* Corresponding author. Tel.: #49-30-6392-1476; fax: #49- 30-6392-1489. E-mail address: lienau@mbi-berlin.de (C. Lienau) Physica B 272 (1999) 96}100 Picosecond and femtosecond near-"eld optical spectroscopy of carrier dynamics in semiconductor nanostructures Ch. Lienau*, V. Emiliani, T. Gu K nther, F. Intonti, T. Elsaesser Max-Born-Institut fu ( r Nichtlineare Optik und Kurzzeitspektroskopie, Rudower Chaussee 6, D-12489 Berlin, Germany Abstract Quasi-two-color femtosecond pump}probe spectroscopy and near-"eld scanning optical microscopy are combined to study the carrier dynamics in single semiconductor nanostructures. In temporally, spectrally and spatially resolved measurements with a time resolution of 200 fs and a spatial resolution of 200 nm, the nonlinear change in re#ectivity of a single quantum wire is mapped in real space and time. The experiments show that carrier relaxation into a single quantum wire occurs on a 100 fs time scale at room temperature. First experimental evidence is given for carrier transport along the wire axis on a picoseond time and 100 nm length scale.  1999 Elsevier Science B.V. All rights reserved. Keywords: Quantum wire; Near-"eld microscopy; Femtosecond spectroscopy During the last few years there has been a con- tinuously growing interest in the application of optical techniques o!ering subwavelength spatial resolution, such as near-"eld scanning optical microscopy [1,2], to the spectroscopy of semicon- ductor nanostructures. By bringing the resolution of all-optical techniques closer to the nanometer size of these objects, the investigation of single nanostructures becomes possible and new and de- tailed insights into their optical and electronic properties are obtained. Moreover, the combina- tion of nanometer scale optics with the powerful tools of ultrafast spectroscopy } providing tem- poral resolution in the range of a few femtoseconds } o!ers exciting new perspectives for gaining de- tailed microsopic insights into dynamic processes in nanostructures. Speci"cally, real-space transfer, trapping and relaxation of photogenerated carriers in low-dimensional semiconductors are of interest from the viewpoint of fundamental physics as well as device applications. First experimental activities in this novel "eld of research have focused on combining ultrafast spectroscopy and near-"eld microscopy to studies of GaAs microdisks [3] and quantum wells [4], carrier di!usion in ion- implanted GaAs quantum wells [5] and exciton spin dynamics in magnetic heterostructures [6]. In this paper, we apply for the "rst time femtosecond near-"eld pump}probe spectroscopy to one-dimen- sional nanostructures. The nonlinear optical re- sponse from a single (3 1 1) GaAs quantum wire (QWR) structure is resolved in real space and time. We present evidence for ultrafast relaxation of car- riers into the quantum wire at room temperature and for carrier transport along the wire axis. 0921-4526/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 2 5 2 - 5