Non-volatile silicon photonic devices enabled by phase change material

We review our recent research progress on silicon-Ge 2 Sb 2 Te 5 hybrid photonic devices. The optical transmission can be varied by 20 dB using micron-length GST. The inherent "self-holding" feature of GST make it attractive for low-power applications

Tunable Silicon integrated photonics based on functional materials

This thesis is concerned with the design, fabrication, testing and development of tunable silicon photonic integrated circuits based on functional materials. This tunability is achieved by integrating liquid crystals, 2D materials and chalcogenide phase-change materials with silicon and silicon nitride integrated circuits. Switching the functional materials between their various states results in dramatic changes in the optical properties, with consequent changes in the optical response of the individual devices. Furthermore, such changes are volatile or non-volatile depending on the materials.Engineering and Physical Sciences Research Council (EPSRC

Enhanced Performance and Diffusion Robustness of Phase-Change Metasurfaces via a Hybrid Dielectric/Plasmonic Approach

This is the final version. Available on open access from MDPI via the DOI in this recordMaterials of which the refractive indices can be thermally tuned or switched, such as in chalcogenide phase-change alloys, offer a promising path towards the development of active optical metasurfaces for the control of the amplitude, phase, and polarization of light. However, for phase-change metasurfaces to be able to provide viable technology for active light control, in situ electrical switching via resistive heaters integral to or embedded in the metasurface itself is highly desirable. In this context, good electrical conductors (metals) with high melting points (i.e., significantly above the melting point of commonly used phase-change alloys) are required. In addition, such metals should ideally have low plasmonic losses, so as to not degrade metasurface optical performance. This essentially limits the choice to a few noble metals, namely, gold and silver, but these tend to diffuse quite readily into phase-change materials (particularly the archetypal Ge2Sb2Te5 alloy used here), and into dielectric resonators such as Si or Ge. In this work, we introduce a novel hybrid dielectric/plasmonic metasurface architecture, where we incorporated a thin Ge2Sb2Te5 layer into the body of a cubic silicon nanoresonator lying on metallic planes that simultaneously acted as high-efficiency reflectors and resistive heaters. Through systematic studies based on changing the configuration of the bottom metal plane between high-melting-point diffusive and low-melting-point nondiffusive metals (Au and Al, respectively), we explicitly show how thermally activated diffusion can catastrophically and irreversibly degrade the optical performance of chalcogenide phase-change metasurface devices, and how such degradation can be successfully overcome at the design stage via the incorporation of ultrathin Si3N4 barrier layers between the gold plane and the hybrid Si/Ge2Sb2Te5 resonators. Our work clarifies the importance of diffusion of noble metals in thermally tunable metasurfaces and how to overcome it, thus helping phase-change-based metasurface technology move a step closer towards the realization of real-world applications.Engineering and Physical Sciences Research Council (EPSRC

Reconfigurable phase-change optical metasurfaces: novel design concepts to practicable devices

Optical metasurfaces have been proven to be capable of controlling amplitude, phase and polarization of optical beams without the need of bulky geometries, making them really attractive for the development of compact photonic devices. Recently, their combination with chalcogenide phase-change materials (traditionally employed in non-volatile optical and electrical memories), whose refractive index can be reversibly and repeatedly controlled, has been proposed to yield low power consumption tunable metasurfaces having several functionalities in a single device. However, despite phase-change memories are commercially available since various decades now, the unification of phase-change materials with metasurfaces towards real life applications is becoming a formidable task, mainly due to the several engineering branches involved in this technology, which sometimes compromise each other in a non-trivial way. This includes thermo/optical, thermo/electric, and chemical incompatibilities which are typically not taken into account by researchers working in the field, resulting in devices having exciting reconfigurable properties, but at the same time, lack of practicability. This thesis is therefore dedicated to the development of novel phase-change metasurface architectures which could partially or totally address such engineering problems. Particular emphasis has been put in the realization of reconfigurable metasurfaces for active wavefront control, as such a functionality remains relatively unexplored. The first part of this thesis focuses in the first experimental demonstration of active, reconfigurable non-mechanical beam steering devices working the near-infrared. This was achieved via integration of ultra-thin films of chalcogenide phase-change materials (in this case, the widely employed alloy Ge2Sb2Te5) within the body of a dielectric spacer in a plasmonic metal/insulator/metal metasurface architecture. Active, and optically reversible beam steering between two different angles with efficiencies up to 40% were demonstrated. The second part of this work shows the work carried out in metal-free metasurfaces as a way to manipulate optical beams with high efficiency in both transmission and/or reflection. This was achieved via combination of all-dielectric silicon nanocylinders with deeply-subwavelenght sized Ge2Sb2Te5 inclusions. By strategic placement of the phase-change inclusions in the regions of high electric field density, independent and active control of the metasuface resonances is demonstrated, with modulations depths as high as 70% and 65% in reflection and transmission respectively. Multilevel, and fully reversible optically-induced switching of the phasechange layer is also reported, with up to 11 levels of tunability over 8 switching cycles. Finally, the last section of this thesis introduces the concept of hybrid dielectric/plasmonic phase-change metasurfaces having key functional benefits when compared to both purely dielectric and plasmonic approaches. The proposed architectures showed great versatility in terms of both active amplitude and phase control, offering the possibility of designing devices for different purposes (i.e. such as active absorbers/modulators or beam steerers with enhanced efficiency) employing the same unit-cell configuration with minor geometry re-optimizations. Initial device experimental demonstrations of such an approach are discussed, as well as their potential in terms of delivering in-situ electrical switching capabilities using a metallic ground plane as a resistive heater.Engineering and Physical Sciences Research Council (EPSRC

Toward a new generation of photonic devices based on the integration of metal oxides in silicon technology

[ES] La búsqueda de nuevas soluciones e ideas innovadoras en el campo de la fotónica de silicio mediante la integración de nuevos materiales con prestaciones únicas es un tema de alta actualidad entre la comunidad científica en fotónica y con un impacto potencial muy alto. Dentro de esta temática, esta tesis pretende contribuir hacia una nueva generación de dispositivos fotónicos basados en la integración de óxidos metálicos en tecnología de silicio. Los óxidos metálicos elegidos pertenecen a la familia de óxidos conductores transparentes (TCO), concretamente el óxido de indio y estaño (ITO) y el óxido de cadmio (CdO), y materiales de cambio de fase (PCM) como el dióxido de vanadio (VO2). Dichos materiales se caracterizan especialmente por una variación drástica de sus propiedades optoelectrónicas, tales como la resistividad o el índice de refracción, frente a un estímulo externo ya sea en forma de temperatura, aplicación de un campo eléctrico o excitación óptica. De esta forma, nuestro objetivo es diseñar, fabricar y demostrar experimentalmente nuevas soluciones y dispositivos clave tales como dispositivos no volátiles, desfasadores y dispositivos con no linealidad óptica. Tales dispositivos podrían encontrar potencial utilidad en diversas aplicaciones que comprenden las comunicaciones ópticas, redes neuronales, LiDAR, computación, cuántica, entre otros. Las prestaciones clave en las que se pretende dar un salto disruptivo son el tamaño y capacidad para una alta densidad de integración, el consumo de potencia, y el ancho de banda.[CA] La recerca de noves solucions i idees innovadores al camp de la fotònica de silici mitjançant la integració de nous materials amb prestacions úniques és un tema d'alta actualitat entre la comunitat científica en fotònica i amb un impacte potencial molt alt. D'aquesta temàtica, aquesta tesi pretén contribuir cap a una nova generació de dispositius fotònics basats en la integració d'òxids metàl·lics en tecnologia de silici. Els òxids metàl·lics elegits pertanyen a la família d'òxids conductors transparents (TCO), concretament l'òxid d'indi i estany (ITO) i l'òxid de cadmi (CdO), i materials de canvi de fase (PCM) com el diòxid de vanadi (VO2). Aquests materials es caracteritzen especialment per una variació dràstica de les propietats optoelectròniques, com ara la resistivitat o l'índex de refracció, davant d'un estímul extern ja siga en forma de temperatura, aplicació d'un camp elèctric o excitació òptica. D'aquesta manera, el nostre objectiu és dissenyar, fabricar i demostrar experimentalment noves solucions i dispositius clau com ara dispositius no volàtils, desfasadors i dispositius amb no-linealitat òptica. Aquests dispositius podrien trobar potencial utilitat en diverses aplicacions que comprenen les comunicacions òptiques, xarxes neuronals, LiDAR, computació, quàntica, entre d'altres. Les prestacions clau en què es pretén fer un salt disruptiu són la grandària i la capacitat per a una alta densitat d'integració, el consum de potència i l'amplada de banda.[EN] The search for new solutions and innovative ideas in the field of silicon photonics through the integration of new materials featuring unique optoelectronic properties is a hot topic among the photonics scientific community with a very high potential impact. Within this topic, this thesis aims to contribute to a new generation of photonic devices based on the integration of metal oxides in silicon technology. The chosen metal oxides belong to the family of transparent conducting oxides (TCOs), namely indium tin oxide (ITO) and cadmium oxide (CdO), and phase change materials (PCMs) such as vanadium dioxide (VO2). These materials are characterized by a drastic variation of their optoelectronic properties, such as resistivity or refractive index, in response to an external stimulus either in the form of temperature, application of an electric field, or optical excitation. Therefore, our objective is to design, fabricate and experimentally demonstrate new solutions and key devices such as non-volatile devices, phase shifters, and devices with optical nonlinearity. Such devices could find potential utility in several applications, including optical communications, neural networks, LiDAR, computing, and quantum. The key features in which we aim to take a leapfrog are footprint and capacity for high integration density, power consumption, and bandwidth.This work is supported in part by grants ACIF/2018/172 funded by Generaliltat Valenciana, and FPU17/04224 funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future”.Parra Gómez, J. (2022). Toward a new generation of photonic devices based on the integration of metal oxides in silicon technology [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/19088

바나듐 이산화물 기반 상변이 격자들을 이용한 능동 열광학 변조

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2019. 2. 이병호.지난 약 20년간 다양한 형태의 나노광학 소자들이 플라즈모닉스와 메타물질 기술들을 통해 개발되어 왔다. 이러한 소자들의 주된 목표는 금속 표면에 빛을 집속함으로써 기존 벌크 광학 소자들의 기능을 파장 한계 이하로 소형화된 스케일에서 구현하거나, 자연에 없는 특성을 보이는 광학적 물질을 개발하는 것이다. 이를 통하여 궁극적으로 차세대 초소형 고집적 광학 시스템을 만드는 것이 두 분야의 주요한 목표이다. 특히, 유전 함수 스펙트럼이 외부 자극에 의해 조절이 가능한 능동 물질과 나노광학 소자의 결합을 통해 능동적으로 외부 자극에 의해 기능이 변조되는 나노광학 소자들에 대한 연구들이 큰 주목을 받아왔다. 최근에 대표적인 절연체-금속 상변이 물질인 바나듐이산화물을 이용한 나노광학 소자 개발이 주목 받고 있다. 바나듐이산화물은 가시광선, 근적외선 및 적외선 파장 대역에서 온도에 따라 큰 폭의 유전 함수 변화가 일어난다는 장점이 있다. 본 박사학위 논문에서는 주기적으로 배열된 광 격자 구조들에 바나듐이산화물을 결합하여 입사광의 산란 양상을 큰 폭으로 조절하는 새로운 공학적 응용 방법들을 제안한다. 바나듐이산화물의 상변이를 이용하여 산란된 가시광선 및 근적외선의 운동량, 세기, 스펙트럼, 그리고 편광을 효율적으로 조절하는 새로운 방법들을 개발하였다. 각 장 별로 제안한 광소자의 목적 및 동작 대역에 따라서 격자의 주기, 구성 물질 및 구조를 설계하는 이론 및 전산모사 연구를 수행하고 이를 실험적으로 검증한다. 첫번째 장에서는, 능동 나노광학에 대한 간략한 소개와 함께 바나듐이산화물의 유전 함수 변화를 중심으로 동작 파장 대역에 따른 상변이 현상의 특성을 논한다. 다음으로는 금속 표면 위에 집속된 빛의 형태인 표면 플라즈몬 폴라리톤을 여기하고 그 방향을 뒤바꾸는 상변이 광 스위치를 제안한다. 열에 의해 뒤바뀌는 비대칭 나노안테나와 파장 이하 광학 격자에 의한 운동량 보상에 기반하여 표면 플라즈몬 폴라리톤의 방향성이 크게 변함을 보인다. 세번째 장에서는, 광대역에서 비공진 작동이 가능한 직진 광 투과율 조절기를 바나듐 이산화물 격자를 통해 구현한다. 근적외선 대역에서 나타나는 유전체-플라즈모닉 상변이 특성과 나노 도파로에 의한 광대역 초소형 간섭계 구조에 기반하여 고성능 능동 투과율 변조가 가능함을 보인다. 네번째 장에서는, 가시광선의 색 스펙트럼과 편광 방향을 효율적으로 조절하는 바나듐이산화물 메타필름 연구를 소개한다. 반사형 구조의 완전 흡수 현상과 유효 매질 이론을 통한 편광 의존적 유효 굴절률의 설계를 활용한, 가시광선의 효율적인 능동 변조 프레임워크를 제시한다. 마지막으로 결론에서는 연구 결과들의 요약과 그 가치에 대한 논의로 학위논문을 마무리 한다. 본 박사학위 논문의 결과는 능동적 변조가 요구되는 차세대 초소형 광 스위치, 초소형 디스플레이 픽셀, 공간 광 변조기 등에 기여할 수 있을 것으로 기대된다.For the last two decades, various nanophotonic devices have been developed in the fields of plasmonics and metamaterials. There are two main goals of the nanophotonic devices in the fields. One is to confine light on metallic surface and manipulate it in subwavelength scale beyond the diffraction limit. The second is to design artificial materials with extraordinary optical properties that do not exist in the nature. Nanophotonics community has been trying to achieve such phase goals for next-generation miniaturized and integrated optical systems. In particular, dynamically tunable nanophotonic devices have been in spotlight and demonstrated by combining dynamically reconfigurable optical materials and periodic optical grating structures. Optical responses of such devices can be tuned dynamically by application of external stimuli such as heat, voltage, current, and optical pulse. Recently, the representative insulator-to-metal phase-transition material responsive to thermal stimulus, vanadium dioxide, has been thoroughly studied for optical applications. Vanadium dioxide is advantageous in manipulating the visible, near-infrared, and infrared light by virtue of the large change of the dielectric function. This dissertation proposes the novel nanophotonic engineering methods of combining optical grating structures and vanadium dioxide for dynamic, extraordinary thermo-optic modulation of light scattering. Specifically, momentum, intensity, color spectrum, and polarization state of scattered light are modulated based on the transition characteristics of vanadium dioxide and periodic grating functionalities in the visible and near-infrared. Every main chapters present the theoretical and numerical studies involving effects of optical grating geometry for the certain intended modulation functions. In the first chapter, the concepts of dynamic nanophotonics are introduced. In addition, wavelength-dependent optical properties of vanadium dioxide are discussed with experimental studies. In the second chapter, nanophotonic phase-transition switch is proposed for asymmetric excitation and dynamic directivity switching of surface plasmon polaritons. The vanadium dioxide metagrating is demonstrated as the directional switch using thermally switchable asymmetric unit cell and grating period-assisted momentum matching of surface plasmon polaritons. The third chapter introduce the broadband efficient modulation method of transmissivity with vanadium dioxide diffraction grating. Rather than using subwavelength-spaced resonant metasurfaces, diffractive large period grating is designed for broadband operation. Based on such diffraction modulation, forward transmissivity is largely modulated with high efficiency over the broad bandwidth in the near-infrared. In the fourth chapter, the deep subwavelength-spaced gratings, reflective metafilms, are proposed by introducing mirror and effective medium approximation for efficient operation in the visible spectrum. For high contrast modulation of visible light color spectrum, near-unity absorption is designed in noble metal-vanadium dioxide metafilm and tuned owing to large anisotropy and boosted phase-transition effect. Moreover, dynamic polarization modulation is achieved by virtue of extremely anisotropic tunability of absorptions dips. At last, conclusion is presented with brief summary and discussions. The results of the dissertation would help developing and improving next-generation optical switch, ultracompact display pixel, and high performance spatial light modulators in the future.Abstract i Contents iv List of Tables vi List of Figures vii Chapter 1 Introduction 1 1.1 Overview of dynamic nanophotonics 1 1.2 VO2: a volatile phase-transition material 7 1.3 Overview of this dissertation 12 Chapter 2 Dynamic directional switching of surface plasmon polaritons with metal-VO2 metagrating 16 2.1 Introduction 16 2.2 Principles of metal-VO2 metagrating 20 2.2.1 Tunable near-field scattering near VO2 nanoantenna 20 2.2.2 Design of tunable asymmetric launching of surface plasmon polaritons 23 2.3 Experimental demonstration 32 2.3.1 Fabrication of metal-VO2 metagrating 32 2.3.2 Measurement 34 2.4 Summary and discussions 37 Chapter 3 Broadband efficient modulation of forward transmission with VO2 diffraction grating 39 3.1 Introduction 39 3.2 Principles of VO2 diffraction grating 43 3.2.1 Photonic modes in VO2 waveguide 43 3.2.2 Design of VO2 grating modulator 55 3.3 Experimental demonstration 61 3.3.1 Fabrication of VO2 grating modulator 61 3.3.2 Measurement 62 3.4 Summary and discussions 66 Chapter 4 Tunable multifunctional phase-transition effect with noble metal-VO2 metafilm in the visible 67 4.1 Introduction 67 4.2 Absorbing Ag-VO2 metafilm in the visible 69 4.2.1 Role of metallic mirror 69 4.2.2 Role of effective medium approximation for dynamic metafilm 75 4.2.3 Experimental demonstration 85 4.3 Summary and discussions 96 Chapter 5 Conclusion 97 Bibliography 100 Appendix 113 초 록 114Docto

Reconfigurable Phase-Change Metasurface Absorbers for Optoelectronics Device Applications

This thesis is concerned with the design and development of dynamically reconfigurable optical metasurfaces. This reconfigurability is achieved by integrating chalcogenide phase-change materials with plasmonic resonator structures of the metal-insulator-metal type. Switching the phase-change material between its amorphous and crystalline states results in dramatic changes in its optical properties, with consequent dramatic changes in the resonant behaviour of the plasmonic metasurface with which it is integrated. Moreover, such changes are non-volatile, reversible and potentially very fast, in the order of nanoseconds. The first part of the thesis is dedicated to the design, fabrication and characterisation of metasurface devices working at telecommunications wavelengths, specifically at wavelengths corresponding to the C-band (1530 to 1565 nm), and that act as a form of perfect absorber when the phase-change layer (in this case Ge2Sb2Te5) is amorphous but reflect strongly when switched to the crystalline state. Such behaviour can be used, for example, to provide a form of optical amplitude modulator. Fabricated devices not only showed very good performance, including a large modulation depth of ~77% and an extinction ratio of ~20 dB, but also incorporated a number of practicable design features often overlooked in the literature, including a means for protecting the phase-change layer from environmental oxidation and, importantly, an electrically-driven in-situ switching capability. In the second part of the thesis a method, based on eigenmode analysis and critical coupling theory, is developed to allow for the design and fabrication of perfect absorber type devices in a simple and efficient way, while at the same time maintaining design control over the key performance characteristics of resonant frequency, reflection coefficient at resonance and quality factor. Validation of this new method was carried out via the design and fabrication of a family of absorbers with a range of ‘on-demand’ quality factors, all operating at the same resonant frequency and able to be fabricated simply and simultaneously on the same chip. The final part of the thesis is concerned with the design and development of a switchable phase-change metamaterial type absorber working in the visible part of the spectrum and with non-volatile colour generating capability. With the phase-change layer, here GeTe, in the crystalline phase, the absorber can be tuned to selectively absorb the red, green and blue spectral bands, so generating vivid cyan, magenta and yellow pixels. When the phase-change layer is switched into the amorphous phase, the resonant absorption is suppressed and a flat, pseudo-white reflectance results. This potentially opens up a route to the development of non-volatile, phase-change metamaterial colour displays and colour electronic signage.Engineering and Physical Sciences Research Council (EPSRC

Active Extraordinary Optical Transmission Metasurfaces Using Phase-Change Materials

The key question that this thesis aims to answer is “can a tuneable bandpass optical filter for the mid-infrared regime be made by combining extraordinary optical transmission (EOT) arrays and phase-change materials (PCMs)?”. It is proposed that such devices may be useful for a wide range of applications where the ability to dynamically change the transmissive (or reflective) properties of a filter is required, include multispectral sensing/imaging and signal modulation amongst others. Current multispectral imaging systems are mainly dependant on either multiple sets of lenses and sensors, or multiple mechanically-exchanged filters exposed through in-sequence; the use of a single, dynamically tuneable phase-change EOT-based filter opens avenues to reducing systems’ size, cost and complexity. The EOT effect is observed with arrays of sub-wavelength-sized holes in thin plasmonic metal (e.g. gold) films, with the transmission peak position dependent on the array geometry and surrounding materials’ optical properties: a PCM layer on top of the array allows shifting of the transmission peak position by switching the PCM phase (and its refractive index) via heat pulses. Specific areas studied in this thesis include the use of different fabrication methods to make phase-change EOT transmission filters for the mid-infrared regime, including electron-beam lithography-based techniques and a novel (and much faster) approach of direct patterning via laser ablation. Tuneable filters for use in various parts of the optical spectrum, especially the mid and long-wave infrared range, were designed, simulated, fabricated and characterised. Good performance was obtained for phase-change EOT filters over a wide range of array pitch sizes. EOT arrays designed for the mid-infrared range and fabricated via wet-etching and measured with FTIR spectroscopy produced very similar spectra to those of finite-element simulations with peak transmittance of Q-factors between 5-6 and a peak transmittance of ~0.8. Laser-ablated arrays showed a similar (though not quite so good) performance, due mainly to slight irregularities in the positioning of holes in the array. The addition of a phase-change layer, specifically Ge2Sb2Te5, to the EOT arrays resulted in a shift in the wavelength of the peak transmission, with the amount of shift depending on the phase-state (crystalline, amorphous, or mixed-state) of the phase-change layer, so demonstrating the ability for dynamic tuning of the filter response by switching of the phase-change layer. An important requirement for proper and prolonged operation of the filter devices was found to be the use of a thin dielectric barrier layer (here Si3N4, between the plasmonic film and the phase-change layer, to prevent inter-diffusion between the two: reflection cavities of Ge2Sb2Te5 on unpatterned gold layers were created to investigate this effect, with resonance features of a 20 nm layer being destroyed without a barrier layer present and the complete assimilation of gold and phase-change layers evident with cross-section TEM imaging

LOW INSERTION-LOSS NANOPHOTONIC MODULATORS THROUGH EPSILON-NEAR-ZERO MATERIAL-BASED PLASMON-ASSISTED APPROACH FOR INTEGRATED PHOTONICS

Electro-optic/absorption Modulators (EOM/EAMs) encode high-frequency electrical signals into optical signals. With the requirement of large packing density, device miniaturization is possible by confining light in a sub-wavelength dimension by utilizing the plasmonic phenomenon. In plasmon, energy gets transferred from light to the form of oscillation of free electrons on a surface of a metal at an interface between the metal and a dielectric. Plasmonic provides increased light-matter interaction (LMI) and thus making the light more sensitive to local refractive index change. Plasmonic-based integrated nanophotonic modulators, despite their promising features, have one key limiting factor of large Insertion Loss (IL) which limits their practical potential. To combat this, this research utilizes a plasmon-assisted approach through the lens of surface-to-volume ratio to realize a 4-slot-based EAM with an extinction ratio (ER) of 2.62 dB/μm and insertion loss (IL) of 0.3 dB/μm operating at ~1 GHz and a single slot design with ER of 1.4 dB/ μm and IL of 0.25 dB/ μm operating at ~20 GHz, achieved by replacing the traditional metal contact with heavily doped Indium Tin Oxide (ITO). Furthermore, the analysis imposes realistic fabrication constraints, and material properties, and illustrates trade-offs in the performance that must be carefully optimized for a given scenario. Besides the research investigates optical and electrical properties of constituent materials through techniques such as atomic layer deposition (ALD) for depositing thin films, spectroscopic ellipsometry (SE), and Hall measurements for optical and electrical characterization respectively