Bradley, Daniel Joseph ('Dan') (1928–2010), physicist, was born in Derry city on 18 January 1928, the son of John Columba Bradley, a Post Office worker, and his wife Margaret (née Keating), one of ten daughters of farming stock, originally from near Cashel, Co. Tipperary. He was the second-born of four surviving children, with an elder sister and two younger brothers. The family home was at 12 Ewing Street, Derry. Dan's grandfather, William John Bradley, was head postman for the Derry postal area and an alderman (1920–23) in the first catholic-majority Derry Corporation.
After completing the first and second parts of the technical school examinations in electrical engineering, Dan worked for three months as a telegraph boy, during which time he failed the clerkship entry examination for the Post Office. He subsequently enrolled at Lumen Christi high school, Derry, where he obtained his secondary-school senior certificate (1945), surprisingly not taking physics, but with a satisfactorily high standard to warrant a university scholarship. As the scholarship was insufficient to cover all costs, he did not take up a place at QUB but instead with a king's scholarship enrolled at St Mary's teacher training college in Belfast.
After graduation in 1947, he taught at the Long Tower primary school in Derry. While in various teaching posts in Derry (1947–53), he studied part time for a B.Sc. special degree in mathematics with the University of London. On receipt of his degree he moved to London, where he taught science and mathematics at various independent and London County Council schools. In October 1953 he enrolled at Birkbeck College to study part time for the B.Sc. special physics degree, and was a Ravenscroft exhibitioner (1954–7) and college exhibitioner (1956), obtaining his degree in 1957 at the top of the class.
At the start of the 1957–8 academic year, he obtained an assistant lectureship at Royal Holloway College, enabling him to undertake a Ph.D. research programme under the supervision of Professor Sam Tolansky, FRS. The topic of his chosen research programme was the scanning Fabry-Perot interferometer. In the late 1950s there was considerable interest in developing such scanning techniques to add versatility and simplify the recording of emission spectra for application in spectroscopy. Many of the approaches were slow and cumbersome, and resulted in reduced finesse and resolution. Bradley developed a simple mechanical scheme where one plate was fixed and the other moved, with the moving plate mounted on a brass tube and moved parallel to this on springs. He also used photomultiplier detection, and with scanning rates of up to 1 kHz allowed real time detection of spectral line emission with high resolution.
At Royal Holloway, he met Winifride O'Connor, a 1951 botany graduate from the University of Liverpool, who was also an assistant lecturer. They married in 1958 and had five children: Sean (b. 1959), Mairead (b. 1960), Donal (b. 1960) (who became a distinguished physicist and was elected FRS (2004) and appointed CBE (2010)), Ronan (b. 1965) and Martin (b. 1972).
Bradley submitted his Ph.D. thesis, 'A high resolution interference spectroscope', in August 1961, and alluded to the potential of the device in earth satellite observations. At the beginning of the 1961–2 academic year he was appointed to a lectureship in the instrument technology group in the physics department at Imperial College in London. Broadening his research on the applications of the Fabry-Perot, with the coming of the laser era he was quick to realise that, although suitable for diagnosis of continuous operating lasers, the scanning Fabry-Perot configurations he had developed were totally unsuitable for investigations of nanosecond Q-switched lasers and the laser-generated plasmas they produced. He rapidly expanded his research programme to utilise gated or swept electro-optic image intensifiers in combination with fixed-gap Fabry-Perots, and widely investigated new, efficient, highly reflecting coatings for operation in the ultraviolet. With the superior angular dispersion of the Fabry-Perot and the increased luminosity over grating dispersers, he realised that these compact devices were highly suitable for astronomical spectroscopy from both balloons and rockets.
With an increasing international reputation, Bradley was promoted to a readership at Royal Holloway in 1964. He continued research and development on Fabry-Perot devices and their applications, and introduced a particularly novel, narrow-gap, hence exceedingly broad free-spectral range device, which was later used to tune short-pulse, broad-band lasers and was successfully commercialised. Although he maintained a major interest and published widely on solar spectroscopy, his principal interests were increasingly directed towards ultrafast electron-optical technology, laser development, diagnostics and applications.
His appointment as professor and head of the department of physics at QUB, remarkably only five years after obtaining his Ph.D., afforded him the opportunity to establish one of the most influential and largest laser research groups worldwide, with research outputs punctuated by many world firsts. In addition to innovative advances in tuneable narrow-line lasers for spectroscopy, Dan was particularly noted for the development of the passively mode-locked, flashlamp-pumped dye laser, which allowed wavelength tuneable pulses of picosecond duration to be generated. He clarified the role of the saturable absorber (ultrafast light gate) in the mode-locking process, showing that with solid-state lasers it was the lifetime or open time of the gate that determined the pulse width, while with the dye laser, pulses substantially shorter than the absorber lifetime were generated because saturable gain played a vital role in reducing the tail of the pulse after saturable absorption had removed the front. Pulses of a few picoseconds (10-12s) were routinely achieved and Bradley was also instrumental in the formation of one of the first laser spin-out companies, helping establish Electro Photonics Ltd in Belfast, who rapidly commercialised the outputs from the research group. Bradley was also internationally distinguished for his innovation and development of the pulsed electron-optical streak camera that allowed the only technique for the direct measurement of picosecond and sub picosecond (femtosecond) optical events.
Bradley had lobbied for and had hoped that a government-funded national laser facility could be established in Northern Ireland. However, the increasing instability and political violence of the early 1970s effectively negated any possibility of that. By 1973 he also had concerns for the well-being of his young family, leading him to accept the chair of optics at Imperial College that October.
At Imperial, Bradley set about establishing another laser group of major international importance from small beginnings. He initiated major programmes on e-beam excitation of gaseous excimer and exciplex lasers for vuv generation, as well as mode-locked, semiconductor lasers for application in communication and switching. He expanded his dye laser work to mode-locked, continuously operating systems and at various times held the record for the shortest pulses achieved. To record and use the pulses from continuously operating mode-locked lasers, he developed the concept of the synchroscan streak camera, a continuously sweeping streak camera, where the high voltage, sinusoidal sweeping of the electron image of the fast optical pulses was synchronised with the pulses emitted by the laser. When recorded on a phosphor screen, much like the concept of the scanning system of an old television tube, the pulses were precisely located on top of each other, greatly increasing the sensitivity of the device, while exhibiting sub picosecond resolution. Together, the streak camera and the mode-locked continuous dye laser represented a powerful piece of ultrafast technology and diagnostics that Bradley applied extensively to investigating the picosecond relaxation dynamics of various molecular species. He also saw great potential in application to systems of biological importance.
Bradley's time was increasingly being called upon outside his research, and in 1976 he was appointed head of the department of physics at Imperial. He was also chairman of the Science Research Council (SRC) steering committee for the Joint Laser Centre, an institution where he had always been a vociferous proponent. He was delighted when the facility was approved in October 1975, with him serving as chairman of the SRC laser facility committee (1976–9). Throughout 1979, however, Bradley had become increasingly frustrated by administrative problems, in particular those relating to the processes and limits on promotion and appointment of junior staff. He also had wished to be more actively involved in laboratory investigations and was planning his future research strategy when his term as head of department ended. Despite this, it was surprising when he announced he would take up the newly established chair in optical electronics at TCD in October 1980.
Bradley had firm ideas on his research directions as he set out to establish his new group and research facility at Trinity. He planned application of mode-locked semiconductor lasers to optical communication and optical logic and a major effort on the application of ultrafast laser technology to investigate dynamical processes in biological molecular species. Although he succeeded in obtaining considerable external funding to support this, internal funding for major projects was difficult to come by. Nominated by the provost, Professor Bill Watts (qv), in 1981, he became bursar of Trinity. By the latter part of 1983, Bradley's group was growing, was in well-established laboratories, and was beginning to publish in the areas designated above, but on 21 October 1983 Bradley suffered a severely debilitating stroke.
As a result, his mobility was restricted and his speech severely impaired, and he retired in 1984. He still continued to travel extensively and participate at numerous national and international conferences. In time, illness restricted his travels and in his latter years he was cared for in a residential home in Dublin, where he died on 7 February 2010.
In a relatively short but stellar career, Dan Bradley published more than 160 scientific papers and co-wrote more than 200 conference presentations. He supervised more than sixty Ph.D. students. His extensive contributions to the field were recognised through numerous international awards. He received the Thomas Young medal and prize of the Institute of Physics (1975), was elected a fellow of the Royal Society (1976), received the royal medal of the Royal Society (1983), the C. H. Townes medal of the Optical Society of America (1989), and the Cunningham medal of the RIA (2002). Awarded honorary D.Sc. degrees from the University of Ulster (1983) and QUB (1986), he was a fellow of the Institute of Electronic and Electrical Engineers, the Optical Society of America, and the Institute of Physics, and a member of the RIA. He also presented the prestigious Scott lectures in physics at the Cavendish Laboratory at the University of Cambridge in 1977. A bench was placed in his memory outside the Fitzgerald Building at TCD.