Electromagnetic radiation in the terahertz (THz) frequency range is a fascinating spectroscopic tool that provides resonant access to fundamental modes, including the motions of free electrons, the rotations of molecules, the vibrations of crystal lattices and the precessions of spins. Consequently, THz waves have been extensively used to probe such responses with high sensitivity. However, owing to recent developments in high-power sources, scientists have started to abandon the role of pure observers and are now exploiting intense THz radiation to engineer transient states of matter. This Review provides an overview and illustrative examples of how the electric and magnetic fields of intense THz transients can be used to control matter and light resonantly and nonresonantly

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 electronic properties of graphene can give rise to a range of nonlinear optical responses. One of the most desirable nonlinear optical processes is high-harmonic generation (HHG) originating from coherent electron motion induced by an intense light field. Here, we report on the observation of up to ninth-order harmonics in graphene excited by mid-infrared laser pulses at room temperature. The HHG in graphene is enhanced by an elliptically polarized laser excitation, and the resultant harmonic radiation has a particular polarization. The observed ellipticity dependence is reproduced by a fully quantum mechanical treatment of HHG in solids. The zero-gap nature causes the unique properties of HHG in graphene, and our findings open up the possibility of investigating strong-field and ultrafast dynamics and nonlinear behavior of massless Dirac fermions.

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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 investigated ultrafast carrier dynamics in graphene with near-infrared transient absorption measurement after intense half-cycle terahertz pulse excitation. The terahertz electric field efficiently drives the carriers, inducing large transparency in the near-infrared region. Theoretical calculations using the Boltzmann transport equation quantitatively reproduce the experimental findings. This good agreement suggests that the intense terahertz field should promote a remarkable impact ionization process and increase the carrier density.

Single crystals of the {Fe (II)(pyrazine)[Pt(CN) 4]} spin crossover complex were synthesized by a slow diffusion method. The crystals exhibit a thermal spin transition around room temperature (298 K), which is accompanied by a 14 K wide hysteresis loop. X-ray single-crystal analysis confirms that this compound crystallizes in the tetragonal P4/ mmm space group in both spin states. Within the thermal hysteresis region a complete bidirectional photoconversion was induced between the two phases (high spin right arrow over left arrow low spin) when a short single laser pulse (4 ns, 532 nm) was shined on the sample.