Oelmann, Jannis
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Investigation of the laser cooling cycle in the time domain (Conference Presentation)
Upon excitation of a material below its fundamental transition, cooling of the lattice results if the subsequent emission is predominantly radiative. Despite overwhelming experimental success, it remains a challenge to understand the microscopic nature of detrimental processes that can even prevent cooling. We apply ultrafast spectroscopy to resolve the laser refrigeration cycle in the time domain. Strong evidence for lattice cooling on picosecond timescales in bulk GaAs/InGaP double-heterostructures and GaAs/AlGaAs quantum wells establishes the non-local nature of the parasitic mechanisms. Further precision measurements investigating long-time dynamics are currently underway to resolve detrimental heating in bulk GaAs for the first time.
Tunneling breakdown of a strongly correlated insulating state in VO2 induced by intense multiterahertz excitation
We directly trace the near- and midinfrared transmission change of a VO2 thin film during an ultrafast insulator-to-metal transition triggered by high-field multiterahertz transients. Nonthermal switching into a metastable metallic state is governed solely by the amplitude of the applied terahertz field. In contrast to resonant excitation below the threshold fluence, no signatures of excitonic self-trapping are observed. Our findings are consistent with the generation of spatially separated charge pairs and a cooperative transition into a delocalized metallic state by THz field-induced tunneling. The tunneling process is a condensed-matter analog of the Schwinger effect in nonlinear quantum electrodynamics. We find good agreement with the pair production formula by replacing the Compton wavelength with an electronic correlation length of 2.1 Ã….
Ultrafast Insulator-Metal Transition in VO2 Driven by Intense Multi-THz Pulses
We demonstrate sub-100 fs metallization of VO2 induced by few-cycle electric transients at frequencies around 25 THz. Interband tunneling is identified as an instantaneous excitation mechanism.
Femtosecond Insulator-Metal Transition in VO2 Induced by Intense Multi-THz Transients
Tunneling Breakdown of a Strongly Correlated Insulating State in VO2 Induced by Intense Multi-Terahertz Excitation
We directly trace the near- and mid-infrared transmission change of a VO2 thin film during an ultrafast insulator-to-metal transition triggered by high-field multi-terahertz transients. Non-thermal switching into a metastable metallic state is governed solely by the amplitude of the applied terahertz field. In contrast to resonant excitation below the threshold fluence, no signatures of excitonic self-trapping are observed. Our findings are consistent with the generation of spatially separated charge pairs and a cooperative transition into a delocalized metallic state by THz field-induced tunneling. The tunneling process is a condensed-matter analogue of the Schwinger effect in nonlinear quantum electrodynamics. We find good agreement with the pair production formula by replacing the Compton wavelength with an electronic correlation length of 2.1 A.