In THz-gated Scanning Tunneling Microscopy (THz-STM), the electric field of a single-cycle THz pulse acts as a transient bias modulating the STM-junction, enabling control of the tunneling current on femtosecond time scales. Optimal operation of a THz-STM requires exact knowledge and precise control of the THz near field waveform. In this regard, we demonstrate THz near field sampling via THz-induced modulation of ultrafast photocurrents in a metal-metal junction, and characterize in detail the coupling of broadband (1-30 THz) single-cycle THz pulses generated from a spintronic emitter to the STM tip. Specifically, we show that employing NIR laser pulses with a curved wavefront for THz generation allows for precise control of the phase, amplitude and bandwidth of the THz near field. Depending on the excitation conditions, THz near fields with frequencies up to 10 THz and peak voltages of several volts can be achieved at 1 MHz repetition rate.
In laser-excited ferromagnetic heterostructures, both ultrafast local magnetization dynamics and spin-transport processes can lead to a THz emission. Here, we demonstrate that the THz emission spectroscopy is a powerful tool to investigate ultrafast magnetization dynamics in laser-excited magnetic systems. The polarity of emitted THz can be used to distinguish which process, local or non-local, dominates the emission of THz in a ferromagnetic heterostructure. The measured THz radiation can be used for rigorous reconstruction of ultrafast magnetization process in the laser-excited magnetic material.
Coherently excited phonons are unique tools to control material properties, drive phase transitions or even access hidden phases on ultrafast time scales. The increasing availability of high-field THz and mid-infrared sources facilitates targeting of specific resonantly driven phonon modes, while leaving the material in its electronic ground state. Nevertheless, this direct excitation is restricted to IR-active modes, whereas purely Ramanactive modes are symmetry protected against even intense resonant THz fields. Thus, for Raman-active modes, nonlinear excitation mechanisms must be employed, which are either mediated through modulation of the electric polarizability or through anharmonicities of the crystal lattice. Respectively, these difference-frequency processes are conventional impulsive stimulated Raman scattering (ISRS) and, more recently, ionic Raman scattering (IRS), which both lack the precise selectivity of resonant excitation. Here, we present the THz sum-frequency counterparts of these two mechanisms, which are more selective, nonimpulsive, and provide direct control over the phonon phase. We demonstrate THz sum-frequency excitation of the archetypal Raman-active phonon in diamond. This two-photon absorption process, the upconversion counter part of ISRS, directly imprints the carrier-envelope phase of the light field onto the coherent phonon’s phase. Additionally, our theoretical formalism based on first-principles calculations in combination with phenomenological modeling predicts an efficient sum-frequency counterpart for IRS, which was subsequently confirmed by other experimental research groups. In summary, we complete the map of photonic and ionic Raman excitation mechanisms with their sumfrequency counterparts, providing a comprehensive guide for selective excitation of coherent phonons and other Raman-active modes by strong THz fields.
Terahertz (THz) electromagnetic radiation is located between the realms of electronics and optics and has successfully been used to probe and even control numerous low-energy excitations including phonons, excitons and Cooper pairs. Here, we show that THz spectroscopy is also a highly useful tool in the field of ultrafast spinbased electronics (spintronics) and consider the resonant manipulation of the magnetization of an antiferromagnet (through the THz Zeeman torque) and the probing of the tailored transport of spin density from a ferromagnetic into a nonmagnetic metal (through the THz inverse spin Hall effect).
Ultrashort pulses in the terahertz (THz) spectral range allow us to study and control spin dynamics on time scales faster than a single oscillation cycle of light. In a first set of experiments, we harness an optically triggered coherent lattice vibration to induce a transient spin-density wave in BaFe2As2, the parent compound of pnictide superconductors. The time-dependent multi-THz response of the non-equilibrium phases shows that the ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. The results suggest important implications for unconventional superconductivity. In a second step, we utilize the magnetic field component of intense THz transients to directly switch on and off coherent spin waves in the antiferromagnetic nickel oxide NiO. A femtosecond optical probe traces the magnetic dynamics in the time domain and verifies that the THz field addresses spins selectively via Zeeman interaction. This concept provides a universal ultrafast handle on magnetic excitations in the electronic ground state.
We present a table-top source of extremely intense multi-THz transients covering the spectral region between 0.1 and
140 THz. Electric field amplitudes of up to 108 MV/cm and pulse durations as short as a single cycle are demonstrated
with our hybrid Er:fiber-Ti:sapphire laser system. All THz waveforms are electro-optically detected. This source opens
the door to a regime of non-perturbative THz nonlinearities in condensed matter. First examples range from coherent
control of excitons, via a breakdown of the power expansion of the nonlinear polarization in bulk semiconductors to twodimensional
multi-wave mixing and direct femtosecond spin control by magnetic field excitation.
By performing an ultrafast pump-probe experiment, we demonstrate the adiabatic frequency conversion of a
telecommunications pulse in a silicon photonic crystal waveguide. By using slow-light modes to spatially compress the
pulse, a 1.3 ps long pulse is blue-shifted by 0.3 THz with 80% efficiency in a waveguide just 19 μm long. We also
present the results of an adiabatic model of the process, which agrees excellently with the experimentally measured data.
Phase-locked electromagnetic transients in the terahertz (THz) spectral domain have become a unique contact-free probe
of the femtosecond dynamics of low-energy excitations in semiconductors. Access to their nonlinear response, however,
has been limited by a shortage of sufficiently intense THz emitters. Here we introduce a novel high-field source for THz
transients featuring peak amplitudes of up to 108 MV/cm. This facility allows us to explore the non-perturbative
response of semiconductors to intense fields tailored with sub-cycle precision. In a first experiment intense transients
drive Rabi-oscillations between excitonic states in Cu2O, implying exciting perspectives for future THz quantum optics.
At electric fields beyond 10 MV/cm, we observe the breakdown of the power expansion of the nonlinear polarization in
bulk semiconductors. Furthermore, we employ the intense magnetic field components of our transients to coherently
control spin waves in antiferromagnetically ordered solids. Finally, intersubband cavity polaritons in semiconductor
microcavities are exploited to push light-matter coupling to an unprecedented ultrastrong and sub-cycle regime.
We discuss the performance of slow-light enhanced optical switches and modulators fabricated in silicon. The switch is
based on photonic crystal waveguides in a directional coupler geometry, and the dispersion of the device is engineered to
allow a switching length as short as 5 μm and rerouting of optical signals within 3 ps. The 3 ps switching time is
demonstrated using free carriers in the silicon generated by the absorption of a femtosecond pump pulse. The modulator
is based on a Mach-Zehnder interferometer configuration, with photonic crystal waveguides in each arm to act as phase-shifters.
A flat-band slow-light region has been engineered in the phase-shifters to provide an extinction ratio in excess
of 15 dB over the entire 11 nm bandwidth of the modulator device.
The optical properties of single-wall carbon nanotube sheets in the far-infrared (FIR) spectral range from few THz to several tens of THz have been investigated with terahertz spectroscopy both with static measurements elucidating the absorption mechanism in the FIR and with time-resolved experiments yielding information on
the charge carrier dynamics after optical excitation of the nanotubes. We observe an overall depletion of the
dominating broad absorption peak at around 4THz when the nanotubes are excited by a short visible laser pulse.
This finding excludes particle-plasmon resonances as absorption mechanism and instead shows that interband transitions in tubes with an energy gap of ~10meV govern the far-infrared conductivity. A simple model based on an ensemble of two-level systems naturally explains the weak temperature dependence of the far-infrared conductivity by the tube-to-tube variation of the chemical potential. Furthermore, the time-resolved measurements do not show any evidence of a distinct free-carrier response which is attributed to the photogeneration of strongly bound excitons in the tubes with large energy gaps. The rapid decay of a featureless background with pronounced dichroism is associated with the increased absorption of spatially localized charge carriers before thermalization is completed.