SEA TADPOLE Promises Metrology Improvements

Alexander E. Braun, Senior Editor -- Semiconductor International, 5/29/2008 7:35:00 AM

The utility of ultrashort laser pulses in metrology and other applications is diminished by spatio-temporal distortions. To become ultrashort, these pulses must be massively manipulated, becoming dispersed, amplified, elongated and compressed. Unless the devices producing them are precisely aligned — a complicated undertaking — spatio-temporal distortions are introduced.

Physics professon Rick Trebino and his group at the Georgia Institute of Technology (Atlanta) have developed techniques for measuring ultrashort laser pulses. “Twenty years ago, all one could do was measure an autocorrelation of the pulse in time, and that just gave a rough idea of the pulse length,” Trebino said, adding that the method ignores things like structure in the intensity of the pulse and variation in the phase of the color of the pulse in time. It also neglects these effects in space.

For a long time it has been possible to measure an integrated beam’s spatial profile over time, but there is a multiplicity of important distortions that these pulses can experience, especially at their foci that come under the heading of spatial-temporal distortions; thus, it becomes necessary to know the pulse intensity and phase as a function not just of time or space separately, but of space and time simultaneously.

Two of these important distortions are pulse front tilt and spatial chirp. In pulse front tilt, the pulse intensity fronts are not perpendicular to the propagation vector, and the pulse’s left half arrives before the right half. This kind of distortion would not be discovered by just making an autocorrelation and then simultaneously making a spatial profile of the beam because that requires that the pulse be viewed in space and time. In spatial chirp, the pulse’s average wavelength varies spatially across the beam, causing the different colors to propagate in various directions.

Tightly focused pulses are measured using near-field scanning optical microscopy (NSOM) probes with apertures as small as 30 nm. The NSOM probe currently used is a 500-nm-diameter tapered fiber probe, identical to the reference pulses’ fiber. (Source: Rick Trebino)

Until now, these distortions had been unmeasurable, complicating their control. “Over time, most ultrashort pulses are contaminated by these distortions,” Trebino said. “In 1981, I co-invented a technique called FROG [frequency-resolved optical gating] that allowed us to at least measure pulse intensity and phase against time. However, it wouldn’t achieve any spatial resolution of the pulse.” Years later, Trebino came up with a simple version of FROG called GRENOUILLE (French for “frog”), which allowed these simple distortions — spatial chirp and pulse front tilt — to be measured, but that was as far as the technique could go.

More recently, the research has led to doing composite measurements. “Just as when people want to make shaped or intense pulses of light, they have a laser that puts out a pulse and another device that shapes and amplifies it; we do the same thing with measurements. We measure the pulse directly out of the first laser and then use that as a reference pulse for the second device, which we then use to measure the spatio-temporal distortions and some of the pulse’s advance characteristics,” Trebino said.

The pulse emitted by the first laser is usually simple and clean, and can be measured using FROG. However, the later pulse, which has been amplified, shaped and distorted, displays some or all of these spatio-temporal distortions — even just focusing can be a nightmare. This is not only because of aberrations with which every photographer is familiar, such as chromatic and spherical aberrations, but also other unusual and quite unintuitive temporal problems in the pulse. For instance, when these pulses focus, they can turn into oddly shaped crescents and develop fringes. There can also be a “forerunner pulse,” which is a pulse that precedes the main one; it is very minor and typically submicron in size, but it can be several pulse lengths ahead of the main one. Although this had been predicted in the 1990s, no one had been able to measure them.

Trebino and his group have a complete suite of spatio-temporal pulse measurement techniques that measure even out-of-focus pulses. The first one is STRIPED FISH (spatially and temporally resolved intensity and phase evaluation device), which can measure the full spatio-temporal field of an ultrashort unfocused laser pulse; it is holography-based and simultaneously measures several holograms, one for each frequency in the pulse. The second is a single-pulse technique, SEA TADPOLE (spatial encoded arrangement for temporal analysis by dispersing a pair of light e-fields), which can measure a very complex pulse even if it is out of focus — its complete spatio-temporal intensity and phase. Although interferometry-like in its operation, its crossed beams avoid the need to align collinear beams, providing a measured 2-D trace with spatial, not spectral, fringes and no reduced spectral resolution. It is a multi-pulse technique that must average over several pulses.

SEA TADPOLE has applications for anything using an ultrashort pulse focus: spectroscopy, microscopy and even lithography, which sometimes uses short pulses. Most applications require either high intensity or very good spatial resolution; the tighter the pulse focus, the better the spatial resolution, and the shorter the focused pulse is the more its energy can be concentrated into a small spatio-temporal region with higher sensitivity. With the capability of determining which aberrations — caused by lenses and other optical components — are present, it becomes possible to create a desired distortionless pulse at the focus, improving practically any short-pulse system.

There may also be some e-beam device applications. Presently, the device is intended to work only with light beams. However, often the way an e-beam is measured is by shining a light beam off it and determining what happens to the light beam, which might open it up to this technique.