The energy of picosecond pulses emitted by single-mode stripe Q-switched and gain-switched laser diodes typically does not exceeed 10 pJ. To obtain high-brightness picosecond pulses with energies as high as 50 pJ, more complex specialist constructions such as bow-tie lasers [ 11 or master laser -power amplifier systems [2] are required. Here, we propose to increase the energy of a picosecond pulse in a non-tapered, single-lobe laser by using an ion-implantation induced saturable absorber in the form of separate sections 3-5 pm long, positioned at regular intervals within the laser cavity. Such geometry allows the volume fraction of the absorber to be increased compared with the tandem construction with a lumped absorber whilst preserving the Qswitching efficiency, makes for higher injected carrier density, and enables the use of longer lasers, all of which result in a higher pulse energy.To create the 1.5-pm single-lobed Q-switched picosecond lasers with pulse energies over 50 pJ, we used a simple InGaAsP/InP double heterostructure with a 0.25 pm thick active layer. This choice of active layer thickness was a compromise between the capacity for storing large carrier densities in the active layer and preserving single transverse mode operation. The ultrafast saturable absorber was created by high-energy heavy ion implantation; this method has been established as highly suitable for structures with thick active layers [3]. To fabricate the multisection absorber, we used the surface (as opposed to facet) implantation method developed in our earlier work for quantum-well structures [4]. The implantation by Ar ions of an energy of 3.1 MeV using a 7 pm thick photoresist layer as the protective mask enabled us to fabricate absorbers of a virtually arbitrary geometry.In the experiments, we studied gain-guided lasers with a stripe width of 10 pm. Saturable absorber sections were located symmetrically in the laser cavity, and their number was varied from one to 20 (Fig. 1). The laser was pumped by current pulses of 3 ns FWHM. Fig. 2 shows the light-current characteristics of the 500pm long implanted lasers, with the total volume fraction of the absorber fixed at 20% and different absorber geometry. The curves for lasers with several absorber sections clearly display a characteristic kink which is the signature of the saturable absorber action and corresponds to a single pulse generation. For lasers with a single, lumped absorber, no pronounced Q-switching was observed in the entire range of currents studied. The variation in the (first) pulse energy with the number of absorber sections is shown in Fig.3, demonstrating pulse energies in excess of 100 pJ for some samples with multiple absorber sections. Auto-correlation measurements give the pulse width of 30-35 ps for a 500 pm long laser with 20 absorber sections. The lateral far field of such lasers has a half width of -9-10.5 O, which implies a pulse power of 3-5 W in a single lobe.We used the earlier developed [5] distributed time-domain model to simulate the variation...

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Objective.
Current-controlled neurostimulation is increasingly used in the clinical treatment of neurological disorders and widely applied in neural prostheses such as cochlear implants. Despite its importance, time-dependent potential traces of electrodes during microsecond-scale current pulses, especially with respect to an electrochemical reference scale, are not precisely understood. However, this knowledge is critical to predict contributions of chemical reactions at the electrodes, and ultimately electrode stability, biocompatibility, and stimulation safety and efficacy.
Approach.
We assessed the electrochemistry of neurostimulation protocols with Pt microelectrodes from millisecond (classical electroanalysis) to microsecond (neurostimulation) timescales. We developed a dual-channel instrumentation amplifier to include a reference electrode in neurostimulation setups. Uniquely, we combined potential measurements with potentiostatic prepolarization to control and investigate the surface status, which is not possible in typical stimulation setups.
Main results.
We thoroughly validated the instrumentation and highlighted the importance of monitoring individual electrochemical electrode potentials in different configurations of neurostimulation. We investigated electrode processes such as oxide formation and oxygen reduction by chronopotentiometry, bridging the gap between milli- and microsecond timescales. Our results demonstrate how much impact on potential traces the electrode’s initial surface state and electrochemical surface processes have, even on a microsecond scale.
Significance.
Our unique use of preconditioning in combination with stimulation reveals that interpreting potential traces with respect to electrode processes is misleading without rigorous control of the electrode’s surface state. Especially in vivo, where the microenvironment is unknown, simply measuring the voltage between two electrodes cannot accurately reflect the electrode’s state and processes. Potential boundaries determine charge transfer, corrosion, and alterations of the electrode/tissue interface such as pH and oxygenation, particularly in long-term in vivo use. Our findings are relevant for all use-cases of constant-current stimulation, strongly advocating for electrochemical in situ investigations in many applications like the development of new electrode materials and stimulation methods.