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An active disordered medium, i.e., a diffusive material where a population inversion is established by a pumping system, can emits a optical radiation known as random laser. Since such a optical source is very sensitive to the scattering, a sensor based on random lasing can be developed to investigate diffusive samples. Here we report a design of a sensor with non-invasive characteristics achieved by means of separation between the active medium and the investigated sample.

A random laser is an optical system where the light is amplified by stimulated emission along random paths in a disordered medium. In recent years, a new kind of non-invasive sensor based on random lasing has been proposed. The striking point is that a sensor based on random lasing has an emission "fed" by the feedback due to the scattering properties of the medium, making such a system a natural candidate for studying materials with strong disorder. Here, we report the recent advances in the sensor structure and performances.

Optical sensing is a very important method for investigating different kinds of samples. Recently, we proposed a new kind of optical sensor based on random lasing [ Sci. Rep.6, 35225 (2016)], that couples the advantages of stimulated emission in detecting small variations on scattering properties of a sensed material, to the needs of no alteration of the sample under investigation. Here, we present a method to achieve a quantitative measurement of the scattering properties of a material. The results on samples of calibrated microspheres show a dependence of the peak intensity of the emission spectrum on the transport mean free path of the light within the sample, whatever the dimension (down to ≈100 nm of particle diameter) and the concentration of scatterers dispersed in the sensed material. A direct and fast measurement of the scattering properties is obtained by calibration with a well-known and inexpensive reference medium.

We provide theoretical and experimental evidence of statistical outliers in random laser emission that are not accounted for by the, now established, power-law tailed (L\'evy) distribution. Such outliers manifest themselves as single, large isolated spikes over an otherwise smooth background. A statistical test convincingly shows that their probability is larger than the one extrapolated from lower-intensity events. To compare with experimental data, we introduced the anomaly parameter that allows for an identification of such rare events from experimental spectral measurements and that agrees as well with the simulations of our Monte Carlo model. A possible interpretation in terms of Black Swans or Dragon Kings, large events having a different generation mechanism from their peers, is discussed.

We report experimental results on the propagation temporal characteristics of the precursor in an inhomogeneous sample. The transient behavior of a step-like pulse in an atomic hot medium is two orders of magnitude faster than the radiative broadened case up to now presented in the literature. Moreover, we show the dependence on the resonant or nonresonant condition. Numerical simulations compare favorable to experimental results.

Optical sensing has been subject to a great interest for the moderate intrusiveness of its operation. The introduction of random lasers in ’90s has opened the door for developing a new kind of optical sensors. In such a source, disorder is introduced within an inverted medium, increasing the lifetime of the radiation without the presence of an optical cavity. The striking point is that the spectral characteristics of the output emission are strongly dependent on the scattering properties of the medium, suggesting new methods to investigate disordered materials. Recently, a novel concept for optical sensing based on the physics of random laser has been reported,1 overcoming the limits due to the alteration of the investigated sample by injecting an active material. Here we present a characterization of such a kind of sensor, suggesting non-invasive and also in-vivo applications.

Random lasers are optical sources where light is amplified by stimulated emission along random paths through an amplifying scattering medium. Connections between their physics and the one of quenched disordered nonlinear systems, notably spin glasses, have been recently suggested. Here we report a first experimental study of correlations of spectral fluctuations intensity in a random laser medium where the scatterers displacement significantly changes among consecutive shots. Remarkably, our results reveal that the replica symmetry breaking (RSB) phenomenology is robust with respect to an averaging over different realizations of the disorder. Moreover, besides opening new intriguing questions about the understanding of such a phenomenon, this work aims to clarify the connection between the RSB with the onset of the Lévy regime, i.e. the fluctuations regime that is a peculiar feature of the random lasing under critical conditions. Our results suggest that the former occurs independently of the latter and then the RSB phenomenology is a generic feature linked to the random laser threshold.

In a random laser the optical feedback is provided by scattering rather than by an optical cavity. Then, since its emission characteristics are very susceptible to the scattering details, it is a natural candidate for making active sensors to use as a diagnostic tool for disordered media like biological samples. However, the methods reported up to now, requiring the injection of toxic substances in the sample, have the drawback of altering the physical-chemical composition of the medium and are not suitable for in-vivo measurements. Here we present a random laser based sensor that overcomes these problems by keeping gain and diffusion separated. We provide an experimental characterisation of the sensor by using a reference diffusive liquid phantom and we show that, compared to a passive method, this sensor takes advantage of the gain and spectral properties of the random laser principle.

We show how the velocity of an optical pulse propagating through a dispersive medium depends on the pulse duration. A transition from the group velocity for long pulses to the in-vacuum velocity for short pulses is shown both in experimental results and in theoretical predictions. The temporal duration of the experimental pulses are 150 ps and 3.5 ns. A description of the pulse propagation in terms of the time “center of mass” of the energy flow allows an intuitive overview of the results.

We present a laser source with several desirable characteristics, such as tunable wavelength in the near infrared, single-longitudinal-mode emission and variable pulse temporal duration in the nanosecond regime. The laser is based on an injection-seeded Titanium-doped-sapphire ring cavity. Our experiments show how the pulse duration can be varied in a controllable fashion either by changing the cavity length or by changing the pump energy. We present a theoretical model which successfully reproduces the experimental results by treating the operation of this type of laser as a gain-switching technique. As far as the stabilization of the laser cavity, we also present a novel solution involving the use of an avalanche photodiode.

We built and characterized a high-energy, injection-seeded, single-longitudinal-mode pulsed titanium-doped-sapphire laser with tuneable wavelength in the near-infrared and variable pulse temporal duration in the nanosecond regime. We show experimentally how the pulse duration can be easily varied either by changing the cavity length or by changing the pump energy. We successfully interpreted these results on the basis of a theoretical model which treats the operation of this type of laser as a gain-switching technique. Also, as far as the stabilization of the laser cavity, compared to the traditional techniques we employed a novel simplified solution involving the use of an avalanche photodiode.

Unlike a conventional optical cavity laser, a random laser system is generally characterized by a nondirectional output emission. In this work we report an experimental and theoretical study on the angular properties of the random laser emission and its dependence on the diffusive properties of the sample and the spatial gain profile, showing the possibility of controlling it by the total amount of available energy. We show that the directional characteristics are associated with the statistical regime of fluctuations in the emission spectrum and, in particular, with a L\'evy statistical regime. A model based on a phase-insensitive feedback mechanism has been used in order to explain the experimental data and two different L\'evy subregimes have been identified.

We present an experimental realization of slow and fast light schemes for a few ns long optical pulses that makes use of incoherent interactions in an atomic medium. The combination of such different schemes allows us to demonstrate that the propagation delay acquired in the slow light stage, can be completely recovered in a fast light one. The use of an incoherent interactions scheme makes the control of the propagation dynamics of light pulses easer to realize. Delays up to 13 ns, in slow light regime, and advances up to 500 ps, in fast light regime, are reported when the stages work individually for a 3 ns long pulse. When both stages are switched-on the fast light stage is able to recover a previously induced delay and even to produce an extra advance, with an overall advance up to 1 ns. Since every optical transmission line needs an amplification system to overcome the unavoidable losses, the results suggest the opportunity and perspective of a proper tailoring of the amplification stage for data timing purposes.

The random laser has represented an important topic in photonics and an opportunity to develop new potential practical applications. Our work, both experimental and theoretical, has been devoted to characterize, understand and control the emission in random laser systems in weakly scattering media. Three statistical regimes of the emission spectrum have been detected, besides modifications in the output directionality. Moreover, here we report our preliminary results in the field of sensing, that can open new promising perspectives.

We present a theoretical and experimental study aimed at characterizing statistical regimes in a random laser. Both the theoretical simulations and the experimental results show the possibility of three region of fluctuations increasing the pumping energy. An initial Gaussian regime is followed by a L\'evy statistics and Gaussian statistic is recovered again for high pump pulse energy. These different statistical regimes are possible in a weakly diffusive active medium, while the region of L\'evy statistics disappears when the medium is strongly diffusive presenting always a Gaussian regime with smooth emission spectrum. Experiments and theory agree in identification of the key parameters determining the statistical regimes of the random laser.

We present an experimental study of temporal retardation combined with temporal compression of weak laser pulses. The effect is optically controlled by the presence of another light pulse, although no coherent control is required. Probe pulse delays up to 13 ns and maximum compression factor of 3 were observed using few-ns-long laser pulses. We also present a theoretical model of pulse propagation that is able to reproduce the experimental results with fairly good accuracy.

We present an experimental and theoretical investigation, performed on hot sodium atoms in a ladder scheme, showing the control of the absorption and of the propagation velocity of a probe light pulse with a spectral bandwidth as large as 1.8 GHz. The predictions of the theoretical model compare favorably with the experimental results.

We present a theoretical and experimental study on the possibility of spectral manipulation of weak probe-laser pulses in the presence of dynamical electromagnetically induced transparency. We predict a spectral enlargement or narrowing process depending on whether the probe-laser pulse is overlapped by the rising or the falling edge of the coupling pulse, respectively. The results of an experiment in sodium atomic vapors confirm the theoretical predictions.

We present recent results on different schemes (involving both coherent and incoherent interactions) that allow all-optical control of several properties of a large-spectral-bandwidth (up to 3.3 GHz) laser pulse propagating in an atomic medium.

We present recent theoretical and experimental results concerning both retardation and acceleration of light pulses in schemes involving a second ‘control’ laser field but that do not involve any coherent preparation of the atomic medium.

We report the experimental observation of an optically controllable shift of the central frequency of a laser pulse, using a scheme based on dynamical electromagnetically induced transparency. This is evidence of frequency shift controllable by a coherent process. Original theoretical results are in agreement with the experimental data.

In this letter we report the first experimental observation of temporal delay control of large-spectral-bandwidth multimode laser pulses by means of electromagnetically induced transparency (EIT). We achieved controllable retardation with limited temporal distortion of optical pulses with an input spectral bandwidth of 3.3 GHz. The experimental results compare favorably with theoretical predictions., Submitted to Optics Letters (January 2009)

Causality and special relativity pose an upper limit to the amount of advance that an optical pulse can acquire during a superluminal propagation. Such a limit can be circumvented if the pulse, before entering the superluminal medium, is retarded by letting it propagate under normal dispersion. We present an experimental evidence of this fact by showing that a laser pulse propagating in an atomic vapor, quasi resonant with an inverted transition and in conditions of anomalous dispersion, moves faster if it is previously retarded in a cell containing the same medium with no population inversion. Optical transmission lines often need an amplification stage to overcome the signal attenuation and the unavoidable delay respect to propagation at c; in this paper we tailor such stage to provide also an optical controlled recover of such delay. We believe that our results can open exciting prospects for real-life optical data processing and communication.