation between the one-dimensional model and the topological edge states in the two-dimensional nanoribbons. It deepens our understanding of topological behaviors and the Bloch theorem. Finally, in Chapter 3, I investigate the bound-state-in-continuum, which is a topological singularity in the photonic crystals. Then I show the photonic integrated circuits that utilize this concept and result in a versatile optical filter. All the devices proposed are made of silicon, which is a promising material choice in terms of fabrication and scalability.

In general, I introduce the essential concepts in topological photonics and explain the physical pictures to my best knowledge, in the hope of inspiring readers to explore this field and design novel photonic devices.

The recent discoveries of ferroic properties, such as ferromagnetism and ferroelectricity, in two-dimensional van der Waals (2D vdW) materials have generated a surge of interest in the exploration of fundamental interaction in reduced dimensions and functional device applications.

On one hand, ferromagnetism in 2D vdW crystals enables better understanding of fundamental interactions in ultra-thin condensed matter systems. However, existing material candidates face limitations, such as the lack of room-temperature magnetic ordering. Here we report the room-temperature ferromagnetism in Co-doped graphene-like ZnO, a novel chemically stable 2D material, down to single-atom thickness. Through magneto-optic Kerr effect and superconducting quantum interference device measurements, we identify the spontaneous magnetization in such systems at room temperature and above. X-ray characterization shows that the Co atoms form Co2+ states in graphitic ZnO. By varying the Co doping level, we observe an exotic manipulation among paramagnetic, ferromagnetic, and disordered phases. Our development of Co-doped 2D ZnO opens an appealing path to 2D ferromagnetism persisting at room temperature with unique controllability.

On the other hand, the stabilization of 2D ferroelectricity overcomes the thickness limit. In contrast with conventional 3D ferroelectrics, 2D ferroelectrics show great advantages, including but not limited to, high mobility, perfect interface, allowing for non-destructive reading, being fatigueless. In this dissertation, we report the physical vapor deposition (PVD) of 2D vdW SnSe directly onto the silicon substrate. The synthesis process occurs at relatively low temperatures; the deposition temperature is 250 ℃. By varying the time of synthesis, we are able to manipulate the thickness of SnSe single crystals, which spans in a large range from 6 nm to the bulk. Out-of-plane ferroelectricity is detected in SnSe at room temperature (300 K) by means of piezoresponse force microscopy (PFM). More importantly, the ferroelectric switching voltage scales linearly with the sample thickness. A 6-nm-thick SnSe sheet displays sub-0.3 V switching, which is substantially smaller than those of previously reported 2D ferroelectrics and hafnium oxides. Moreover, the ferroelectric mechanisms are unraveled by transmission electron microscopy (TEM); AA stacking of vdW layers and interlayer sliding give rise to the vertical ferroelectric polarization. The ferroelectric dipole, as a new degree of freedom, can effectively manipulate the optical and electrical behaviors in SnSe. These results indicate that 2D SnSe is a promising candidate for future energy-efficient and non-volatile electronics or optoelectronics, which is highly compatible with the emergent complementary metal oxide semiconductor (CMOS) technologies.

The physical properties of crystalized materials are closely related to the symmetry elements of the point group of the crystal. To the contrary, engineered symmetry breaking would allow the emergence of new physical phenomena which are originally forbidden. With the successful exfoliation of two-dimensional Van der Waals materials, the restricted stacking sequence between layers is relaxed and thus, the symmetry of artificially stacked heterostructure can be readily controlled.

In this dissertation, we present two approaches to engineer the symmetry of solid-state and photonic two-dimensional systems respectively. The resulting optical and topological property changes are explored and related back to the underlying symmetry breaking. Chapter 1 gives an overview of the various symmetry breaking mechanisms and their distinct consequences. The emphasis is given to the inversion-symmetry breaking which can be easily implemented in two-dimensional systems via structural or electrical means. In Chapter 2, we show that introducing a twisting between two layers of graphene, the symmetry of bilayer structure is decisively changed, and new nonlinear optical effects are allowable. We further verify that the observed nonlinear signals are controlled by the composited bilayer instead of the linear combination of two individual layers experimentally and confirmed by DFT calculations. The methodology proposed is generic and can be applied to other composite systems. In Chapter 3, we break the symmetry of two-dimensional photonic lattice by ‘tilting’ the constitutive parameter tensors and demonstrate spin degeneracy lifting via electromagnetic simulations. The approach allows us to enlarge the scale of spin-orbit coupling effect in a photonic platform and at the same time, the controllability of spin-polarized energy bands associated with symmetry is revealed.

To sum up, we propose two methods to engineer the symmetry of the 2D system in a controllable way and show their potential for new physical properties and applications.

Abstract

Graphite Oxide Template Based Synthesis and Characterization of Metal Oxide Nanosheets

by

Kyle B. Tom

Doctor of Philosophy in

Engineering – Materials Science and Engineering

University of California, Berkeley

Professor Jie Yao, Chair

Two dimensional materials and their composite structures show unique and favorable properties for many different kinds of applications and physical studies. Isolated 2D materials have shown a unique array of physical phenomena, including the room temperature observation of the quantum Hall effect, as well as enhanced physical properties, including large exciton energies and extremely high carrier mobilities. Composites have shown enhanced performance in traditional applications, including enhanced photocatalysis and battery performance, that stems from a synergistic effect between the individual materials in the composite. However, there is a limited number of naturally layered materials, an even smaller subset that is stable in ambient conditions. As such, it is beneficial to find 2D materials with better properties that may originate from other types of crystals. It has been predicted certain types of crystal structures, such as wurtzite, will form a two-dimensional phase similar to αBN below a particular thickness threshold. ZnO, and its graphitic form (gZnO) is of particular interest due to its favorable optical properties and oxidation resistance. gZnO has also been shown to have superior piezoelectric performance over similarly structured most other 2D materials, including BN and transition metal dichalcogenides (TMDCs). It also has shown excellent properties for many other applications that range from magnetism to catalysis. However, forming such confined systems is difficult and has limited the lateral size significantly. This has limited experimental work on this system, prohibiting application and basic understanding.

This dissertation explores the use of a graphite oxide template technique that confines the growth direction to synthesize multiple metal oxide nanosheets, with specific focus on sub-nm ZnO. Graphite oxide, a chemically functionalized form of graphene, has many properties that are favorable for use as a template. The interlayer distance ranges from 0.6 – 1.2 nm, creating a nanosized reactor. Additionally, graphite oxide templates are hydrophilic; this allows templates to be easily to synthesized in water by sonication and helps aqueous precursors be drawn in between layers. Because of these advantages, as well as the large template size and uniformity, graphite oxide offers a unique opportunity for nanosheet synthesis.

This method shows growth of nanosheets in the tens of microns size. The structure of these composites and the nanosheets are presented here, including TEM side view images of the stacking and fine geometry verification of the gZnO transition using XANES and thorough simulation work. Changes in the optoelectronic properties are determined through various measurements including XAS, EELS, XPS, and electronic transport. In particular, gZnO has a massive increase in the bandgap and a strong enhancement in the Zn 4s state availability. Improvements and advantages of this method are shown, including the compatibility with plasma cleaning and doping.

In addition to van der Waals materials and thickness dependent structures, a significant amount of work has been focused on taking typical 3D crystals and growing them into two dimensions. This technique offered a way to synthesize some of these nanosheets. Fe2O3 was used as a test case and showed similar lateral sizes to the gZnO. Composites of the synthesized Fe2O3 nanosheets and the rGO showed strong interactions between the two, leading to some unique properties. The rGO transfers a significant amount of charge to the Fe2O3, most likely as a way to passivate the dangling bonds in the crystal. The XAS spectra of composites show significant formation of Fe+2 like states, making the hematite appear more like magnetite or maghemite. The magnetic properties of Fe2O3 are measured using SQUID and XMCD. Composites show enhanced coercivity and saturation, but no change in the overall magnetic structure.

Other materials were also grown using this method, including doped and alloyed gZnO, NiO, and MgO. Various changes were observed, including strong changes in the core level shifts by XPS and the absence of an interlayer between SiO2 and rGO. Some limits of this technique were also found, as Al2O3 and CuO showed distinct nanoparticle formation instead of nanosheet formation. Finally, a rule of thumb is presented for materials that are compatible with this technique in hopes that it can be further refined and used as a guide for future work in this direction.

Light can carry spin angular momentum (SAM) and orbital angular momentum (OAM). OAM, as a new degree of freedom, has played an important role in lots of fields and leads to promising applications. Understanding how the SAM and OAM interact and convert to each other can pave the road of OAM application. To understand the mechanism, I explored spin to orbital angular momentum conversion in:1. self-assembled concentric ring of gold nanoparticles and block co-polymer system, where the circularly polarized light is converted to light carry OAM by the plasmonic effect of Au. 2. the photonic crystal mimicking hexagonal structure system, where we try to understand the spin-orbit interaction from K space and real space point of view. 3. valleytronic system to see whether the involvement of OAM will change the selection rule. 4. twisted waveguide nanowires generating OAM. These fundamental research projects will help with a better understanding about the new twist degree of freedom OAM in the nano-photonics system and will help pave the road of OAM applications.

Besides the optical wave, I also explored the acoustic wave in one of my projects by using interdigital transducer generated surface acoustic wave. Surface acoustic wave has similarity with respect to surface plasmonic polariton. And in this project, I explore how the surface acoustic wave can interact with matter and mediate the assembly of nanoparticles and polymer system. We found the surface acoustic wave can help form micro-sized structure and help with the formation of hierarchical structure. The research project reveals the potential power of using surface acoustic wave to control assembly and to explore the fundamental material science topics.

Properly structuring materials at subwavelength scale allows for strong light-matter interaction, thereby enhancing near-field effects and engineering far-field scattering through intermodal interference. A majority of such effects are associated with plasmonics where electromagnetic waves created in the vicinity of metallic nanostructures is able to give rise to a variety of novel phenomena and fascinating applications. In the recent years, dielectric nanoparticles with high refractive index based on optically induced electric and magnetic Mie resonances attract a plethora of attention. In this rapidly developing field, dissipative loss in optical materials is considered one of the major challenges. Here, in this dissertation, we show that, counter-intuitively, it contributes positively to sub-wavelength scale light enhancement and confinement, and also improves scattering efficiency in the far field.

In the first part of this dissertation, near field enhancement in dissipative dielectric antennas is demonstrated to be orders of magnitude higher than their lossless dielectric counterparts, which is particularly favorable in deep UV applications where metals are plasmonically inactive and transparent dielectrics always have low index. The loss facilitated field enhancement is the result of large material permittivity contrast and electric field discontinuity. These dissipative dielectric nanostructures can be easily achieved with a great variety of dielectrics at their Lorentz oscillation frequencies, thus having the potential to build a completely new material platform boosting light-matter interaction over broader frequency ranges, with advantages such as bio-compatibility, CMOS compatibility and harsh environment endurance.

Additionally, manipulation of ultra-violet light through metasurface in the far field utilizing the silicon loss is then presented. We experimentally demonstrate Si metasurfaces working effectively over a broad band down to 290nm, with efficiencies comparable to plasmonic metasurface performance in the infrared regime. And for the first time, we show photolithography enabled by metasurface-generated ultraviolet holograms. We attribute such performance enhancement to the large scattering cross-sections of Si antennas in the ultraviolet range, which is adequately modeled via a circuit model. Our new platform will deepen our understanding of the role of material dissipation and introduce even more material options to broadband metaphotonic applications, including those in integrated photonics and holographic lithography technologies.

Dynamically tunable far field with subwavlength nanostructures is always desired for practical applications. In the last section of this dissertation, we introduce a lithography free and field-programmable photonic metacanvas. Previous attempts of realizing such idea used micro-mechanical metamaterials or amorphous-crystalline phase transition materials, which are limited in terms of the functionalities, efficiency, cost, and high working temperature (> 600oC). It is much desired to reconfigure photonic devices in a fast, large-scale, cost-effective, reliable, and free-style way at or near room temperature. Here, we present a completely rewritable meta-canvas on which arbitrary photonic devices can be rapidly written, erased and rewritten. The writing is with a low-power (1 mW) continuous laser and the entire process stays below ~ 90oC. Using these devices we demonstrate dynamical manipulation of optical waves for light propagation, reconstruction and polarization. Such meta-canvas supports physical (re)compilation of photonic operators akin to that of FPGA, opening up possibilities where a single photonic element can be field-programmed to deliver complex, system-level functionalities.