Using the tilted-pump-pulse-front scheme, we generate single-cycle terahertz ͑THz͒ pulses by optical rectification of femtosecond laser pulses in LiNbO 3 . In our THz generation setup, the condition that the image of the grating coincides with the tilted-optical-pulse front is fulfilled to obtain optimal THz beam characteristics and pump-to-THz conversion efficiency. By using an uncooled microbolometer-array THz camera, it is found that the THz beam leaving the output face of the LN crystal can be regarded as a collimated rather than point source. The designed focusing geometry enables tight focus of the collimated THz beam with a spot size close to the diffraction limit, and the maximum THz electric field of 1.2 MV/cm is obtained. © 2011 American Institute of Physics. ͓doi:10.1063/1.3560062͔Recent successful developments in efficient high-power terahertz ͑THz͒ pulse generation has created many promising applications such as in large-scale object imaging, medical diagnosis and treatment, and remote sensing techniques for security issues.1,2 In addition, intense THz pulses allow for study of unexplored nonlinear phenomena such as coherent THz manipulation of quantum states, 3,4 high-order harmonic generation, 5 nonlinear optical processes, and nonlinear transport phenomena in solids. [6][7][8][9][10][11][12] Since Hebling et al.13 ͑2002͒ proposed a tilted-pumppulse-front scheme for efficient phase-matched THz pulse generation using LiNbO 3 crystals, the technique has been rapidly developing. This technique has demonstrated the possibility of THz pulse generation with energies on the scale of 10 J by using an amplified Ti:sapphire laser with low repetition frequencies.14,15 To make the technique versatile for applications and useful for study of unexplored nonlinear phenomena, a generation setup to obtain optimal THz beam characteristics and a maximized THz peak field is required. A recent detailed analysis of the scheme predicted that the imaging errors in the setup consisting of a grating and lenses can lead to distortion in THz intensity profile after the LiNbO 3 output surface. 16 This distortion could create strong and ambiguous divergence in the THz beam, causing inaccuracy in optimal optis design for THz measurement, thereby limiting its applications.In this paper, we report the generation of single-cycle THz pulses using the tilted-pump-pulse-front scheme with a 1.3 mol % MgO-doped stoichiometric LiNbO 3 ͑LN͒ crystal. In the THz generation setup, the condition that the image of the grating coincides with the tilted-optical-pulse front is fulfilled to obtain optimal THz beam characteristics and pumpto-THz conversion efficiency. The propagation characteristics of the THz beam leaving the output face of the LN crystal were measured by an uncooled microbolometer-array THz camera. The results show that the THz beam had divergence of 52Ϯ 5 mrad in the horizontal direction for 1 THz.The designed focusing geometry for the collimated THz beam enables tight focus onto the electro-optic ͑EO͒ crystal with a spot size ...

The study of carrier multiplication has become an essential part of many-body physics and materials science as this multiplication directly affects nonlinear transport phenomena, and has a key role in designing efficient solar cells and electroluminescent emitters and highly sensitive photon detectors. Here we show that a 1-MVcm−1 electric field of a terahertz pulse, unlike a DC bias, can generate a substantial number of electron–hole pairs, forming excitons that emit near-infrared luminescence. The bright luminescence associated with carrier multiplication suggests that carriers coherently driven by a strong electric field can efficiently gain enough kinetic energy to induce a series of impact ionizations that can increase the number of carriers by about three orders of magnitude on the picosecond time scale.

We evaluated the radiative lifetimes and the one-dimensional exciton coherence lengths in single-walled carbon nanotubes ͑SWNTs͒. The radiative lifetimes determined from simultaneous measurements of photoluminescence ͑PL͒ lifetimes and PL quantum yields range from ϳ3 to 10 ns, and slightly increase with the tube diameter. The exciton coherence lengths in SWNTs are of the order of 10 nm, as deduced from the experimentally obtained radiative lifetimes, and they are about ten times larger than the exciton Bohr radius along the tube axis.