Samuel A. March

Ultrafast optical control of coercivity in GaMnAs

Femtosecond optical control of the magnetization and coercive field is demonstrated in GaMnAs using time-resolved magneto-optical Kerr effect techniques. These experiments reveal a near-complete, subpicosecond collapse of the hysteresis loop, consistent with femtosecond demagnetization. On longer time scales (∼300ps) an increase in coercivity is observed, attributed to hole-mediated enhancement of the domain wall energy.

Simultaneous observation of free and defect-bound excitons in CH 3 NH 3 PbI 3 using four-wave mixing spectroscopy

Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM) spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their longer coherence decay times than unbound electron- hole pairs, we show that the exciton resonance energies can be directly observed from the nonlinear optical spectra. Our results indicate low-temperature binding energies of 13 meV (29 meV) for the free (defect-bound) exciton, with the 16 meV localization energy for excitons attributed to binding to point defects. Our findings shed light on the wide range of binding energies (2–55 meV) reported in recent years.

Four-Wave Mixing in Perovskite Photovoltaic Materials Reveals Long Dephasing Times and Weaker Many-Body Interactions than GaAs

Perovksite semiconductors have shown promise for low-cost solar cells, lasers and photodetectors, yet their fundamental photophysical properties are not well understood. Recent observations of a low exciton binding energy and evidence of hot phonon effects in the room temperature phase suggest that perovskites are much closer to inorganic semiconductors than the absorber layers in traditional organic photovoltaics, signaling the need for experiments that shed light on the placement of perovskite materials within the spectrum of semiconductors used in optoelectronics and photovoltaics. Here we use four-wave mixing (FWM) to contrast the coherent optical response of CH3NH3PbI3 thin films and crystalline GaAs. At carrier densities relevant for solar cell operation, our results show that carriers interact surprisingly weakly via the Coulomb interaction in perovskite, much weaker than in inorganic semiconductors. These weak many-body effects lead to a dephasing time in CH3NH3PbI3 ∼ 3× longer than in GaAs. Our r...

Interband dephasing and photon echo response in GaMnAs

Coherent carrier dynamics are studied in GaMnAs using time-integrated and time-resolved four-wave mixing techniques. Dephasing is observed to be dominated by spin-flip scattering between the optically injected holes and Mn ions, revealing the rapid time scale of this scattering process in the III-Mn-V diluted magnetic semiconductors. The optical response is shown to exhibit the characteristic signatures of a simple photon echo, despite the complexity of band tail contributions and strong exchange coupling in this system.

Ultrafast studies of carrier and magnetization dynamics in GaMnAs

We have investigated the carrier and magnetization dynamics in a GaMnAs structure with perpendicular uniaxial anisotropy using time-resolved pump probe techniques. Experiments were performed over two orders of magnitude variation in pump fluence, revealing an ultrafast demagnetization response that saturates at fluence values larger than 1 mJ/cm2. Dichroic bleaching contributions exhibit no dependence on the circular polarization state of the pump beam, indicating no signature of electron spin dynamics, in contrast to experiments at similar pump pulse fluence in other III-Mn-V semiconductors. We observe no evidence of a transient hole spin depolarization despite the strong demagnetization effects in our experiments, suggesting that more studies are needed to elucidate the influence of hot holes on the nonlinear optical response of diluted magnetic semiconductors. Differential reflectivity experiments indicate an electron trapping time of 1 ps, followed by carrier recombination on a time scale of several n...

Observation of the exciton and Urbach band tail in low-temperature-grown GaAs using four-wave mixing spectroscopy

Four-wave mixing (FWM) spectroscopy reveals clear signatures associated with the exciton, free carrier inter-band transitions, and the Urbach band tail in low-temperature-grown GaAs, providing a direct measure of the effective band gap as well as insight into the influence of disorder on the electronic structure. The ability to detect (and resolve) these contributions, in contrast to linear spectroscopy, is due to an enhanced sensitivity of FWM to the optical joint density of states and to many-body effects. Our experiments demonstrate the power of FWM for studying the near-band-edge optical properties and coherent carrier dynamics in low-temperature-grown semiconductors.

Carrier diffusion in thin-film CH3NH3PbI3 perovskite measured using four-wave mixing

We report the application of femtosecond four-wave mixing (FWM) to the study of carrier transport in solution-processed CH3NH3PbI3. The diffusion coefficient was extracted through direct detection of the lateral diffusion of carriers utilizing the transient grating technique, coupled with simultaneous measurement of decay kinetics exploiting the versatility of the boxcar excitation beam geometry. The observation of exponential decay of the transient grating versus interpulse delay indicates diffusive transport with negligible trapping within the first nanosecond following excitation. The in-plane transport geometry in our experiments enabled the diffusion length to be compared directly with the grain size, indicating that carriers move across multiple grain boundaries prior to recombination. Our experiments illustrate the broad utility of FWM spectroscopy for rapid characterization of macroscopic film transport properties.

Influence of Rashba splitting on carrier dynamics in organic-inorganic perovskites (Conference Presentation)

The lead halide hybrid perovskites have gained considerable attention in recent years due to their stellar performance as absorber layers in solution-processed solar cells, with efficiencies recently reaching over 22 percent [1]. Owing to their large spin-orbit coupling, these materials are also of interest for spintronic applications, in which the presence of lead may be less of an impediment to their adoption [2]. Measurements of spin dynamics in bulk CH3NH3PbI3-xClx have been reported in recent years [3,4,5], the spin-dependent optical Stark effect was demonstrated in 4F-PEPI [6], and a large Rashba effect has been predicted in both bulk and 2D perovskites [2], highlighting the need for further studies of the spin-related properties of these materials. Here we report spin-dependent measurements of carrier kinetics in butylammonium methylammonium lead iodide 2D perovskite and measurements of the coherent carrier response in 3D CH3NH3PbI3. Both experiments provide direct evidence of the impact of Rashba on the carrier kinetics in these systems, further supporting the potential for developing spin-optoelectronic devices using these materials. [1] https://www.nrel.gov/pv/assets/images/efficiency_chart.jpg. [2] M. Kepenekian and J. Even, J. Phys. Chem. Lett. 8, 3362 (2017). [3] D. Giovanni et al. Nano Lett. 15, 1553 (2015). [4] C. Zhang et al. Nat. Phys. 11, 427 (2015). [5] P. Odenthal et al. Nat. Phys. 13, 894 (2017). [6] D. Giovanni et al. Science Advances 2, e1600477 (2016).

Four-wave mixing response of solution-processed CH3NH3PbI3 thin films

The interest in perovskite-based solar cell absorber materials has skyrocketed in recent years due to the rapid rise in solar cell efficiency and the potential for cost reductions tied to solution-processed device fabrication. Due to complications associated with the presence of strong static and dynamic disorder in these organic-inorganic materials, the fundamental photophysical behavior of photo-excited charge carriers remains unclear. We apply four-wave mixing spectroscopy to study the charge carrier dynamics in CH3NH3PbI3 thin films. Our experiments reveal two discrete optical transitions below the band gap of the semiconductor with binding energies of 13 meV and 29 meV, attributed to free and defect-bound excitons respectively.

Spin dynamics and manipulation in GaMnAs alloys

We review recent experiments on spin excitation and manipulation in the ferromagnetic semiconductor GaMnAs. Spin dynamics in GaMnAs have been studied by two complementary approaches - by frequency-domain techniques, such as Brillouin light scattering (BLS) and ferromagnetic resonance (FMR); and by optical real-time techniques, such as ultrafast pump-probe magneto-optical spectroscopy. Using BLS and FMR, magnon frequencies (or resonance fields), were investigated as a function of Mn concentration, temperature and direction of magnetization, leading to information on magnetic anisotropy. Time-resolved magneto-optical Kerr effect, on the other hand, was used to study photo-induced femtosecond magnetization rotation, ultrafast optical demagnetization, and collective magnetization precession. Optically-induced transient changes in magnetization of GaMnAs produced by femtosecond laser pulses are analyzed and discussed in terms of the Landau-Lifshitz-Gilbert model. Finally, for completeness, we also discuss carrier-mediated nonthermal and thermal (lattice-heating) contributions to spin dynamics.