Illustration of the ultrafast all-optical modulator. (Top) The probe beam at right (in blue) carries no information when entering the modulator. The switching beam (in red) encodes its information onto the probe beam. (Bottom) Schematic of the experimental setup. The probe beam, operating at a telecommunications frequency, is shown in blue and the trigger beam in red. Clock rate: 1.4 THz
http://www.osa-opn.org/home/newsroom/2013/february/optical_switching_at_terahertz_rates/#.URplWtuF_kE
Patricia Daukantas
A new optical microcavity switch that can operate 350 times faster than its electronic counterpart could speed up future telecommunications systems and even quantum computers (Opt. Lett. 38, 374).
Scientists at the University of Twente (Netherlands) and the Institute for Nanoscience and Cryogenics (France) made their semiconductor planar microcavity switch on and off at a reproducible rate of 1.4 THz. By contrast, today's typical electronic switch operates at 4 GHz.
Emre Yüce, a junior scientist at Twente's MESA+ Institute for Nanotechnology, and his collaborators fabricated the cavity out of layers of gallium arsenide and aluminum arsenide and measured its resonance frequency at 1,284.1 nm. In the experiment, the team used two optical parametric amplifiers, pumped by a near-infrared Ti:sapphire laser, to shoot probe and trigger light beams into the tiny cavity.
The trigger beam creates the so-called electronic Kerr effect, which changes the refractive index of the cavity material on a subpicosecond time scale. The researchers lengthened the wavelength of the trigger beam to 2,400 nm to reduce two-photon absorption within the cavity; they estimated that the cavity absorbed only about one millionth of the light input.
The cavity storage time of roughly 300 fs determines the fundamental “speed limit” of the switching rate, according to the authors. The phenomenon is not dependent on the geometry of the microcavity.
Besides ultrafast on-chip photonic modulation, the new switch could find applications in fundamental studies of cavity quantum electrodynamics.
Photonic crystal for a 6 gigahertz laser. The regular arrangement of the rods provides the unique properties that manipulate the laser beam. (Credit: Image courtesy of University of Twente)
http://www.sciencedaily.com/releases/2012/12/121219101227.htm
Dec. 19, 2012 — A laser capable of working in the terahertz range – that of long-wavelength light from the far infrared to 1 millimeter – takes a better 'fingerprint' of, say, a drug under investigation, than a traditional chemical analysis. PhD student Thomas Denis of the University of Twente's MESA+ Institute for Nanotechnology has now combined a free electron source with photonic crystals. The result: greater flexibility and a compact laser.
A terahertz laser is capable of showing the molecular structure of, say, a drug, because the laser beam it produces is at wavelengths suitable for examining molecular and atomic bonds. This enables more spatial information to be obtained than from chemical analysis, a detailed fingerprint. To date, however, the limitation has been that lasers of this type are restricted to particular wavelengths, e.g. because the source of the laser light is a semiconductor, in which electrons can only take on fixed energy states, hence only a limited number of 'colours' of light can be produced.
Free electrons
In a free electron laser the electrons are not restricted to fixed states, as are electrons in a classic cathode ray tube. So Denis thought, why not combine a free electron source with a 'photonic crystal'? This is a structure with lot of tiny 'posts' that together slow down the incident light and turn it into a coherent beam. Photonic crystals can be created at micro level, e.g. for a lab-on-a-chip, or on a much larger scale. The dimensions and shape of the crystal determine the rough wavelength region, and the precise wavelength can be set and adjusted by changing the speed of the electrons being fired at it. This combination is known as a 'photonic free-electron laser' or pFEL.
Looking inside the crystal
Existing terahertz lasers also have the disadvantage that they are very large, big enough to fill a room. Thanks to the use of photonic crystals the pFEL that Denis has designed is not much bigger than a domestic microwave oven and can still provide high power despite its small size. He has also found a special way of 'looking' inside a photonic crystal -- something that is not normally possible. By interfering slightly with the wavelength pattern in the crystal using a tiny metal ball the actual pattern can be measured.
Thomas Denis (Ahaus, 1981) received his PhD on 14 December for his thesis Theory and Design of Microwave Photonic Free-Electron Lasers. He carried out his research in Prof. Klaus Boller's Laser Physics and Non-linear Optics Group. The thesis, or the summary, is available in digital form on request.
