Quantum-confined Franz-Keldysh Effect in CdTe Quantum Dots in Glass

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Abstract
Microcrystallites of CdTe were grown in a special-formula glass, yielding the first reported CdTe dots (ro ≅ 36Å). As many as six quantum-confinement peaks1 have been seen in the linear absorption spectrum. These quantum dots are especially suited for electric-field effects because of CdTe′s large Bohr radius (aB ≅ 73 Å). The quantum-dots glass sample was arranged in a waveguide configuration with the electric field applied transverse to the light propagation direction. The samples were 200 to 250 µm thick, and measurements were taken at 8 K. Single photon counting enabled measurement of differential signals as small as 10−5. The observed changes in the absorption spectrum are interpreted to arise mainly from a red shift of the lowest transition and the redistribution of the oscillator strength as forbidden transitions are opened up by the symmetry-breaking action of the external field. This interpretation is made plausible by a strong-confinement Franz-Keldysh calculation similar to that of Miller, Chemla, and Schmitt-Rink2 which predicts a differential absorption spectrum in good qualitative agreement with the data. We also report the first nonlinear optical measurements on CdTe quantum dots, observing the differential absorption during 2-µs pump pulses.