Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

As noted by Khurgin [16], the intraband transition has an oscillatorstrength that is the same order of magnitude as the value for the interband1S 3/2 – 1S e transition. For a spherical box and using the parabolic approximation,the 1S e –1P e transition carries most of the oscillator strength of theelectron in the 1S e state (96%), with little left for transitions to higher P estates. There should then be a large region of transparency between the IRintraband absorption and the band-edge absorption. In the parabolic approximation,the oscillator strength of the 1S e –1P e transition is proportionalto 1/m*, where m* is the effective electron mass. The magnitude of theoscillator strength is close to 10 for CdSe (m*f0.12), and it can be as high as100 for other materials. In practice, as the quantum-dot size decreases, the 1S eand 1P e states occupy a higher-energy region of the conduction band and havea larger effective mass. Consequently, the 1S e –1P e oscillator strength becomessmaller, although it is still large by molecular standards.III.EXPERIMENTAL OBSERVATIONS OF THE INTRABANDABSORPTION IN COLLOIDAL QUANTUM DOTSIntraband absorption in quantum dots was first observed in lithographicallydefined quantum dots in the far IR below the optical phonon frequency band[17]. For the more strongly confined epitaxial quantum dots and colloidalquantum dots, the intraband absorption lies in the mid-IR above the opticalphonon bands. In epitaxial quantum dots, the intraband absorption was firstdetected by the infrared spectroscopy of n-doped materials, [18,19], whereasfor the quantum dots grown using colloidal chemistry routes, the intrabandspectra were initially recorded by IR probe spectroscopy after interbandphotoexcitation [12,20,21].Figure 1 shows the peak position of the IR absorption for CdSenanocrystals of various sizes measured under different experimental conditions.The experimental intraband spectra obtained using Fourier transformIR (FTIR) measurements on n-type colloidal CdSe nanocrystals are shown inFig. 2 [14]. The peak position of the IR absorption is only weakly sensitive tothe surface chemistry of the nanocrystals. This position is also not significantlydependent on whether the electrons are placed in the 1S e by photoexcitationor by electron transfer. This result indicates that in the case ofphotoexcited nanocrystals, the electron–hole Coulomb interaction is weak orthat on the fast timescale of these measurements (>6 ps), the hole has beenlocalized. Time transients on the order of 1 ps in the mid-IR absorption regionhave been attributed to hole cooling dynamics [22,23]. It would be of interestto perform spectroscopic IR transient measurements of the intraband spectrumto monitor spectral shape changes on the subpicosecond timescale.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.