Thomas Han
Senior Lecturer

Research

Research activities in recent years have developed into new areas and have expanded from fundamental research to more application-oriented research, primarily based on applications of spectroscopic techniques. Particular areas of interest include the static and dynamic properties of transition-metal and lanthanide ions in disordered crystals and low phonon hosts, and the applications of linear and non-linear properties of optical materials in guided-wave geometries. Dynamic features dealing with interline and intra-line energy transfer processes arising from ion-ion interactions, electron-phonon and phonon-phonon interactions can now be probed directly giving a better understanding of the relaxation processes, pumping cycle and phonon dynamics in laser gain hosts and other optically active materials. The research areas of energy transfer and energy upconversion lead logically to fluorescence cooling which has potential applications in the optoelectronic industry and this theme is being developed. The premise that anti-Stokes fluorescence of a solid material can form the basis of a refrigeration cycle is not new. However, an incomplete understanding of the spectroscopy of these solid-state systems, plus the probable non-radiative quenching of excited states by impurities are some of the issues that need to be addressed. By pumping a fluorescent cooling element with a high-efficiency diode laser, it should be possible to construct a compact, wireless, all-solid-state optical cryo-cooler. This would enable widespread deployment of cryogenic electronics and detectors in space and elsewhere.

Combining the linear and nonlinear optical properties of crystalline materials with the wave-guiding property of the fibre geometry has been for many years a goal of optoelectronics research. It has long been recognised that the resulting increased efficiency will lead to miniaturisation and/or increased output power and versatility. This theme is being pursued to develop new modular photonic devices based on crystalline fibres. Modular devices such as these are expected to find wide acceptance by the optical fibre community in general. It is envisage that they will be recognised for their novelty and versatility as well as for their improved efficiency and ruggedness.

Current research interests follows the natural extension to applications of spectroscopy in sensing and the studies of dynamic interactions and energy transfer between optically active ions in solid state materials. Investigation of the optical properties of Lanthanide and Transition-metal ions in insulating host remains the back bone of the research activities, in particular Two-Photon Fluorescence and order/disorder structures. The former led to the development of optical fibre sensor based on the process of two-photon fluorescence. This work has led to two patents and is in the process of sorting external partners for the next step of the development and applications to EPSRC and other funding bodies.

The interface between the two phases is anticipated to be a gradual change from ordered to disordered structure. The volume/size effect is expected to have significant influence on the ion-ion, ion-host interactions in the crystalline and amorphous phase and between the two phases. It is of fundamental interest to investigate the interaction and energy transfer between the crystalline and amorphous phase as a function of cluster size. To this ends, complementary spectroscopic techniques in frequency and time domain will enable a greater understanding of this expected complicated system. The dopant of choice is transition-metal ions, such as Cr^3+/4+, the partially filled outer 3d electron shell is particularly sensitive to its surrounding environment and is a good probe for optical spectroscopic technique. The dopant could reside in the crystalline phase, the amorphous phase or both depending on the driving mechanism of nucleation process. Judicial choice of dopant or nucleation agent will enable investigation of each case independently. A good candidate for this investigation is the borate family, which can be found in both amorphous glass and crystalline phase. Controlling the size of the crystalline phase is crucial to this investigation and this is a function of annealing time, temperature, and nucleation processes. If fine control of the crystallisation process is achieved, potentially this could leads to nano-size structure, which could leads to another new area of research interest. Some of the crystalline borates have nonlinear properties and it would be of interest to investigate the size effect of these microcrystalline nonlinear structures. Understanding the interaction between dopant in the amorphous phase and the nonlinear property of the crystalline phase could lead to potential useful novel devices. The ultimate purpose of the proposed research is a study of non-radiative vibrational relaxation of photo-excited impurities ions and the interaction between crystalline and amorphous phase. This will have direct benefit for researchers into novel laser systems in both academic and industrial research.

Highlights

Working together with Hugh G. Gallagher and under the mentor of Prof. . Henderson we have developed the Optical Materials Research Centre (OMRC) to an international recognised centre of excellence. Since 1990, OMRC has attracted a total research grant funding in excess of £2.5M. In 2002, the crystal growth facilities of the OMRC was commercialised into The Crystal Consortium Ltd. (TCC) absorbing the Bulk Optical Materilas Group at DERA in Malvern. HGG was seconded as TCC Technical Director whilst TPJH continues the research work into the laser spectroscopy of inorganic photonic materials within the university.