Optical tweezers are a micromanipulation technology in which the gradient forces associated with tightly focussed laser light are exploited to exert forces on dielectric particles. By using a high numerical aperture objective lens, a laser beam is brought to a focus in a liquid sample chamber, forming a trap for micron-sized particles (see image below). These traps make an ideal environment for the investigation of systems such as dielectric colloidal particles and single cells.
Multiple optical traps can be generated and controlled by imposing a diffractive optical element (DOE) in a plane optically conjugate with the back-aperture of the objective lens. We use liquid crystal spatial light modulators (LCSLMs) for this purpose, with a 'gratings and lenses' algorithm running on a graphics card generating the necessary patterns to produce arbitrary arrays of laser focii in the microscope's image plane.
Further details are available on the Nanophysics and Soft Matter research group pages.
Optical tweezers can be exploited to study the behaviour of single cells in suspension, as well as the behaviour of small populations of cells as they are brought into close proximity with one another. The 'light touch' of optical tweezers are ideal for investigations in physiologically relavent in vitro conditions.
The Access Grid is an advanced video conference system where people in different places meet in a 'virtual venue' using audio and video tools and other shared applications, such as presentations. The Access Grid is particularly suitable for group-to-group collaboration, potentially between many sites.
In the School of Physics we currently have one access grid room able to accommodate around 20 people.
The Access Grid room is more commonly known as a node. The node has the following equipment specifications:
There is a charge of £100 per hour (pro-rata after the first hour) for use of the room by users from outside the School of Physics.
When booking a room, if you are unfamiliar with the equipment or would prefer the meeting to be setup/established by the support staff, please allow at least 15 minutes of setup time when booking the node.
For Access Grid IT support please contact Duncan Baldwin.
If you have any problems or questions please send an email to firstname.lastname@example.org.
In Bristol, positrons are used to probe the structure and electronic structure of materials. The Bristol Two-Dimensional Angular Correlation of Annihilation Radiation (or 2D-ACAR) spectrometer is a unique instrument for probing the Fermi surfaces of metallic systems.
When a positron enters the sample under study, it quickly comes into thermal equilibrium (typically within a few picoseconds) and then, on a timescale of order of hundreds of picoseconds, it annihilates with an electron resulting predominantly in two gamma-rays travelling in (almost) opposite directions (conserving the energy and momentum of the annihilated electron-positron pair).
The gamma rays are detected by a pair of position-sensitive detectors, known as HIDACS (high-density avalanche chambers) operating in coincidence which allows Dr. Dugdale and Prof. Alam of the CES Group to measure the occupied momentum states and hence the Fermi surface of the material under study.
Understanding the Fermi surface is vital for understanding the different behaviours of metals.
The most visible sign of the research carried out by the Astrophysics Group is the impressive six metre radio telescope on the roof of the School of Physics. The Coldrick Observatory was funded through the generosity of William P Coldrick, after whom the telescope is named.
Coldrick was a graduate of the University, who also endowed the Chair in Astrophysics and Cosmology presently held by Mark Birkinshaw. The telescopewill be used to survey our Galaxy for sources of maser emission, the (natural) radio equivalents of lasers.
Its Cassegrain geometry has been designed to operate between 4.5 GHz and 25 GHz, making it very suitable for studying water masers at 22.235 GHz.
The 22 GHz front-end receiver is a copy of a MERLIN K-band receiver which is about to be (re-) installed on the telescope. The receiver operates in both circular polarisations and the polariser and the first amplifiers are cooled to 10 K by a closed-cycle helium refrigeration system. This apparatus was funded by a JREI grant to MD Gray.
The front-end converts the signals to an intermediate frequency (IF) near 4.2 GHz. These will be further converted to a second IF at 150 MHz in a mixer stage on the telescope using a local oscillator whose frequency can be adjusted by the supervising computer to allow for the Doppler effect of the Earth's velocity. This part of the receiver has been developed and built in-house.
The back end is a fully-digital, real-time Fast Fourier Transform Spectrometer, developed by the Group in partnership with AphaData Parallel Systems Ltd (Edinburgh) and Beam Ltd (Bristol). Fourier transforms are taken in real time by FFT code written for field programmable gate arrays (FPGA). The data are therefore integrated in the frequency domain. This very flexible arrangement will be used to provide a pair of power spectra with 4096 channels over 25 MHz.
The observatory also has the use of a 25-cm Meade optical telescope with CCD camera, which is mounted in a dome on the roof of the Physics building.
The Coldrick Observatory on the roof of the School of Physics is used to survey our Galaxy for sources of maser emission.