CNST Nanotechnology Seminar Series Archive - 2006

Microphotonic devices based on planar fabrication technology have the potential to significantly impact the development of quantum optical devices for quantum information processing and computing. In this talk, I will address some of the opportunities and challenges in this field, and describe our efforts in developing chip-based, semiconductor optical microcavities for observing coherent quantum interactions between photons and semiconductor quantum dots. In particular, I will present work that addresses the following topics: 1) the demonstration of low optical losses in a wavelength-scale microcavity, 2) the development of an efficient fiber optic channel through which the sub-micron-scale microcavity field can be accessed and interrogated, and 3) the integration of fiber-coupled microcavities into the high-vacuum, cryogenic environment necessary for the current generation of cavity QED experiments with quantum dots. The design, fabrication, and characterization of high quality factor photonic crystal and microdisk cavities in the InP, Si, and AlGaAs material systems will be presented, as will application of this work to room-temperature, fiber-coupled, microdisk-quantum-dot lasers. Extensions of this work to areas outside of quantum optics will also be considered.

The rapidly developing field of plasmonics has captured the imagination of physicists, chemists and engineers because of the unique ability to control optical dispersion and localize light in metallodielectric structures at nanoscale dimensions. Many ideas are currently being generated by researchers, which may ultimately enable plasmonic components to form building blocks of a chip-based optical device technology with potential imaging, spectroscopy and interconnection applications in ultramicroscopy, computing, communication and chemical/biological detection.
In this talk I will describe recent opportunities presented by new plasmonic components including: i) design of metal-insulator-metal plasmon waveguide ‘heterostructures’ that facilitate dispersion control to enable very high positive as well as negative effective refractive index in the visible and near infrared; ii) Si CMOS compatible light near-infrared light sources for coupling into plasmonic networks; iii) plasmon-enhanced emission from quantum dots; and iv) active plasmonic devices based on electro-optic and all-optical modulation of plasmon propagation.
Finally, amid the exuberance currently felt by plasmonics researchers, it is worthwhile to ponder the potential technological and scientific limitations that we currently face, and how we might take the next steps toward integrated plasmonic circuit and system technologies with compelling applications.

Nanomaterials provide capabilities to introduce new functionalities as well as enhance bulk functionalities thereby enabling new and/or improved technologies. Nanomaterials, create opportunities for emerging, multidisciplinary new fields of nanophotonics, nanoplasmonics, nanomagnetics and nanomedicine. Their impact in areas of global priorities will be discussed. Examples chosen are from on-going research and development at the Institute for Lasers, Photonics and Biophotonics.

Nanophotonics deals with manipulation of optical excitation and dynamics on a nanoscale, opening opportunities for many optical and optoelectronic technolgies. Nanoplasmonics is an area of nanophotonics that deals with optically generated interfacial electromagnetic excitations in metallic nanostructures. Nanomagnetics deals with control, manipulation and utilization of magnetic interactions on nanoscale. Nanomedicine is a new field holding tremendous promise for the 21^st Century health care, where nanoparticles are used to provide nanoprobes for imaging and sensing aimed towards early detection of diseases, as well as nanotherapeutics for targeted, more effective treatment. Opportunities in these fields will be highlighted.

Metamaterials are engineered composite media with unconventional electromagnetic and optical properties. They can be formed by embedding sub-wavelength inclusions as artificial molecules in host media in order to exhibit specific desired response functions that are not readily available in nature, but physically realizable. These metamaterials have exciting characteristics in manipulating and processing RF, microwave, IR and optical signal information. In my group, we have been interested in various features of these media and have been developing some of the fundamental concepts, theories, and modeling of wave interaction with a variety of structures and systems involving these material media. From our analyses and simulations, we have found that the devices and components formed by these media may be ultracompact and subwavelength, while supporting resonant and propagating modes. This implies that in such structures RF, microwave, IR and optical signals can be controlled and reshaped beyond the diffraction limits, leading to the possibility of miniaturization of optical interconnects and design and control of near-field devices. There has been a growing interest in the field of plasmonics in recent years due to the significant activities in the areas of nanotechnology and nanooptics and exciting potentials for merging of the two fields of nanooptics and nanoelectronics. Localization and enhancement of optical fields in sub-wavelength regions, and transport of optical energy along guided-wave plasmonic structures with lateral confinement beyond diffraction limits provide possibilities for unprecedented miniaturization and integration of nanoscale optical elements and components. We are also interested in nano-optics of metamaterial/plasmonic structures that effectively act as lumped nanocircuit elements at optical frequencies. These may provide nano-inductors, nano-capacitors, and nano-resistors as part of field nanocircuits in the optical regimes – or optical-field nanoelectronics--, and can provide roadmaps to more complex optical nanocircuits and systems formed by collection of such nanostructures. Nanoplasmonic structures can open doors to a new paradigm for manipulation and processing of optical signals in ultracompact sub-wavelength regions, and may offer various potential applications in high-resolution near-field imaging and microscopy, enhancement or reduction of wave interaction with nano-particles and nano-apertures, nanoantennas and arrays, far-field sub-diffraction optical microscopy (FSOM), optical nano-circuit-filters, optical data storage, nano-beam patterning and spectroscopy, optical-molecular signaling and optical coupling and interfacing with cells, to name a few.
In this talk, I present an overview of the concepts, salient features, recent developments, and some of the potential applications of these metamaterials and nanoplasmonic structures, and will forecast some futures ideas and directions in this area.

High-resolution force microscopy is a tool not only for imaging surfaces with high resolution but also for measuring force and dissipation at the nanometer scale. We will discuss two examples: A study of atomic friction with high bandwidth has revealed the role of thermal fluctuations in atomic jump processes. The combination of quasi-static indentation experiments with dynamic non-contact imaging allows to study the elementary steps of plasticity. Atomic-scale yield has been correlated to the nucleation of dislocations which penetrate the surface close to the indentation point. Finally, we will generally comment on future prospects of a dissipation force microscopy with atomic resolution.

Over the past decade, atomic force microscopy (AFM) utilizing an oscillating cantilever and the frequency modulation (FM) technique was developed as a versatile tool to image virtually all kinds of surfaces independent of their conductivity with high resolution. Initially, it was mainly performed under ultra-high vacuum conditions in the attractive non-contact regime (NC-AFM) of the tip-sample interaction to obtain true atomic resolution. Since the snap-to-contact can be avoided in this mode of operation, the distance dependence of the tip-sample force F(z) can be recorded continuously, even deep into the repulsive regime. By recording such curves on every (x,y)-image point, complete 3-dimensional force fields F(x,y,z) could be acquired with atomic resolution.
Due to the high Q-values in vacuum, this technique has also increased the sensitivity of magnetic force microscopy (MFM), which detects the long-range magnetostatic tip-sample interaction. The most recent breakthrough is the demonstration of magnetic exchange force microscopy (MExFM). It combines the high magnetic sensitivity of MFM with the atomic resolution capability of AFM to image the arrangement of the magnetic moments at surfaces via the short-range magnetic exchange interaction between tip and sample spins.

Despite the challenges associated with the cost and technological innovation, the semiconductor industry has introduced nanoelectronics into the market place. By extending traditional CMOS (complimentary metal oxide semiconductor transistors) through new materials in both the devices and on-chip interconnect, planar CMOS is already manufactured with transistor gate lengths less than 50 nm. Further extending CMOS will not only require new materials, but it may also drive the use of new non-planar device structures for both transistors and interconnection. At some point, CMOS extension may no longer be possible, and then a new switch would be necessary. The transition between CMOS extension and planar CMOS could involve the gradual incorporation of Beyond CMOS technology. The number of candidates continues to grow even as some possible technology is finds itself removed from consideration. Characterization and Metrology is finding each step in this transistor to be a great challenge. This presentation overviews the characterization and metrology necessary for both CMOS extension and beyond CMOS semiconductor technology. The International Technology Roadmap for Semiconductors is used a guide that provides a roadmap for both metrology and process technology that extends to beyond CMOS materials and devices.

The lack of high-speed quantitative measurements of mechanical properties at the nanoscale has been identified as a major bottleneck for the development of new materials. To achieve high fidelity mechanical measurements with temporal resolution of nanoseconds and spatial resolution of nanometers, we have explored three approaches, namely, tapping harmonics; contact resonance and quasi-static indenting. Combination of these methods allowed us to achieve highly consistent stiffness measurements that are scalable for materials from a few MPa to tens of GPa. Various models used to extract elastic modulus and hardness have been developed and validated through experiments and finite elements analysis. This presentation will discuss critical factors that lead to reliable mechanical measurements with special emphasis on innovative experimental techniques that address some of the main barriers currently preventing AFMs from being widely used for mechanical measurement. We will also address some of the key unresolved issues.

The STM has rapidly found its place in most surface science laboratories as a tool to obtain atomically resolved images of conductive samples. Obtaining atomic resolution by Atomic force microscopy (AFM) took almost a decade because of the special challenges faced by AFM with respect to STM AFM is a method that now allows to routinely image conductive and insulating surfaces with atomic resolution. One of the challenges faced by AFM researchers originates in the physics of measuring the small forces that act between the tip of a force sensing cantilever and the sample. Frequency modulation AFM, where the cantilever’s oscillation frequency is used to determine the forces acting between tip and sample is the method of choice for atomic AFM imaging as frequency measurements are among the most precise physical measurements possible. Early measurements used silicon cantilevers with a stiffness of k 20 N/m and oscillation amplitudes of A 10 nm. From 1999, we used cantilevers made from quartz with k approx. 2 kN/m with sub-nm amplitudes. The stiff cantilever/small oscillation amplitude allows imaging at much smaller tip-sample distances which greatly improves spatial resolution. Even greater resolution can be obtained by monitoring the higher harmonics that occur when a cantilever vibrates in a non-harmonic tip-sample potential, and the simultaneously recorded STM signal shows much less contrast (Fig. c). If forces and currents were proportional or related by power laws, we would expect to see similar images in combined STM/AFM experiments, but experimentally we find large differences in some cases. This could be explained by the energy range of the electrons that are involved in tunneling currents and forces - STM only probes electrons at the Fermi level, while the bonds that form in AFM operation may involve lower energy states as well.

We have recently completed a series of experiments in which we have pushed the electrical detection and manipulation of ferromagnetic resonance (FMR) into the nanoscale regime, by embedding a single, nanoscale ferromagnetic element in an on-chip, lateral microwave transmission line device. Strong, resonant driving of large-angle magnetization precession in the element is achieved by locating the element in close proximity to the shorted end of a coplanar strip waveguide (CSW), which generates an intense, local microwave magnetic field. The FMR uniform precession mode is detected by measuring the microwave transmission across an inductively coupled detection CSW. Moreover, we demonstrate a method by which the cone angle of the precessing magnetization is precisely measured via the anisotropic magnetoresistance effect of the ferromagnet. I will conclude with some new results from a novel spin electronics device (the spin battery), whose realization was made possible by our advances in the control and understanding of FMR at the nanoscale.

Molecular quantum-dot cellular automata (QCA) is an approach to electronic computing at the single-molecule level which encodes binary information using molecular charge configuration. This approach differs fundamentally from efforts to reproduce conventional transistors and wires using molecules. A QCA molecular cell has multiple redox centers which act as quantum dots. The arrangement of mobile charge among these dots represents the bit. The interaction from one molecule to the next is through the Coulomb coupling-no charge flows from cell to cell. Prototype single-electron QCA devices have been built using small metal dots and tunnel junctions. Logic gates and shift registers have been demonstrated, though at cryogenic temperatures. Molecular QCA would work at room temperature. Molecular implementations have been explored and the basic switching mechanism confirmed. Clocked control of QCA device arrays is possible and requires creative rethinking of computer architecture paradigms. By not using molecules as current switches, the QCA paradigm may offer a solution to the fundamental problem of excess heat dissipation in computation.

The use of magnetic materials to probe biological systems offers a number of intriguing possibilities. Because most biomaterials do not interact with magnetic fields, it is possible to remotely manipulate and detect magnetic particles in vivo. I will review some of the emerging approaches for using magnetic micro- and nanoparticles in biological systems, and describe in more detail specific examples from my own research. In particular, I will discuss how we combine magnetoelectronics and microfluidics for biosensing applications, and our attempts to create a nanomagnetic bioreporter based on a magnetotactic bacterium.

Conventional electronic devices exploit the charge of the electron. Recently, devices have started to exploit the electron's spin. A particular success is the read head used in high-density magnetic recording, where only ten years passed between the basic laboratory discovery and its application in commercial devices. In this talk, I survey the basic physics of spintronics in metallic systems, where most applications have been made. The basic ingredients are newly-discovered and non-classical effects of current flow on magnetism and of magnetism on current flow.

The development of DNA-based sensors and micro-arrays for genetic assays has been an area of intense research over the last decade. DNA is also being studied as a material for the programmed self-assembly of nanostructures because of its unique base-pairing ability, variable base sequence, and ease of synthetic manipulation. All of these applications rely on the hybridization of complementary oligonucleotides with surface-immobilized, single-stranded DNA or RNA, so-called nucleic acid probes. Understanding and controlling the surface structure of bound nucleic acid probes is critical for optimizing hybridization reactions; however, the molecular scale structure of immobilized probes in most DNA sensors is typically poorly defined. The structure of DNA probes will depend on the how the probe interacts with itself, other surrounding probes, and the substrate surface. To elucidate the nature of these interactions, we have studied the adsorption of thiol-derivatized DNA molecules on gold surfaces as a model system. We have used various surface analytical methods to determine the coverage of DNA probes, their conformation, and the nature and extent of probe-surface and probe-probe interactions. This talk will review some of our recent studies including the finding that the four different nucleotides of DNA exhibit dramatically different affinities for gold, a property that can be exploited to form surface-confined DNA brushes of controlled graft density and length.

Collagen type I is the most abundant extracellular matrix protein in the body. In vivo and in vitro, it polymerizes into nanometer size supramolecular fibrils to form an adhesion scaffold for cells in tissues. To develop a model system for understanding the effects of collagen on cellular behavior, we prepared thin film mimics of collagen gels by adsorbed native type I collagen to substrates covered in alkanethiol self-assembled monolayers. Ellipsometric and atomic force microscopy analysis indicated the films were on average 30 nm thick and composed of collagen fibrils that are microns long and as large as several hundred nanometers in diameter. Interestingly, we found that we could dramatically change the supramolecular structure of the adsorbed collagen by varying the surface energy of the adsorbing surface. This feature allows us to systematically control the structural nature of collagen when it is presented to cells as an adhesion matrix. The use of thin film analytical techniques to investigate the structural properties of adsorbed collagen has provided valuable insights into how real extracellular matrices influence cellular behavior. The stability and reproducibility of these protein films suggests they will be useful as reference biological materials for develop of new nanoscale analytical techniques applicable to biological systems.