Research Projects

Optical data storage materials

Not unlike the quest for high temperature superconductors, many research groups worldwide are in the search for a spectral hole-burning material that can be used in frequency and/or time domain optical storage at non-cryogenic temperatures with a high figure-of-merit (ratio of inhomogeneous line width to homogeneous line width). The next decade will be dominated by optical computing and optical data storage . An example of current laser diode based technology is the re-writeable 50-cent CD-RW disc and the $6 DVD-RW disc with 700 Megabyte and 4.7 Gigabyte storage capacity, respectively. There is a growing demand for cheap, very high-density data storage media (e.g. a CD that can hold several Terabyte {1 Terabyte=1000 Gigabyte}). A possible way to approach this is by means of frequency and/or time domain optical data storage (FDOS/TDOS) with potential storage densities of ~10¹⁵ bits/cm³ (~125,000 Gigabyte/cm³).

Practical FDOS/TDOS devices may evolve from my recent discovery of a remarkable enhancement (1000x) of spectral hole-burning in inorganic hydrates upon partial deuteration (see below). This discovery represents a major breakthrough in the hole-burning spectroscopy of inorganic crystals. We have assigned this extraordinary isotope effect to photoinduced 180^o flip motions of the water molecules of crystallization: each DHO flip is accompanied by a variation of the transition energy due to a change of the difference in zero point vibrational-librational energies of the ground and excited state. Many other systems will exhibit the effect and it will be possible to optimise parameters for the design of FDOS/TDOS materials for use at liquid nitrogen temperatures. We will also continue our work on samarium(II) based systems that may exhibit persistent spectral hole-burning at room temperature. Samarium(II) may be the key to hole-burning materials that can be used for commercial applications.

Materials for laser stabilization schemes and portable frequency standards

Spectral holes can be used to stabilize diode lasers to 1 part in 10¹³. We are in the process of extending such stabilization schemes by taking advantage of the Zeeman (magnetic field) and Stark (electric field) effects. In particular, inorganic hydrates may again be crucial in developing a stabilization scheme that works at liquid nitrogen temperatures. Spectral holes can also be used as portable frequency standards if they are reasonably narrow at liquid nitrogen temperatures. We will pursue partially deuterated inorganic hydrates containing rare earth ions, such as Eu³⁺, for their suitability in this area of application.

Massive enhancement of electronic spectral hole-burning by partial deuteration of lattice water molecules

This project builds on my recent discovery of a dramatic enhancement (1000x) of laser selective spectral hole-burning in electronic excitations of inorganic crystals upon partial substitution of hydrogen by deuterium in crystalline water molecules. We have assigned this observation to 180^o flip motions of water molecules. A rigorous understanding of the optimal parameters for the effect may enable us to design materials for applications in optical data storage with enormous storage densities), laser stabilization schemes and for frequency standards at liquid nitrogen temperatures.

The efficiency of spectral hole-burning of the R-lines in partially deuterated samples of NaMgAl(oxalate)₃.9H2O doped with Cr³⁺ increases by a factor of 1000. This is the largest increase of hole-burning efficiency upon a minor modification that has ever been observed. For the semi-deuterated water molecule DHO, the difference of zero-point energies in the excited state and ground state changes (the environment of the chromium(III) centre changes) upon the flip, which can be induced by photoexcitation. Thus an extremely efficient (0.1%) hole-burning mechanism is provided. Spectral holes are stable up to 120 K and hence may be used in devices, such as optical storage, all optical RF spectrum analysers, pulse shapers, frequency standards etc at liquid nitrogen temperatures. We are currently searching for other inorganic hydrates that display the effect and we are confident that this hole-burning mechanism will be found in many other systems.

Water flips in inorganic hydrates probed by low temperature solid state dueterium NMR

With Professor M. Mizuno, Kanazawa University, Japan

With Professor Mizuno we are performing low temperature solid state deuterium NMR at 40 MHz to fully characterise the dynamics of lattice water molecules in a range of inorganic hydrates. Line shape and T₁ measurements are used to determine the 180^o flip rates and the activation barriers. These experiments are complementary to the spectral hole-burning work discussed above.

Enhanced hole-burning properties in exchange coupled systems

We have recently demonstrated that sharp spectral holes can be burnt in a crystal of a neat binuclear chromium(III) compound (Chemical Physics Letters 383, 2004, pp. 512-517). Usually fast interactions between the optical centres prevent hole-burning in neat materials. This unusual observation is caused by the exchange interactions between the spin systems of the two chromium(III) centres which can be described by a simple Heisenberg-Dirac-vanVleck operator.

As a consequence of this interaction, the lowest-excited state has no magnetic moment (total spin S=0) and hence is not subject to, and cannot lead to, magnetic fluctuations. We are currently looking at a range of other systems to investigate this remarkable property, which may be used in the design of materials for future applications.

Spectral diffusion and optical line widths in coordination compounds. Nephelauxetic versus ligand field effects

Spectral diffusion (change of line width with time) of electronic excitations in chromium(III) complexes have been systematically investigated as a function of temperature, the structure of the ligand and the amorphous host, and the nephelauxetic effect (delocalisation of d-electrons onto the ligands). Investigations were carried out by the laser techniques of time resolved persistent and transient spectral-hole-burning and fluorescence line narrowing. The project has enhanced the conceptual understanding of line widths, spectral diffusion and the electronic structure of coordination compounds and provides directions for the future design of materials which can be used in optical storage of digital data.

Persistent memory of (low) magnetic fields

We have demonstrated that persistent spectral hole-burning can provide a memory of (low) magnetic fields: the spectral hole pattern read out in zero field in particular systems reflects the magnetic field the hole was burnt in. This memory effect can be used to record the temporal profiles of pulsed or fluctuating magnetic fields. Other applications include the recording of the magnetic fields on space probes.

Direct measurement of spin-lattice and cross relaxation in the ground and excited state by transient spectral hole-burning experiments

We have shown the outstanding power of transient hole-burning experiments: g-factors (used to quantify magnetic moments) and spin-lattice (interaction of electronic/nuclear spin system with crystal lattice) relaxation times can be measured simultaneously for the ground and the excited state. "Classically", four different experiments were required to obtain the same information.

The hole-burning experiment can also deliver the dependence of the relaxation times as a function of the energy splitting. In contrast, measurements based on electron paramagnetic resonance (EPR) are performed at fixed frequencies. We are currently studying a range of systems with varying relaxation times.

Spectral hole-burning studies of impurities in ring and chain silicates

We are revisiting the spectroscopy of a range of natural gemstones with unusual properties such as blue maxixe (beryl with radiation induced colour centres), hiddenite, tourmaline, spinel and emerald from a range of locations. We have been able to measure subtle details of the electronic structure of hiddenite and are currently progressing well on hole-burning investigations of maxixe, emerald and spinel. The results will provide a better understanding of the electronic properties and peculiarities of impurities within a range of silicates. The studies of emerald may be used in the design of highly efficient hole-burning materials.

Diode lasers in single molecule spectroscopy

Diode lasers are inherently more stable than Ti:sapphire or dye lasers. Consequently, they may facilitate single molecule spectroscopy of systems that have proven to be difficult or impossible so far. We aim to perform single molecule experiments in inorganic systems, both in ligand and metal centred transitions. To increase the sensitivity, we use multiple modulation schemes e.g. frequency and amplitude modulation. We are confident that, with our expertise with weak transitions, we will be able to significantly extend the range of compounds that can be investigated by single molecule spectroscopy.

Optical properties of oxide nanoparticles

The continuation of electronic device miniaturization is mainly limited by the availability of suitable functional materials. Due to their rich physical properties, crystalline oxides in the form of thin films or particulate media can be used in many applications, offering new solutions to a number of challenging problems in the rapidly evolving field of microelectronics. However, the quality and functionality of metal oxide materials depend critically on the type of manufacturing process applied. We have started to investigate upconversion and hole-burning properties of rare earth doped yttria nanoparticles produced by wet chemistry. The properties may be used in sensor technology such as thermometry, oxygen and hydrogen detection etc.

High efficiency X-ray storage phosphor based on nanoparticles

With Dr Kazcmarkek I have discovered an extremely efficient X-ray storage phosphor. This phosphor may be used in medical imaging, minimizing the harmful exposure to X-rays (which can cause cancer). The phosphor shows a remarkable efficiency, the image is persistent, but can be reversibly bleached, and the resolution is unprecedented due to the small grain size. We have lodged a International PCT (patent) application and are currently optimizing the specifications for this novel phosphor family.