Momentum exchange between light and nanostructured matter

An object\u27s translational and rotational motion is associated with linear and angular momenta. When multiple objects interact the exchange of momentum dictates the new system\u27s motion. Since light, despite being massless, carries both linear and angular momentum it too can partake in this momentum exchange and mechanically affect matter in tangible ways. Due to conservation of momentum, any such exchange must be reciprocal, and the light therefore acquires an opposing momentum component. Hence, light and matter are inextricably connected and one can be manipulated to induce interesting effects to the other. Naturally, any such effect is facilitated by having strongly enhanced light-matter interaction, which for visible light is something that is obtained when nanostructured matter supports optical resonances. This thesis explores this reciprocal relationship and how nanostructured matter can be utilised to augment these phenomena.Once focused by a strong lens, light can form optical tweezers which through optical forces and torques can confine and manipulate small particles in space. Metallic nanorods trapped in two dimensions against a cover glass can receive enough angular momentum from circularly polarised light to rotate with frequencies of several tens of kilohertz. In the first paper of this thesis, the photothermal effects associated with such optical rotations are studied to observe elevated thermal environments and morphological changes to the nanorod. Moreover, to elucidate upon the interactions between the trapped particle and the nearby glass surface, in the thesis\u27 second paper a study is conducted to quantify the separation distance between the two under different trapping conditions. The particle is found to be confined ~30-90 nm away from the surface.The momentum exchange from a single nanoparticle to a light beam is negligible. However, by tailoring the response of an array of nanoparticles, phase-gradient metasurfaces can be constructed that collectively and controllably alter the incoming light\u27s momentum in a macroscopically significant way, potentially enabling a paradigm shift to flat optical components. In the thesis\u27 third paper, a novel fabrication technique to build such metasurfaces in a patternable polymer resist is investigated. The technique is shown to produce efficient, large-scale, potentially flexible, substrate-independent flat optical devices with reduced fabricational complexity, required time, and cost.At present, optical metasurfaces are commonly viewed as stationary objects that manipulate light just like common optical components, but do not themselves react to the light\u27s changed momentum. In the last paper of this thesis, it is realised that this is an overlooked potential source of optical force and torque. By incorporating a beam-steering metasurface into a microparticle, a new type of nanoscopic robot – a metavehicle – is invented. Its propulsion and steering are based on metasurface-induced optical momentum transfer and the metavehicle is shown to be driven in complex shapes even while transporting microscopic cargo

Optical manipulation and heating of gold nanoparticles near interfaces

By focusing laser light to small volumes, its momentum can be used to trap and manipulate objects in the size range from cells down to single atoms. Devices using this effect are called optical tweezers, and have found use in measuring and applying minuscule forces and torques, contributed to deepening our knowledge of molecular motors, unraveling the mechanics of cells and DNA, and better understand statistical mechanics and hydrodynamic interactions at the nanoscale. In short, the optical tweezer is a crucial component in our aspiration to understand and unlock the potential of nano-scaled objects.One class of nano-elements worth devoting special attention to are nanoparticles supporting plasmonic resonances. These present strongly enhanced light-matter interactions and may find use in as diverse fields as high-density data storage, single molecule detection, and personalized medicine. One potential use of plasmonic nanorods is as rotary nanomotors. These are capable of reaching record rotation frequencies of several tens of kilohertz when optically trapped in water against a glass surface. This thesis focuses on studying vital questions related to such rotary nanomotors, which are interesting to resolve from both a fundamental and from an applied point of view.It is well-known that metallic nanoparticles are efficiently heated by light. This will give rise to several photothermal effects affecting the nanoparticle and its surrounding. How these influence the performance of the nanomotor is evaluated. Through spectroscopic measurements, morphological changes induced by atomic migration is observed. Moreover, the elevated thermal environment around the nanoparticle is probed using two separate techniques, and temperatures above $200^\circ$C are routinely reached, but could be kept as low as a few degrees above ambient under the right circumstances.The gold nanoparticle is trapped at a small, but hitherto unknown, distance from a glass interface. The vicinity to a surface can affect several of a nanoparticles properties, including its diffusion and thermal environment, and knowing this distance is hence critical for any claims about the nanomotors\u27 performance. Therefore, total internal reflection microscopy is performed on the trapped nanoparticles and it is found that they are confined less than 100 nm from the surface. The distance can be controlled by altering the radiation pressure, or Coulomb repulsion.In summary, the work performed in this thesis present important building blocks towards a complete understanding of this highly promising rotary motor system

Large-Scale Metasurfaces Made by an Exposed Resist

Phase-gradient metasurfaces have the potential to revolutionize photonics by offering ultrathin alternatives to a wide range of common optical elements, including bulky refractive optics, waveplates, and axicons. However, the fabrication of state-of-the-art metasurfaces typically involves several expensive, time-consuming, and potentially hazardous processing steps. To address this limitation, a facile methodology to construct phase-gradient metasurfaces from an exposed standard electron beam resist is developed. The method dramatically cuts the required processing time and cost as well as reduces safety hazards. The advantages of the method are demonstrated by constructing high-performance flat optics based on the Pancharatnam-Berry phase gradient concept for the entire visible wavelength range. Manufactured devices include macroscopic (1 cm diameter) positive lenses, gratings exhibiting anomalous reflection, and cylindrical metalenses on flexible plastic substrates

Aberration-corrected large-scale hybrid metalenses

Hybrid components combining the optical power of a refractive and a diffractive optical system can form compact doublet lenses that correct various aberrations. Unfortunately, the diffraction efficiency of these devices decreases as a function of the deflection angle over the element aperture. Here, we address this issue, compensating for chromatic dispersion and correcting for monochromatic aberrations with centimeter-scale hybrid-metalenses. We demonstrate a correction of at least 80% for chromatic aberration and 70% for spherical aberration. We finally present monochromatic and achromatic images that clearly show how these hybrid systems outperform standard refractive lenses. The possibilities to adjust arbitrary spatial amplitude, phase, polarization, and dispersion profiles with hybrid metasurfaces offer unprecedented optical design opportunities for compact and broadband imaging, augmented reality/virtual reality, and holographic projection

High index dielectric metasurfaces and colloidal solutions: From fabrication to application

High index dielectric nanoparticles and meta-materials have been proposed for many different applications, including light harvesting, sensing and metalenses. However, widespread utilization in practice also requires large-scale fabrication methods able to produce homogeneous structures with engineered optical properties in a cost effective manner. Here, it is presented a facile fabrication method for silicon nanoparticles which is scalable to 4-inch wafers and can produce a wide range of nanoparticle shapes on demand. We also show that the fabricated nanoparticles can be detached from their support using a simple substrate removal technique and then transferred to colloidal suspension. We will finally discuss some uses of the fabricated systems. For the metasurfaces, we will demonstrate complete absorption due to far field interference effects. For the nanoparticles colloids we will show the possibility of realizing an intrinsically chiral structure composed of a low-loss dielectric resonator and we will study optical trapping phenomena for different particle sizes and shapes

Population and fertility by age and sex for 195 countries and territories, 1950–2017: a systematic analysis for the Global Burden of Disease Study 2017

Background: Population estimates underpin demographic and epidemiological research and are used to track progress on numerous international indicators of health and development. To date, internationally available estimates of population and fertility, although useful, have not been produced with transparent and replicable methods and do not use standardised estimates of mortality. We present single-calendar year and single-year of age estimates of fertility and population by sex with standardised and replicable methods. Methods: We estimated population in 195 locations by single year of age and single calendar year from 1950 to 2017 with standardised and replicable methods. We based the estimates on the demographic balancing equation, with inputs of fertility, mortality, population, and migration data. Fertility data came from 7817 location-years of vital registration data, 429 surveys reporting complete birth histories, and 977 surveys and censuses reporting summary birth histories. We estimated age-specific fertility rates (ASFRs; the annual number of livebirths to women of a specified age group per 1000 women in that age group) by use of spatiotemporal Gaussian process regression and used the ASFRs to estimate total fertility rates (TFRs; the average number of children a woman would bear if she survived through the end of the reproductive age span [age 10–54 years] and experienced at each age a particular set of ASFRs observed in the year of interest). Because of sparse data, fertility at ages 10–14 years and 50–54 years was estimated from data on fertility in women aged 15–19 years and 45–49 years, through use of linear regression. Age-specific mortality data came from the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2017 estimates. Data on population came from 1257 censuses and 761 population registry location-years and were adjusted for underenumeration and age misreporting with standard demographic methods. Migration was estimated with the GBD Bayesian demographic balancing model, after incorporating information about refugee migration into the model prior. Final population estimates used the cohort-component method of population projection, with inputs of fertility, mortality, and migration data. Population uncertainty was estimated by use of out-of-sample predictive validity testing. With these data, we estimated the trends in population by age and sex and in fertility by age between 1950 and 2017 in 195 countries and territories. Findings: From 1950 to 2017, TFRs decreased by 49\ub74% (95% uncertainty interval [UI] 46\ub74–52\ub70). The TFR decreased from 4\ub77 livebirths (4\ub75–4\ub79) to 2\ub74 livebirths (2\ub72–2\ub75), and the ASFR of mothers aged 10–19 years decreased from 37 livebirths (34–40) to 22 livebirths (19–24) per 1000 women. Despite reductions in the TFR, the global population has been increasing by an average of 83\ub78 million people per year since 1985. The global population increased by 197\ub72% (193\ub73–200\ub78) since 1950, from 2\ub76 billion (2\ub75–2\ub76) to 7\ub76 billion (7\ub74–7\ub79) people in 2017; much of this increase was in the proportion of the global population in south Asia and sub-Saharan Africa. The global annual rate of population growth increased between 1950 and 1964, when it peaked at 2\ub70%; this rate then remained nearly constant until 1970 and then decreased to 1\ub71% in 2017. Population growth rates in the southeast Asia, east Asia, and Oceania GBD super-region decreased from 2\ub75% in 1963 to 0\ub77% in 2017, whereas in sub-Saharan Africa, population growth rates were almost at the highest reported levels ever in 2017, when they were at 2\ub77%. The global average age increased from 26\ub76 years in 1950 to 32\ub71 years in 2017, and the proportion of the population that is of working age (age 15–64 years) increased from 59\ub79% to 65\ub73%. At the national level, the TFR decreased in all countries and territories between 1950 and 2017; in 2017, TFRs ranged from a low of 1\ub70 livebirths (95% UI 0\ub79–1\ub72) in Cyprus to a high of 7\ub71 livebirths (6\ub78–7\ub74) in Niger. The TFR under age 25 years (TFU25; number of livebirths expected by age 25 years for a hypothetical woman who survived the age group and was exposed to current ASFRs) in 2017 ranged from 0\ub708 livebirths (0\ub707–0\ub709) in South Korea to 2\ub74 livebirths (2\ub72–2\ub76) in Niger, and the TFR over age 30 years (TFO30; number of livebirths expected for a hypothetical woman ageing from 30 to 54 years who survived the age group and was exposed to current ASFRs) ranged from a low of 0\ub73 livebirths (0\ub73–0\ub74) in Puerto Rico to a high of 3\ub71 livebirths (3\ub70–3\ub72) in Niger. TFO30 was higher than TFU25 in 145 countries and territories in 2017. 33 countries had a negative population growth rate from 2010 to 2017, most of which were located in central, eastern, and western Europe, whereas population growth rates of more than 2\ub70% were seen in 33 of 46 countries in sub-Saharan Africa. In 2017, less than 65% of the national population was of working age in 12 of 34 high-income countries, and less than 50% of the national population was of working age in Mali, Chad, and Niger. Interpretation: Population trends create demographic dividends and headwinds (ie, economic benefits and detriments) that affect national economies and determine national planning needs. Although TFRs are decreasing, the global population continues to grow as mortality declines, with diverse patterns at the national level and across age groups. To our knowledge, this is the first study to provide transparent and replicable estimates of population and fertility, which can be used to inform decision making and to monitor progress. Funding: Bill & Melinda Gates Foundation

Construction and operation of a light-driven gold nanorod rotary motor system

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and artificial nanomotors and may provide new routes towards single cell analysis, studies of non-equilibrium thermodynamics, and mechanical actuation of nanoscale systems. A facile way to drive rotation is to use focused circularly polarized laser light in optical tweezers. Using this approach, metallic nanoparticles can be operated as highly efficient scattering-driven rotary motors spinning at unprecedented rotation frequencies in water. In this protocol, we outline the construction and operation of circularly-polarized optical tweezers for nanoparticle rotation and describe the instrumentation needed for recording the Brownian dynamics and Rayleigh scattering of the trapped particle. The rotational motion and the scattering spectra provides independent information on the properties of the nanoparticle and its immediate environment. The experimental platform has proven useful as a nanoscopic gauge of viscosity and local temperature, for tracking morphological changes of nanorods and molecular coatings, and as a transducer and probe of photothermal and thermodynamic processes

Ultrafast Modulation of Thermoplasmonic Nanobubbles in Water

Thermo-optically generated bubbles in water provide a powerful means for active matter control in microfluidic environments. These bubbles are often formed via continuous-wave illumination of an absorbing medium resulting in bubble nucleation via vaporization of water and subsequent bubble growth from the inward diffusion of gas molecules. However, to date, such bubbles tend to be several microns in diameter, resulting in slow dissipation. This limits the dynamic rate, spatial precision, and throughput of operation in any application. Here we show that isolated plasmonic structures can be utilized as highly localized heating elements to generate thermoplasmonic nanobubbles that can be modulated at frequencies up to several kilohertz in water, orders of magnitude faster than previously demonstrated for microbubbles. The nanobubbles are envisioned as advantageous localized active manipulation elements for high throughput microfluidic applications. \ua

Counter-Propagating Optical Trapping of Resonant Nanoparticles Using a Uniaxial Crystal

Laser tweezing of optically resonant nanostructures, such as plasmonic nanoparticles and high-index dielectric nanoresonators, is extremely challenging because the enhanced light–matter interaction usually amplifies radiation pressure to an extent where conventional single beam gradient trapping in three dimensions becomes impossible. Such particles are therefore typically trapped off resonance or in two dimensions only. To extend the application potential of optical tweezers to the resonant case, focus splitting inside a uniaxial birefringent crystal and reflection from a mirror is used to develop a counter-propagating beam configuration based on a single microscope objective. The setup allows one to trap and rapidly rotate resonant gold nanorods in water far from any interface, thereby opening a range of possibilities for novel studies of resonantly enhanced optical forces and interactions in uniform environments

Photothermal Heating of Plasmonic Nanoantennas: Influence on Trapped Particle Dynamics and Colloid Distribution

Plasmonic antennas are well-known and extremely powerful platforms for optical spectroscopy, sensing, and manipulation of molecules and nanoparticles. However, resistive antenna losses, resulting in highly localized photothermal heat generation, may significantly compromise their applicability. Here we investigate how the interplay between plasmon-enhanced optical and thermal forces affects the dynamics of nanocolloids diffusing in close proximity to gold bowtie nanoantennas. The study is based on an anti-Stokes thermometry technique that can measure the internal antenna temperature with an accuracy of ∼5 K over an extended temperature range. We argue that Kapitza resistances have a significant impact on the local thermal landscape, causing an interface temperature discontinuity of up to ∼20% of the total photothermal temperature increase of the antenna studied. We then use the bowties as plasmonic optical tweezers and quantify how the antenna temperature influences the motion and distribution of nearby fluorescent colloids. We find that colloidal particle motion within the plasmonic trap is primarily dictated by a competition between enhanced optical forces and enhanced heating, resulting in a surprising insensitivity to the specific resonance properties of the antenna. Furthermore, we find that thermophoretic forces inhibit diffusion of particles toward the antenna and drive the formation of a thermal depletion shell that extends several microns. The study highlights the importance of thermal management at the nanoscale and points to both neglected problems and new opportunities associated with plasmonic photothermal effects in the context of nanoscale manipulation and analysis