No abstract

No abstract

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.

We perform a comprehensive comparison between terahertz ͑THz͒ time-domain spectroscopy and conventional far-infrared Fourier transform spectroscopy, including radiation source, detector, signal to noise ratio, bandwidth, availability, applications, and their own uniqueness. In terms of signal to noise ratio, THz time-domain spectroscopy is advantageous at low frequencies under 3 THz, while Fourier transform spectroscopy works better at frequencies above 5 THz. In addition, we provide a detailed discussion of the unique features of THz time-domain spectroscopy and its application to dynamic and time-resolved processes.

Exciton relaxation in poly(phenylenevinylene), PPV, has been probed by femtosecond luminescence-up-conversion. We report on excitonic luminescence profiles that depend on the spectral position of the detection window (ε̂=2.7,2.6,2.5, and 2.4 eV, respectively). In an attempt to reveal the transient steps implied in fs relaxation, we present a quantitative forward reconvolution fit procedure that is based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations. Special emphasis has been placed (i) on the analysis of luminescence lifetime distributions 〈φ(τ;ε̂,τ0)〉 which directly map out the spectra of hopping modes of energy-cascading neutral excitations, and (ii) on the rigorous evaluation of (radiationless) transfer population from high-energy subensembles to low-energy tail states of the DOS. We quantitatively show that the absence of significant rise terms in the S0ν=0←S1ν=0 luminescence transition of low-lying tail states is due to the spectral superposition of the S0ν=1←S1ν=0 luminescence decay from states near the center of the DOS which, owing to the predominantly positive amplitudes of their ultrafast decay channels, destructively interfere with the negative amplitudes (luminescence rise terms) of migratively prepared bottom states. Finally, the asymptotic branch of the luminescence lifetime distribution 〈φ(τ;ε̂=2.4 eV,τ0)〉 has been reconstructed in an unbiased fashion by inverting the high-precision, picosecond, single-photon counting luminescence data at ε̂=2.4 eV with the help of a regularized exponential series methodology. The measurements combined by numerical computing are in accord with the molecular picture of exciton relaxation in PPV in which neutral, localized excitations undergo predominantly incoherent excitation energy transfer among sites of fluctuating self-energies related to segmental subunits with distributed π-bond conjugation lengths.

No abstract

No abstract

No abstract