Kerr-like nonlinearity induced via terahertz generation and the electro-optical effect in zinc blende crystals

Enhanced spectral broadening in an optical parametric oscillator based on a PPLN crystal

We report a chirped-pulse optical parametric oscillator (OPO) generating light pulses with an instantaneous-bandwidth much wider than the parametric gain-bandwidth of nonlinear crystals. Our numerical simulations show that a relatively high residual second-order-dispersion within the OPO cavity is required in order to achieve the maximum signal-bandwidth from an OPO system. Based on this principle, we constructed an OPO using a 3-mm-long PPLN crystal, which produced a signal wave with an instantaneous-bandwidth of 20 THz (at -20 dB) covering 1447-1600 nm, roughly twice as much as the phase-matching bandwidth of the nonlinear crystal. This scheme represents a promising technical route for generating high-repetition-rate, ultrashort optical pulses with a wide bandwidth at various wavelengths, which may benefit many applications, including optical coherence tomography, pulse synthesis and spectroscopy.

Chirped-pulse optical parametric oscillators

We demonstrated that by introducing an extra optical material with a high nonlinear refractive index into a conventional optical parametric oscillator (OPO) cavity and managing the intra-cavity dispersion, a chirped-pulse OPO (CPOPO) can be constructed, generating optical pulses with an instantaneous bandwidth much wider than the pump bandwidth. Experimentally, from a CPOPO pumped by a Yb-fiber laser system centered at 1058 nm with a pulse duration of 230 fs and a -20  dB bandwidth of 8.7 THz, we obtained a signal wave with a -20  dB bandwidth of 28.4 THz and an idler wave covering 2900-3960 nm with a -20  dB bandwidth of 27.7 THz.

A cascading nonlinear magneto-optical effect in topological insulators

Topological insulators (TIs) are characterized by possessing metallic (gapless) surface states and a finite band-gap state in the bulk. As the thickness of a TI layer decreases down to a few nanometers, hybridization between the top and bottom surfaces takes place due to quantum tunneling, consequently at a critical thickness a crossover from a 3D-TI to a 2D insulator occurs. Although such a crossover is generally accessible by scanning tunneling microscopy, or by angle-resolved photoemission spectroscopy, such measurements require clean surfaces. Here, we demonstrate that a cascading nonlinear magneto-optical effect induced via strong spin-orbit coupling can examine such crossovers. The helicity dependence of the time-resolved Kerr rotation exhibits a robust change in periodicity at a critical thickness, from which it is possible to predict the formation of a Dirac cone in a film several quintuple layers thick. This method enables prediction of a Dirac cone using a fundamental nonlinear optical effect that can be applied to a wide range of TIs and related 2D materials.

Terahertz-field-induced second harmonic generation through Pockels effect in zinc telluride crystal

We report on the second harmonic generation (SHG) of a near-infrared pulse in a zinc telluride crystal through the Pockels effect induced by an intense terahertz pulse. The temporal and angular behaviors of the SHG have been measured and agree well with theoretical predictions. This phenomenon, so far overlooked, makes it possible to generate second harmonic through cascading of two second-order nonlinear phenomena in the near-infrared and terahertz ranges. We also show how this cascading process can be used to sample terahertz pulses.

Nonlinear cross-phase modulation with intense single-cycle terahertz pulses

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100 microJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.

Optical generation of narrow-band terahertz packets in periodically inverted electro-optic crystals: conversion efficiency and optimal laser pulse format

We explore optical-to-terahertz conversion efficiencies which can be achieved with femto- and picosecond optical pulses in electro-optic crystals with periodically inverted sign of second-order susceptibility. Optimal crystal lengths, pulse durations, pulse formats and focusing are regarded. We show that for sufficiently short (femtosecond) optical pulses, with a pulsewidth much shorter than the inverse terahertz frequency, conversion efficiency does not depend on pulse duration. We also show that by mixing two picosecond pulses (bandwidth-limited or chirped), one can achieve conversion efficiency, which is the same as in the case of femtosecond pulse with the same pulse energy. Additionally, when the group velocity dispersion of optical pulses is small, one can substantially exceed Manley -Rowe conversion limit due to cascaded processes.

Efficient generation of high-power quasi-single-cycle terahertz pulses from a single infrared beam in a second-order nonlinear medium

It is shown that the coherence lengths for terahertz (THz) generation based on difference-frequency generation within an ultrafast infrared pulse can be sufficiently long for a wide bandwidth about each phase-matching wavelength owing to a slight dispersion in the THz region. As a result, quasi-single-cycle THz pulses can be efficiently generated. An efficient conversion for the parametric process is made possible not only by use of the wide phase-matching bandwidth but also by optimization of the pulse width for each peak THz frequency. I have investigated the strong-pump regime and found the limits to the conversion efficiencies to generate high peak intensities efficiently for THz waves with which to explore nonlinear regimes of the THz interactions.

Cascaded nonlinear difference-frequency generation of enhanced terahertz wave production

Cascaded nonlinear optical interactions are analyzed for their potential to overcome quantum-defect related limitations on the efficiency of terahertz wave difference-frequency generation. The dispersion of ZnTe permits phase-matched production of a series of Stokes lines from two initial near-infrared beams. As the pump beams run down the Stokes ladder, the number of terahertz photons continually increases. A potential improvement by a factor of 5 is demonstrated in a 0.26-cm-long crystal by use of 25-MW/mm2 pumps at a wavelength of 824 nm.

The birth of a quasiparticle in silicon observed in time-frequency space

The concept of quasiparticles in solid-state physics is an extremely powerful tool for describing complex many-body phenomena in terms of single-particle excitations. Introducing a simple particle, such as an electron, hole or phonon, deforms a many-body system through its interactions with other particles. In this way, the added particle is 'dressed' or 'renormalized' by a self-energy cloud that describes the response of the many-body system, so forming a new entity--the quasiparticle. Using ultrafast laser techniques, it is possible to impulsively generate bare particles and observe their subsequent dressing by the many-body interactions (that is, quasiparticle formation) on the time and energy scales governed by the Heisenberg uncertainty principle. Here we describe the coherent response of silicon to excitation with a 10-femtosecond (10(-14) s) laser pulse. The optical pulse interacts with the sample by way of the complex second-order nonlinear susceptibility to generate a force on the lattice driving coherent phonon excitation. Transforming the transient reflectivity signal into frequency-time space reveals interference effects leading to the coherent phonon generation and subsequent dressing of the phonon by electron-hole pair excitations.