Andres H. La Rosa

Micro-photonic cylindrical waveguide based protein biosensor

In this paper we experimentally demonstrate the fabrication and operation of a rapidly prototyped optical cylindrical micro-waveguide based biosensor. This device works on the principle of variation to the light intensity and path of coupled input light due to the binding of protein bio-molecules onto the micro-waveguide surface as a method of physical transduction. The variation to the coupled light intensity and path is dependent on the nature of the bio-molecule and the density of the bio-molecules. This technique has been used to identify protein biomarkers for inflammation and thrombosis, namely myeloperoxidase (MPO) and C-reactive protein (CRP). The detection limit that has been demonstrated is pg?ml?1. The detection speed is of the order of seconds from the time of injection of the bio-molecule. The optical signature that is obtained to identify a protein bio-molecule is entirely dependent on the nature of adsorption of the bio-molecule on to the cylindrical cavity surfaces. This in turn is dependent on the protein conformation and the surface charge of the bio-molecules. Hence a specific protein bio-molecule generates a unique optical identifier based on the nature of binding/adsorption to the cavity surface. This physical phenomenon is exploited to identify individual proteins. This technique is a demonstration of detection of nano-scale protein bio-molecules using the optical biosensor technique with unprecedented sensitivity.

Compact method for optical induction of proximal probe heating and elongation

A tapered, metal-coated, optical fiber probe will elongate when heated by light input through a fiber. The induced motion can be used for data storage or nanostructuring of a surface. The elongation produced by this alignment-free system is measured with force feedback in a near-field scanning optical microscope (NSOM). The input light intensity controls the elongation magnitude, which ranges from a few nanometers to more than 100 nm. A 0.5-mW input energy yields approximately 20 nm of probe elongation. The elongation quantified here can create artifacts in any experiment using pulsed laser light with a NSOM or an atomic force microscope.

Time-resolved contrast in near-field scanning optical microscopy of semiconductors

We demonstrate the ability of near-field scanning optical microscopy (NSOM) technique to detect inhomogeneities of the dynamics of excess carriers in oxidized silicon wafers. NSOM is used to improve the spatial resolution of a standard IR-scattering optical technique, which is carried out in a noncontact fashion. Continuous wave infrared light is used as a detector of the time dependent carrier population produced by a pulsed visible laser. We will show high resolution images of carrier lifetime, and discuss some aspects of the NSOM measurement that differentiate it from its far field counterpart.

Investigation of the probe-sample interaction in the ultrasonic/shear-force microscope: The phononic friction mechanism

The dissipative and conservative interactions between a sharp probe and a flat Si sample in the ultrasonic/shear-force microscope are investigated. It is shown that, when working in the ambient condition, there are two distinct probe-sample interaction regions: the pure dissipative interaction region in the relatively far probe-sample distance, and the highly correlated dissipative and conservative interaction region in the close probe-sample distance. The ultrasonic data suggest that the phonon generation is a dissipative channel for the probe-sample interaction in the shear force microscope. A shaking potential model is proposed to explain the phononic friction mechanism.

Nano-structure Formation Driven by Local Protonation of Polymer Thin Films

We report the creation of nano-structures via Dip Pen Nanolithography by locally exploiting the mechanical response of polymer thin films to an acidic environment. Protonation of cross linked poly(4-vinylpyridine) (P4VP) leads to a swelling of the polymer. We studied this process by using an AFM tip coated with a pH 4 buffer. Protons migrate through a water meniscus between tip and sample into the polymer matrix and interact with the nitrogen of the pyridyl group forming a pyridinium cation. The increase in film thickness, which is due to Coulomb repulsion between the charged centers, was investigated using Atomic Force Microscopy. The smallest structures achieved had a width of about 40 nm. Different control experiments support our claim that the protonation is the reason for the swelling and therefore the formation of the structures. Kelvin probe force microscopy measurements suggest the presence of counter ions which compensate the positively charged pyridinium ions. We investigated the influence of the water meniscus on the structure formation by varying the relative humidity in the range from 5% to 60% for different dwell times. The diffusion of protons and counter ions is humidity-dependent and requires a water meniscus.

Whispering-gallery acoustic sensing: characterization of mesoscopic films and scanning probe microscopy applications.

Full understanding of the physics underlying the striking changes in viscoelasticity, relaxation time, and phase transitions that mesoscopic fluid-like films undergo at solid-liquid interfaces, or under confinement between two sliding solid boundaries, constitutes one of the major challenges in condensed matter physics. Their role in the imaging process of solid substrates by scanning probe microscopy (SPM) is also currently controversial. Aiming at improving the reliability and versatility of instrumentation dedicated to characterize mesoscopic films, a noninvasive whispering-gallery acoustic sensing (WGAS) technique is introduced; its application as feedback control in SPM is also demonstrated. To illustrate its working principle and potential merits, WGAS has been integrated into a SPM that uses a sharp tip attached to an electrically driven 32-kHz piezoelectric tuning fork (TF), the latter also tighten to the operating microscopes frame. Such TF-based SPMs typically monitor the TFs state of motion by electrical means, hence subjected to the effects caused by the inherent capacitance of the device (i.e., electrical resonance differing from the probes mechanical resonance). Instead, the novelty of WGAS resides in exploiting the already existent microscopes frame as an acoustic cavity (its few centimeter-sized perimeter closely matching the operating acoustic wavelength) where standing-waves (generated by the nanometer-sized oscillations of the TFs tines) are sensitively detected by an acoustic transducer (the latter judiciously placed around the microscopes frame perimeter for attaining maximum detection). This way, WGAS is able to remote monitoring, via acoustic means, the nanometer-sized amplitude motion of the TFs tines. (This remote-detection method resembles the ability to hear faint, but still clear, levels of sound at the galleries of a cathedral, despite the extraordinary distance location of the sound source.) In applications aiming at characterizing the dynamics of fluid-like mesoscopic films trapped under shear between the TF probe and the solid substrate, WGAS capitalizes on the well-known fact that the TFs motion is sensitively affected by the shear-forces (the substrate and its adsorbed mesocopic film playing a role) exert on its tip, which occurs when the latter is placed in close proximity to a solid substrate. Thus, WGAS uses a TF as an efficient transducer sandwiched between (i) the probe (that interact with the substrate and mesoscopic film), and (ii) the acoustic cavity (where an assessment of the probe mechanical motion is obtained). In short, WGAS has capability for monitoring probe-sample shear-force interactions via remote acoustic sensing means. In another application, WGAS can also be used as feedback control of the probes vertical position in SPM. In effect, it is observed that when the microscopes probe stylus approaches a sample, a monotonic change of the WGAS acoustic signal occurs in the last ~20 nm before the probe touches the solid samples surface, which allows implementing an automated-control of the probe-sample distance for safely scanning the tip across the sample surface. This principle is demonstrated by imaging the topographic features of a standard sample. Finally, it is worth to highlight that this alignment-free acoustic-based method offers a very direct assessment of the probes mechanical motion state (the mechanical and the WGAS acoustic frequency responses coincide), which makes the WGAS a convenient metrology tool for studying surface interactions, including interfacial friction at the nanometer scale.

Thermal/temporal response of the NSOM probe/sample system

In measurements of sample temporal response with a near-field scanning optical microscope, or NSOM, one must account for the temporal response of the probe. The coupling of thermal and temporal effects in an NSOM fitted with a coated tapered fiber probe is considered. Study of the perturbation of cw infrared light by a pulse of visible light simultaneously sent through an illumination mode NSOM allows one to separate the relatively slow thermal response of the probe from the appreciably faster response of a silicon sample imaged with the probe. Temporal and thermal contrast in NSOM imaging are discussed in terms of the results.

Acousto characterization of fluid-like mesoscopic films under shear

Full understanding of the physics underlying the striking changes—in viscoelasticity, relaxation time, and phase transitions—that mesoscopic fluid-like systems undergo when placed under confinement or when adsorbed at solid surfaces constitutes a long standing scientific challenge. One of the methods used to characterize these films consists of bringing a solid boundary closer to another solid boundary (while in relative lateral periodic motion) with a liquid trapped in between. In addition, using a tapered probe (∼ 50 nm apex diameter) as one of the boundaries improves the lateral resolution of the measurement. In this scenario, the dynamics of the fluid is inferred from the changes in the tapered probes motion. However, due to the complexity of the films dynamics, different and sometimes conflicting experimental results are reported; in particular, for example, whether the motion of the probe changes due to its interaction with the fluid alone, or due to its intermittent mechanical contact with the solid substrate. Newer analytical methods would be highly desirable. Herein we report the monitoring of mesoscopic film dynamics from an acoustic measurements perspective (complemented with other more conventional sensing methods for control and comparison purposes). More specifically, two acoustic-based methods, Whispering-Gallery Acoustic Sensing or WGAS (that uses an acoustic sensor attached to a tapered probe) and Shear-force/Acoustic Near-field Microscopy or SANM (that uses an acoustic sensor attached to the solid substrate), monitor the effects that shear-force interactions exert not only on the laterally oscillating probe but also on the trapped mesoscopic fluid itself (as acoustic waves engendered at the fluid film couple into the static substrate and subsequently reaching the SANM acoustic transducer). One significant result of these measurements constitute the supporting evidence that the probes motion is affected even when not in mechanical contact with the solid substrate, hence highlighting the role played by the adsorbed mesoscopic fluid layer as the source of the shear force interactions. On the other hand, to further support the SANM working principle (i. e. the measurement of acoustic waves engendered at adsorbed films of nanometer-sized) control experiments have also been performed for interrogating the dynamics of small millimeter-sized drops of water.

The Ultrasonic/Shear-Force Microscope: A Metrology Tool for Surface Science and Technology

This paper describes recent results obtained with the Ultrasonic/Shear-Force Microscope (SUNM), an analytical tool suitable for investigating the quite different dynamic displayed by fluid-like films when subjected to mesoscopic confinement and while in intimate contact with two sliding solid boundaries. The SUNM uses two sensory modules to concurrently but independently monitor the effects that fluid-mediated interactions exert on two sliding bodies: the microscopes sharp probe (attached to a piezoelectric sensor) and the analyzed sample (attached to an ultrasonic transducer). This dual capability allows correlating the fluid-like films viscoelastic properties with changes in the probes resonance frequency and the generation of sound. A detailed monitoring of sliding friction by ultrasonic means and with nanometer resolution is unprecedented, which opens potential uses of the versatile microscope as a surface and subsurface material characterization tool. As a surface metrology tool, the SUNM presents a potential impact in diverse areas ranging from fundamental studies of nanotribology, confinement-driven solid to liquid phase transformation of polymer films, characterization of industrial lubricants, and the study of elastic properties of bio-membranes. As a sub-surface metrology tool, the SUNM can be used in the investigation of the elastic properties of low- and high-k dielectric materials, piezoelectric and ferroelectric films, as well as quality control in the construction of micro- and nano-fluidics devices.

Near-field acousto monitoring shear interactions inside a drop of fluid: The role of the zero-slip condition

A full understanding of nanometer-range (near-field) interactions between two sliding solid boundaries, with a mesoscopic fluid layer sandwiched in between, remains challenging. In particular, the origin of the blue-shift resonance frequency experienced by a laterally oscillating probe when approaching a substrate is still a matter of controversy. A simpler problem is addressed here, where a laterally oscillating solid probe interacts with a more sizable drop of fluid that rests on a substrate, aiming at identifying interaction mechanisms that could also be present in the near-field interaction case. It is found that the inelastic component of the probe-fluid interaction does not constitute the main energy-dissipation channel and has a weak dependence on fluid’s viscosity, which is attributed to the zero-slip hydrodynamic condition. In contrast, the acoustic signal engendered by the fluid has a stronger dependence on the fluid’s viscosity (attributed also to the zero-slip hydrodynamic condition) and correlates well with the probe’s resonance frequency red-shift. We propose a similar mechanism happens in near field experiments, but a blue-shift in the probe’s resonance results as a consequence of the fluid molecules (subjected to the zero-slip condition at both the probe and substrate boundaries) exerting instead a spring type restoring force on the probe.