Ultrafast carrier dynamics and confined acoustic phonons in CdSe nanorods

Dichroic Photoelasticity in Black Phosphorus Revealed by Ultrafast Coherent Phonon Dynamics

Coherent longitudinal lattice vibrations of black phosphorus provide unique access to the out-of-plane strain coupled in-plane optical properties. In this work, polarization-resolved femtosecond transient absorption microscopy is applied to study the anisotropic coherent phonon responses. Multiorder phonon harmonics were observed with thickness dependence well explained by the linear chain model, allowing rapid optical mapping of phonon frequency distributions. More interestingly, exotic coherent phonon oscillations occourred with a π-phase jump between the armchair and zigzag polarizations, which reveals opposite signs of photoelasticity under the longitudinal strain. Specifically, compressive strain reduces the imaginary refractive index in the armchair polarization but increases the real refractive index in the zigzag polarization, as confirmed by the ab initio calculations and thin film model. These fundamental properties of black phosphorus hold potential for applications in ultrafast and polarization-sensitive photoacoustic/photoelastic modulators.

Energy relaxation in CdSe nanocrystals: the effects of morphology and film preparation

Ultrafast time-resolved absorption spectroscopy is used to investigate exciton dynamics in CdSe nanocrystal films. The effects of morphology, quantum-dot versus quantum-rod, and preparation of nanocrystals in a thin film form are investigated. The measurements revealed longer intraband exciton relaxation in quantum-rods than in quantum-dots. The slowed relaxation in quantum-rods is due to mitigation of the Auger-relaxation mechanism from elongating the nanocrystal. In addition, the nanocrystal thin film showed long-lived confined acoustic phonons corresponding to the ellipsoidal breathing mode, contrary to others work on colloidal systems of CdSe nanocrystals.

Particle-level engineering of thermal conductivity in matrix-embedded semiconductor nanocrystals

Known manipulations of semiconductor thermal transport properties rely upon higher-order material organization. Here, using time-resolved optical signatures of phonon transport, we demonstrate a "bottom-up" means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. Growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap.

Controlling piezoelectric response in semiconductor quantum dots via impulsive charge localization

By direct observation of coherent acoustic phonons, we demonstrate a novel extrinsic piezoelectric response in colloidal CdSe semiconductor quantum dots. This response is driven by the migration of charges to the surface of the quantum dot on a vibrationally impulsive time scale. Surface- and fluence-dependent studies reveal that the observed carrier capture based piezo response is controllable and is at least an order of magnitude larger than the intrinsic piezo response of wurtzite CdSe.

The influence of shell parameters on phonons in core-shell nanoparticles: a resonant Raman study

The effect of shells of various thicknesses on the vibrational resonant Raman spectra of CdSe/ZnS core-shell nanoparticles is studied. The dependence of the core-shell structure on the method of shell deposition is derived from a comparison of the vibrational and photoluminescence spectra of nanoparticles. Along with the appearance of peaks attributed to the shell, the phonon spectrum of the core undergoes significant changes upon shell growth. The change of the CdSe LO peak lineshape in core-shell nanoparticles is discussed with respect to possible changes in the spectrum of both optical and acoustical phonons upon shell formation. Based on the observed decrease of the CdSe 2LO/LO peak intensity ratio, a weakening of exciton coupling to the CdSe LO phonon upon ZnS shell deposition is supposed. The change in the carrier localization volumes upon shell formation is discussed as a possible reason for the reduced coupling.