Silvia Viciani

Single-photon excitation of a coherent state : Catching the elementary step of stimulated light emission

When a single quantum of electromagnetic field excitation is added to the same spatiotemporal mode of a coherent state, a new field state is generated that exhibits intermediate properties between those of the two parents. Such a single-photon-added coherent state is obtained by the action of the photon creation operator on a coherent state and can thus be regarded as the result of the most elementary excitation process of a classical light field. Here we present and describe in depth the experimental realization of such states and their complete analysis by means of a novel ultrafast, time-domain, quantum homodyne tomography technique clearly revealing their nonclassical character.

Lineshape of a vertical cavity surface emitting laser

We report on the experimental study of the lineshape of an air-post GaAs/AlGaAs Vertical-Cavity Surface-Emitting Laser (VCSEL). The signal is obtained by heterodyne with a narrow linewidth extended-cavity laser. This technique allows to fully take into account the entire spectrum of frequency fluctuations. In this way we can distinguish a Lorentzian and a Gaussian contributions to the lineshape, obtaining a clear picture of the dependence of the emission spectrum on the pump current. The study also includes the effects on the laser lineshape of the secondary transverse modes fluctuations.

Frequency noise and lineshape of VCSELs

The study of the lineshape of semiconductor lasers is very interesting, being related to phase and frequency noise sources which are usually hidden in other kinds of laser. The importance of this topic for Vertical Cavity Surface Emitting Lasers (VCSELs) is further increased by their large impact in communication applications, since frequency and phase noise limits the performances of different optical communication techniques. We have performed such a study on an air-post AlAs/AlGaAs VCSEL. We have recorded the lineshape at different injection current levels by heterodyne with a narrow linewidth extended-cavity laser, while the frequency noise spectrum is investigated using a Fabry-Perot cavity as frequency discriminator. In single-mode emission conditions, the lineshape is Lorentzian at low pump current, while a Gaussian contribution is evident for higher pump level. The Lorentzian linewidth is inversely proportional to the laser power and can be compared with the results of the Schawlow-Townes-Henry theory properly modified to consider the particular structure of VCSELs. The study of the frequency noise shows that a quasi-Gaussian lineshape is due to an excess low frequency noise. This contribution has a 1/f^n dependence, with n around 1, for frequencies higher than 20 kHz and is flat at lower frequencies. This peculiar power spectrum has been observed in the electric noise of AlGaAs Bragg reflectors. The current noise generated in the Bragg mirrors is the source of the frequency noise through the fluctuations of the cavity optical length. The results are extended to other kinds of VCSELs.

Noise characterization of a coherent tunable far infrared spectrometer

The different noise contributions in a coherent tunable far infrared spectrometer were measured. On the basis of this quantitative analysis, several experimental schemes to enhance the sensitivity for spectroscopy in this spectral region are proposed. In particular, a full theoretical analysis for differential detection is developed. A double-detector scheme is theoretically analyzed and it is shown that a noise reduction of up to 17 dB is achievable with this configuration. The present work should make possible quantitative forecasting of sensitivity enhancement for new, different experimental configurations.

Recurrent fourth-order interference dips and peaks with a comblike two-photon entangled state

We demonstrate full selective control over the constructive or destructive character of fourth-order recurrent interferences in a modified version of a Hong-Ou-Mandel interferometer using comblike two-photon states. The comb spectral/temporal structure is obtained by inserting an etalon cavity in the signal path of an entangled photon pair obtained by pulsed spontaneous parametric down-conversion. Both a simple qualitative discussion and a complete theoretical derivation are used to explain and analyze the experimental data.

Spectroscopic observation of the Faraday effect in the far infrared

The Faraday effect was observed in the far-infrared spectrum of NO(2) for what is believed to be the first time. A systematic investigation of the magneto-optical signal as a function of the uncrossing angle between the polarizers was performed. Differences from similar experiments performed in the infrared and the visible regions were encountered. Moreover, a fit of the experimental data allowed calculation of the difference in the real and the imaginary terms of the refractive index for DM(F)=+1 and DM(F)=-1 multiplets.

Disorder and dephasing as control knobs for light transport in optical fiber cavity networks

Transport phenomena represent a very interdisciplinary topic with applications in many fields of science, such as physics, chemistry, and biology. In this context, the possibility to design a perfectly controllable experimental setup, where to tune and optimize its dynamics parameters, is a challenging but very relevant task to emulate, for instance, the transmission of energy in light harvesting processes. Here, we experimentally build a scalable and controllable transport emulator based on optical fiber cavity networks where the system noise parameters can be finely tuned while maximizing the transfer efficiency. In particular, we demonstrate that disorder and dephasing noise are two control knobs allowing one to play with constructive and destructive interference to optimize the transport paths towards an exit site. These optical setups, on one side, mimic the transport dynamics in natural photosynthetic organisms and, on the other, are very promising platforms to artificially design optimal nanoscale structures for novel, more efficient, clean energy technologies.

Characteristic vibrational frequencies of toxic polychlorinated dibenzo-dioxins and -furans.

The possibility to monitor in real-time the emission of dioxins produced by incineration of waste or by industrial processes is nowadays a necessity considering the high toxicity of these compounds, their persistence in the environment and their ability to bio-accumulate in the food chain. Recently it has been demonstrated the potentiality of detecting dioxins in carbon tetrachloride via MIR Quantum Cascade Lasers. A fundamental step in real time monitoring of dioxins emission is the possibility to recognize the most toxic congeners within complex mixtures and at low concentrations. Taking into account the lack of spectroscopic data about these very toxic environmental pollutants and the necessity to monitor their emissions we have recorded infrared spectra of 13 of the 17 most toxic congeners of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs) dissolved in carbon tetrachloride. In this way we have obtained a small database that we have used to test the ability of a linear regression algorithm to recognize each congener and its relative concentration in complex mixtures of these compounds.

Characterization of the HCl-HBr-HI gas absorption cell for GIANO-TNG

GIANO is an high resolution cross-dispersed spectrometer operating at near IR wavelengths (0.9-2.5 microns) which will be soon commissioned at the 3.6m TNG Italian telescope in La Palma. One of its most ambitious aims is searching for earth-like planets with habitable conditions around very cool main sequence stars. This requires measurements of radial velocities with accuracies of a few m/s which can be achieved by means of a gas absorption cell containing a mixture of the halogen-hydrates HCl, HBr and HI. We present here the results of the laboratory work for the construction and characterization of such cell.

A Portable Quantum Cascade Laser Spectrometer for Atmospheric Measurements of Carbon Monoxide

Trace gas concentration measurements in the stratosphere and troposphere are critically required as inputs to constrain climate models. For this purpose, measurement campaigns on stratospheric aircraft and balloons are being carried out all over the world, each one involving sensors which are tailored for the specific gas and environmental conditions. This paper describes an automated, portable, mid-infrared quantum cascade laser spectrometer, for in situ carbon monoxide mixing ratio measurements in the stratosphere and troposphere. The instrument was designed to be versatile, suitable for easy installation on different platforms and capable of operating completely unattended, without the presence of an operator, not only during one flight but for the whole period of a campaign. The spectrometer features a small size (80 × 25 × 41 cm3), light weight (23 kg) and low power consumption (85 W typical), without being pressurized and without the need of calibration on the ground or during in-flight operation. The device was tested in the laboratory and in-field during a research campaign carried out in Nepal in summer 2017, onboard the stratospheric aircraft M55 Geophysica. The instrument worked extremely well, without external maintenance during all flights, proving an in-flight sensitivity of 1–2 ppbV with a time resolution of 1 s.