Volkmar Langer - Academia.edu (original) (raw)

Papers by Volkmar Langer

Research paper thumbnail of Freie Online-Angebote für Selbstlernende. Lebenslanges Lernen mit dem Internet

Research paper thumbnail of 6. Improvement of Self-directed Learning by Using the Inverted Classroom Model (ICM) for a Basic Module in Business Computer Sciences

De Gruyter eBooks, Oct 13, 2014

Research paper thumbnail of Propagation Beats of Quadrupole Polaritons in Cu2O

physica status solidi (b), 1992

ABSTRACT Time-resolved measurements of transmission in resonance with the 1 S quadrupole polarito... more ABSTRACT Time-resolved measurements of transmission in resonance with the 1 S quadrupole polariton in Cu2O reveal a long-lasting signal which is modulated by pronounced oscillations with time-dependent period. These oscillations are interpreted as “propagation beats”, which are caused by superposition of excited states of both polariton branches which propagate coherently through the crystal. At higher excitation intensities the decay becomes faster and the beats disappear. Magneto-quantum beats are also influenced significantly by propagation effects in transmission measurements.

Research paper thumbnail of Transform-limited picosecond resonant light scattering of excitons

Journal of Luminescence, 1991

ABSTRACT The relaxation processes of electronic states in solids are determined by energy-relaxat... more ABSTRACT The relaxation processes of electronic states in solids are determined by energy-relaxation, changing the occupation of the levels, and by phase-relaxation, destorying the coherence of the excited states. By calculating quantum-mechanically the time-resolved resonant light scattering spectrum for an exciton system interacting with a thermal bath, it is shown how these processes can be investigated. The presented theory clarifies the implications of the measurement process and demonstrates how the time dependence and polarization properties of the scattered light can be exploited to discriminate between Raman scattering proceeding without loss of coherence and hot luminescence occurring after phase-relaxation. Experimental results for time-resolved light sattering in resonance with the Γ+6 ⊗ L-4,5 exciton states in AgBr are found in good agreement with the calculated time-dependence.

Research paper thumbnail of Quantum beat spectroscopy: a new tool to study phase relaxation of excitons

Journal of Luminescence, 1992

ABSTRACT We briefly review our recent work in which picosecond quantum beat spectroscopy was empl... more ABSTRACT We briefly review our recent work in which picosecond quantum beat spectroscopy was employed for the study of excitons. The principle of the method and the necessary experimental technique are described. As examples, results are presented that were obtained for the indirect free exciton in AgBr, for bound excitons in CdS, and for the exciton-polariton in Cu2O.

Research paper thumbnail of Quantum beats in picosecond time-resolved resonant light scattering of free excitons in AgBr

Journal of Luminescence, 1990

Abstract Experiments are reported in which quantum beats are observed in the resonance fluorescen... more Abstract Experiments are reported in which quantum beats are observed in the resonance fluorescence of a free exciton in a solid. The beats are generated in AgBr at 2 K by pulsed excitation of the Γ+6 ⊗ L-4,5 indirect singlet-triplet exciton state split in magnetic fields up to 4 T. Originating from the coherent superposition of the split states, damped oscillations occur superimposed on the decay of the 2TO(L) scattered intensity. The quantum-beat signals calculated by means of the density matrix formalism are found to be in good agreement with the experiment. From the beat frequencies the splitting of the exciton states is obtained with high accuracy. Quantum dephasing times of several hundreds of picoseconds are determined dependent on exciton energy and magnetic field strength.

Research paper thumbnail of Magneto-Quantum Beats and Coherence in Resonant Light Scattering from Quadrupole Polaritons in Cu 2 O

Europhysics Letters (EPL), 1992

Using picosecond time-resolved light scattering, we observe quantum beats of the 1S yellow excito... more Using picosecond time-resolved light scattering, we observe quantum beats of the 1S yellow exciton-polariton in Cu2O, split by a magnetic field, both in one- and two-phonon scattering. From these measurements Raman and hot-luminescence-like contributions to these processes can be discriminated. A quantitative analysis leads to exciton-polariton coherence times up to 800 ps which strongly decrease at larger wave vectors and

Research paper thumbnail of Observation of quantum beats in the resonance fluorescence of free excitons

Physical Review Letters, 1990

Research paper thumbnail of Coherent propagation and quantum beats of quadrupole polaritons in Cu2O

Journal of Luminescence, 1992

Coherent propagation of quadrupole polaritons in CuzO is demonstrated by time-resolved spectrosco... more Coherent propagation of quadrupole polaritons in CuzO is demonstrated by time-resolved spectroscopy. This manifests itself in a strong distortion of the temporal shape of a picosecond optical pulse which is in resonance with the 1S exciton polariton. Oscillations with time-dependent period in the transmitted light intensity are quantitatively explained as resulting from quantum beats between the two branches of the exciton polariton. The analysis yields the homogeneous linewidth and oscillator strength.

Research paper thumbnail of Leadership Coaching Challenge Orientation Guide

Research paper thumbnail of Picosecond quantum-beat spectroscopy of bound excitons in CdS

Physical Review Letters, 1991

We report the first observation of quantum beats in the resonance fluorescence of a bound exciton... more We report the first observation of quantum beats in the resonance fluorescence of a bound exciton in a semiconductor, the ionized-donor exciton complex in CdS. Measurements of the polarization behavior and magnetic-field dependence of the beats permitted a detailed investigation of strain effects and magneto-optical parameters of the states. From the damping of the beats, coherence times of the order

Research paper thumbnail of Freie Online-Angebote für Selbstlernende. Lebenslanges Lernen mit dem Internet

Noch nie in der Geschichte der Menschheit war der Zugang zu Wissen so unkompliziert und so unbegr... more Noch nie in der Geschichte der Menschheit war der Zugang zu Wissen so unkompliziert und so unbegrenzt moglich wie heute: Wer sich neues Wissen aneignen will (oder muss), findet im Internet einen nahezu unerschopflichen Pool an Lernangeboten fur die unterschiedlichsten Themenbereiche und Niveaustufen sowie in den verschiedensten Formaten. Dieser Beitrag betrachtet die mit dieser Entwicklung einher gehenden Chancen und Herausforderungen aus der Perspektive der Lernenden. Zunachst wird ein Uberblick uber die unterschiedlichen offenen Online-Lernangebote bzw. Lernformen gegeben, die Selbstlernenden zur Verfugung stehen; Schwerpunkte sind freie Bildungsressourcen, Lernportale und offene Online-Kurse. Dabei wird zunachst dargestellt, wie mit diesen Angeboten gelernt werden kann; daruber hinaus geht es aber auch darum, wie Lernende qualitativ hochwertige Lernangebote finden konnen, die fur sie und ihre individuellen Bedurfnisse geeignet sind, und welche Moglichkeiten es gibt, selbstgesteue...

Research paper thumbnail of 1 Improvement of Self-directed Learning by Using the Inverted Classroom Model (ICM) for a Basic Module in Business Com- puter Sciences

How can the teaching and learning method Inverted Classroom Model (ICM) support students during t... more How can the teaching and learning method Inverted Classroom Model (ICM) support students during their learning activities? How can the time inside the classroom be used more sustainably by teachers and students for their learning activities? Based on these questions, this paper provides examples from the usage of the ICM in the module ‘Computer Networks’, which serves as a fundamental module in the Business Computer Sciences studies at the HSW University of Applied Sciences. The ICM was used in this context to develop a new educational concept for the mentioned module, aiming to provide a cooperative learning environment during the time of attendance in class. In our scenario, the lecturers became learning facilitators to a greater extent than the traditional teacher typically does. The main target of this new concept is to increase the practical orientation based on the learners’ requirements and to provide them with a higher level of sustainable and well-trained decision-making co...

Research paper thumbnail of Freie Online-Angebote für Selbstlernende. Lebenslanges Lernen mit dem Internet

Research paper thumbnail of Improvement of Self-directed Learning by Using the Inverted Classroom Model (ICM) for a Basic Module in Business Computer Sciences

How can the teaching and learning method Inverted Classroom Model (ICM) sup-port students during ... more How can the teaching and learning method Inverted Classroom Model (ICM) sup-port students during their learning activities? How can the time inside the class-room be used more sustainably by teachers and students for their learning activi-ties? Based on these questions, this paper provides examples from the usage of the ICM in the module 'Computer Networks', which serves as a fundamental module in the Business Computer Sciences studies at the HSW -University of Applied Sciences. The ICM was used in this context to develop a new educational concept for the mentioned module, aiming to provide a cooperative learning environment during the time of attendance in class. In our scenario, the lecturers became learn-ing facilitators to a greater extent than the traditional teacher typically does. The main target of this new concept is to increase the practical orientation based on the learners' requirements and to provide them with a higher level of sustainable and well-trained d...

Research paper thumbnail of 6. Improvement of Self-directed Learning by Using the Inverted Classroom Model (ICM) for a Basic Module in Business Computer Sciences

The 3rd German ICM-Conference - Proceedings, 2014

Research paper thumbnail of Picosecond quantum-beat spectroscopy of quadrupole polaritons in Cu2O

Physical review. B, Condensed matter, Jan 15, 1995

Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the ... more Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the yellow 1S quadrupole exciton polariton. In order to obtain information on the coherence of the resonant intermediate polariton states, we have studied the electric-dipole allowed lightscattering processes involving one and two I'3 phonons. A quantitative analysis provides excitonpolariton coherence times of about 1 ns at 2 K, decreasing with higher temperatures and larger wave vectors due to phonon scattering. From the magnetic-field dependence, low-field g values are determined with high accuracy. The measurements permit a clear distinction between Raman and hot-luminescence-like contributions to the scattering intensity. The strong eKect of the spectral width of the excitation pulse on the time behavior of the scattering, as well as the temperature dependence of the coherence time, indicate that it is essential to take into account the polariton character.

Research paper thumbnail of Picosecond quantum-beat spectroscopy of quadrupole polaritons inCu2O

Physical Review B

Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the ... more Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the yellow 1S quadrupole exciton polariton. In order to obtain information on the coherence of the resonant intermediate polariton states, we have studied the electric-dipole allowed lightscattering processes involving one and two I'3 phonons. A quantitative analysis provides excitonpolariton coherence times of about 1 ns at 2 K, decreasing with higher temperatures and larger wave vectors due to phonon scattering. From the magnetic-field dependence, low-field g values are determined with high accuracy. The measurements permit a clear distinction between Raman and hot-luminescence-like contributions to the scattering intensity. The strong eKect of the spectral width of the excitation pulse on the time behavior of the scattering, as well as the temperature dependence of the coherence time, indicate that it is essential to take into account the polariton character.

Research paper thumbnail of Freie Online-Angebote für Selbstlernende. Lebenslanges Lernen mit dem Internet

Research paper thumbnail of 6. Improvement of Self-directed Learning by Using the Inverted Classroom Model (ICM) for a Basic Module in Business Computer Sciences

De Gruyter eBooks, Oct 13, 2014

Research paper thumbnail of Propagation Beats of Quadrupole Polaritons in Cu2O

physica status solidi (b), 1992

ABSTRACT Time-resolved measurements of transmission in resonance with the 1 S quadrupole polarito... more ABSTRACT Time-resolved measurements of transmission in resonance with the 1 S quadrupole polariton in Cu2O reveal a long-lasting signal which is modulated by pronounced oscillations with time-dependent period. These oscillations are interpreted as “propagation beats”, which are caused by superposition of excited states of both polariton branches which propagate coherently through the crystal. At higher excitation intensities the decay becomes faster and the beats disappear. Magneto-quantum beats are also influenced significantly by propagation effects in transmission measurements.

Research paper thumbnail of Transform-limited picosecond resonant light scattering of excitons

Journal of Luminescence, 1991

ABSTRACT The relaxation processes of electronic states in solids are determined by energy-relaxat... more ABSTRACT The relaxation processes of electronic states in solids are determined by energy-relaxation, changing the occupation of the levels, and by phase-relaxation, destorying the coherence of the excited states. By calculating quantum-mechanically the time-resolved resonant light scattering spectrum for an exciton system interacting with a thermal bath, it is shown how these processes can be investigated. The presented theory clarifies the implications of the measurement process and demonstrates how the time dependence and polarization properties of the scattered light can be exploited to discriminate between Raman scattering proceeding without loss of coherence and hot luminescence occurring after phase-relaxation. Experimental results for time-resolved light sattering in resonance with the Γ+6 ⊗ L-4,5 exciton states in AgBr are found in good agreement with the calculated time-dependence.

Research paper thumbnail of Quantum beat spectroscopy: a new tool to study phase relaxation of excitons

Journal of Luminescence, 1992

ABSTRACT We briefly review our recent work in which picosecond quantum beat spectroscopy was empl... more ABSTRACT We briefly review our recent work in which picosecond quantum beat spectroscopy was employed for the study of excitons. The principle of the method and the necessary experimental technique are described. As examples, results are presented that were obtained for the indirect free exciton in AgBr, for bound excitons in CdS, and for the exciton-polariton in Cu2O.

Research paper thumbnail of Quantum beats in picosecond time-resolved resonant light scattering of free excitons in AgBr

Journal of Luminescence, 1990

Abstract Experiments are reported in which quantum beats are observed in the resonance fluorescen... more Abstract Experiments are reported in which quantum beats are observed in the resonance fluorescence of a free exciton in a solid. The beats are generated in AgBr at 2 K by pulsed excitation of the Γ+6 ⊗ L-4,5 indirect singlet-triplet exciton state split in magnetic fields up to 4 T. Originating from the coherent superposition of the split states, damped oscillations occur superimposed on the decay of the 2TO(L) scattered intensity. The quantum-beat signals calculated by means of the density matrix formalism are found to be in good agreement with the experiment. From the beat frequencies the splitting of the exciton states is obtained with high accuracy. Quantum dephasing times of several hundreds of picoseconds are determined dependent on exciton energy and magnetic field strength.

Research paper thumbnail of Magneto-Quantum Beats and Coherence in Resonant Light Scattering from Quadrupole Polaritons in Cu 2 O

Europhysics Letters (EPL), 1992

Using picosecond time-resolved light scattering, we observe quantum beats of the 1S yellow excito... more Using picosecond time-resolved light scattering, we observe quantum beats of the 1S yellow exciton-polariton in Cu2O, split by a magnetic field, both in one- and two-phonon scattering. From these measurements Raman and hot-luminescence-like contributions to these processes can be discriminated. A quantitative analysis leads to exciton-polariton coherence times up to 800 ps which strongly decrease at larger wave vectors and

Research paper thumbnail of Observation of quantum beats in the resonance fluorescence of free excitons

Physical Review Letters, 1990

Research paper thumbnail of Coherent propagation and quantum beats of quadrupole polaritons in Cu2O

Journal of Luminescence, 1992

Coherent propagation of quadrupole polaritons in CuzO is demonstrated by time-resolved spectrosco... more Coherent propagation of quadrupole polaritons in CuzO is demonstrated by time-resolved spectroscopy. This manifests itself in a strong distortion of the temporal shape of a picosecond optical pulse which is in resonance with the 1S exciton polariton. Oscillations with time-dependent period in the transmitted light intensity are quantitatively explained as resulting from quantum beats between the two branches of the exciton polariton. The analysis yields the homogeneous linewidth and oscillator strength.

Research paper thumbnail of Leadership Coaching Challenge Orientation Guide

Research paper thumbnail of Picosecond quantum-beat spectroscopy of bound excitons in CdS

Physical Review Letters, 1991

We report the first observation of quantum beats in the resonance fluorescence of a bound exciton... more We report the first observation of quantum beats in the resonance fluorescence of a bound exciton in a semiconductor, the ionized-donor exciton complex in CdS. Measurements of the polarization behavior and magnetic-field dependence of the beats permitted a detailed investigation of strain effects and magneto-optical parameters of the states. From the damping of the beats, coherence times of the order

Research paper thumbnail of Freie Online-Angebote für Selbstlernende. Lebenslanges Lernen mit dem Internet

Noch nie in der Geschichte der Menschheit war der Zugang zu Wissen so unkompliziert und so unbegr... more Noch nie in der Geschichte der Menschheit war der Zugang zu Wissen so unkompliziert und so unbegrenzt moglich wie heute: Wer sich neues Wissen aneignen will (oder muss), findet im Internet einen nahezu unerschopflichen Pool an Lernangeboten fur die unterschiedlichsten Themenbereiche und Niveaustufen sowie in den verschiedensten Formaten. Dieser Beitrag betrachtet die mit dieser Entwicklung einher gehenden Chancen und Herausforderungen aus der Perspektive der Lernenden. Zunachst wird ein Uberblick uber die unterschiedlichen offenen Online-Lernangebote bzw. Lernformen gegeben, die Selbstlernenden zur Verfugung stehen; Schwerpunkte sind freie Bildungsressourcen, Lernportale und offene Online-Kurse. Dabei wird zunachst dargestellt, wie mit diesen Angeboten gelernt werden kann; daruber hinaus geht es aber auch darum, wie Lernende qualitativ hochwertige Lernangebote finden konnen, die fur sie und ihre individuellen Bedurfnisse geeignet sind, und welche Moglichkeiten es gibt, selbstgesteue...

Research paper thumbnail of 1 Improvement of Self-directed Learning by Using the Inverted Classroom Model (ICM) for a Basic Module in Business Com- puter Sciences

How can the teaching and learning method Inverted Classroom Model (ICM) support students during t... more How can the teaching and learning method Inverted Classroom Model (ICM) support students during their learning activities? How can the time inside the classroom be used more sustainably by teachers and students for their learning activities? Based on these questions, this paper provides examples from the usage of the ICM in the module ‘Computer Networks’, which serves as a fundamental module in the Business Computer Sciences studies at the HSW University of Applied Sciences. The ICM was used in this context to develop a new educational concept for the mentioned module, aiming to provide a cooperative learning environment during the time of attendance in class. In our scenario, the lecturers became learning facilitators to a greater extent than the traditional teacher typically does. The main target of this new concept is to increase the practical orientation based on the learners’ requirements and to provide them with a higher level of sustainable and well-trained decision-making co...

Research paper thumbnail of Freie Online-Angebote für Selbstlernende. Lebenslanges Lernen mit dem Internet

Research paper thumbnail of Improvement of Self-directed Learning by Using the Inverted Classroom Model (ICM) for a Basic Module in Business Computer Sciences

How can the teaching and learning method Inverted Classroom Model (ICM) sup-port students during ... more How can the teaching and learning method Inverted Classroom Model (ICM) sup-port students during their learning activities? How can the time inside the class-room be used more sustainably by teachers and students for their learning activi-ties? Based on these questions, this paper provides examples from the usage of the ICM in the module 'Computer Networks', which serves as a fundamental module in the Business Computer Sciences studies at the HSW -University of Applied Sciences. The ICM was used in this context to develop a new educational concept for the mentioned module, aiming to provide a cooperative learning environment during the time of attendance in class. In our scenario, the lecturers became learn-ing facilitators to a greater extent than the traditional teacher typically does. The main target of this new concept is to increase the practical orientation based on the learners' requirements and to provide them with a higher level of sustainable and well-trained d...

Research paper thumbnail of 6. Improvement of Self-directed Learning by Using the Inverted Classroom Model (ICM) for a Basic Module in Business Computer Sciences

The 3rd German ICM-Conference - Proceedings, 2014

Research paper thumbnail of Picosecond quantum-beat spectroscopy of quadrupole polaritons in Cu2O

Physical review. B, Condensed matter, Jan 15, 1995

Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the ... more Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the yellow 1S quadrupole exciton polariton. In order to obtain information on the coherence of the resonant intermediate polariton states, we have studied the electric-dipole allowed lightscattering processes involving one and two I'3 phonons. A quantitative analysis provides excitonpolariton coherence times of about 1 ns at 2 K, decreasing with higher temperatures and larger wave vectors due to phonon scattering. From the magnetic-field dependence, low-field g values are determined with high accuracy. The measurements permit a clear distinction between Raman and hot-luminescence-like contributions to the scattering intensity. The strong eKect of the spectral width of the excitation pulse on the time behavior of the scattering, as well as the temperature dependence of the coherence time, indicate that it is essential to take into account the polariton character.

Research paper thumbnail of Picosecond quantum-beat spectroscopy of quadrupole polaritons inCu2O

Physical Review B

Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the ... more Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the yellow 1S quadrupole exciton polariton. In order to obtain information on the coherence of the resonant intermediate polariton states, we have studied the electric-dipole allowed lightscattering processes involving one and two I'3 phonons. A quantitative analysis provides excitonpolariton coherence times of about 1 ns at 2 K, decreasing with higher temperatures and larger wave vectors due to phonon scattering. From the magnetic-field dependence, low-field g values are determined with high accuracy. The measurements permit a clear distinction between Raman and hot-luminescence-like contributions to the scattering intensity. The strong eKect of the spectral width of the excitation pulse on the time behavior of the scattering, as well as the temperature dependence of the coherence time, indicate that it is essential to take into account the polariton character.