In quantum-dot tunnel-injection lasers, the excited charge carriers are efficiently captured from the bulk states via an injector quantum well and then transferred into the quantum dots via a tunnel barrier. The alignment of the electronic levels is crucial for the high efficiency of these processes and especially for the fast modulation dynamics of these lasers. In particular, the quantum mechanical nature of the tunneling process must be taken into account in the transition from two-dimensional quantum well states to zero-dimensional quantum dot states. This results in hybrid states, from which the scattering into the quantum-dot ground states takes place. We combine electronic state calculations of the tunnel-injection structures with many-body calculations of the scattering processes and insert this into a complete laser simulator. This allows us to study the influence of the level alignment and limitations due to inhomogeneous quantum-dot distributions. We find that the optimal alignment deviates from a simple picture in which the of the quantum-dot ground state energies are one LO-phonon energy below the injector quantum well ground state. \author{Frank Jahnke

SignificanceOptical-resolution optoacoustic microscopy (OR-OAM) enables label-free imaging of the microvasculature by using optical pulse excitation and acoustic detection, commonly performed by a focused optical beam and an ultrasound transducer. One of the main challenges of OR-OAM is the need to combine the excitation and detection in a coaxial configuration, often leading to a bulky setup that requires physically scanning the ultrasound transducer to achieve a large field of view.AimThe aim of this work is to develop an OR-OAM configuration that does not require physically scanning the ultrasound transducer or the acoustic beam path.ApproachOur OR-OAM system is based on a non-coaxial configuration in which the detection is performed by a silicon-photonics acoustic detector (SPADE) with a semi-isotropic sensitivity. The system is demonstrated in both epi- and trans-illumination configurations, where in both configurations SPADE remains stationary during the imaging procedure and only the optical excitation beam is scanned.ResultsThe system is showcased for imaging resolution targets and for the in vivo visualization of the microvasculature in a mouse ear. Optoacoustic imaging with focal spots down to 1.3 μm, lateral resolution of 4 μm, and a field of view higher than 4 mm in both lateral dimensions were demonstrated.ConclusionsWe showcase a new OR-OAM design, compatible with epi-illumination configuration. This setup enables relatively large fields of view without scanning the acoustic detector or acoustic beam path. Furthermore, it offers the potential for high-speed imaging within compact, miniature probe and could potentially facilitate the clinical translation of OR-OAM technology.

Dual-comb spectroscopy (DCS) is a powerful technique for broadband spectroscopy with high precision. High-frequency resolution requires long data acquisition times, limiting the temporal resolution in time-resolved measurements. We overcome this limitation by engineering the interaction between the sample under test and the DCS pulsed laser. The DCS interferogram is measured in steps with every step comprising a different number of pulses that interact with the sample. The sample's complex properties (absorption and phase) are extracted from the Fourier transform of the interferogram as a function of the number of pulses; this maps the temporal evolution of the excited state population. A two-dimensional spectrum is generated from which the system time evolution is deduced. We benchmark this method by measuring the two-dimensional spectrum of a room-temperature rubidium vapor. The measured population dynamics of the excited state show a square dependence on the number of interacting pulses due to the coherent accumulation of population. Rabi oscillations are observed under intense excitation conditions. This is the first demonstration of DCS with high frequency and high temporal resolutions (which is given by the inverse of the repetition rate of the comb laser) without invoking pump-probe spectroscopy, combining the pulsed laser spectral and temporal properties. This method allows one to detect simultaneously the kinetics of different chemical species and hence the pathway for chemical reactions.

Dual comb spectroscopy (DCS) is a broadband technique offering high resolution and fast data acquisition. Current state-of-the-art designs are based on a pair of fiber or solid-state lasers, which allow broadband spectroscopy but require a complicated stabilization setup. Semiconductor lasers are tunable, cost-effective, and easily integrable while limited by a narrow bandwidth. This motivates a hybrid design combining the advantages of both systems. However, establishing sufficiently long mutual coherence time remains challenging. This work describes a hybrid dual-comb spectrometer comprising a broadband fiber laser (FC) and an actively mode-locked semiconductor laser (MLL) with a narrow but tunable spectrum. A high mutual coherence time of around 100 seconds has been achieved by injection locking the MLL to a continuous laser (CW), which is locked on a single line of the FC. We have also devised a method to directly stabilize the entire spectrum of FC to a high finesse cavity. This results in a long term stability of 5 × 10⁻¹² at 1 second and 5 × 10⁻¹⁴ at 350 seconds. Additionally, we have addressed the effect of cavity dispersion on the locking quality, which is important for broadband comb lasers.

Controlling optical fields on the subwavelength scale is at the core of nanophotonics. Laser-driven nanophotonic particle accelerators promise a compact alternative to conventional radiofrequency-based accelerators. Efficient electron acceleration in nanophotonic devices critically depends on achieving nanometer control of the internal optical nearfield. However, these nearfields have so far been inaccessible due to the complexity of the devices and their geometrical constraints, hampering the design of future nanophotonic accelerators. Here we image the field distribution inside a nanophotonic accelerator, for which we developed a technique for frequency-tunable deep-subwavelength resolution of nearfields based on photon-induced nearfield electron-microscopy. Our experiments, complemented by 3D simulations, unveil surprising deviations in two leading nanophotonic accelerator designs, showing complex field distributions related to intricate 3D features in the device and its fabrication tolerances. We envision an extension of our method for full 3D field tomography, which is key for the future design of highly efficient nanophotonic devices.

Dual comb spectroscopy (DCS) is a broadband technique offering high resolution and fast data acquisition. We describe a hybrid dual-comb spectrometer comprising a broadband commercial fiber laser and an actively mode-locked semiconductor laser with a tunable and relatively narrow spectrum. We have devised a direct locking method, which leads to high long-term absolute stability (5 x 10⁻¹² at 1 second). Our hybrid DCS system exhibits a mutual coherence time of roughly 100 seconds, a significant improvement over current schemes. We demonstrate this by performing DCS on rubidium atoms at 313 K with different data integration times. Finally, we used the phase stable DCS system to perform time-resolved measurements, where the resolution is limited only by the laser repetition rate (4 ns) and observed Rabi oscillations of the rubidium atoms at room temperature. The high time resolution is achieved by performing a series of DCS measurements with varying numbers of laser pulses incident on the atoms. The temporal evolution of the population of atoms in the excited state is recovered by suitably combining the measurement data. This is the first demonstration of the DCS method with a few nanosecond resolutions without using pump-probe spectroscopy.

We present a comprehensive study of the temperature dependent electronic and optoelectronic properties of a tunnelling injection quantum dot laser. The optical power-voltage ( P opt – V ) characteristics are shown to be correlated with the current-voltage ( I – V ) and capacitance-voltage ( C – V ) dependencies at low and elevated temperatures. Cryogenic temperature measurements reveal a clear signature of resonant tunnelling manifested in periodic responses of the I – V and P opt – V characteristics, which diminish above 60 K. The C – V characteristics reveal a hysteresis stemming from charging and de-charging of the quantum dots, as well as negative capacitance. The latter is accompanied by a clear peak that appears at the voltage corresponding to carrier clamping, since the clamping induces a transient-like effect on the carrier density. C – V measurements lead also to a determination of the dot density which is found to be similar to that obtained from atomic force microscopy. C – V measurements enable also to extract the average number of trapped electrons in each quantum dot which is 0.95. As the important parameters of the laser have signatures in the electrical and electro-optical characteristics, the combination serves as a powerful tool to study intricate details of the laser operation.

Coherent control is a key experimental technique for quantum optics and quantum information processing. We demonstrate a new degree of freedom in coherent control of semiconductor quantum dot (QD) ensembles operating at room temperature using the tunneling injection (TI) processes in which charge carriers tunnel directly from a quantum well reservoir to QD confined states. The TI scheme was originally proposed and implemented to improve QD lasers and optical amplifiers, by providing a direct injection path of cold carriers thereby eliminating the hot carrier injection problem which enhances gain nonlinearity. The impact of the TI processes on the coherent time of the QDs was never considered, however. We show here that since the cold carriers that tunnel to the oscillating QD state are incoherent, the rate of injection determines the coherent time of the QDs thereby controlling coherent light–matter interactions. Coherent interactions by means of Rabi oscillations were demonstrated in absorption and for weak excitation pulses in the gain regime. However, Rabi oscillations are totally diminished under strong excitation pulses which increase the rate of stimulated emission, causing the tunneling processes to dominate what shortens the coherence time significantly. Since the tunneling rate, and hence, the coherence time, were controlled by the optical excitation and electrical bias, our finding paves the way for TI-based coherence switching on a sub-picosecond time scale in room-temperature semiconductor nanometric structures.

Tunneling-injection structures are incorporated in semiconductor lasers in order to overcome the fundamental dynamical limitation due to hot carrier injection by providing a carrier transport path from a cold carrier reservoir. The tunneling process itself depends on band alignment between quantum-dot levels and the injector quantum well, especially as in these devices LO-phonon scattering is dominant. Quantum dots with their first excited state near the quantum well bottom profit most from tunnel coupling. As inhomogeneous broadening is omnipresent in quantum dot structures, this implies that individual members of the ensemble couple differently to the injector quantum well. Quantum dots with higher energy profit less, as the phonon couples to higher, less occupied states. Likewise, if the energy difference between ground state and quantum well exceeds the LO phonon energy, scattering becomes increasingly inefficient. Therefore, within 20-30meV we find Quantum Dots that benefit substantially different from the tunnel coupling. Furthermore, in quantum dots with increasing confinement depth, excited states become sucessively confined. Here, scattering gets more efficient again, as subsequent excited states reach the phonon resonance with the quantum well bottom. Our results provide guidelines for the optimization of tunnel-injection lasers. Theoretical results for electronic state caluluations in connection with carrier-phonon and carrier-carrier scattering are compared to experimental results of the temporal gain recovery after a short pulse perturbation.