title: Direct measurement of lattice dynamics and optical phonon excitation in semiconductor nanocrystals using femtosecond stimulated Raman spectroscopy.
abstract: We report femtosecond stimulated Raman spectroscopy measurements of lattice dynamics in semiconductor nanocrystals and characterize longitudinal optical (LO) phonon production during confinement-enhanced, ultrafast intraband relaxation. Stimulated Raman signals from unexcited CdSe nanocrystals produce a spectral shape similar to spontaneous Raman signals. Upon photoexcitation, stimulated Raman amplitude decreases owing to experimentally resolved ultrafast phonon generation rates within the lattice. We find a ∼600  fs, particle-size-independent depletion time attributed to hole cooling, evidence of LO-to-acoustic down-conversion, and LO phonon mode softening.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Hannah
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Kristen E
authors.initials: KE
authors.lastname: Brown
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Ryan M
authors.initials: RM
authors.lastname: Young
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: George C
authors.initials: GC
authors.lastname: Schatz
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Dick T
authors.initials: DT
authors.lastname: Co
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Richard D
authors.initials: RD
authors.lastname: Schaller