About the UEC Lab
At the heart of the entire experimental setup is the Femtosecond(fs) LASER system. It is a mode-locked Ti:Sapphire Laser capable of generating extremely intense laser pulses at 800nm, ~45fs, 2.5W and 1kHz repetition rate. Thus each Laser pulse carries along with it a peak power of almost 55GW - strong enough to cut through Steel!
This intense laser beam or 'primary' beam enters the Frequency Tripler where it undergoes frequency upconversion, first to 400nm and then subsequently to 266nm. The frequency doubling is achieved via a non-linear optical process inside a Lanthanum TriBorate or 'LBO' crystal housed inside the Tripler. Due to the non-Linear nature of the process, the efficiency of generating frequency unconverted Laser photons increases dramatically with the Laser intensity, which is one of the reasons for the intense primary beam.
The output of the Tripler has 3 separate beam lines - 800, 400 and 266nm. These beams are directed towards the Ultra High Vacuum (UHV) chamber, using a combination of dichroic mirrors, beam splitters, lenses and irises placed on the optics bench. At any given time, only one of these three beams is chosen as the excitation beam. This allows us to control the wavelength at which the sample is excited.
Simultaneously, a part of the 266nm beam at the Tripler output is split and directed, via optics, towards the electron gun cathode. There, the beam impinges on a thin silver film to generate photoelectrons via the Photoelectric effect. This electron pulse is then accelerated to 30kV and focused using a magnetic lens onto the sample surface. The Electron gun was built at the Physics Machine Shop and is designed so as to minimize the cathode-to-sample distance. This is because the longer the electrons travel, their inherent coulombic repulsion serves to broaden the pulse temporally, thus diminishing the temporal resolution possible in the experiment. Our current gun has a cathode-to-sample distance of about 5cm.
The actual experiment occurs inside the UHV Chamber, which is maintained at extremely low pressures of less than a few nanoTorrs. Such high vacuum is necessary to ensure that the sample is maintained and the experiment performed under an ultra-clean environment, particularly for nanoscale material investigations. When the Laser excites the Nanomaterials, they are kicked into an excited state, where they are highly reactive and have a high propensity to react with any contaminants which might be inside the chamber, thus destroying the original sample. Thus, an ultra-clean environment is essential to preserve the sample integrity. These UHV conditions are achieved using a cascade of vacuum pumps (dry rotary, Turbomolecular and Ion Pumps), each designed to achieve lower pressures than the previous.
The sample is placed at the center of this UHV chamber. The excitation laser (shown in red in the figure) enters the chamber at 45 degrees relative to the electron beam. The electron beam is then allowed to diffract off from the sample. Each electron packet, being only a few 100 fs long, only see the instantaneous excited state of the system. The diffraction pattern thus corresponds to an excited state structure of the sample. This diffraction patttern is captured on the imaging system consisting of an Intensifier CCD Camera. The intensifier serves to magnify the otherwise weak diffraction signal.
The arrival of the electron beam at the sample, relative to excitation laser, is controlled using a transnational stage. Translating the stage forward, in effect, decreases the path length of the excitation laser. Hence, the excitation laser arrives much earlier than before - or equivalently, the electron beam is delayed relative to the excitation laser. Thus, by delaying the electron pulse more and more relative to the excitation, one can monitor the evolution of the diffraction pattern in time following the excitation, which in turn, provides insight into the structural evolution and bonding dynamics. One can literally see how the atoms are moving!
The entire setup is computer controlled. Once the laser and the electron beam have been optimized, the data acquisition is automatic, controlled by the computer. It is typical for experiments to run unaided overnight, requiring only regular checks at periodic intervals to ensure smooth progress of the experiment. i.e. under normal circumstances, sleeping bags aren't required in the lab, except under rare (yet to occur) critical situations!
Currently, all our experiments are performed at room temperature. However, we are in the final stages of testing our design of a new, Cryogenic Sample Holder capable of reaching 40K or lower with an atmosphere-to-UHV sample transfer capability. Once incorporated into the chamber, it will allow us to perform experiments at very low temperatures and also insert / remove samples in and out of the chamber without venting it to atmosphere, thus minimizing its exposure to atmospheric contaminants.
In addition, we are currently building a more powerful electron gun at the Physics Machine Shop, which will be able to focus the electron beam to less than 1mm in size onthe sample surface. The reduced size will enable us to perform Time Resolved Local Probe Experiments and in conjunction with the cryogenic sample holder, will help us achieve significantly higher signal-to-noise ratio in our experiments, thus improving our spatial resolution.
The Lab is also in possession of a Cambridge Instruments Scanning Electron Microscope which will be made operational in the near future.
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