Landmarks: Lasing with Electrons

Traditional lasers have been essential in science and technology, but each one is limited in the wavelengths at which it can operate. Two papers published in Physical Review Letters in 1976 and 1977 described a wholly new kind of laser that could in principle operate over a wide range of wavelengths. Today, these so-called free electron lasers provide high intensity from microwaves all the way up to x rays, with applications ranging across biology, chemistry, and physics.

Conventional lasers rely on the process of stimulated emission, described by Albert Einstein. Electromagnetic radiation of the correct wavelength triggers atoms or molecules in an excited state to emit more radiation of the same wavelength, and the emitted radiation is in phase (or "coherent") with the triggering radiation [see 2005 Focus story, Invention of the Maser and Laser].

In 1971, John M. J. Madey of Stanford University in California showed theoretically that stimulated emission can also occur with bremsstrahlung, the radiation that a charged particle emits when forced to follow a curved path [1]. He considered a beam of electrons traveling close to the speed of light through a magnetic field oriented perpendicular to its path. He assumed a field that varies periodically in a way that forces the electrons to wiggle from side to side (or up and down), and as they wiggle, they emit radiation, concentrated in the forward direction.

Madey showed that if radiation of the right frequency travels along the same axis as the electron beam, it can stimulate additional bremsstrahlung radiation with the same frequency and phase. Unlike a conventional laser, however, where the operating frequency is a fixed property of the atoms or molecules at hand, the frequency of stimulated emission from "free electrons" depends only on the energy of the electrons and the magnetic field periodicity. So the wavelength can in principle be adjusted over a very wide range, just by varying the electron beam energy.

It was five years before Madey and several Stanford colleagues demonstrated the effect and reported it in Physical Review Letters. To provide a periodic magnetic field with the necessary strength and structure, the team built a superconducting electromagnet just over 5 meters long, with a period of 3.2 centimeters. Through the center of this magnet they directed a pulsed beam of electrons with energy of about 24 MeV from the Stanford Linear Accelerator Center (SLAC), along with laser light with a 10.6-micron wavelength. They observed a 7 percent amplification of the laser light when the electron beam energy had the right value to allow stimulated emission at the laser frequency.

The following year, Madey and colleagues published their account of the first free electron laser (FEL). It used the same apparatus as the earlier experiment, with a beam energy of 43.5 MeV, but now enclosed between two mirrors and without an external light source. Radiation generated spontaneously by the electron beam reflected back and forth between the mirrors, stimulating further emission to form a strong beam at an infrared wavelength of 3.4 microns.

Although Madey couched his theory in quantum mechanical terms, the working of FELs is now almost always described classically, says Joe Frisch of SLAC's Linac Coherent Light Source (LCLS), an FEL operating at x-ray wavelengths. In this picture, there is an interaction between the sideways motion of electrons and the radiation's sideways electric field that ultimately causes the electrons to form into "microbunches" spaced just a wavelength apart. Each bunch is at the same point in the wave, so their wiggling creates additional waves that are guaranteed to be in synch with the radiation that is already present.

At the LCLS, says Frisch, the electron-wiggling magnets (the undulators) have a 3-centimeter period, and electrons passing through them form into microbunches over about the first 100 periods. During the rest of their trip, the bunches generate coherent x rays with enough intensity that mirrors aren't necessary--a good thing, as no such mirrors exist for x rays. LCLS was the first FEL to generate x rays of sub-nanometer wavelength, with an energy of about 10 keV. Brief, intense pulses of x rays can image individual atoms, making it possible to follow the progress of chemical reactions. FELs are also invaluable in generating high power at infrared frequencies and below, with uses ranging from studies of molecular structure to medical diagnostics.

References:

[1] John M. J. Madey, "Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field," J. Appl. Phys. 42, 1906 (1971).

Graphene makes ultrafast laser

Researchers at the University of Cambridge in the UK and CNRS in Grenoble, France, have fabricated an ultrafast "mode-locked" graphene laser. The result – which comes as quite a surprise, given the absence of a band gap in graphene – paves the way to photonic devices based on the material.

Since its discovery in 2004, graphene has continued to amaze scientists thanks to its unique electronic and mechanical properties that make it useful for a host of device applications. The "wonder material", as it is called, might even replace silicon as the electronic material of choice in the future. Graphene consists of a planar single sheet of carbon arranged in a honeycombed lattice and electrons travel through the material at extremely high speeds thanks to the fact that they behave like relativistic, or "Dirac", particles with no rest mass.

Now, Andrea Ferrari and colleagues say that graphene might be used in optoelectronics applications too, by demonstrating an ultrafast laser made from the material.

Ultrafast lasers are widely used in science and technology, and there is an increasing demand for compact, tunable laser sources. Today, the dominating technology in so-called mode-locked lasers – that is, lasers that produce ultrashort pulses at a very high repetition rate – is based on semiconductor saturable absorber mirrors (SESAMs). However, such devices are complicated and expensive to make, and are severely limited in their bandwidth.

Graphene mode-lockers

The new ultrafast laser exploits graphene and graphene layers as mode-lockers.

"In principle, this is quite a surprising result because graphene has no band gap, which is a key requirement for mode-locking in SESAMs," said Ferrari.

The team studied how light is absorbed in graphene and how photo-excited charge carriers behave in the material. In particular, they highlighted the key role of "Pauli blocking" in saturating the light absorption. Because of the Pauli exclusion principle, when pumping of electrons in the excited state is quicker than the rate at which they relax, the absorption saturates. This is because no more electrons can be excited until there is "space" available for them in the excited state.

Since the Dirac electrons in graphene linearly disperse, this means that it is the most wideband saturable light absorber ever, far out-passing the bandwidth provided by any other known material.

The researchers made their laser by starting with a graphene-polymer composite, obtained from a solution of graphene. Next, they placed this composite between two optical fibres in a laser cavity.

"Graphene is the ideal wideband saturable absorber, able to operate from the UV to visible and far-infrared wavelengths," Ferrari told our sister website nanotechweb.org. "Our graphene-based ultrafast laser, which harnesses the wideband optical nonlinearity of graphene, with no need for band gap engineering, extends the practical application of this novel material from nanoelectronics to optoelectronics and integrated photonics."

The team are now in the process of optimizing a fully functioning wideband tunable laser based on graphene, as well as trying similar experiments with graphene oxide.

The work was reported in ACS Nano.