Rosalía Serna

3D Nanostructures in Hydrogen Silsesquionxane Achieved by Proton Beam Writing

MRS BULLETIN • VOLUME 31 • MAY 2006 367 laser spot, and the information from scans of the sample is later converted to a two-dimensional image. One problem inherent in large-NA systems is a reduction in the depth of focus, causing a deterioration of image quality in the out-of-focus region. The synthetic-aperture focusing technique (SAFT), which employs a point detector such as a needle hydrophone, was developed to reduce this phenomenon. In the February 15 issue of Optics Letters (p. 474), M.L. Li of Texas A&M University and co-researchers have introduced a virtual detector for PAM that allows the SAFT to be applied to the large-NA transducer without losing depth of focus and eliminates the need for the point detector. In this system, the focal point of the transducer is considered to be a virtual point detector. Li and colleagues said, “Photoacoustic (PA) waves that are generated within a certain solid angle are assumed to be detected by the virtual detector.” Linear scanning of both the laser and the transducer creates a superposition of the PA waves produced by the biological tissue. This superposition, coupled with a coherence factor, facilitates synthetic-aperture focusing and is the basis for improved lateral resolution and signal-to-noise ratio. The researchers offered a brief description of the PAM system, but focused largely on advances made using the new virtual point detector. First, a proof-of-principle study of a 6-μm carbon fiber immersed in 1% Intralipid solution was performed. In this study, “depth-independent lateral resolution of the carbon fiber” was achieved, “without affecting the axial resolution.” Second, in vivo experiments were carried out on the scalps of rats. According to the researchers, the technique provides a much clearer vascular distribution that is attributed to improved lateral resolution and signal-to-noise ratio. KEVIN P. HERLIHY