Experimental investigation of the electrical and optical discharge characteristics is performed in the barrier discharge in nitrogen at 50 Hz. The time dependences of the current and the applied voltage are measured. Short-time photographs of the light intensity distribution in the discharge are obtained. When the frequency of the external voltage is low and dielectric barriers made of ‘Mylar’ are used, it is possible to obtain a homogeneous form of the discharge. During the pulse, the discharge current reaches the value of some amperes, and the maximum power is of the order of some tens of kilowatts. The appearance of a homogeneous discharge is interpreted as the widening of a single-electron avalanche due to photoemission. On the basis of a two-dimensional fluid model, the possibility of the initiation of the discharge by a narrow avalanche is demonstrated. It is shown that the observed discharge is a glow discharge, where the space charge plays a crucial role. The discharge passes successively through the Townsend phase, the streamer phase, the phase of radial expansion and, finally, the afterglow phase. The theory predicts that the distribution of the light intensity in the phase of discharge expansion must have the form of a ring widening in time. The properties of the power supply play an important role in the model. A comparison between the theoretical and experimental results shows good agreement.
In the past two decades high precision optical astronomical interferometry has benefited from the use of photonic technologies. Today, near-infrared interferometric instruments deliver high-resolution, hyperspectral images of astronomical objects and combine up to 4 independent telescopes at a time thanks to integrated optics (IO). Following the success of IO interferometry, several initiatives aim at developing components which could combine simultaneously more telescopes and extend their operation beyond the near-infrared bands. Here we report on the development of multi-telescope IO beam combiners for mid-infrared interferometry exploiting the three-dimensional (3D) structuring capabilities of ultrafast laser inscription. We characterise the capability of a 2-telescope and a 4-telescope beam combiner to retrieve the visibility amplitude and phase of monochromatic light fields at a wavelength of 3.39 µm. The combiner prototypes exploit different 3D architectures and are written with a femtosecond laser on substrates of Gallium Lanthanum Sulfide. Supporting numerical simulations of the performance of the beam combiners show that there is still room for improvement and indicate a roadmap for the development of future prototypes.
Planar microcavities with distributed Bragg reflectors (DBRs) host, besides confined optical modes, also mechanical resonances due to stop bands in the phonon dispersion relation of the DBRs. These resonances have frequencies in the 10- to 100-GHz range, depending on the resonator’s optical wavelength, with quality factors exceeding 1,000. The interaction of photons and phonons in such optomechanical systems can be drastically enhanced, opening a new route towards the manipulation of light. Here we implemented active semiconducting layers into the microcavity to obtain a vertical-cavity surface-emitting laser (VCSEL). Thereby, three resonant excitations—photons, phonons and electrons—can interact strongly with each other providing modulation of the VCSEL laser emission: a picosecond strain pulse injected into the VCSEL excites long-living mechanical resonances therein. As a result, modulation of the lasing intensity at frequencies up to 40 GHz is observed. From these findings, prospective applications of active optomechanical resonators integrated into nanophotonic circuits may emerge.
Context. Optical long baseline interferometry is a unique way to study astronomical objects at milli-arcsecond resolutions not attainable with current single-dish telescopes. Yet, the significance of its scientfic return strongly depends on a dense coverage of the uv-plane and a highly stable transfer function of the interferometric instrument. In the last few years, integrated optics (IO) beam combiners have facilitated the emergence of 4-telescope interferometers such as PIONIER or GRAVITY, boosting the imaging capabilities of the VLTI. However, the spectral range beyond 2.2 µm is not ideally covered by the conventional silica based IO. Here, we consider new laser-written IO prototypes made of gallium lanthanum sulfide (GLS) glass, a material that permits access to the mid-infrared spectral regime. Aims. Our goal is to conduct a full characterization of our mid-IR IO two-telescope coupler in order to measure the performance levels directly relevant for long-baseline interferometry. We focus in particular on the exploitation of the L and M astronomical bands. Methods. We use a dedicated Michelson-interferometer setup to perform Fourier transform spectroscopy on the coupler and measure its broadband interferometric performance. We also analyze the polarization properties of the coupler, the differential dispersion and phase degradation, as well as the modal behavior and the total throughput. Results. We measure broadband interferometric contrasts of 94.9% and 92.1% for unpolarized light in the L and M bands. Spectrally integrated splitting ratios are close to 50%, but show chromatic dependence over the considered bandwidths. Additionally, the phase variation due to the combiner is measured and does not exceed 0.04 rad and 0.07 rad across the L and M band, respectively. The total throughput of the coupler including Fresnel and injection losses from free-space is 25.4%. Furthermore, differential birefringence is low (<0.2 rad), in line with the high contrasts reported for unpolarized light. Conclusions. The laser-written IO GLS prototype combiners prove to be a reliable technological solution with promising performance for mid-infrared long-baseline interferometry. In the next steps, we will consider more advanced optical functions, as well as a fiberfed input, and we will revise the optical design parameters in order to further enhance the total throughput and achromatic behavior.
We use a picosecond acoustics technique to modulate the laser output of electrically pumped GaAs/AlAs micropillar lasers with InGaAs quantum dots. The modulation of the emission wavelength takes place on the frequencies of the nanomechanical extensional and breathing (radial) modes of the micropillars. The amplitude of the modulation for various nanomechanical modes is different for every micropillar which is explained by a various elastic contact between the micropillar walls and polymer environment.
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