Person: Schmidt, Jan
Upgrade technologies for silicon photovoltaics : Part I: industrial solution to minimize the negative impact of light induced degradation
2020, Pernau, Thomas, Derricks, Christian, Hahn, Giso, Helmich, Lailah, Herguth, Axel, Schmidt, Jan, Walter, Dominic
This paper is to inform about Part I of a joint research project on optimum reduction of boron-oxygen related degradation and passivated contacts for PERC-based solar cells. It includes the application of investigated technologies into potential future cell designs such as PERT and bifacial solar cells. The joint project was merged from two individual project suggestions and therefore had two parts. This paper focuses on an industrial solution to minimize the negative impact of light induced degradation (LID) by: • Investigating and understanding the nature of the degradation effect • Applying the experimental results and concepts to industrially suppress LID • Developing an optimized integrated firing-regeneration process for industrial application The second part of this project is also covered in this conference . The detailed project report (in German) has been published .
Laser cooling of semiconductors : ultrafast carrier and lattice dynamics
2018, Schmidt, Jan
Correlated fluorescence blinking in two-dimensional semiconductor heterostructures
2016, Xu, Weigao, Liu, Weiwei, Schmidt, Jan, Zhao, Weijie, Lu, Xin, Raab, Timo, Diederichs, Carole, Gao, Weibo, Seletskiy, Denis V., Xiong, Qihua
'Blinking', or 'fluorescence intermittency', refers to a random switching between 'ON' (bright) and 'OFF' (dark) states of an emitter; it has been studied widely in zero-dimensional quantum dots and molecules, and scarcely in one-dimensional systems. A generally accepted mechanism for blinking in quantum dots involves random switching between neutral and charged states (or is accompanied by fluctuations in charge-carrier traps), which substantially alters the dynamics of radiative and non-radiative decay. Here, we uncover a new type of blinking effect in vertically stacked, two-dimensional semiconductor heterostructures, which consist of two distinct monolayers of transition metal dichalcogenides (TMDs) that are weakly coupled by van der Waals forces. Unlike zero-dimensional or one-dimensional systems, two-dimensional TMD heterostructures show a correlated blinking effect, comprising randomly switching bright, neutral and dark states. Fluorescence cross-correlation spectroscopy analyses show that a bright state occurring in one monolayer will simultaneously lead to a dark state in the other monolayer, owing to an intermittent interlayer carrier-transfer process. Our findings suggest that bilayer van der Waals heterostructures provide unique platforms for the study of charge-transfer dynamics and non-equilibrium-state physics, and could see application as correlated light emitters in quantum technology.
Analytical model for the optimization of locally contacted solar cells
2005, Plagwitz, Heiko, Schaper, Meilin, Schmidt, Jan, Terheiden, Barbara, Brendel, Rolf
Multicolor femtosecond pump-probe system with single-electron sensitivity at low temperatures and high magnetic fields
2019-12-01, Traum, Christian, Henzler, Philipp, Lohner, Stefan, Becker, H., Nabben, David, Gumbsheimer, Pascal, Hinz, Christopher, Schmidt, Jan, Seletskiy, Denis V., Leitenstorfer, Alfred
We present an ultrafast spectroscopy system designed for temporal and spectral resolution of transient transmission changes after excitation of single electrons in solid-state quantum structures. The system is designed for optimum long-term stability, offering the option of hands-off operation over several days. Pump and probe pulses are generated in a versatile Er:fiber laser system where visible photon energies may be tuned independently from 1.90 eV to 2.51 eV in three parallel branches. Bandwidth-limited pulse durations between 100 fs and 10 ps are available. The solid-state quantum systems under investigation are mounted in a closed-cycle superconducting magnet cryostat providing temperatures down to 1.6 K and magnetic fields of up to 9 T. The free-standing cryomagnet is coupled to the laser system by means of a high-bandwidth active beam steering unit to eliminate residual low-frequency mechanical vibrations of the pulse tube coolers. High-NA objective lenses inside the sample chamber are employed for focusing femtosecond laser pulses onto the sample and recollection of the transmission signal. The transmitted probe light is dispersed in a grating monochromator equipped with a liquid nitrogen-cooled CCD camera, enabling a frame rate of 559 Hz. In order to eliminate spurious background effects due to low-frequency changes in the thermal equilibrium of the sample, we operate with a lock-in scheme where, instead of the pump amplitude, the pump-probe timing is modulated. This feature is provided without any mechanical action by an electro-optic timing unit inside the femtosecond Er:fiber system. The performance of the instrument is tested with spectrally resolved pump-probe measurements on a single negatively charged CdSe/ZnSe quantum dot under a magnetic field of 9 T. Selective initialization and readout of charge and spin states is carried out via two different femtosecond laser pulses. High-quality results on subpicosecond intraband relaxation dynamics after single-electron excitation motivate a broad variety of future experiments in ultrafast quantum optics and few-fermion quantum dynamics.
High-speed asynchronous optical sampling based on GHz Yb:KYW oscillators
2017-04-17, Li, Changxiu, Krauß, Nico, Schäfer, Gerhard, Ebner, Lukas, Kliebisch, Oliver, Schmidt, Jan, Winnerl, Stefan, Hettich, Mike, Dekorsy, Thomas
A low-cost scheme of high-speed asynchronous optical sampling based on Yb:KYW oscillators is reported. Two GHz diode-pumped oscillators with a slight pulse repetition rate offset serve as pump and probe source, respectively. The temporal resolution of this system is limited to 500 fs mainly by the pulse duration of the oscillators and also by relative timing jitter between the oscillators. A near-shot-noise noise floor around 10−6 (∆R/R) is obtained within a data acquisition time of a few seconds. The performance of the system is demonstrated by measurements of coherent acoustic phonons in a semiconductor sample that resembles a semiconductor saturable absorber mirror or an optically pumped semiconductor chip.
Direct sampling of electric-field vacuum fluctuations
2015-10-23, Riek, Claudius, Seletskiy, Denis V., Moskalenko, Andrey S., Schmidt, Jan, Krauspe, Philipp, Eckart, Sebastian, Eggert, Stefan, Burkard, Guido, Leitenstorfer, Alfred
The ground state of quantum systems is characterized by zero-point motion. Those vacuum fluctuations are generally deemed an elusive phenomenon that manifests itself only indirectly. Here, we report direct detection of the vacuum fluctuations of electromagnetic radiation in free space. The ground-state electric field variance is found to be inversely proportional to the four-dimensional space-time volume sampled electro-optically with tightly focused few-femtosecond laser pulses. Sub-cycle temporal readout and nonlinear coupling far from resonance provide signals from purely virtual photons without amplification. Our findings enable an extreme time-domain approach to quantum physics with nondestructive access to the quantum state of light. Operating at multi-terahertz frequencies, such techniques might also allow time-resolved studies of intrinsic fluctuations of elementary excitations in condensed matter.
Laser cooling of semiconductors traced in the time domain
2019, Schmidt, Jan, Leitenstorfer, Alfred, Seletskiy, Denis V.
Despite tremendous progress in optical refrigeration of rare-earth-doped crystals, laser cooling in III-V semiconductors has not been demonstrated to date. Here we report first observation of cooling in GaAs/lnGaP double heterostructures on a sub-nanosecond timescale.
Investigation of the laser cooling cycle in the time domain (Conference Presentation)
2017, Schmidt, Jan, Raab, Timo, Oelmann, Jannis, Seletskiy, Denis V.
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.
Direct and Ultrabroadband Detection of the Vacuum Field
2014, Riek, Claudius, Schmidt, Jan, Eckart, Sebastian, Seletskiy, Denis V., Leitenstorfer, Alfred
Electro-optic sampling at center frequency and bandwidth around 70 THz allows measuring the vacuum fluctuations of the electric field in free space. Distinction from detector shot noise is accomplished by modifying the space-time volume probed.