Overview of Ultrafast Optical Science
Ultrafast science is the study and subsequent implementation of physical phenomena that occur at the shortest time scales known in science, from picoseconds to femtoseconds to attoseconds. Snapshots in this time domain reveal the most fundamental mechanisms of molecular, atomic, and electron interactions. Ultrafast science enables us to answer such questions as how molecules move in liquids or gases, how electrons collide in semiconductors and superconductors, and how light initiates the vision process.
The Center for Ultrafast Optical Science (CUOS) at the University of Michigan was chartered to advance the scientific and technological applications of ultrashort optical pulses. The heart of the Center is its unique combination of state-of-the-art short-pulse and high-intensity lasers.
For example, the laser featured in the photograph is a compact, high-average power system capable of generating peak powers of 10 billion Watts. Other lasers at CUOS are 1000 times more powerful even than this. Their operating principle, Chirped-Pulse Amplification (CPA), was developed by CUOS scientists. The tremendous peak power, generated by a laser which rests on a single table-top, is contained in bursts of laser light that are less than 100 femtoseconds in duration. That's only .0000000000001 seconds, or far less time that it takes for light to travel the thickness of a sheet of paper. A supersonic aircraft travels less than the diameter of an atom in this time!
Ultrafast is Ultrashort. These unique lasers enable us to examine and to understand phenomena that occur on "ultrashort" time scales. For example, these lasers can freeze the motion of atoms in a vibrating molecule. They can even resolve the rapid tiny motion of electrons within a single atom, which form the foundation of quantum chemistry and physics. Powerful ultrashort light pulses can modify surfaces in new ways. This may lead to new materials, and new opto-electronic devices which operate at unprecedented speeds.
CUOS has also developed techniques to sculpt these short laser pulses, to alter their color and amplitude to high precision. The shaped pulses of light are tools that can be used to engineer new quantum structures in atoms and molecules. The ultimate aim is to control atomic and molecular processes in gases, liquids, and plasmas for purposes ranging from materials processing to light-source production to particle acceleration.
Ultrafast is Ultra-precise. Short bursts of energetic laser light have many surprising properties that can lead to a variety of novel applications. For instance, materials which are normally optically transparent become opaque. Thus, micro-machining or incisions can be made beneath the surface of some materials, without damaging the surface of the medium. This concept of "ultra-precise" materials processing led researchers at the Center to consider using ultrafast lasers as surgical tools. Specifically, CUOS scientists are working on Ophthalmologic applications. Various types of eye surgery may be significantly improved by replacing the traditional scalpel or laser, with an ultrafast laser. Researchers have already demonstrated that the quality of an incision made by an ultrafast laser is greater than those produced by conventional lasers. New surgical techniques which are only possible using ultrafast lasers are presently under investigation. This research may lead to significantly improved eye care -- surgery that once had to be performed in an operating room, could now place in a doctor's office.
Not only can these lasers cut, but they are powerful imaging tools as well. At least three different areas of microscopy are presently pursued at the Center. These microscopes can be used to image and examine important biological systems or materials, or to study the path of electrical current through microchips operating at high speeds.
High speeds is not the only advantage of ultrafast pulses in microscopy. For example, the photo to the right shows how ultrafast pulses can significantly increase the resolution in confocal microscopy, by taking advantage of the nonlinear optical response of tissue to very short pulses. It shows a beautiful confocal microscope image of pollen grains, captured by the second harmonic light scattered when an ultrafast laser beam scans the optical field.
Ultrafast is Ultra-broad. Yet another advantage of ultrafast technology is the development of very broad-band light sources, to permit investigations in the far-infrared, visible, ultraviolet, and extreme ultraviolet spectral regions. Developments at CUOS are extending the useful range of wavelengths available for research of ultrafast dynamics in physics, chemistry, and biology.
We have developed a novel ultrafast broadband radiation source in the extreme ultraviolet that was measured to be a million times brighter, and a thousand times shorter in pulse duration than any existing synchrotron source. It is produced by a table-top laser that is relatively inexpensive, compared to the synchrotron, so it should be more readily available to a wider research community. And, since the x-rays are absolutely synchronized to the laser that produced them, they have the additional advantage of being useful in measurements where the light must be precisely synchronized in time.
This new ultrafast XUV "white light" source can be used to study material structural dynamics such as melting, temporal back-lighting of dense plasmas, imaging of live biological cells, time-resolved absorption spectroscopy of either quantum-controlled photo-initiated chemical reactions or transient energy states of laser-ablated materials (for materials processing and thin-film deposition), photosynthesis dynamics, photo-electron spectroscopy (for condensed-matter surface studies), inner-shell atomic ionization, and nonlinear optics with x-rays.
The figure to the right shows a time-resolved spectrum of the light emitted by this new source. This radiation is produced when an ultrafast (0.5 picosecond pulse duration) infrared (1 micron wavelength) laser is focused to high intensity (I) onto a gold-coated target, creating a hot solid-density plasma. It can be seen that the x-ray pulse width can be arbitrarily adjusted by simply adjusting this single parameter (I), and thus the peak temperature of the plasma.
Ultrafast is Ultra-intense. Chirped-Pulse Amplification is a method for producing lasers of unprecedented intensity. Our most intense table-top lasers produce pulses which exceed 10 Terawatts (10 trillion Watts). During its short duration, the power in each pulse exceeds the total power generated in the United States. The light can be focused to 1022 watts per square centimeter (1 with 20 zeroes or 10000 billion billion Watts), which considerably exceeds the intensity of any other radiation source on earth, other than another CPA laser. These are sources for studying fundamental physics in a new regime. Beyond that, there are important technological benefits to these new light sources. CUOS scientists are studying advanced problems such as compact accelerators, broad-band light sources, and nonlinear plasma channeling effects.