Ultrafast manipulation of magnetic anisotropy in a uniaxial intermetallic heterostructure TbCo2/FeCo

We study experimentally and theoretically the dynamics of spin relaxation motion excited by a femtosecond pulse in the TbCo2/FeCo multilayer structures with different ratios of TbCo2 to FeCo thicknesses rd = dTbCo2 / dFeCo. The main attribute of the structure is in-plane magnetic anisotropy artificially induced during sputtering under DC magnetic field. The optical pump-probe method revealed strongly damped high-frequency oscillations of the dynamical Kerr rotation angle, followed by its slow relaxation to the initial state. Modeling experimental results using the Landau-Lifshitz-Gilbert (LLG) equation showed that the observed entire dynamics is due to destruction and restoration of magnetic anisotropy rather than to demagnetization. For the pumping fluence of 7 mJ/cm2, the maximal photo-induced disruption of the anisotropy field is about 14% for the sample with rd = 1 and decreases when rd increases. The anisotropy relaxation is a three-stage process: the ultrafast one occurs within several picoseconds, and the slow one occurs on a nanosecond time scale. The Gilbert damping in the multilayers is found one order of magnitude higher than that in the constituent monolayers.

Towards Understanding Excited-State Properties of Organic Molecules Using Time-Resolved Soft X-ray Absorption Spectroscopy

The extension of the pump-probe approach known from UV/VIS spectroscopy to very short wavelengths together with advanced simulation techniques allows a detailed analysis of excited-state dynamics in organic molecules or biomolecular structures on a nanosecond to femtosecond time level. Optical pump soft X-ray probe spectroscopy is a relatively new approach to detect and characterize optically dark states in organic molecules, exciton dynamics or transient ligand-to-metal charge transfer states. In this paper, we describe two experimental setups for transient soft X-ray absorption spectroscopy based on an LPP emitting picosecond and sub-nanosecond soft X-ray pulses in the photon energy range between 50 and 1500 eV. We apply these setups for near-edge X-ray absorption fine structure (NEXAFS) investigations of thin films of a metal-free porphyrin, an aggregate forming carbocyanine and a nickel oxide molecule. NEXAFS investigations have been carried out at the carbon, nitrogen and oxygen K-edge as well as on the Ni L-edge. From time-resolved NEXAFS carbon, K-edge measurements of the metal-free porphyrin first insights into a long-lived trap state are gained. Our findings are discussed and compared with density functional theory calculations.

Superposition of 2ω and Electrostatic Field Induced Terahertz Waveforms in DC-Biased Two-Color Filament

Increase in conversion efficiency from a femtosecond optical pump into broadband terahertz (THz) radiation is currently an essential issue since it boosts THz source performance for medicine and security applications. An air-plasma based THz radiation from a two-color femtosecond filament is the most efficient gas-based THz emitter, with a dipole local source having a maximum on the beam propagation axis. In this work, we show the novel advancement to THz yield increase with preservation of the forwardly directed dipole radiation. The two-color THz source can be enhanced if the filament plasma channel is placed into an external electrostatic field (DC bias), which is parallel to the second harmonic polarization direction. In the experiment, we produce a plasma channel from 800-nm, ∼50-fs, 2-mJ pulse with 200 μJ of 400-nm, ∼50-fs mixed with the pump, and allocate it between the electrodes carrying 7-kV/cm static field. Time-domain measurements and 3D+time simulations of THz waveforms from the two-color DC-biased filament show that the THz emission is the superposition of the THz waveforms generated in the 800+400-nm filament without a DC-bias and in the 800-nm (without 400-nm) plasma channel biased by 7-kV/cm static field. The additivity of the two local dipole THz sources is possible if the majority of free electrons are produced by the pump pulse.

Optical Pump–Terahertz Probe Study of HR GaAs:Cr and SI GaAs:EL2 Structures with Long Charge Carrier Lifetimes

The time dynamics of nonequilibrium charge carrier relaxation processes in SI GaAs:EL2 (semi-insulating gallium arsenide compensated with EL2 centers) and HR GaAs:Cr (high-resistive gallium arsenide compensated with chromium) were studied by the optical pump–terahertz probe technique. Charge carrier lifetimes and contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of surface and volume Shockley–Read–Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that, in most cases for HR GaAs:Cr and SI GaAs:EL2, Auger recombination mechanisms make the largest contribution to the recombination rate of nonequilibrium charge carriers at injection levels above ~(0.5–3)·1018 cm−3, typical of pump–probe experiments. At a lower photogenerated charge carrier concentration, the SRH recombination prevails. The derived charge carrier lifetimes, due to the SRH recombination, are approximately 1.5 and 25 ns in HR GaAs:Cr and SI GaAs:EL2, respectively. These values are closer to but still lower than the values determined by photoluminescence decay or charge collection efficiency measurements at low injection levels. The obtained results indicate the importance of a proper experimental data analysis when applying terahertz time-resolved spectroscopy to the determination of charge carrier lifetimes in semiconductor crystals intended for the fabrication of devices working at lower injection levels than those at measurements by the optical pump–terahertz probe technique. It was found that the charge carrier lifetime in HR GaAs:Cr is lower than that in SI GaAs:EL2 at injection levels > 1016 cm−3.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Single Shot Time-Resolved Terahertz Spectroscopy for Optoelectronic Materials

<p>Optoelectronic materials and devices, such as LEDs and solar cells, are ubiquitous in the modern, technologically driven world. Understanding the fundamental physical process in optoelectronic materials is essential for the design and development of new devices which are more efficient, cheaper, printable, as well as environmentally friendly. Two particularly important material properties for device performance are charge mobility and photoconductivity, as they increase charge separation and extraction efficiencies, and thus give specific insight into device efficiency. The best suited technique for measuring mobility and conductivity on ultrafast timescales is Terahertz spectroscopy. Terahertz spectroscopy is a non-invasive, contact-free probe of the mobility of charges in optoelectronic materials. Terahertz time-domain spectroscopy allows for the direct determination of the entire complex-valued conductivity. As a result, important optical properties such as the complex refractive index and dielectric function of a material can be measured directly. The short duration of THz pulses, on the order of 1 ps, also allows for time-resolved studies of the transient photoconductivity in optically-excited materials with sub-picosecond time resolution, i.e. Time-Resolved Teraherz Spectroscopy (TRTS). Traditionally, only the peak of the THz pulse signal is measured with TRTS, due to the time constraints of a two-delay experiment. This does not allow for frequency-resolved THz spectra. As a result, it discards a lot of the information Terahertz-TDS spectroscopy contains, as well as its advantages over other spectroscopic techniques. Frequency-resolved TRTS would allow for the calculation of transient conductivity at each pump-probe delay time and can differentiate between signals of excitons and free charge carriers. This would allow for robust interpretations of charge mobility in novel materials. However, frequency-resolved TRTS is not practically feasible in a dual-delay configuration. We develop in this thesis a novel single-shot method based on angle-to-time mapping of a rotating probe. This method is applied to build a single-shot Terahertz-TDS spectrometer. A transmissive grating applies pulse front tilt which allows for the measurement of the entire THz transient (over a 5.7 ps window) in a single laser shot on a CMOS multichannel detector, thus alleviating the need for delay stage sampling of the THz transient, and leading to a reduction of experimental time by several orders of magnitude. An optical pump excitation is incorporated to allow a time-resolved measurement (TRTS) of the entire terahertz time-domain spectrum, and thus frequency-resolved TRTS. We show qualitative agreement between the THz time domain spectra obtained with the single shot technique and the standard free-space electro-optic (EO) sampling with balanced photodiodes, with an order of magnitude increased signal sensitivity. A proof-of-concept single shot TRTS study of a Si semiconductor sample is also given, showing we are able to resolve the TRTS signal of the entire THz pulse in a single shot, in time. This technique allows us to obtain significantly more information than traditional TRTS methods without any compromise in experiment time. However we find that the implemented single shot technique seems to suffer at higher frequencies (above 2 THz), which must be addressed to confirm the viability of a full spectrum single shot TRTS experiment. Further improvements, such as tighter focusing of the THz radiation, must be made to both the single-shot spectrometer as well as to the optical pump, for a quantitative single shot measurement. However, the proof-of-concept results in this thesis prove frequency-resolved TRTS is viable by using the developed single-shot detection method. As such it directly allows a novel spectroscopic tool which can lead to new insights into charge mobilities in optoelectronic materials, and may encourage wider application of TRTS.</p>

Single Shot Time-Resolved Terahertz Spectroscopy for Optoelectronic Materials

<p>Optoelectronic materials and devices, such as LEDs and solar cells, are ubiquitous in the modern, technologically driven world. Understanding the fundamental physical process in optoelectronic materials is essential for the design and development of new devices which are more efficient, cheaper, printable, as well as environmentally friendly. Two particularly important material properties for device performance are charge mobility and photoconductivity, as they increase charge separation and extraction efficiencies, and thus give specific insight into device efficiency. The best suited technique for measuring mobility and conductivity on ultrafast timescales is Terahertz spectroscopy. Terahertz spectroscopy is a non-invasive, contact-free probe of the mobility of charges in optoelectronic materials. Terahertz time-domain spectroscopy allows for the direct determination of the entire complex-valued conductivity. As a result, important optical properties such as the complex refractive index and dielectric function of a material can be measured directly. The short duration of THz pulses, on the order of 1 ps, also allows for time-resolved studies of the transient photoconductivity in optically-excited materials with sub-picosecond time resolution, i.e. Time-Resolved Teraherz Spectroscopy (TRTS). Traditionally, only the peak of the THz pulse signal is measured with TRTS, due to the time constraints of a two-delay experiment. This does not allow for frequency-resolved THz spectra. As a result, it discards a lot of the information Terahertz-TDS spectroscopy contains, as well as its advantages over other spectroscopic techniques. Frequency-resolved TRTS would allow for the calculation of transient conductivity at each pump-probe delay time and can differentiate between signals of excitons and free charge carriers. This would allow for robust interpretations of charge mobility in novel materials. However, frequency-resolved TRTS is not practically feasible in a dual-delay configuration. We develop in this thesis a novel single-shot method based on angle-to-time mapping of a rotating probe. This method is applied to build a single-shot Terahertz-TDS spectrometer. A transmissive grating applies pulse front tilt which allows for the measurement of the entire THz transient (over a 5.7 ps window) in a single laser shot on a CMOS multichannel detector, thus alleviating the need for delay stage sampling of the THz transient, and leading to a reduction of experimental time by several orders of magnitude. An optical pump excitation is incorporated to allow a time-resolved measurement (TRTS) of the entire terahertz time-domain spectrum, and thus frequency-resolved TRTS. We show qualitative agreement between the THz time domain spectra obtained with the single shot technique and the standard free-space electro-optic (EO) sampling with balanced photodiodes, with an order of magnitude increased signal sensitivity. A proof-of-concept single shot TRTS study of a Si semiconductor sample is also given, showing we are able to resolve the TRTS signal of the entire THz pulse in a single shot, in time. This technique allows us to obtain significantly more information than traditional TRTS methods without any compromise in experiment time. However we find that the implemented single shot technique seems to suffer at higher frequencies (above 2 THz), which must be addressed to confirm the viability of a full spectrum single shot TRTS experiment. Further improvements, such as tighter focusing of the THz radiation, must be made to both the single-shot spectrometer as well as to the optical pump, for a quantitative single shot measurement. However, the proof-of-concept results in this thesis prove frequency-resolved TRTS is viable by using the developed single-shot detection method. As such it directly allows a novel spectroscopic tool which can lead to new insights into charge mobilities in optoelectronic materials, and may encourage wider application of TRTS.</p>

Spontaneous symmetry breaking in persistent currents of spinor polaritons

We predict the spontaneous symmetry breaking in a spinor Bose–Einstein condensate of exciton-polaritons (polaritons) caused by the coupling of its spin and orbital degrees of freedom. We study a polariton condensate trapped in a ring-shaped effective potential with a broken rotational symmetry. We propose a realistic scheme of generating controllable spinor azimuthal persistent currents of polaritons in the trap under the continuous wave optical pump. We propose a new type of half-quantum circulating states in a spinor system characterized by azimuthal currents in both circular polarizations and a vortex in only one of the polarizations. The spontaneous symmetry breaking in the spinor polariton condensate that consists in the switching from co-winding to opposite-winding currents in opposite spin states is revealed. It is characterized by the change of the average orbital angular momentum of the condensate from zero to non-zero values. The radial displacement of the pump spot and the polarization of the pump act as the control parameters. The considered system exhibits a fundamental similarity to a superconducting flux qubit, which makes it highly promising for applications in quantum computing.