R. J. Stephenson

A femtojoule calorimeter using micromechanical sensors

We describe a highly sensitive new type of calorimeter based on the deflection of a ‘‘bimetallic’’ micromechanical sensor as a function of temperature. The temperature changes can be due to ambient changes, giving a temperature sensor or, more importantly, due to the heat absorbed by a coating on the sensor, giving a heat sensor. As an example we show the results of using the sensor as a photothermal spectrometer. The small dimensions and low thermal mass of the sensor make it highly sensitive and we demonstrate a sensitivity of roughly 100 pW. By applying a simple model of the system the ultimate sensitivity is expected to be of the order of 10 pW. The thermal response time of the cantilever can also be determined, giving an estimate of the minimum detectable energy of the sensor. This we find to be 150 fJ and again from our model, expect a minimum value of the order of 20 fJ.

Micromechanical thermal sensors: Comparison of experimental results and simulations

In this article the results of a numerical simulation of the thermal and mechanical properties of micromechanical sensors are described. Finite element analysis is used to model the response of the composite cantilever to temperature variations. The simulations consider triangular shaped cantilevers in air and water environments and under different configurations of thermal loading. The models utilize conditions from actual experiments which are conducted in parallel. Good agreement between numerical and experimental results studied so far allows us to use our numerical simulation to confidently predict the performance of the sensor under novel operating conditions and to optimize the sensor design for specific applications.

Scanning near-field optical microscopy in reflection mode imaging in liquid

A combined atomic force microscope (AFM) and scanning near-field optical microscope (SNOM) has been constructed to obtain images under liquids. In this combined AFM-SNOM an inverted ac mode configuration is used where the AFM cantilever is driven by exciting acoustic modes of the liquid cell. Optical images of latex spheres on a cantilever have been obtained under perfluorononane, butanol, and water with a lateral resolution of 100 nm. Fluorescence images of latex spheres have been obtained under perfluorononane. The ability to image under liquids opens up a wide range of applications of near field optical microscopy in chemistry and biology.

A normal force distance regulation scheme for near‐field optical microscopy

A near‐field optical microscope (NFOM) has been developed that combines the features of a near‐field optical microscope and an atomic force microscope. Improved control over tip‐sample separation has led to improved optical imaging and independent surface topography information. The tip oscillation is normal to the sample plane thereby reducing lateral forces—important for nonperturbative imaging of soft samples. Both topographic images and reflection near‐field optical images are presented which demonstrate the capability of the system.

A near-field optical microscope with normal force distance regulation

Near‐field imaging is a means of exceeding the diffraction limit in optical microscopy to yield subwavelength resolution optical images of a sample surface. In order to achieve such high resolution, it is necessary to scan the measurement probe above the surface at a height of only a few nanometers which requires careful control of the separation between tip and sample. In the implementation of the near‐field optical microscope (NFOM) reported here, the distance regulation scheme is based on an inverted noncontact atomic force microscope (AFM) in which a cantilever is used as the sample substrate and imaging is performed with a fiber optic tip. In this way, both the benefits of AFM and NFOM are realized simultaneously.

Progress in probe microscopy

The development of scanning probe microscopies over the last 15 years has been a major success story. It has given a new lease of life to research areas that had been limited by the inability to observe features at the atomic scale, and it has even created new lines of investigation. While the technology of scanning probe microscopy (SPM) has advanced extremely rapidly, a remarkable feature of the commercial sector is that it has matched this progress by continually developing new products for SPM. This is partly due to the close alliance between manufacturers and specific research groups, but also reflects the demands of SPM users for the latest development.