Second Harmonic Generation and Confined Acoustic Phonons in Highly Excited Semiconductor Nanocrystals

Abstract

The photo-induced enhancement of second harmonic generation, and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals, has been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron-hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement.Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes.The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.

Visualization of nanocrystal breathing modes at extreme strains

“…Using known values for bulk CdSe 27 , this gives a period of B1.1 ps in qualitative agreement with the observed response. This simple model does not account for the effects of the surrounding matrix but is in reasonable agreement with prior measurements 17 . The inset to Fig.…”

“…Estimates based on prior theoretical models 35 suggest that at temperatures of B1,000 K (comparable to those induced within each nanocrystal-Supplementary Note 1, and Supplementary Figs 5 and 6), these scattering rates are of order 1 THz (with scattering mean free paths comparable to the nanocrystal diameter), which is consistent with the strongly damped coherent response observed. Similar acoustic damping effects in strongly photoexcited semiconductor thin films have been observed by X-ray techniques 36 and by optical pump-probe techniques 17 . The observed damping is inconsistent with dephasing associated with the finite size distribution of the nanocrystals (Supplementary Note 3).…”

“…Optical pump-probe studies indirectly reflect structural responses and show evidence for breathing and extensional mode excitations, with typical periods of order 1 ps associated with sound-velocity-limited strains at the nanoscale (1 nm/(1,000 m s À 1 )B1 ps; refs [16][17][18][19]. X-rays have been used to map the acoustic response of few hundred nanometre particles on hundreds of picosecond timescales [20][21][22] .…”

Controlling Piezoelectric Response in Semiconductor Quantum Dots via Impulsive Charge Localization

“…Since the higher energy states of a quantum dot converge toward an excitonic continuum, one might anticipate a similar e-ph response to intense excitation into these nonquantized states. 5,6,22 In contrast to such expectations, we show here a completely new manifestation of e-ph interactions due to spatial migration of charges in the quantum dot: extrinsic piezoelectric coupling. Since CdSe has a wurtzite lattice and is intrinsically piezoelectric, we expect that there would be a piezo response due to the screening of the intrinsic field by the excited electron-hole plasma, as in the case of quantum wells.…”

“…A similar fluence dependence was obtained by Alivisatos and co-workers. 22 In reality, the untreated dots most likely have some polarization accumulated during the course of the experiment. We related the peak FFT intensity to the number of surfacetrapped holes, Figure 4b.…”