- Photoinduced Reflectivity Creates Near-Field Probes
Daniel S. Burgess
Transient-aperture probes generated with light promise high resolutions and near-video rates for near-field infrared microscopy. The technique, under development at the W.W. Hansen Experimental Physics Laboratory at Stanford University in Stanford, Calif., relies on photoinduced reflectivity in semiconductors.
Researchers are using photoinduced reflectivity to produce probes for near-field infrared microscopy. In the setup, a masked laser pulse exposes a silicon substrate to visible light, creating a transient mirror and aperture with which they can image a sample on the back side of the substrate, using infrared light. Courtesy of Dmitrii Simanovskii.
Near-field microscopy has made rapid inroads over the last two decades into biology, materials science and surface chemistry applications that require subdiffraction-limit resolution. But the same scanning approaches that enable near-field imaging lead to long acquisition times.
To demonstrate the transient-aperture technique, the researchers imaged four adjacent 1.7-µm-diameter holes in a gold film. Courtesy of Dmitrii Simanovskii.
Typically, near-field setups use atomic force feedback to control the separation between the optical probe and the sample, said Dmitrii Simanovskii, a physical science research associate at the lab and member of a team developing the transient-aperture technique. Such mechanical scanning – effectively a three-dimensional approach – is inherently slow. The group, which includes Daniel Palanker, Keith Cohn and Todd Smith, decided to dispense with the physical probe, electing to create one with light.
Simanovskii explained that electron-hole plasmas form briefly when a semiconductor material absorbs a photon of visible light with energy greater than its bandgap. If the plasma is sufficiently dense, it will be reflective, much like the free electrons in a metal. Because the critical density of the plasma is inversely proportional to the square of the wavelength of the light to be reflected, he said, it is easiest to create a surface that is temporarily reflective of infrared light.
To make a probe, the researchers shaded part of the semiconductor from the visible pump light with shadow masks, which yielded 200-µm-diameter reflective regions with 0.5- to 3-µm-diameter transmissive areas in the center. The 400-nm second harmonic from a Ti:sapphire laser served as the pump source, and a Ti:sapphire-pumped optical parametric amplifier produced 1-ps pulses of 4- to 10-µm light for imaging a sample on the other side of the semiconductor.
They investigated a 0.5-mm-thick silicon wafer and a 2-µm-thick crystalline silicon film as potential substrates, because the materials are transparent to red light and display a high damage threshold at 400 nm. The wafer remained reflective of the IR light for 100 ps after pumping, and the film for 40 ps.
The researchers demonstrated the functionality of the technique by imaging four adjacent 1.7-µm-diameter holes in gold film on a silicon wafer, raster scanning the sample and substrate in the focal plane of the system with 6.25-µm light. The scanning time was 15 s to produce a 128 x 128-pixel image, and the system displayed a resolution of better than λ/5.
Simanovskii said the approach works with wavelengths of 4 µm through the microwave region. Other semiconductor substrates and pump wavelengths may extend the range of suitable imaging wavelengths, and the researchers hope to demonstrate resolution of better than λ/10 at near-video rates.
They are investigating development of other photoinduced near-field probes and application of the technique to the imaging and microspectroscopy of living cells and cellular structures. They have no plans to commercialize the technology.
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