Semiconductor doped glasses
Glasses doped with nonosized inclusions of metals or semiconductors are known since a very long time. The first glasses containing metal nanoparticles were fabricated by Roman glassmakers in the forth century A. D. Mediaval cathedral windows through several European countries witness the attention drawn by stained glasses containing metal aggregates as artistic work. They exhibit great varieties of beautiful colors owing to the nanosized metal particles which were embedded in the glass matrix.
Semiconductor-doped glasses (SDG) were, however, not so widespreadly used. One very important application of SDG are sharp cut-off glass filters. In most cases, Cd(S,Se) nanocrystals are used for this purpose. By varying x the position of the cut-off wavelength can be precisely tuned between ~ 400 and 1000 nm. These glass filters are easily available from different glassmakers, such as Schott, Corning, Hoya, or Toshiba. This is an import point, as it allowed a great number of optics groups to study this kind of composite material without the need for skills in material preparation.
The optical properties of semiconductor nanocrystals are fundamentally different with respect to the corresponding bulk semiconductors. The fact, that the free electron and holes are confined inside the nanocrystal, changes the available energy levels and the interaction with photons. One of the most striking effect of this quantum confinement is the so-called blue shift of the linear optical absorption edge. But also other effects can be observed, e.g. The optical properties of semiconductor nanocrystals are fundamentally changed with respect to the corresponding bulk semiconductors. The fact, that the free electron and holes are confined inside the nanocrystal, changes the available energy levels and the interaction with photons. One of the most striking effect is the so-called blue shift of the linear optical absorption edge. But also other effects can be observed, e.g. the modification of the density of states induces distinguishable absorption lines instead of the broad absorption bands of bulk semiconductors. These unique optical properties of SDGs makes them a very interesting class of material, which not only allows the study of quantum confinement effects, but which can also be exploited in different applications.
In 1963 Bret and Gires were the first using SDG in nonlinear optics . They introduced a commercial SDG glass filter into a ruby laser cavity. In this way, they exploited the saturable absorption of SDGs for passive Q-switching and nanosecond laser pulses could be obtained. The starting point of the optical study of SDG was, however, the publication of degenerate four-wave mixing results by Jain and Lind in 1983 . This was for the first time that the high optical third-order nonlinearities of SDG were observed. It is worth to notice, that the main reason, they investigated SDG was not the search for new, highly nonlinear materials, but problems they found to elaborate suitable bulk semiconductor samples. In fact, at that time, it was quite difficult to grow semiconductor crystals of appropriate dimensions with precisely desired band edge, absorption coefficient, and overall optical quality. In the eighties and nineties many groups studied the linear and nonlinear optical properties of semiconductor-doped glasses and observed original features such as the blue shift of the absorption edge or the high nonlinearities. The theoretical work, which accompanied the experimental one, allowed to explained most of these features by the quantum confinement inside the nanocrystals.
One aim of this research was to find a good nonlinear material in order to realize nonlinear optical devices, which were already theoretically predicted. But very soon, contradictory results of the strength of the third-order nonlinear refractive index, its sign and particularly the nonlinear response time were published. Electronic states located at the semiconductor particle surface and inside the glass matrix were found to be the main reason for the observed discrepancies. In fact, the high light intensities used in nonlinear optics (~ 1- 1000 MW/cm2) are inducing permanent changes of the optical properties of SDGs. Roussignol et al. were the first to outline the great importance of this phenomena and studied it systematically . They also introduced the term photodarkening. In fact, these non-reversible effects are one of the main reasons why the commercial SDG were not used for nonlinear applications.
In parallel with the characterization of the commercial SDG samples, great progress was made concerning the quality of the SDGs. The development of new elaboration processes such as sol-gel allowed further improvements. Now, it was possible to use purer glass-matrices, which reduce significantly the influence of the particle-matrix interface, e.g. it could be shown that photodarkening is vanishing for SDG samples made by sol-gel. Another very important point was the possibility to produce samples with very narrow particle size distribution. Also the development of more precise characterization technique helped to improve the understanding of these materials. The very important progress made in the fabrication and the understanding of the physics which governs SDG, resulted already in better materials. But this process is not yet finished and the observation of new and interesting features can be expected. This should also lead to new applications of these materials, particularly in the very fast increasing fields of integrated or nonlinear optics.
Very good introductions to the field can be found in two recent books [4-6]
1. G. Bret and F. Gires, Appl. Phys. Lett. 4, 175 (1964).
2. R. K. Jain and R. C. Lind, J. Opt. Soc. Am. 73, 647 (1983).
3. P. Roussignol, D. Ricard, J. Lukasik, and C. Flytzanis, J. Opt. Soc. Am. B 4, 5 (1987).
4. S. V. Gaponenko, Optical Properties of Semicoductor nanocrystals (Cambridge University Press, Cambridge, 1998).
5. U. Woggon, Optical Properties of Semiconductor Quantum Dots (Springer-Verlag, Berlin, 1997).
6. J. Fick in "Handbook of Surfaces and Interfaces of Materials" edited H. S. Nalwa, Academic Press, San Diego (2001) Vol. 3, p 311-350.