Mike van der Poel

Slow light in a semiconductor waveguide at gigahertz frequencies

We experimentally demonstrate slow-down of light by a factor of three in a 100 microm long semiconductor waveguide at room temperature and at a record-high frequency of 16.7 GHz. It is shown that the group velocity can be controlled all-optically as well as through an applied bias voltage. A semi-analytical model based on the effect of coherent population oscillations and taking into account propagation effects is derived and is shown to well account for the experimental results. It is shown that the carrier lifetime limits the maximum achievable delay. Based on the general model we analyze fundamental limitations in the application of light slowdown due to coherent population oscillations.

Controllable delay of ultrashort pulses in a quantum dot optical amplifier

Optical and electrical tuning of the propagation time of 170 fs pulses in a quantum dot semiconductor amplifier at room temperature is demonstrated. Both pulse slowdown and advancement is possible and we achieve fractional delays (delay divided with pulse duration) of up to 40%. The results are explained by a simple gain saturation model.

Measuring the dynamics of second-order photon correlation functions inside a pulse with picosecond time resolution

We present a detailed discussion of a recently demonstrated experimental technique capable of measuring the correlation function of a pulsed light source with picosecond time resolution. The measurement involves a streak camera in single photon counting mode, which is modified such that a signal at a fixed repetition rate, and well defined energy, can be monitored after each pulsed laser excitation. The technique provides further insight into the quantum optical properties of pulsed light emission from semiconductor nanostructures, and the dynamics inside a pulse, on the sub-nanosecond time scale.

Non-invasive bleaching of the human lens by femtosecond laser photolysis.

Background Globally, cataract is the leading cause of blindness and impaired vision. Cataract surgery is an attractive treatment option but it remains unavailable in sufficient quantity for the vast majority of the world population living in areas without access to specialized health care. Reducing blindness from cataract requires solutions that can be applied outside operating theatres. Cataract is a protein conformational disease characterized by accumulation of light absorbing, fluorescent and scattering protein aggregates. The aim of the study was to investigate whether these compounds were susceptible to photobleaching by a non-invasive procedure and whether this would lead to optical rejuvenation of the lens. Methodology/Principal Findings Nine human donor lenses were treated with an 800 nm infra-red femtosecond pulsed laser in a treatment zone measuring 1×1×0.52 mm. After laser treatment the age-induced yellow discoloration of the lens was markedly reduced and the transmission of light was increased corresponding to an optical rejuvenation of 3 to 7 years. Conclusions/Significance The results demonstrate that the age-induced yellowing of the human lens can be bleached by a non-invasive procedure based on femtosecond laser photolysis. Cataract is a disease associated with old age. At the current technological stage, lens aging is delayed but with a treatment covering the entire lens volume complete optical rejuvenation is expected. Thus, femtosecond photolysis has the potential clinical value of replacing invasive cataract surgery by a non-invasive treatment modality that can be placed in mobile units, thus breaking down many of the barriers impeding access to treatment in remote and poor regions of the world.

Statistical surface recovery: a study on ear canals

We present a method for surface recovery in partial surface scans based on a statistical model. The framework is based on multivariate point prediction, where the distribution of the points are learned from an annotated data set. The training set consist of surfaces with dense correspondence that are Procrustes aligned. The average shape and point covariances can be estimated from this set. It is shown how missing data in a new given shape can be predicted using the learned statistics. The method is evaluated on a data set of 29 scans of ear canal impressions. By using a leave-one-out approach we reconstruct every scan and compute the point-wise prediction error. The evaluation is done for every point on the surface and for varying hole sizes. Compared to state-of-the art surface reconstruction algorithm, the presented methods gives very good prediction results.

Control of ultrafast pulse propagation in semiconductor components

Time shifting of optical pulses with duration in the range from 100 fs to a few ps represents one extreme of slow light, where THz bandwidth for the slow down or speed up is necessary. The physics of the time shifting of such very short pulses involves the gain saturation of the optical medium and is different from the slow-light mechanisms responsible for time shifting of pulses of narrower bandwidth. Experimental and theoretical results with semiconductor components are presented, emphasizing the physics as well as the limitations imposed by the dynamical processes.

Photobleaching of a human donor lens using an 800 nm femtosecond Ti:Sapphire laser

Photobleaching of a 58 year old human donor lens was demonstrated using an infra-red, femtosecond Ti:Sapphire laser. Pulse duration was 300 femtoseconds, pulse energy was 0.1 μJ, and the focal spotsize of the laser was approximately 14 μm in diameter. The lens was treated in a 1x1 mm large area by scanning the laser beam. Significant photobleaching was seen after laser treatment. Light transmission increased by 7%. The greatest effect was seen in the blue-green part of the visible spectrum.

Experimental Observation of Pulse Delay and Speed-up in Cascaded Quantum Well Gain and Absorber Media

Slow-down and speed-up of 180 fs pulses in semiconductor waveguides beyond the existing models is observed. Cascaded gain and absorbing sections is shown to provide significant temporal pulse shifting at near constant output pulse energy.

Femtosecond laser-induced cavitations in the lens of the human eye

Ultrafast femtosecond lasers are used increasingly for a wide range of medical purposes. The immediate tissue response to pulses above a certain threshold is optically or laser induced breakdown, which is often visible as gas-filled cavities that persist for some time. In the present study, we attempted to define the cavitation threshold in the human lens in vitro using multiphoton effects based on radiation from a femtosecond 800 nm Ti:Sapphire laser. Cavitations were observed from pulse energy densities exceeding 16 mJ/cm2, but only after several minutes of exposure and not as a result of a single laser pulse. This suggests that cavitations were caused by a process which differs from the single-pulse cavitations observed at higher intensities. To evaluate whether the release of gas was caused by ionization and plasma formation or by thermal effects, we introduced pauses into the pulse train, which did not change the total exposure time needed to form a cavitation. This suggests that local heating did not play a significant role in producing the observed phenomenon, suggesting that photochemical reactions may be involved. These results demonstrate that there are several types of ultrafast laser effects in the lens that have a potential for therapeutic application and treatment of eye disease though further studies are needed to shed light on the nature of the formation of delayed cavitations.

Coherent modulation of microcavity‐polaritons by acoustic phonons

We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. The results are in good agreement with model calculations.