We report on the application of scanning near-field optical microscopy for terahertz imaging. We demonstrate a spatial resolution of 150 nm for 2.0 THz pulses. Our experiments show the feasibility of submicron THz microscopy for imaging of biologic tissues on the cell level or for the investigation of individual submicron semiconductor devices.

Abstract. The impulsive excitation and phase-sensitive detection of coherent phonons and phonon-polaritons provide a detailed insight i n to the dynamical properties of matter. The experiments are based on optical pump-probe techniques with femtosecond time resolution. These techniques enable the detection of amplitude and phase of the coherent lattice motion simultaneously. F requencies in the THz range and dephasing times in the picosecond range can be obtained with high accuracy. Especially in semiconductors and semiconductor heterostructures, where a coherent phonon mode and free carriers are excited simultaneously, important information about carrier-phonon interaction far away from equilibrium is obtained. This paper presents an overview of recent a c hievements in this vivid eld of condensed matter physics.

Terahertz near-field microscopy may serve as a novel tool to measure the high-frequency permittivity of dielectric surfaces on submicrometre semiconductor structures. We present an apertureless THz near-field microscope, which allows for spatial resolutions as small as 150 nm. A new model has been developed that considers the field coupling the scanning probe with a sample and reproduces the image data qualitatively and quantitatively.

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We report on the time-resolved observation of coherent optical phonons in GaAs by the Franz-Keldysh effect. This effect leads to a modulation of the optical interband transitions with a nonlinear dependence on the macroscopic electric field associated with the coherent phonons. We find that the nonlinear electro-optic effect forms a far more efficient detection mechanism for coherent phonons than the linear electro-optic effect near the band gap.In recent years, the study of coherent phenomena with ultrashort laser pulses has become a matter of growing interest. The use of subpicosecond laser pulses allows a direct observation of coherent vibrations in molecules, 1 phonons in solid-state materials, 2-4 and electronic wave-packet oscillations in quantum-well structures in the time domain. 5 In previous studies the excitation mechanisms and the dephasing of coherent oscillations have been addressed.Recent investigations of coherent phonons in solid-state materials have shown that the excitation results from the impulsive interaction of the material with laser pulses whose durations are shorter than the period of the oscillation. In molecules, coherent vibrations can be excited by impulsivestimulated Raman scattering processes. 1 In narrow-band-gap semiconductors, the strong interband excitation of carriers leads to the excitation of coherent phonons by the impulsive elongation of the bonds. 3 It was found that in these materials the breathing mode of the coherent lattice vibrations modifies the band gap via the deformation potential. 6 In III-V semiconductors, the femtosecond excitation of free carriers results in the impulsive screening of the surface depletion field due to an ultrafast charge separation. This mechanism generates coherent longitudinal optical ͑LO͒ phonons, which are associated with an oscillating macroscopic electric field. 2,7 These field changes have been observed in timeresolved reflectivity measurements via the linear electrooptic effect. 2 In principle, the reflectivity changes induced by coherent phonons in III-V semiconductors arise from the modulation of the optical susceptibility at the phonon frequency, which is expressed by the coupling of the Raman tensor with the coherent motion of the atomic displacement. In a first-order approximation the phonon-induced reflectivity changes ⌬R can be described bywhere is the real part of the optical susceptibility, Q the atomic displacement, and F the macroscopic electric field. The first and second terms at the right-hand side in Eq. ͑1͒ are the deformation potential and the electro-optic contribution, respectively. This first-order Raman tensor determines the usual allowed Raman scattering with its specific polarization selection rules. 8,9 However, in the vicinity of critical points in the electronic band structure, nonlinear effects such as forbidden Raman scattering and electric-field-induced Raman scattering by LO phonons become strongly enhanced. 8,9 The polarization selection rule for these types of Raman tensors is different from that of a...