Optical properties of periodic, quasi-periodic, and disordered one-dimensional photonic structures

Photonic crystals are characterized by a spatial modulation of the dielectric constant on the length scale of the wavelength of light giving rise to energy ranges where light cannot propagate through the crystal - the photonic band gap. While mostly photonic crystals are referred to as periodic arrangements, in this review we aim to highlight as well how aperiodicity and disorder affects light modulation. In this review article, we introduce the concepts of periodicity, quasi-periodicity, and disorder in photonic crystals, focussing on the one-dimensional case. We discuss in detail the physical peculiarities, the fabrication techniques, and the applications of periodic, quasi-periodic, and disorder photonic structures, highlighting how the degree of crystallinity matters in the manipulation of light. We report different types of disorder in 1D photonic structures and we discuss their properties in terms of light transmission. We discuss the relationship between the average total transmission, in a range of wavelengths around the photonic band gap of the corresponding photonic crystal, and the homogeneity of the photonic structures, quantified by the Shannon index. Then we discuss the light transmission in structures in which the high refractive index layers are aggregated in clusters following a power law distribution. Finally, in the case of structures in which the high refractive index layers are aggregated in clusters with a truncated uniform distribution, we discuss: i) how different refractive index contrast tailors the total light transmission; ii) how the total light transmission is affected by the introduction of defects made with a third material.Comment: 42 pages, 24 figure

Ultrafast broadband optical modulation in indium tin oxide/titanium dioxide 1D photonic crystal

Photonic crystals can integrate plasmonic materials such as Indium Tin Oxide (ITO) in their structure. Exploiting ITO plasmonic properties, it is possible to tune the photonic band gap of the photonic crystal upon the application of an external stimuli. In this work, we have fabricated a one-dimensional multilayer photonic crystal alternating ITO and Titanium Dioxide (TiO2) via radio frequency sputtering and we have triggered its optical response with ultrafast pump-probe spectroscopy. Upon photoexcitation, we observe a change in the refractive index of ITO. Such an effect has been used to create a photonic crystal that changes its photonic bandgap in an ultrafast time scale. All optical modulation in the visible region, that can be tuned by designing the photonic crystal, has been demonstrated

One-dimensional disordered photonic structures with two or more materials

Here we would like to discuss the light transmission modulation by periodic and disordered one dimensional (1D) photonic structures. In particular, we will present some theoretical and experimental findings highlighting the peculiar optical properties of: i) 1D periodic and disordered photonic structures made with two or more materials; ii) 1D photonic structures in which the homogeneity or the aggregation of the high refractive index layers is controlled. We will focus also on the fabrication aspects of these structures.Comment: 6 pages, 4 figure

Effective medium optical modelling of indium tin oxide nanocrystal films

: Doped semiconductor nanocrystal-based thin films are widely used for many applications, such as screens, electrochromic windows, light emitting diodes, and solar cells. Herein, we have employed spectroscopic ellipsometry to measure and model the complex dielectric response of indium tin oxide films fabricated by nanocrystal deposition and sintering. The films could be modelled as Bruggemann effective media, allowing estimation of the nanoscale interstitial porosity of the structure. The effective dielectric constants show the possibility of tuning the plasma frequency and the epsilon-near zero condition of the film

High quality factor 1-D Er 3+ -activated dielectric microcavity fabricated by RF-sputtering

Rare earth-activated 1-D photonic crystals were fabricated by RF-sputtering technique. The cavity is constituted by an Er3+-doped SiO2 active layer inserted between two Bragg reflectors consisting of ten pairs of SiO2/TiO2 layers. Scanning electron microscopy is employed to put in evidence the quality of the sample, the homogeneities of the layers thickness and the good adhesion among them. Near infrared transmittance and variable angle reflectance spectra confirm the presence of a stop band from 1500 nm to 2000 nm with a cavity resonance centered at 1749 nm at 0° and a quality factor of 890. The influence of the cavity on the 4I13/2 -> 4I15/2 emission band of Er3+ ion is also demonstrated

Electrochromism in Electrolyte-Free and Solution Processed Bragg Stacks

Achieving an active manipulation of colours has huge implications in optoelectronics, as colours engineering can be exploited in a number of applications, ranging from display to lightning. In the last decade, the synergy of the highly pure colours of 1D photonic crystals, also known as Bragg stacks, with electro-tunable materials have been proposed as an interesting route to attain such a technologically relevant effect. However, recent works rely on the use of liquid electrolytes, which can pose issues in terms of chemical and environmental stability. Here, we report on the proof-of-concept of an electrolyte free and solution-processed electrochromic Bragg stack. We integrate an electro-responsive plasmonic metal oxide, namely indium tin oxide, in a 1D photonic crystal structure made of alternating layers of ITO and TiO2 nanoparticles. In such a device we observed 15 nm blue-shift upon application of an external bias (5 V), an effect that we attribute to the increase of ITO charge density arising from the capacitive charging at the metal oxide/dielectric interface and from the current flowing throughout the porous structure. Our data suggest that electrochromism can be attained in all-solid state systems by combining a judicious selection of the constituent materials with device architecture optimisation

Amorphous WO3 as transparent conductive oxide in the near-IR

The demand for transparent conductive films (TCFs) is dramatically increasing. In this work tungsten oxide (WO3-x) is studied as a possible option additional to the existed TCFs. We introduce WO3-x thin films fabricated by a non-reactive magnetron RF-sputtering process at room temperature, followed by thermal annealing in dry air. Films are characterized morphologically, structurally, electrically, optically, and dielectrically. Amorphous WO3-x thin films are shown to be ntype conductive while the transparency extends to the near-IR. By evaluating a figure of merit for transparent-conductive performance and comparing to some most-widely used TCFs, WO3-x turns out to outperform in the near-IR optical range

Thermal Decomposition of Silicon-rich Oxides Deposited by the LPCVD Method

Silicon-rich oxide (SiOx, 0 < x < 2) thin films were deposited using the Low Pressure Chemical Vapor Deposition (LPCVD) method at temperature of 570 °C using silane (SiH4) and oxygen as the reactant gasses. The films were annealed at temperatures of 800, 900, 1000, and 1100 °C to induce the separation of excess silicon in the SiOx films into nanosized crystalline silicon particles inside an amorphous SiOx matrix. The size of the silicon particles was determined using Raman spectroscopy. (doi: 10.5562/cca1969

Novel Sol-Gel Route to Prepare Eu3+-Doped 80SiO2-20NaGdF4 Oxyfluoride Glass-Ceramic for Photonic Device Applications

Oxyfluoride glass-ceramics (OxGCs) with the molar composition 80SiO2-20(1.5Eu3+: NaGdF4) were prepared with sol-gel following the “pre-crystallised nanoparticles route” with promising optical results. The preparation of 1.5 mol % Eu3+-doped NaGdF4 nanoparticles, named 1.5Eu3+: NaGdF4, was optimised and characterised using XRD, FTIR and HRTEM. The structural characterisation of 80SiO2-20(1.5Eu3+: NaGdF4) OxGCs prepared from these nanoparticles’ suspension was performed by XRD and FTIR revealing the presence of hexagonal and orthorhombic NaGdF4 crystalline phases. The optical properties of both nanoparticles’ phases and the related OxGCs were studied by measuring the emission and excitation spectra together with the lifetimes of the 5D0 state. The emission spectra obtained by exciting the Eu3+-O2− charge transfer band showed similar features in both cases corresponding the higher emission intensity to the 5D0→7F2 transition that indicates a non-centrosymmetric site for Eu3+ ions. Moreover, time-resolved fluorescence line-narrowed emission spectra were performed at a low temperature in OxGCs to obtain information about the site symmetry of Eu3+ in this matrix. The results show that this processing method is promising for preparing transparent OxGCs coatings for photonic applications.This project received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 739566. The authors acknowledge financial support from MICINN under projects PID2020-115419GB-C21-C22/AEI/10.13039/501100011033 and University of the Basque Country (Project GIU21/006)

Glass-based 1-D dielectric microcavities

We have developed a reliable RF sputtering techniques allowing to fabricate glass-based one dimensional microcavities, with high quality factor. This property is strongly related to the modification of the density of states due to the confinement of the gain medium in a photonic band gap structure. In this short review we present some of the more recent results obtained by our team exploiting these 1D microcavities. In particular we present: (1) Er3+ luminescence enhancement of the 4I13/2 → 4I15/2 transition; (2) broad band filters based on disordered 1-D photonic structures; (3) threshold defect-mode lasing action in a hybrid structure