Intentional nonlinearity in the small scale with applications to atomic force microscopy (AFM) and mass sensing

Over the past several decades, the development of ultra-sensitive nano/micromechanical sensor technology has had a transformative effect on the field of nanoscience. These devices are currently used in many different applications including biological, chemical and inertial sensing; atomic force microscopy and infrared spectroscopy; and precise time keeping and synchronization. Traditionally, these systems were studied within the framework of linear dynamics, and incidental nonlinearity was suppressed by design. More recently, researchers have intentionally incorporated nonlinearity in the design of such devices in order to exploit the rich nonlinear behavior. Some of the nonlinear phenomena that researchers aim to utilize include internal resonance, resonant bandwidth expansion, ultra-sensitive bifurcation frequencies associated with sudden jumps in the response, coexistence of multiple solution branches and higher harmonic generation. In this dissertation, I investigate further ways in which intentional nonlinearity can be leveraged to enhance micromechanical resonant sensing techniques. In particular, I focus on applications to AFM and mass sensing. Within the area of AFM, the performance of a new cantilever design during multi-frequency tapping mode AFM is studied. The system consists of a base cantilever with an inner paddle which, under harmonic excitation, vibrates like a system of linearly coupled oscillators engaging simultaneously a lower, in-phase and a higher, out-of phase resonant mode. The cantilever is designed so that the 2nd mode frequency (i.e., the out-of-phase eigenfrequency) coincides with an integer multiple of the fundamental mode frequency, providing the necessary conditions for realization of internal resonance. During tapping mode, the nonlinear tip-sample force activates the internal resonance and thereby amplifies the out-of-phase resonant mode. In contrast to other multi-frequency AFM techniques, the advantage of this approach is that multiple harmonics with strong signal-to-noise ratios (SNR) are excited while maintaining the simplicity of a single excitation frequency. The ability of this inner-paddled cantilever to measure compositional properties of polymers and bacteria was studied, and it was found that the internal resonance-based design results in enhanced sensitivity to Young’s modulus. In another study, a new micromechanical resonant mass sensor design is introduced consisting of a doubly clamped beam having a concentrated mass at its center, subjected to harmonic base excitation. The resonator is specifically designed to exhibit geometric nonlinearity due to midplane stretching. The reduced order model of the system’s fundamental bending mode is that of a Duffing oscillator (i.e., an oscillator having cubic stiffness in addition to linear stiffness) under harmonic base excitation. For positive cubic stiffness, it is well known that the Duffing oscillator exhibits hardening in the frequency response curve resulting in a broadband resonance. The bandwidth of the resonator is determined by the linear resonant frequency (lower bound) and the jump-down bifurcation frequency (upper bound). Under harmonic excitation at a fixed forcing level, the jump down bifurcation frequency is proportional to the forcing level, and at each forcing level there indeed exists a jump down bifurcation. In the proposed system, the forcing level is not fixed; rather, it is proportional to the square of the driving frequency of the base excitation. Interestingly, analytical and computational analyses predict the existence of a critical excitation amplitude above which there is no theoretically predicted jump down bifurcation. It is shown that the effect of the concentrated mass is to lower the threshold of the critical excitation amplitude to a realizable level. In practice, there must inevitably be a jump down bifurcation and this bifurcation may be triggered by the excitation of internal resonances, shrinking domain of attraction of the upper solution branch, variations in the initial state due to noise and/or the presence of nonlinear damping. However, the critical excitation amplitude appears to correspond to sudden and significant bandwidth expansion. Experimental results from a Duffing-like oscillator provide some verification of the powerful theoretical predictions. Ultimately, by operating at an excitation amplitude above the critical level, the ultra-wide resonant bandwidth can be exploited in a mass detection scheme based on amplitude tracking. In comparison to other micromechanical mass sensors, this technique and design offers a wide range of operational frequencies and amplitudes with strong SNR, eliminates the need for frequency sweeping and sophisticated feedback control, and requires relatively simple actuation and microfabrication methods

Resonant Cantilever Sensing: From Model Systems to Application

Micro and nanomechanical resonators are highly sensitive, label-free analyte sensors in a range of environments. Resonant cantilevers, i.e. those operated in dynamic mode, can be considered as mechanical oscillators, with analyte adsorption creating a shift in cantilever resonance. Cantilever sensors work via a purely mechanical approach, transducing an analyte binding event into a nanomechanical signal. This response is governed by changes in sensor mass and stiffness due to adsorbed analytes, with previous theoretical work predicting the latter to produce significant effects on measured frequency shifts, counteracting effects of adsorbed mass. This highlights a particularly unsatisfactory feature of micro/nano-mechanical sensors, as an accurate interpretation of the sensor response must depend on both adsorbate mass and rigidity, which for nanometer-scale coverage can only be guessed, rather than derived from independent measurements. In this thesis, procedures to disentangle such effects in air and liquid are discussed and tested on a range of surface coatings, offering a novel method of analyte detection and analysis. The dynamic characteristics of cantilever beams are strongly dependent on the mass density and viscosity of the fluid in which the beams are immersed. The application of cantilevers in accurately determining such rheological properties is also presented, first via the use of model solutions, and then extending measurements to a range of commercial alcoholic and non-alcoholic drinks. A method to quantify alcohol content is also discussed, further demonstrating the commercial applications of cantilever sensors

Nanostructuring of indium tin oxide with sub-15 femtosecond laser pulses for technical and biomedical applications

Modern laser technologies enable the production of nanostructures on a relatively large area for a variety of applications. Nonlinear effects occurring at high light intensities, such as multiphoton absorption, allow structuring in dimensions below the diffraction limit. While existing applications for 2D confined nanostructures are essentially based on selforganizing structures without a well definable profile or in combination with complex processing steps, precisely flexible defined nanostructures were fabricated in this work. It is about the interaction of a tightly focused near infrared sub-15 femtosecond laser (repetition rate: 85 MHz) with sputtered polycrystalline ITO thin films exhibiting different electrical conductivities. Depending on the choice of parameters, total ablation, periodic nano-cuts or else crystal modifications can be generated, rendering the ITO layer resistant to a subsequent etching step. Using this innovative approach, nanowires attached to the substrate or even freestanding nanowires (in combination with a selectively etchable sacrificial layer) with a length-to-width ratio of more than one hundred were prepared. The applicability of such nanowires for self-heating resistive gas sensors for detection of oxidizing gases was demonstrated. As another demonstrator, ITO bioelectrodes were trimmed with respect to their impedance by coherent sub-20 nm wide and several microns long nano-cuts.Moderne Lasertechnologien ermöglichen eine relativ großflächige Herstellung von Nanostrukturen für eine Vielzahl von Anwendungen. Bei hohen Lichtintensitäten auftretende nichtlineare Effekte, wie z.B. Multiphotonenabsorption, erlauben das Unterschreiten der Beugungsgrenze. Während existierende Anwendungen für räumlich 2D beschränkte Nanostrukturen wesentlich auf selbstorganisierenden, wenig steuerbaren Profile in Kombination mit aufwändigen Prozessen basieren, konnten in der vorliegenden Arbeit präzise definierbare Nanostrukturen hergestellt werden. Zentraler Punkt dieser Arbeit ist die Wechselwirkung eines eng fokussierten nahinfraroten Sub-15-Femtosekunden-Laserstrahls (Wiederholrate: 85 MHz) mit gesputterten ITO-Dünnschichten, die unterschiedliche elektrische Leitfähigkeiten aufweisen. Je nach Wahl der Parameter können totale Ablation, periodische Nanoschnitte aber auch Kristallmodifikationen erreicht werden, die die ITO-Schicht resistent gegenüber einem nachfolgenden Ätzschritt machen. Mit diesem innovativen Prozessansatz konnten substratgebundene bzw. freistehende (in Kombination mit einer selektiv ätzbaren Opferschicht) Nanodrähte mit einem Längen-zu-Breiten-Verhältnis von mehr als einhundert hergestellt werden. Die Verwendbarkeit solcher selbstheizbarer Drähte zur resistiven Detektion von oxidierenden Gasen konnte demonstriert werden. Als weiterer Demonstrator wurden ITO-Bioelektroden mit kohärenten sub-20 nm-breiten und mehrere Mikrometer langen Nanoschnitten bezüglich ihrer Impedanz modifiziert

Cooling and sensing using whispering gallery mode resonators

This thesis reports on a detailed exploration of the optomechanical interaction between a tapered optical fibre and a silica microsphere mounted on a cantilever. The amount of light evanescently coupled from the fibre into the optical whispering gallery mode of the sphere is exquisitely sensitive to their separation allowing fast measurement of picometre displacements of both the microsphere-cantilever and the fibre. By exploiting this enhanced transduction, strong active feedback damping/cooling of the thermal motion of both the fibre and microsphere-cantilever have been demonstrated to the noise limit of the system. The cavity enhanced optical dipole force between the fibre and the sphere was used to damp multiple mechanical modes of the tapered fibre, while a piezo-stack at the clamped end of the microsphere-cantilever allowed for cooling of its centre-of-mass motion and the second mechanical eigenmode. The effect of noise within the feedback loop was shown to invert the measured mechanical mode spectrum at high feedback gain as the noise itself is fed into the resonator. A rich variety of feedback induced spring stiffening and softening of the mode is measured when time delays are introduced. Cooling of the mechanical modes of the taper, which are ubiquitous to many whispering gallery mode experiments and are considered as unwanted noise, has not been achieved previously. Simultaneous operation of both feedback schemes was demonstrated for the first time, providing stabilization of the system. By using the microsphere-cantilever as an inertial test mass, measurement of its displacement induced by acceleration can resolve micro-g accelerations at high bandwidth

Nanostructuring of indium tin oxide with sub-15 femtosecond laser pulses for technical and biomedical applications

Modern laser technologies enable the production of nanostructures on a relatively large area for a variety of applications. Nonlinear effects occurring at high light intensities, such as multiphoton absorption, allow structuring in dimensions below the diffraction limit. While existing applications for 2D confined nanostructures are essentially based on selforganizing structures without a well definable profile or in combination with complex processing steps, precisely flexible defined nanostructures were fabricated in this work. It is about the interaction of a tightly focused near infrared sub-15 femtosecond laser (repetition rate: 85 MHz) with sputtered polycrystalline ITO thin films exhibiting different electrical conductivities. Depending on the choice of parameters, total ablation, periodic nano-cuts or else crystal modifications can be generated, rendering the ITO layer resistant to a subsequent etching step. Using this innovative approach, nanowires attached to the substrate or even freestanding nanowires (in combination with a selectively etchable sacrificial layer) with a length-to-width ratio of more than one hundred were prepared. The applicability of such nanowires for self-heating resistive gas sensors for detection of oxidizing gases was demonstrated. As another demonstrator, ITO bioelectrodes were trimmed with respect to their impedance by coherent sub-20 nm wide and several microns long nano-cuts.Moderne Lasertechnologien ermöglichen eine relativ großflächige Herstellung von Nanostrukturen für eine Vielzahl von Anwendungen. Bei hohen Lichtintensitäten auftretende nichtlineare Effekte, wie z.B. Multiphotonenabsorption, erlauben das Unterschreiten der Beugungsgrenze. Während existierende Anwendungen für räumlich 2D beschränkte Nanostrukturen wesentlich auf selbstorganisierenden, wenig steuerbaren Profile in Kombination mit aufwändigen Prozessen basieren, konnten in der vorliegenden Arbeit präzise definierbare Nanostrukturen hergestellt werden. Zentraler Punkt dieser Arbeit ist die Wechselwirkung eines eng fokussierten nahinfraroten Sub-15-Femtosekunden-Laserstrahls (Wiederholrate: 85 MHz) mit gesputterten ITO-Dünnschichten, die unterschiedliche elektrische Leitfähigkeiten aufweisen. Je nach Wahl der Parameter können totale Ablation, periodische Nanoschnitte aber auch Kristallmodifikationen erreicht werden, die die ITO-Schicht resistent gegenüber einem nachfolgenden Ätzschritt machen. Mit diesem innovativen Prozessansatz konnten substratgebundene bzw. freistehende (in Kombination mit einer selektiv ätzbaren Opferschicht) Nanodrähte mit einem Längen-zu-Breiten-Verhältnis von mehr als einhundert hergestellt werden. Die Verwendbarkeit solcher selbstheizbarer Drähte zur resistiven Detektion von oxidierenden Gasen konnte demonstriert werden. Als weiterer Demonstrator wurden ITO-Bioelektroden mit kohärenten sub-20 nm-breiten und mehrere Mikrometer langen Nanoschnitten bezüglich ihrer Impedanz modifiziert

Fifteenth Space Simulation Conference: Support the Highway to Space Through Testing

The Institute of Environmental Sciences Fifteenth Space Simulation Conference, Support the Highway to Space Through Testing, provided participants a forum to acquire and exchange information on the state-of-the-art in space simulation, test technology, thermal simulation and protection, contamination, and techniques of test measurements

Laboratory Directed Research and Development Program FY 2005 Annual Report

The Oak Ridge National Laboratory (ORNL) Laboratory Directed Research and Development (LDRD) Program reports its status to the U.S. Department of Energy (DOE) in March of each year. The program operates under the authority of DOE Order 413.2A, 'Laboratory Directed Research and Development' (January 8, 2001), which establishes DOE's requirements for the program while providing the Laboratory Director broad flexibility for program implementation. LDRD funds are obtained through a charge to all Laboratory programs. This report describes all ORNL LDRD research activities supported during FY 2005 and includes final reports for completed projects and shorter progress reports for projects that were active, but not completed, during this period. The FY 2005 ORNL LDRD Self-Assessment (ORNL/PPA-2006/2) provides financial data about the FY 2005 projects and an internal evaluation of the program's management process. ORNL is a DOE multiprogram science, technology, and energy laboratory with distinctive capabilities in materials science and engineering, neutron science and technology, energy production and end-use technologies, biological and environmental science, and scientific computing. With these capabilities ORNL conducts basic and applied research and development (R&D) to support DOE's overarching national security mission, which encompasses science, energy resources, environmental quality, and national nuclear security. As a national resource, the Laboratory also applies its capabilities and skills to the specific needs of other federal agencies and customers through the DOE Work For Others (WFO) program. Information about the Laboratory and its programs is available on the Internet at <http://www. ornl.gov/>. LDRD is a relatively small but vital DOE program that allows ORNL, as well as other multiprogram DOE laboratories, to select a limited number of R&D projects for the purpose of: (1) maintaining the scientific and technical vitality of the Laboratory; (2) enhancing the Laboratory's ability to address future DOE missions; (3) fostering creativity and stimulating exploration of forefront science and technology; (4) serving as a proving ground for new research; and (5) supporting high-risk, potentially high-value R&D. Through LDRD the Laboratory is able to improve its distinctive capabilities and enhance its ability to conduct cutting-edge R&D for its DOE and WFO sponsors. To meet the LDRD objectives and fulfill the particular needs of the Laboratory, ORNL has established a program with two components: the Director's R&D Fund and the Seed Money Fund. As outlined in Table 1, these two funds are complementary. The Director's R&D Fund develops new capabilities in support of the Laboratory initiatives, while the Seed Money Fund is open to all innovative ideas that have the potential for enhancing the Laboratory's core scientific and technical competencies. Provision for multiple routes of access to ORNL LDRD funds maximizes the likelihood that novel and seminal ideas with scientific and technological merit will be recognized and supported

Functional complex plasmonics : understanding and realizing chiral and active plasmonic systems

The present thesis concerns itself with the theoretical study and experimental realization of complex plasmonic systems for highly integrated nanophotonic devices and enhanced chiroptical spectroscopy. In particular, the two broad topics of active metasurfaces and chiral plasmonic systems are investigated to this end. In this context, the chalcogenide phase change material GeSbTe is utilized to demonstrate, for the first time, metasurface based beam steering and varifocal lensing devices. The versatility of this approach to lending active functionality to plasmonic systems is further evidenced through our realization of a chiral plasmonic system that both exhibits a wavelength tunable and handedness switchable chiroptical response. Furthermore, in order to enable a systematic study of plasmon- enhanced chiroptical spectroscopy, we rst establish and analyze canonical chiral plasmonic building blocks, in particular, the loop wire and chiral dimer structure. The results from this undertaking lead to fundamental insights for understanding complex chiral plas- monic systems. Finally, we implement chiral media in the commercial electromagnetic full- field solver Comsol Multiphysics to carry out rigorous numerical studies of the macroscopic electrodynamic processes involved in plasmon-enhanced circular dichroism spectroscopy revealing both substantial enhancement due to near-field effects as well as upper boundaries to the magnitude of such enhancements