Technical News Summaries - CLEO 2008

PUSHING THE OPTICAL LIMITS OF A MICROSCOPE

Looking at living cells can give extraordinary insight into the intricate microscopic machinery of biology, but ordinary microscopes are limited in resolution by the wavelength of light (400-700 nanometers [nm]). This limitation confounds biologists because some of the most interesting structures found in the cell are at least an order of magnitude smaller than this. Optical fluorescent microscopy, whereby light-emitting dyes are attached to tiny molecules of interest, has been used for pushing the limits of light microscopy, but biologists often hit a wall when they try to use fluorescence microscopy because resolution is only improved in the case of well-separated molecules; if the concentration of these dyes is too high, blurring occurs as the molecules’ images overlap.

Julie Biteen, W. E. Moerner, and colleagues at Stanford University found a way to beat the resolution limit by turning on only a sparse subset of the fluorescent protein labels in a cell and then taking snapshots of the individual molecules that remain. By doing this multiple times, they can image structures inside live bacterial cells with 40-nm resolution, including MreB, a large structural protein that appears to form a helix. (Talk CFT2, "Superresolution Imaging in Live Bacterial Cells by Single-Molecule Active-Control Microscopy.")

AN OPTICAL NMR DETECTOR

As nuclear magnetic resonance (NMR) detectors have become more and more powerful over the last few decades, they have also gotten larger and larger. Improvements in performance follow increases in the field strength of the magnet that encompasses the heart of every NMR instrument. Bigger magnets mean better signals, and many of the latest instruments in laboratories have massive superconducting magnets cooled by a bath of liquid helium surrounded by a dewar of nitrogen.

Now a group of collaborating scientists at the University of California, Berkeley and the National Institute of Standards and Technology is exploring ways of reducing NMR devices into something that would fit in the palm of the hand by using optical detectors to measure the NMR signals. Micah Ledbetter and his colleagues built a small NMR detector that uses a permanent magnet about one-tenth the strength of large electromagnets. Recently they demonstrated their optical detection of NMR technique using plain tap water. The liquid is polarized as it flows through the magnetic field of a small permanent magnet. Subsequently, the fluid flows through an “encoding region” where NMR pulse sequences can be applied, and finally, the fluid flows past the optical detection region. The optical detection region consists of an alkali vapor cell illuminated by a circularly polarized laser beam. The polarized nuclei generate a small magnetic field, which causes a change in the absorption of the circularly polarized light, which is monitored by a photodiode. With further development, this technique could provide a useful way of performing large parallel assays on chemical compounds, for example, in the screening of new compounds for drug development, among other applications. (Talk QTuF7, "Optical Microchip Detection of Nuclear Magnetic Resonance.")

A NOVEL FREQUENCY COMB GENERATOR

A novel frequency comb generator on a microchip has been created by a team of physicists led by T.J. Kippenberg at the Max Planck Institute for Quantum Optics (MPQ) in Garching, Germany. A frequency comb is a laser system that emits a spectrum of many equally spaced laser lines (that is, laser light with frequencies coming at regular intervals) whose frequency is known accurately. The frequency comb can be used like a “ruler” to measure the frequency of an unknown laser source. For this to work, the ruler, or comb, must have exactly equally spaced "teeth." For the new monolithic frequency comb generator, the teeth have been measured to be equidistant with deviations smaller than 7 x 10^-18 relative to the frequency of the light to be measured.

Additionally, the absolute position of the single comb modes could be stabilized to more than 1-Hz-precision, which is very accurate compared to the optical frequency of approximately 2 x 10^14 Hz. Compared to bulky mode-locked lasers that are usually used to generate frequency combs, the novel generation process discovered at the MPQ is much simpler and works by sending a single continuous wave laser beam into a micrometer size glass resonator. Inside this glass ring, so called “four-wave mixing” leads to the comb generation. Since the whole frequency comb sits on a silicon microchip, it could prove very useful for high speed telecommunications and metrology. Particularly interesting is the large spacing of the comb teeth (80 GHz), important for telecommunications and astrophysical spectrometer calibration. (Talk CTuM4, "Full Stabilization of a Frequency Comb Generated in a Monolithic Microcavity.")

HEAT GOES BALLISTIC

Scientists at the University of Colorado have watched as tiny parcels of heat (called phonons) spread in a crystal and how the propagation changes from particle form into the more usual diffusive form as they move. The transport of heat is considered to be "ballistic" if the distance over which the phonon moves is smaller than its mean free path (the characteristic distance it goes before scattering from another phonon). Mark Siemens and his colleagues shoot a beam of soft X-rays at the sample to understand what the phonons are doing, especially at the moment they pass from a nanostructured pathway into a bulk substrate (heat sink). In reality, the heat of the phonon isn’t measured directly. Instead, the heat left behind in the nanostructure is accounted for by measuring the internal temperature of the sample using the diffraction of the X-rays (whose wavelength is 29 nm), a process that can detect the displacement of atoms (brought on by heating) with a picometer (10^-12m) precision. The shortness of the X-ray pulses (10 femtoseconds) also allows one to watch heat transport over short periods. One of the reasons for understanding how heat moves away from a point of origin, says Siemens, is to manage the thermal environment of future advanced high-speed transistors. (Talk CWA6, "Nanoscale Heat Transport Probed with Soft-X-rays.")