Transcript Document
Obtaining Ion and Electron Beams From a source of Laser-Cooled Atoms Alexa Parker, Gosforth Academy Project Supervisor: Dr Kevin Weatherill Department of Physics, Durham University, Durham DH1 3LE, UK Abstract Ion and Electron beams are a fundamental part of nanotechnology and used in a multitude of other scientific applications. In order for advances in this field - faster low power computer processors, more efficient batteries, improved sensors, circuit editing, precision doping in semiconductors etc. - superior beams to those already in use are necessary. Currently sources have large angular and energy spreads causing chromatic aberrations, limiting performance. For a higher resolution beam, a less active source is needed. This is where laser cooling comes into play - reducing motion by reducing the temperature which consequently reduces the energy spread. Beam quality can be greatly improved and this innovative approach means that single ions and electrons can be produced on demand from a greater range of source elements than currently available from existing technology. Laser Cooling The process of laser cooling reduces the temperature of the atoms as low as 100µK - one ten-thousandth of a degree above absolute zero. Scientists have actually gone even colder than that - to temperatures of 1 or 2µK by evaporative cooling, however in this experiment that would cause a loss of too many atoms. How Slow Can We Go? Speed of atom at room temp. - 4000km/h Speed of atom at -270C - 400km/h Speed of atom after Laser Cooling - 25cm/s How Do We Do It? • Mutually orthogonal, counter-propagating • Doppler shifting is used when tuning the lasers to beams. ensure that atoms which have already been slowed down aren’t given momentum kicks to whizz them off in the • Photons collide with atoms and by laws of other direction. Conservation of Momentum, atoms are slowed down. m1u1 + m2u2 = m1v1 + m2v2 • Repeated photon absorption and emission slows down atoms significantly. One absorption and re-radiation can occur in about 30 nanoseconds. • In one second a force 100,000 times the force of gravity can be applied to an atom simply from photon collisions. • One photon slows an atom by 3cm/s (on average). That means you need about 370 photons to slow it down to the desired rate. All the equipment used to tune the lasers. • Lasers need to be tuned to the exact frequency (colour) to be absorbed by the atoms. If not then the photons will pass straight through the atoms. No absorption Absorption • The lasers are tuned slightly below resonance frequency – towards the redder end of the spectrum – so that atoms travelling towards the photon streams see the light Doppler shifted up to resonance frequency. Atoms that have already slowed won’t be as affected by the photons. • With beams coming from different directions, the cold atoms cluster in the intersection of the lasers, like a ‘laser trap’. To stop atoms falling out of the trap under gravity, a Magneto Optical Trap is applied. MOT • Used simultaneously with laser cooling. • Two small coils of wire are placed on either side of the cell. • A small electric current is passed through them to create a magnetic field which varies across the cell. • Pushes atoms into the centre of the trap using a position-dependent magnetic force which is greater than gravity. Rydberg Blockade Ionization • Ionization beams focus in the cloud of atoms. They are focused to a fine point, ~ 5µm, so the overlap region is tiny – it can fit roughly 10 atoms. Ionisation beams: green = 2nd step laser; purple = 3rd step laser • step laser excites the atom to the first Rydberg level (2nd classic energy level) 2nd • 3rd step laser excites atom all the way to the 2nd Rydberg level – just below threshold for ionization. • The atom becomes a Rydberg atom which has very exaggerated properties as the valence electrons have a very high principal quantum numbers which reduces their binding energy greatly. This makes them very susceptible to disturbances from external fields. • If an electric field is applied between the two electrodes the valence electron is released from the orbital, ionizing the atom. A field strength of only about 3.21 x (100/n4) V/cm is needed for ionization. • Ions and electrons are extracted from the cell and picked up by the detectors and can then be studied and used for the creation of ion/electron beams. • Strong and long range dipole-dipole interactions between Rydberg atoms cause a blockade of laser excitation effect. • The interactions cause an energy shift of resonance in the surrounding atoms which prevents their excitation, which is the so called blockade effect. Therefore because of the tiny overlap area in the beams only one atom can be transferred into Rydberg state and ionized at a time. • Ultracold plasma forms around the Rydberg atom and its surrounding neighbours until it is ionized and the process starts again. Rydberg atom and its blockaded neighbours surrounded by an ultracold plasma.