Arash Rahimi-Iman

Low threshold electrically pumped quantum dot-micropillar lasers

We report on low threshold lasing in electrically pumped quantum dot-micropillar cavities. Lasing action associated with threshold currents as low as 8μA at 10K is observed for micropillar cavities with quality factors exceeding 10.000. Due to an optimized contact scheme lasing is achieved for pillar structures with diameters as small as 1.5μm, containing on average less than 100 quantum dots in the active layer. The transition from spontaneous to stimulated emission is confirmed by autocorrelation measurements which reveal pronounced photon bunching near threshold.

Demonstration of strong coupling via electro-optical tuning in high-quality QD-micropillar systems.

We demonstrate electro-optical tuning of single quantum dots (QDs) embedded in high-quality (high-Q) micropillar cavities by exploiting the quantum confined Stark effect (QCSE). Combining electrically contacted high-Q micropillars and large In(0.3) Ga(0.7)As QDs with high oscillator strength facilitates the realization of strong coupling. In our experiments a single QD exciton was electrically tuned on resonance with a cavity mode of a micropillar with 1.9 microm diameter and a quality-factor (Q-factor) of 14,000 enabling the observation of strong coupling with a vacuum Rabi-Splitting of 63 microeV.

From polariton condensates to highly photonic quantum degenerate states of bosonic matter

Bose–Einstein condensation (BEC) is a thermodynamic phase transition of an interacting Bose gas. Its key signatures are remarkable quantum effects like superfluidity and a phonon-like Bogoliubov excitation spectrum, which have been verified for atomic BECs. In the solid state, BEC of exciton–polaritons has been reported. Polaritons are strongly coupled light-matter quasiparticles in semiconductor microcavities and composite bosons. However, they are subject to dephasing and decay and need external pumping to reach a steady state. Accordingly the polariton BEC is a nonequilibrium process of a degenerate polariton gas in self-equilibrium, but out of equilibrium with the baths it is coupled to and therefore deviates from the thermodynamic phase transition seen in atomic BECs. Here we show that key signatures of BEC can even be observed without fulfilling the self-equilibrium condition in a highly photonic quantum degenerate nonequilibrium system.

A 23-watt single-frequency vertical-external-cavity surface-emitting laser

We report on a single-frequency semiconductor disk laser which generates 23.6 W output power in continuous wave operation, at a wavelength of 1013 nm. The high output power is a result of optimizing the chip design, thermal management and the cavity configuration. By applying passive stabilization techniques, the free-running linewidth is measured to be 407 kHz for a sampling time of 1 ms, while undercutting 100 kHz in the microsecond domain.

Self-mode-locking semiconductor disk laser

The development of mode-locked semiconductor disk lasers received striking attention in the last 14 years and there is still a vast potential of such pulsed lasers to be explored and exploited. While for more than one decade pulsed operation was strongly linked to the employment of a saturable absorber, self-mode-locking emerged recently as an effective and novel technique in this field - giving prospect to a reduced complexity and improved cost-efficiency of such lasers. In this work, we highlight recent achievements regarding self-mode-locked semiconductor devices. It is worth to note, that although nonlinear effects in the active medium are expected to give rise to self-mode-locking, this has to be investigated with care in future experiments. However, there is a controversy whether results presented with respect to self-mode-locking truly show mode-locking. Such concerns are addressed in this work and we provide a clear evidence of mode-locking in a saturable-absorber-free device. By using a BBO crystal outside the cavity, green light originating from second-harmonic generation using the out-coupled laser beam is demonstrated. In addition, long-time-span pulse trains as well as radiofrequency-spectra measurements are presented for our sub-ps pulses at 500 MHz repetition rate which indicate the stable pulse operation of our device. Furthermore, a long-time-span autocorrelation trace is introduced which clearly shows absence of a pedestal or double pulses. Eventually, a beam-profile measurement reveals the excellent beam quality of our device with an M-square factor of less than 1.1 for both axes, showing that self-mode-locking can be achieved for the fundamental transverse mode.

Evolution of multi-mode operation in vertical-external-cavity surface-emitting lasers

The longitudinal multi-mode emission in a vertical-external-cavity surface-emitting laser is investigated using both single shot streak camera measurements and interferometric measurement techniques. For this, the laser is operated in the single- and two-color emission regime using both an etalon and a free-running configuration without etalon, respectively. The laser emission is analyzed with respect to pump power and output coupling losses for a long and for a short resonator. We observe a steep increase of emission bandwidth close to the laser threshold and monitor the transition between longitudinal single- and multi-mode operation. Additionally, the results indicate that a stable two-color operation is related to a sufficiently high number of oscillating longitudinal modes within each color.

Single-photon emission at a rate of 143 MHz from a deterministic quantum-dot microlens triggered by a mode-locked vertical-external-cavity surface-emitting laser

We report on the realization of a quantum dot (QD) based single-photon source with a record-high single-photon emission rate. The quantum light source consists of an InGaAs QD which is deterministically integrated within a monolithic microlens with a distributed Bragg reflector as back-side mirror, which is triggered using the frequency-doubled emission of a mode-locked vertical-external-cavity surface-emitting laser (ML-VECSEL). The utilized compact and stable laser system allows us to excite the single-QD microlens at a wavelength of 508 nm with a pulse repetition rate close to 500 MHz at a pulse width of 4.2 ps. Probing the photon statistics of the emission from a single QD state at saturation, we demonstrate single-photon emission of the QD-microlens chip with g(2)(0) < 0.03 at a record-high single-photon flux of (143 ± 16) MHz collected by the first lens of the detection system. Our approach is fully compatible with resonant excitation schemes using wavelength tunable ML-VECSELs, which will optimize ...

Mode-locked semiconductor disk lasers

This paper will review the recent advances in the field of ultrashort pulse generation from optically pumped vertical-external-cavity surface-emitting lasers (OP-VECSELs). In this review, we will summarize the most significant results presented over the last 15 years, before highlighting recent breakthroughs related to mode-locked VECSELs by different research groups. Different mode-locking techniques for OP-VECSELs are described in detail. Previously, saturable absorbers, such as semiconductor saturable absorber mirrors—external, or internal as in mode-locked integrated external-cavity surface emitting lasers (MIXSEL)—, and recently, novel-material-based carbon-nanotube and graphene saturable absorbers have been employed. A new mode-locking method was presented and discussed in recent years. This method is referred to as self-mode-locking or saturable-absorber-free operation of mode-locked VECSELs. In this context, we particularly focus on achievements regarding self-mode-locking, which is considered a promising technique for the realization of high-power, compact, robust and cost-efficient ultrashort pulse lasers. To date, the presented mode-locking techniques have led to great enhancement in average powers, peak powers, and repetition rates that can be achieved with passively mode-locked VECSELs.

High-Power Quantum-Dot Vertical-External-Cavity Surface-Emitting Laser Exceeding 8 W

We report on a record-high output power from an optically pumped quantum-dot vertical-external-cavity surface-emitting laser, optimized for high-power emission at 1040 nm. A maximum continuous-wave output power of 8.41 W is obtained at a heat sink temperature of 1.5°C. By inserting a birefringent filter inside the laser cavity, a wavelength tuning over a range of 45 nm is achieved.

Dual-Wavelength Emission From a Serially Connected Two-Chip VECSEL

We present a compact two-chip vertical-external-cavity surface-emitting laser design, which provides dual-wavelength emission with a wavelength separation of 10 nm. The design is ideal for type-I frequency conversion, since the two wavelengths exhibit the same polarization, and over 600-W intracavity power is generated. Dual-wavelength operation with other desirable wavelength difference can be achieved in this flexible cavity, by using different chip combinations and suitable filters.