Search

The growing interest in the field of nanomechanical resonators stems from their potentialuse as high performance sensors of a variety of physical quantities by means of detecting the resonance frequency shift. With the recent advancements in nanofabrication techniques as well as an abundance of new materials, sensors optimized for the detection of mass, force, and temperature with unprecedented responsivities are within reach. Central to many reported nanomechanical sensor investigations has been to maximize the mechanical quality factor Q, which directly results in an enhanced force sensitivity. Concurrently, high-Q resonators present an opportunity to unite the quantum regime with real-world applications by allowing quantum phenomena to be observed at room temperature, which is a topic of the so-called cavity optomechanics research field. As such, methods employed to reliable enhance the Q are in high demand in several fields employing nanomechanical resonators. In order to properly use high-Q resonators, both as sensors as well as building blocks in optomechanics, an in-depth analysis of their properties needs to be made. The most important questions to answer would be, what are the fundamental limits to the dissipation of mechanical resonators, and what sources of fluctuations determine the maximum sensitivities? This thesis aims to partly answer the above questions using high-stress silicon nitride nanomechanical resonators. Such resonators present a nanomechanical system capable of room temperature Qs approaching 1 billion due to the dissipation dilution effect. The presented results follow a linear pattern, first focusing on methods used to enhance the mechanical Q, then discussing fundamental limits to the frequency stability, and finally presenting a possible application of high-Q resonators as thermal sensors. Two approaches to Q-enhancement are presented, one focusing on the optimization of the dissipation dilution effect and the other on reducing intrinsic losses. The former works by widening the clamping of nanomechanical string resonators, which is numerically shown to reduce the curvature at the string clamping, thus potentially circumventing one of the limiting mechanisms of stressed resonators. Through a systematic investigation of strings with different orientations relative to the surrounding substrate, only marginal Q-enhancements result from the method, attributed to the increased material bending resulting from the clamp-widening. Finite element method simulations corroborate all the presented measurements, thus offering a path to further optimization of the resonator design. For nanomechanical resonators, with increasingly large surface-to-volume ratios, surfacefriction becomes the dominant intrinsic loss mechanism. In the second Q-enhancement approach, an effort is made to reduce the surface friction by means of high-temperature annealing of resonators in an ultrahigh vacuum environment. This procedure is performed on a variety of stressed resonator geometries, resulting in a clear Q-enhancement regardless of the geometry. Increases in the intrinsic Q approaching an order of magnitude are observed, which is shown to be directly related to reduced surface losses. The annealing treatment is shown to be reproducible and nondestructive, only marginally influencing the mechanical properties of the resonator. Given that this approach can be applied to current state-of-the-art high-Q resonators in literature, these results present an important step forward in the pursuit of room-temperature quantum experiments. Moving on to the second part of the thesis concerning applying high-Q resonators as sensors, a detailed investigation into the frequency fluctuations of such resonators is presented. Nanomechanical string resonators are employed as the system under consideration and the frequency stability is characterized using the Allan deviation, both in open-loop and closed-loop tracking configurations. For open-loop tracking, it is shown how the Allan deviation is divided into a regime limited by thermomechanical noise and another limited by the back-ground noise of the detection. Despite the latter noise source potentially lower in level than the thermomechanical noise, the frequency stability is shown to always be limited fundamentally by thermomechanical noise with a response time defined by intrinsic properties of the resonator. Closed-loop tracking is shown to offer a response faster than the intrinsic limit at the expense of increased noise. Additionally, when employing optical detection schemes, the laser power fluctuations are shown to be a potentially limiting factor in the frequency stability. Measurements are supported by theory-based calculations of the Allan deviation, showing great agreement, allowing the design of future sensors with optimized frequency stability.The final part of the thesis demonstrates how localized defect modes of low-stress phononiccrystal (PnC) membranes can be utilized as thermal sensors. Here, the thermal response is quantified through laser heating of the membrane center, which shifts the frequency through the photothermal effect. Owing to the perforation of the membranes and the increased over-lap between the defect modes and the temperature field resulting from the laser heat source, the thermal responsivity of PnC membranes is shown to be potentially an order of magnitude greater than that of uniform membranes of equal size. Even larger thermal response is reported through geometrical stress reduction, achieved by fabricating a PnC membrane inside a nanomechanical trampoline. Lastly, defect mode and bandgap tuning through laser heating are presented, demonstrating how the defect mode can be thermally detuned enough to completely exit the phononic bandgap. These results serve as a basis upon which further optimization of resonator design can be made, paving the way for a new class of thermal sensors with unparalleled thermal response.

Historisk sett har forskning på det okulære systemet vært av stor interesse. Teknologiske nyvinninger har gitt muligheter for å kunne forstå det okulære systemet bedre, noe som har resultert i spesifikke enheter for øyemålinger, mindre inngripende instrumenter, og bruksområder basert på teknologien. Kunnskap om stimuluskontroll og variabler som kan påvirke og etablere stimuluskontroll, er særs viktig innenfor atferdsanalyse. Observeringsresponser står sentralt i vurderingen av spørsmål knyttet til stimuluskontroll. Øyebevegelser kan være en måte å måle observeringsresponser på, og utgjør en av de vanligste måtene å tilegne seg informasjon om omgivelsene. Dette kan skje for eksempel ved å rette blikket mot en stimulus før utførelsen av neste handling. For å registrere hvor en person ser kan man bruke eye-tracking som et verktøy. Det å måle hvor folk ser ved bruk av eye-tracking anvendes innen mange fagfelt, og kan være fruktbart i analysen av relasjoner mellom miljø og atferd. I denne artikkelen beskriver vi historiske hovedtrekk i utviklingen av eye-tracking, samt redegjør for de sentrale målingene ved bruken av eye-tracking. Historically, research on the ocular system has been of great interest. Technological innovations have provided opportunities to better understand the ocular system, which has resulted in specific measuring units of eye movements, less intrusive instruments, and applications based on the technology. Knowledge of stimulus control and variables that can influence and establish stimulus control are particularly important in behavioral analysis. Observing responses are central to the assessment of questions related to stimulus control. Eye movements can be a way of measuring observing responses and are one of the most common ways of acquiring information about the environment. This can be done, for example, by looking at a stimulus before responding. To register where a person sees, one can use eye-tracking as a tool. Measuring where people see using eye-tracking is used in many disciplines and can be fruitful in the analysis of relationships between the environment and behavior. In this article, we describe the main historical features in the development of eye-tracking, as well as explain the key measuring units within eye-tracking.

Master i læring i komplekse systemer One of the approaches towards understanding complex human behavior from the behavior analytic position is the stimulus equivalence paradigm. Understanding of stimulus equivalence is of great importance both in regards of conceptual and application purposes. Moreover, eye-tracking—procedures of using high technological equipment to obtain précis and accurate measuring of eye movements—is increasingly applied across of various scientific disciplines. The union of stimulus equivalence and eye-tracking could provide access and advancement in understanding of the nature of stimulus equivalence. Article 1 is a conceptual piece presenting stimulus equivalence and the field of eye- tracking. The role of stimulus equivalences within behavior analysis is discussed and the different variables in which affect the establishment of stimulus equivalence classes is presented. Eye-tracking is introduced with a focus on, its historical highlights; application areas; the technology; different eye movement measures; and behavior analytic research that have employed eye-tracking technology. Article 2 is an empirical study of stimulus equivalence in conjunction with the use of eye-tracking technology. The utilization of eye-tracking allows for the concept of observing response to be used to expand the investigation of how different training structures (i.e., linear series; many-to-one; the one-to-many) influence the establishment of stimulus equivalence class formation, which is the purpose of the study. Results with the additional measures are presented in the form of reaction time; fixation time; fixation rate; and transition rate, and discussed with respect to previous findings within stimulus equivalence research.

High quality factor ($Q$) nanomechanical resonators have received a lot of attention for sensor applications with unprecedented sensitivity. Despite the large interest, few investigations into the frequency stability of high-$Q$ resonators have been reported. Such resonators are characterized by a linewidth significantly smaller than typically employed measurement bandwidths, which is the opposite regime to what is normally considered for sensors. Here, the frequency stability of high-$Q$ silicon nitride string resonators is investigated both in open-loop and closed-loop configurations. The stability is here characterized using the Allan deviation. For open-loop tracking, it is found that the Allan deviation gets separated into two regimes, one limited by the thermomechanical noise of the resonator and the other by the detection noise of the optical transduction system. The point of transition between the two regimes is the resonator response time, which can be shown to have a linear dependence on $Q$. Laser power fluctuations from the optical readout is found to present a fundamental limit to the frequency stability. Finally, for closed-loop measurements, the response time is shown to no longer be intrinsically limited but instead given by the bandwidth of the closed-loop tracking system. Computed Allan deviations based on theory are given as well and found to agree well with the measurements. These results are of importance for the understanding of fundamental limitations of high-$Q$ resonators and their application as high performance sensors., Comment: 8 pages, 6 figures