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The potential of KD 2 PO 4 (or DKDP) crystal as a terahertz generator is demonstrated, despite its extremely strong terahertz absorption. By combining the Cherenkov radiation scheme and surface-emitting geometry, femtosecond Ti:sapphire laser pulses of ∼ 200 − 300 μ J energy were converted to broadband (∼ 4 THz bandwidth) terahertz transients with the efficiency of ∼ 8 × 10 − 6 . The transients produced electro-optic signals of a ∼ 0.15 modulation depth and ∼ 10 3 dynamic range in a 1-mm thick ZnTe detector crystal. This suggests DKDP as a potential generator crystal for terahertz time-domain spectroscopy. From our measurements, we estimated the nonlinear coefficient d 36 ≈ 3 pm/V of DKDP for optical rectification, which is an order of magnitude larger than the value known from second-harmonic-generation experiments. At the same time, the obtained value of d 36 is an order of magnitude smaller than the value calculated from the clamped electro-optic coefficient r 63 for the MHz frequency range. At high pump intensities, we observed multiple filamentation of the pump beam and white light generation, spatially separated from terahertz generation. [ABSTRACT FROM AUTHOR]

We introduce and investigate a special regime of terahertz generation in electro-optic crystals with ultrashort laser pulses, in which neither phase-matching nor Cherenkov radiation mechanism are efficient. This regime occurs under the conditions that the optical group refractive index is larger than the low-frequency phase refractive index of the crystal, and the spectral bandwidth of the laser pulse is smaller than the frequency of the phase-matched terahertz wave. In this regime, that can be realized for a Ti:sapphire laser in GaP, terahertz generation is provided by transient processes at the crystal boundaries. We study the peculiarities of the terahertz emission from a slab of GaP excited with a Ti:sapphire laser. In particular, we demonstrate a possibility to implement a pointlike source of terahertz radiation that can increase the resolution of terahertz apertureless near-field microscopy. [ABSTRACT FROM AUTHOR]

A scheme for efficient generation of broadband terahertz radiation by a femtosecond laser pulse propagating in a planar sandwichlike structure is proposed. The structure consists of a thin nonlinear core cladded with prisms made of a material with low terahertz absorption. The focused into a line laser pulse propagates in the core as a leaky or waveguide mode and emits Cherenkov wedge of terahertz waves in the cladding. We developed a theory that describes terahertz generation in such a structure and calculated spatial distribution of the generated terahertz field, its energy spectrum and optical-to-terahertz conversion efficiency. The developed theory predicts the conversion efficiency of up to several percent in a 1 cm long and 1 cm wide Si–LiNbO3–Si sandwich structure with a 20 μm thick nonlinear layer pumped by 8.5 μJ Ti:sapphire laser with pulse duration of 100 fs. [ABSTRACT FROM AUTHOR]

A theory that describes the terahertz generation via optical rectification of a femtosecond laser pulse with tilted front in an electro-optic crystal is developed. The theory accounts for the transverse size of the laser pulse and allows us to explore the dependence of the terahertz yield on laser focusing. Two typical experimental situations—LiNbO3 excited with Ti:sapphire laser at room and cryogenic temperatures—are considered. It is shown that depending on temperature the dominant generation mechanism can be either phase-matching or Cherenkov effect. The parameters of the laser pulse (transverse size, tilt angle, and pulse duration) and crystal size maximizing the terahertz yield for LiNbO3 are calculated. [ABSTRACT FROM AUTHOR]

We show that a weakly focused ultrashort laser pulse with tilted intensity front can efficiently excite a terahertz surface wave via phase-matched optical rectification at the surface of a semiconductor. The surface wave is excited by a strip of nonlinear polarization moving along the semiconductor surface with subluminous velocity. The amplitude and power of the radiated surface wave are calculated. For a 90-fs optical pulse with a peak intensity of 2 GW/cm2, we estimate the energy conversion coefficient to be 10-7 at the surface of GaAs. The direct excitation of surface waves at terahertz frequencies can be particularly attractive for terahertz spectroscopy of semiconductor surfaces. [ABSTRACT FROM AUTHOR]

The propagation of light in metal-coated fiber probes used in scanning near-field optical microscopy is studied computationally by means of the cross-section method. In contrast to previous work focusing on the behavior of the HE11 mode in the vicinity of the end aperture of the probe, we trace the propagation of the incident fundamental mode of the optical fiber through the tapered probe. The transformation of the fiber mode into the probe modes, intermode conversion, and modes cutoff are consistently calculated. In particular, it is shown that in the conventional gradual tapered probe only a few percent of the input power are transformed into the HE11 mode which has the smallest cutoff radius. In addition, an easily manufacturable modification of the shape of the probe’s tip that can provide a tenfold enhancement in optical near-field intensity has been proposed. [ABSTRACT FROM AUTHOR]

We analyze the excitation of a surface plasmon-polariton at terahertz frequencies by an optical pulse with tilted intensity front incident on a semiconductor surface. We show that one can achieve an order of magnitude increase in the generated terahertz power by using an optical pulse with frequencies below the band gap compared to the above-band-gap excitation. [ABSTRACT FROM AUTHOR]