All-optical schemes for switching magnetization offer a pathway towards the creation of more advanced data-storage technologies, both in terms of recording speed and energy-efficiency. It has previously been shown that picosecond-long optical pulses with central frequencies ranging between 12 and 30 THz are capable of driving magnetic switching in yttrium-iron-garnet films, provided that the excitation frequency matches the characteristic frequency of longitudinal optical phonons. Here, we explore how the phononic mechanism of magnetic switching in three distinct ferrimagnetic iron-garnet films evolves at optical frequencies below 10 THz, within the so-called terahertz gap. We find that at long wavelengths the magnetic switching rather correlates with phonon modes associated with the substrate. Our results show that the process of phononic switching of magnetization, previously discovered in the mid- to far-infrared spectral range, becomes much more complex at frequencies within the terahertz gap.

The results of observation of Ramsey oscillations in germanium doped with arsenic donors detected by photothermal ionization of Coulomb centers are presented. To excite quantum coherent superpositions of the states at the transition 1 s (A 1 )–2 p 0 the Novosibirsk free electron laser radiation was used. The results are analyzed using a theoretical model using several key parameters of the experiment.

The electrically detected orbital spectrum of a mesoscopic silicon device containing a small number of donors has been investigated. The device was fabricated on silicon-on-insulator with an optically active channel containing 6×105 substitutional bismuth centers introduced by ion implantation. The 1s(A1)→2p± orbital transition at the energy associated with isolated bismuth donors was detected via a change in photocurrent when illuminated by THz light from a free electron laser. The spectral dependence on bias, temperature, and laser intensity is explored to determine optimum conditions for detecting orbital transitions in smaller devices with fewer donors. These results suggest that photo-induced impact ionization can offer a route for the spectroscopic detection of few impurities providing a useful tool for the development of solid-state quantum technologies.

We present a quantitative study of the efficiency of degenerate THz transient four-wave mixing in germanium doped by shallow impurities using the Dutch free electron laser FELIX. We derive the third order nonlinear sheet susceptibility and find values at low temperature that exceed those of any other material and wavelength range reported to date.

Ramsey oscillations have been observed in germanium doped with shallow impurities exposed to terahertz pulses from the NovoFEL facility involving free-electron lasers in experiments performed using the standard method with the action of a sequence of two optical pulses at the frequency close to the frequency of the 1 s ( A 1 ) → 2 p 0 impurity transition. The coherent state of the ensemble of donors has been detected by measuring the photocurrent caused by the thermal ejection of electrons from the 2 p 0 state to the conduction band. The revealed effect is quite stable under experimental conditions, in particular, to the temperature regime, which allows the further improvement in artificial systems based on shallow donors in germanium.

We observe third-harmonic generation (THG) in gallium-doped germanium (Ge:Ga) upon pumping with picosecond sub-THz (0.6-0.7 THz) pulses from the THz free-electron laser FLARE with a peak electric field strength of up to 4 kV/cm (30kW/cm2). The measurements are performed at cryogenic temperatures where the majority of holes are bound to the acceptor dopants. The dependence of the THG on the pump intensity exhibits a threshold-free power-law behavior with an exponent close to 3 as expected.

We have combined ultrafast time-resolved techniques, available at European infrared free-electron laser facilities (HFML-FELIX and ELBE), for the analysis of intracenter relaxation processes in an atomiclike energy spectrum of the double donor magnesium in silicon. We show that complementary techniques enable the accurate analysis of complex transients inherent to strongly absorbing multilevel media. From the combined analysis of degenerate pump-probe, photon echo, and transient grating spectroscopy, we find short decay times of (10 +/- 1) ps and (30 +/- 3) ps for the 2p0 and 2p +/- states, respectively. These high decay rates exceed those obtained earlier for the shallow, one-electron donors in silicon, where intracenter relaxation profits from interaction with a single phonon of a host lattice.

The results of experimental and theoretical investigations of terahertz-emission-spectrum tuning by means of uniaxial stress of a silicon crystal doped with shallow bismuth donors are presented for the case of intracenter optical excitation. The frequency tuning of two emission lines of the bismuth donor is shown for the case of uniaxial strain along the [001] crystallographic direction. The cross sections of stimulated Raman scattering for uniaxially stressed bismuth-doped silicon are calculated.

The results of investigations of relaxation processes of donor bound electrons in the germanium doped by arsenic at cryogenic temperatures are presented. The lifetimes of p-type excited states of a donor were measured by pump-probe technique utilizing THz radiation from free-electron laser NovoFEL. The uniaxial stress along [111] axis was applied to the crystal to modify the energy spectrum of the impurity and intervalley relaxation processes. The analysis of experimental for p-type excited states allows to get the upper limit for the relaxation time of lowest 1s excited state. The comparison of obtained experimental results with the theoretical estimates for the interaction with TA phonons indicates on the enhancement of the interaction with the phonons having wavenumbers corresponding to the flat part of the dispersion.

The relaxation of electrons bound to bismuth donors in silicon and the effect of uniaxial stress have been studied using the time-resolved single color pump-probe technique. In unstressed Si:Bi, an excited 2p0 donor state is resonantly coupled with the donor 1s(A1) ground state via an intervalley f-TO optical phonon. This results in a very short lifetime (a few ps) of the excited state. Even a slight deformation of the silicon crystal leads to modification of particular interstate energies of the donor, which resolves the resonant coupling to intervalley phonons whose energies remain unchanged. We have shown that once the energy gap between the lower stress-split component of the 2p0 state and the ground state becomes less than the energy of the f-TO phonon, the relaxation of electrons from the excited state slows down to about 300 ps. The experimental data are compared with theoretical calculations of the 2p0 state relaxation rate, which are performed assuming that it is dominated by the emission of intervalley phonons.

Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices. While nonlinearities play a central role for cutting-edge functionality, they require strong field amplitudes owing to the weak light-matter coupling of electronic resonances of naturally occurring materials. Here, we ultrastrongly couple intersubband transitions of semiconductor quantum wells to the photonic mode of a metallic cavity in order to custom-tailor the population and polarization dynamics of intersubband cavity polaritons in the saturation regime. Two-dimensional THz spectroscopy reveals strong subcycle nonlinearities including six-wave mixing and a collapse of light-matter coupling within 900 fs. This collapse bleaches the absorption, at a peak intensity one order of magnitude lower than previous all-integrated approaches and well achievable by state-of-the-art QCLs, as demonstrated by a saturation of the structure under cw-excitation. We complement our data by a quantitative theory. Our results highlight a path towards passively mode-locked QCLs based on polaritonic saturable absorbers in a monolithic single-chip design.

Transient-type stimulated emission in the terahertz (THz) frequency range has been achieved from phosphorus doped silicon crystals under optical excitation by a few-picosecond-long pulses generated by the infrared free electron lasers FELIX and CLIO. The analysis of the lasing threshold and emission spectra indicates that the stimulated emission occurs due to combined population inversion based lasing and stimulated Raman scattering. Giant gain has been obtained in the optically pumped silicon due to large THz cross sections of intracenter impurity transitions and resonant intracenter electronic scattering. The transient-type emission is formed under conditions when the pump pulse intervals exceed significantly the photon lifetime in the laser resonator.

Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW) structures are promising candidates for the development of saturable absorbers at terahertz (THz) frequencies. Here, we exploit amplitude and phase-resolved two-dimensional (2D) THz spectroscopy on the sub-cycle time scale to observe directly the saturation dynamics and coherent control of ISB transitions in a metal-insulator MQW structure. Clear signatures of incoherent pump-probe and coherent four-wave mixing signals are recorded as a function of the peak electric field of the single-cycle THz pulses. All nonlinear signals reach a pronounced maximum for a THz electric field amplitude of 11 kV/cm and decrease for higher fields. We demonstrate that this behavior is a fingerprint of THz-driven carrier-wave Rabi flopping. A numerical solution of the Maxwell-Bloch equations reproduces our experimental findings quantitatively and traces the trajectory of the Bloch vector. This microscopic model allows us to design tailored MQW structures with optimized dynamical properties for saturable absorbers that could be used in future compact semiconductor-based single-cycle THz sources.

We report on an optically pumped laser where photons are simultaneously generated by population inversion and by stimulated Raman scattering in the same active medium, namely crystalline silicon doped by bismuth (Si:Bi). The medium utilizes three electronic levels: ground state [vertical bar 1 >: 1s(A(1)) in Si:Bi], upper [vertical bar 3 >: 2p(+/-)] and lower [vertical bar 2 >: 1s(E)] laser levels. The vertical bar 1 > <-> vertical bar 3 > and vertical bar 2 > <-> vertical bar 3 > transitions are optically allowed and the vertical bar 1 > <-> vertical bar 2 > transition is Raman active. Lasing based on population inversion occurs between the states vertical bar 3 > and vertical bar 2 >, while Raman scattering benefits from the Raman-active transition. At high pump power the inversion-based stimulated emission vertical bar 3 > <-> vertical bar 2 > disappears, because electronic scattering from vertical bar 1 > to vertical bar 2 > via a virtual state dominates and the electrons are excited into vertical bar 2 > rather than into vertical bar 3 >. Starting as population inversion-based lasing, it ends as stimulated Raman scattering. Our model shows that such a competition occurs on the timescale of the 10-ps-long pump pulse.

We present the results of the first systematic "round-robin" comparison of far-infrared transmittance spectra measurements, which was performed by five laboratories and piloted by Physikalisch-Technische (PTB). The transmittance spectra of four different samples were measured by the participating laboratories in the 600 cm-1 to 10 cm-1 range (16.67 µm to 1000 µm) in a blind comparison. Different types of instruments, Fourier transform infrared (FT-IR) spectrometers of Michelson type and a laser radiation-based system were used for the transmittance measurements. FT-IR spectrometers are the most popular and commonly used instruments for the spectral characterization of materials in the infrared spectral range, and are well established for quantitative measurements in the mid- and near-infrared spectral ranges. However, obtaining quantitative transmittance measurements in the far-infrared spectral range by means of these instruments is challenging, because it involves weaker radiation sources, stronger diffraction effects, significant radiation originating from the sample itself and temperature gradients inside the spectrometer that may not be given proper consideration. Therefore, this comparison was initiated to test the actual capability of and identify problems with FT-IR transmittance measurements in this spectral region. We discuss the results and the possible reasons for the observed discrepancies.

We analyze the cascade capture of charge carriers due to the interaction with acoustic phonons in highly doped semiconductors using a model that describes the recombination of photo-ionized carriers as a continuous relaxation of carriers in the energy space at both positive and negative energies in the field of a set of impurity ions. Such description enables simultaneous calculation of non-equilibrium carrier distribution formed by interaction with acoustic phonons in the presence of impurity traps, and the time of recombination in a wide range of concentrations of capture centers and phonon temperatures. Additionally, we calculated the time of cascade recombination in the presence of fast scattering processes forming a Maxwellian distribution of free carriers. We show that experimentally observed concentration and temperature dependence of carrier life times in highly doped semiconductors can be described within the model of the cascade capture to uniformly spaced capture centers, and the main factor that determines the regime of cascade capture is the ratio of the thermal energy and the energy of the overlap of impurity potentials.

The Raman Laser Spectrometer (RLS) instrument on board of the future ESAs ExoMars mission will analyze micron-sized powder samples in a low pressure atmosphere. Such micron-sized polycrystalline solid particles might be heated by the laser during the Raman measurements. Here, we report on the temperature-induced alteration of Raman spectra from micron-sized polycrystalline solid particles by comparing Raman spectra on silicon and the rock forming minerals olivine and pyroxene taken at different laser intensities and different ambient temperatures. Our analyses indicate that laser-induced heating results in both broadening and shifting of characteristic Raman lines in the Stokes and anti-Stokes spectral regions. For elementary crystalline silicon a significant local temperature increase and relevant changes in Raman spectra have been observed in particles with median sizes below 250µm. In comparison, significantly weaker laser-induced Raman spectral changes were observed in more complex rock-forming silicate minerals; even for lower grain sizes. Laser power densities realized in the RLS ExoMars instrument should cause only low local heating effects and, thus, negligible frequency shifts of the major Raman lines in common silicate minerals such as olivine and pyroxene.

The relaxation times of localized states of antimony donors in unstrained and strained germanium uniaxially compressed along the [111] crystallographic direction are measured at cryogenic temperatures. The measurements are carried out in a single-wavelength pump–probe setup using radiation from the Novosibirsk free electron laser (NovoFEL). The relaxation times in unstrained crystals depend on the temperature and excitation photon energy. Measurements in strained crystals are carried out under stress bar S > 300, in which case the ground-state wavefunction is formed by states belonging to a single valley in the germanium conduction band. It is shown that the application of uniaxial strain leads to an increase in the relaxation time, which is explained by a decrease in the number of relaxation channels.

The low‐temperature capture processes of non‐equilibrium holes into gallium acceptors in moderately doped p‐germanium (NA ≈ 2 × 1015 cm−3) has been investigated by a single‐color pump–probe experiment using the free electron laser FELBE. The capture time decreases with increasing average photon flux density of the excitation pulse from about 10.9 ns (at ∼1.2 × 1024 cm−2 s−1) to ∼1.2 ns (∼2 × 1026 cm−2 s−1). Relaxation inside the valence band is almost independent on pump light intensity and its characteristic time is about 200 ps. In Addition, the intracenter relaxation times of the lowest excited Ga states were measured. The lifetimes scale with the phonon density of states controlling the bound hole − acoustic phonon interaction. The lifetime of the lowest excited state, , was measured to be ∼275 ps; while the lifetimes of the higher excited states, and , were found to be ∼160 ps.