Defects in inorganic photorefractive materials and their investigations
Defects in inorganic photorefractive materials and their investigations
Defects in inorganic photorefractive materials and their investigations
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8 B. Briat et al.<br />
low temperatures <strong>and</strong> high magnetic fields <strong>in</strong>crease the sensitivity. Also the<br />
heat<strong>in</strong>g of crystal specimens by the resonant absorption of microwaves can<br />
be used, lead<strong>in</strong>g to thermally detected EPR [16]. Double resonance methods,<br />
to be <strong>in</strong>troduced <strong>in</strong> the follow<strong>in</strong>g, constitute further ways to detect the EPR<br />
transitions.<br />
Important <strong>in</strong>formation on the defect wavefunction is furnished by hyperf<strong>in</strong>e<br />
<strong>in</strong>teraction. If a hyperf<strong>in</strong>e splitt<strong>in</strong>g is resolved, the result<strong>in</strong>g (2I i +1) l<strong>in</strong>es<br />
allow the sp<strong>in</strong> I i of the correspond<strong>in</strong>g nucleus to be identified. This gives a<br />
strong h<strong>in</strong>t of the chemical identity of this nucleus. The tensor A i (eq. 1) partly<br />
depends on the density of the wavefunction at the local probe represented by<br />
nucleus i. If hyperf<strong>in</strong>e <strong>in</strong>teraction orig<strong>in</strong>ates from parts of the wavefunction<br />
with low probability density, the correspond<strong>in</strong>g small splitt<strong>in</strong>gs are usually not<br />
resolved. Then the electron nuclear double resonance (ENDOR) [13] technique<br />
(Fig. 4) may help: Here, us<strong>in</strong>g a special experimental scheme, the highly resolved<br />
nuclear magnetic resonances, ly<strong>in</strong>g at radio frequencies, are detected by<br />
changes of the <strong>in</strong>tensities of the correspond<strong>in</strong>g EPR signals. If applicable, this<br />
technique leads to the most detailed <strong>in</strong>formation about a defect wavefunction,<br />
e.g. its spatial distribution <strong>and</strong> the nuclei it encompasses.<br />
We consider now magnetic circular dichroism (MCD), i.e. the differential<br />
absorbance, ∆α = α + − α − , presented by a cubic or uniaxial sample for<br />
left (σ + ) <strong>and</strong> right (σ−) polarized light propagat<strong>in</strong>g along the direction of<br />
an applied magnetic field. In general [17, 18, 19] the MCD signal associated<br />
with an isolated electronic transition conta<strong>in</strong>s two ma<strong>in</strong> contributions. The<br />
diamagnetic term (S-shaped <strong>and</strong> temperature-<strong>in</strong>dependent) results from the<br />
difference <strong>in</strong> energy of the circularly polarized components. Although always<br />
present down to relatively low temperatures <strong>in</strong> the case of very sharp l<strong>in</strong>es (e.g.<br />
lanthanide ions [18]), it can be safely ignored <strong>in</strong> the case of the broad b<strong>and</strong>s at<br />
low temperature. The paramagnetic term (absorption-like shape, temperature<br />
dependent) monitors the magnetization <strong>in</strong> the groundstate. In the case of sp<strong>in</strong><br />
S = 1 2<br />
(Fig. 4) it is proportional to the difference <strong>in</strong> relative populations at<br />
equilibrium (eq. 2) between its two Zeeman sublevels. Experiments at very<br />
low temperatures (pumped helium) thus furnish the largest MCD signals The<br />
technique is very sensitive s<strong>in</strong>ce the smallest detectable absorbance is about<br />
10 −5 , i.e. roughly two orders of magnitude smaller than with a classical spectrometer.<br />
In the case of the sillenites, Fe <strong>and</strong> Cr impurities could be monitored<br />
down to the ppm level.<br />
The term ODMR has often been used <strong>in</strong> connection with the detection of<br />
EPR by various features of photolum<strong>in</strong>escence transitions [13]. S<strong>in</strong>ce a study<br />
of the <strong>photorefractive</strong> effect requires the assignment of the optical absorption<br />
b<strong>and</strong>s, we concentrate rather on the optical detection of magnetic resonance<br />
(ODMR) via the magnetic circular dichroism (MCD). The signal ∆α is measured<br />
as a function of B/T <strong>and</strong> a dip is observed (|∆n| (eq. 2) is reduced) <strong>in</strong><br />
the saturation curve, whenever the resonance conditions are fulfilled.<br />
The great advantage of the MCD-ODMR method is its ability to connect<br />
optical absorption features to <strong>their</strong> microscopic orig<strong>in</strong>s <strong>in</strong> the logically