Application
Photoemission electron microscopy (PEEM) users often employ mercury (Hg) and xenon (Xe) arc-lamps to illuminate their samples for imaging. However, these lamps have significant limitations in spectral range, brightness and stability that greatly impact results.
Hg and Xe arc-lamps have limited output at the shortest UV wavelengths (170nm-250nm), those which are useful in many PEEM applications (UV-PEEM). In addition, the light must be efficiently collected from the lamp and re-imaged through a vacuum window on to the sample. It is desirable to achieve a small sub-millimeter focal spot on the sample at a long working distance. This poses an additional challenge, given the relatively large spot size of arc-lamps and consequently their lower brightness. Finally, the focused spot must be very stable, both temporally and spatially, which also proves challenging for arc-lamps with their typical flicker and arc-wander.
Equipment
These challenges may be addressed and met with the EQ-1500 Laser-Driven Light Source (LDLS™), combined with a pair of off-axis parabolic mirrors to deliver the light to the sample. The EQ-1500 delivers very high brightness in the deep UV down to 170nm (7.3eV), from an emitting plasma spot approximately 125µm wide by 300µm long, full width at half maximum (FWHM). Since the source itself has a diverging beam (0.5NA), the spot is reimaged on to the sample using a pair of UV coated off-axis parabolic mirrors. The first mirror has a focal length of 2” (50mm), the second typically has an 8” (200mm) focal length, although other focal lengths are available from Energetiq. With a 2” and 8” pair, the spot is magnified 4X on the sample, giving a spot size of approximately 500µm by 1200µm. Narrow band filters may be inserted into the beam to select particular wavelengths of interest.
Results
An example of a rapidly achieved result is shown below. Energetiq collaborated with the team at the Arizona State University Physics Department to image samples of periodically polarized lithium niobate. The results were compared with ASU’s standard equipment which utilizes a 100W mercury-xenon arc lamp. Light from the EQ-1500 LDLS was focused on to a spot in the PEEM instrument using the pair of off-axis parabolic mirrors described earlier, filtered by narrow-band filters centered on 254nm, 214nm and 193nm (from Princeton Acton Optics). Images were collected at each wavelength for each light source and compared.
The first two images are the results from the Hg-Xe arc lamp and the EQ-1500 LDLS (respectively) at 254nm. Comparing these two images, it is clear that the EQ-1500 provides significantly greater contrast than the traditional arc lamp. The third and fourth images were created using the EQ-1500 at 214nm and 193nm. The periodic structure in the lithium niobate is shown clearly with good contrast. When imaging with the arc-lamp, the images appeared blank, showing no discernible structure. Finally, when using the LDLS, the spatial stability of the EQ-1500 (<1 µm x-y motion of spot centroid) contributed to more stable, lower noise images.
Conclusion
The EQ-1500 is a high intensity, stable light source that provides deep ultraviolet, visible and near infrared (NIR) light for many PEEM applications. In conjunction with its available coupling optics, the EQ-1500 provides high flux at long working distances on to the sample. Images taken show structure that would have been invisible using conventional arc-lamps.
Acknowledgment
Energetiq would like to thank Gary Hembree and Robert Nemanich of Arizona State University Department of Physics for their collaboration on this project and for providing the images.
© Copyright 2011 Energetiq Technology, Inc.
