Eric Ayars

Surface enhancement in near-field Raman spectroscopy

The intensity and selection rules of Raman spectra change as a metal surface approaches the sample. We study the distance dependence of the new Raman modes with a near-field scanning optical microscope (NSOM). The metal-coated NSOM probe provides localized illumination of a metal surface with good distance control. Spectra are measured as the probe approaches the surface, and the changes elucidated with difference spectra. Comparisons to a theoretical model for Raman excitation by evanescent light near the probe tip indicate that while the general trends are well described, the data show oscillations about the model.

Analysis of the vacuum cannon

We develop a model for the velocity of a projectile in a vacuum cannon. The theoretical maximum velocity is independent of the vacuum cannon diameter and projectile mass and is significantly lower than the speed of sound. Experimental measurements support the theory as an upper limit.

Fundamental differences between micro‐ and nano‐Raman spectroscopy

Electric field polarization orientations and gradients close to near‐field scanning optical microscope (NSOM) probes render nano‐Raman fundamentally different from micro‐Raman spectroscopy. With x‐polarized light incident through an NSOM aperture, transmitted light has x, y and z components allowing nano‐Raman investigators to probe a variety of polarization configurations. In addition, the strong field gradients in the near‐field of a NSOM probe lead to a breakdown of the assumption of micro‐Raman spectroscopy that the field is constant over molecular dimensions. Thus, for nano‐Raman spectroscopy with an NSOM, selection rules allow for the detection of active modes with intensity dependent on the field gradient. These modes can have similar activity as infra‐red absorption modes. The mechanism can also explain the origin and intensity of some Raman modes observed in surface enhanced Raman spectroscopy.

Using XBee transducers for wireless data collection

This article describes how to use XBee transducers to create small and lightweight wireless sensors, which send data to a base station for collection and analysis. Data collection is limited to 10-bit accuracy by the XBee hardware. Depending on the type of XBee used, up to six data channels can be transmitted over a range of up to 15 miles. We describe the technical details of the process using the low-power version of the XBee transducer and a three-axis accelerometer chip.

The effects of probe boundary conditions and propagation on nano-Raman spectroscopy

Raman spectra obtained in the near‐field, with collection of the Raman‐shifted light in reflection, show selective enhancement of vibrational modes. We show that the boundary conditions for an electric field near a metal surface affect propagation of the reflected signal and lead to this selection. The enhancement of certain Raman forbidden vibrations is explained by the presence of an electric field gradient near the metal‐apertured fibre probe.

Two-dimensional heat flow apparatus

We have created an apparatus to quantitatively measure two-dimensional heat flow in a metal plate using a grid of temperature sensors read by a microcontroller. Real-time temperature data are collected from the microcontroller by a computer for comparison with a computational model of the heat equation. The microcontroller-based sensor array allows previously unavailable levels of precision at very low cost, and the combination of measurement and modeling makes for an excellent apparatus for the advanced undergraduate laboratory course.

Finally, a Python-Based Computational Physics Text

Here, Eric Ayars provides an overview and review of Mark Newmans book, Computational Physics.

Measuring the Flight Speed of Fire Bombers from Photos: An In‐Class Exercise in Introductory Kinematics

In the western half of the United States, fire bombers are not an uncommon sight. During the “fire season,” which can extend from June through November, these specially modified aircraft are used to drop fire retardant chemicals or water on wildfires. It can be an entertaining and instructive classroom exercise to use pictures of these planes in action to calculate the speed of the plane during a drop run.

A Versatile Text for the Introductory Computational Physics Course

This book review examines an interesting new textbook on scientific programming languages.

Things One Can Learn by Putting a Quadcopter in a Vacuum Chamber

Quadcopters (also known as “drones”) do not fly in vacuum. This is obvious enough that experimenting on one in a vacuum chamber would seem rather uninteresting, but there is one question that may be usefully addressed by such an experiment: the mechanism for yaw control. Quadcopters control yaw (rotation about the vertical axis) by differential rotor speed, and the question of whether those changes in rotor speed create yaw torque via conservation of angular momentum or via atmospheric drag can be addressed by “flying” a quadcopter in a vacuum where there is effectively zero atmospheric drag.