Stephan W. Koch

We present a passive and robust mode-locking scheme for a Vertical External Cavity Surface Emitting Laser (VECSEL).We placed the semiconductor gain medium and the semiconductor saturable absorber mirror (SESAM) strategically in a ring cavity to provide a stable colliding pulse operation. With this cavity geometry, the two counter propagating pulses synchronize on the SESAM to saturate the absorber together. This minimizes the energy lost and creates a transient carrier grating due to the interference of the two beams. The interaction of the two counter-propagating pulses in the SESAM is shown to extend the range of the modelocking regime and to enable higher output power when compared to the conventional VECSEL cavity geometry. In this configuration, we demonstrate a pulse duration of 195fs with an average power of 225mW per output beam at a repetition rate of 2.2GHz, giving a peak power of 460W per beam. The remarkable robustness of the modelocking regime is discussed and a rigorous pulse characterization is presented.

We demonstrate the highest free running single frequency power from a single chip VECSEL reported to date, with more than 15W in continuous operation at room temperature. The GaAs-based structure presents an emission wavelength of 1020nm and a tuning range 00 output beam exhibits very low transverse phase fluctuations across the entire mode, leading to a beam quality M² <1.2. To identify and reduce the different sources of noise, the relative intensity noise and frequency noise spectral density are investigated and the intensity and the frequency of the laser were independently stabilized. The laser frequency is controlled and tuned varying the cavity length using a high bandwidth piezoelectric element while intensity fluctuations are reduced by varying the pump intensity. Intrinsic and stabilized frequency and intensity noise are compared.

Combining rigorous quantum epitaxial design, highly accurate growth, novel processing and thermal management pushes the output power of single chip vertical‐external‐cavity surface‐emitting lasers (VECSELs) beyond the 100 W milestone.

We investigate the interaction of strong single-cycle THz pulses with a coherent excitonic population, observing the transition from Rabi flopping of the 1s-2p transition to multi THz-photon ionization with increasing field strength.

Interactions of strong few-cycle THz pulses with the induced optical polarization in a QW microcavity reveal that the exciton-polariton modes and the 2p-exciton state form a unique Λ-type three-level system of the coupled light-matter modes.

We report room-temperature high-peak-power operation of an optically pumped semiconductor laser based on the InGaAs/GaAs material system. We present the design of the semiconductor structure and optimization strategies to extract the maximum pulsed peak power. The gain structure was pumped by a 775-nm Alexandrite laser with a pulsewidth adjustable from 400 ns to 1 μs and a repetition rate of 3 Hz. A new record peak power of 400 W at a wavelength of 1020 nm was obtained with a Gaussian-shaped submicrosecond pulse. An optical-to-optical efficiency of 28% is demonstrated at maximum power. The key parameters limiting the output power are discussed.

A tHz external-cavity SEL source delivering mW powers at any targeted frequency across the tHz gap could have many applications.

We characterize chirped excitation on excitonic resonances in GaAs quantum wells using spectrally resolved transient absorption. Positive chirp enhances nonlinearities, despite the increased pulse duration compared with transform-limited excitation.