Adaptive pumping for spectral control of random lasers

A laser is not necessarily a sophisticated device: Pumping energy into an amplifying medium randomly filled with scatterers, a powder for instance, makes a perfect "random laser." In such a laser, the absence of mirrors greatly simplifies laser design, but control over emission directionality or frequency tunability is lost, seriously hindering prospects for this otherwise simple laser. Lately, we proposed a novel approach to harness random lasers, inspired by spatial shaping methods recently employed for coherent light control in complex media. Here, we experimentally implement this method in an optofluidic random laser where scattering is weak and modes extend spatially and strongly overlap, making individual selection a priori impossible. We show that control over laser emission can indeed be regained even in this extreme case by actively shaping the spatial profile of the optical pump. This unique degree of freedom, which has never been exploited, allows selection of any desired wavelength and shaping of lasing modes, without prior knowledge of their spatial distribution. Mode selection is achieved with spectral selectivity down to 0.06nm and more than 10dB side-lobe rejection. This experimental method paves the way towards fully tunable and controlled random lasers and can be transferred to other class of lasers.Comment: 23 pages, 7 figure

Joint Tensor Factorization and Outlying Slab Suppression with Applications

We consider factoring low-rank tensors in the presence of outlying slabs. This problem is important in practice, because data collected in many real-world applications, such as speech, fluorescence, and some social network data, fit this paradigm. Prior work tackles this problem by iteratively selecting a fixed number of slabs and fitting, a procedure which may not converge. We formulate this problem from a group-sparsity promoting point of view, and propose an alternating optimization framework to handle the corresponding $\ell_p$ ($0<p\leq 1$) minimization-based low-rank tensor factorization problem. The proposed algorithm features a similar per-iteration complexity as the plain trilinear alternating least squares (TALS) algorithm. Convergence of the proposed algorithm is also easy to analyze under the framework of alternating optimization and its variants. In addition, regularization and constraints can be easily incorporated to make use of \emph{a priori} information on the latent loading factors. Simulations and real data experiments on blind speech separation, fluorescence data analysis, and social network mining are used to showcase the effectiveness of the proposed algorithm

Towards on-chip generation, routing and detection of non-classical light

We fabricate an integrated photonic circuit with emitter, waveguide and detector on one chip, based on a hybrid superconductor-semiconductor system. We detect photoluminescence from self-assembled InGaAs quantum dots on-chip using NbN superconducting nanowire single photon detectors. Using the fast temporal response of these detectors we perform time-resolved studies of non-resonantly excited quantum dots. By introducing a temporal filtering to the signal, we are able to resonantly excite the quantum dot and detect its resonance uorescence on-chip with the integrated superconducting single photon detector.Comment: 9 pages, 5 figure

Unveiling the population of orphan Gamma Ray Bursts

Gamma Ray Bursts are detectable in the gamma-ray band if their jets are oriented towards the observer. However, for each GRB with a typical theta_jet, there should be ~2/theta_jet^2 bursts whose emission cone is oriented elsewhere in space. These off-axis bursts can be eventually detected when, due to the deceleration of their relativistic jets, the beaming angle becomes comparable to the viewing angle. Orphan Afterglows (OA) should outnumber the current population of bursts detected in the gamma-ray band even if they have not been conclusively observed so far at any frequency. We compute the expected flux of the population of orphan afterglows in the mm, optical and X-ray bands through a population synthesis code of GRBs and the standard afterglow emission model. We estimate the detection rate of OA by on-going and forthcoming surveys. The average duration of OA as transients above a given limiting flux is derived and described with analytical expressions: in general OA should appear as daily transients in optical surveys and as monthly/yearly transients in the mm/radio band. We find that ~ 2 OA yr^-1 could already be detected by Gaia and up to 20 OA yr^-1 could be observed by the ZTF survey. A larger number of 50 OA yr^-1 should be detected by LSST in the optical band. For the X-ray band, ~ 26 OA yr^-1 could be detected by the eROSITA. For the large population of OA detectable by LSST, the X-ray and optical follow up of the light curve (for the brightest cases) and/or the extensive follow up of their emission in the mm and radio band could be the key to disentangle their GRB nature from other extragalactic transients of comparable flux density.Comment: 9 pages, 4 figures, 2 tables. Accepted for publication by Astronomy and Astrophysic

Determination and evaluation of clinically efficient stopping criteria for the multiple auditory steady-state response technique

Background: Although the auditory steady-state response (ASSR) technique utilizes objective statistical detection algorithms to estimate behavioural hearing thresholds, the audiologist still has to decide when to terminate ASSR recordings introducing once more a certain degree of subjectivity. Aims: The present study aimed at establishing clinically efficient stopping criteria for a multiple 80-Hz ASSR system. Methods: In Experiment 1, data of 31 normal hearing subjects were analyzed off-line to propose stopping rules. Consequently, ASSR recordings will be stopped when (1) all 8 responses reach significance and significance can be maintained for 8 consecutive sweeps; (2) the mean noise levels were ≤ 4 nV (if at this “≤ 4-nV” criterion, p-values were between 0.05 and 0.1, measurements were extended only once by 8 sweeps); and (3) a maximum amount of 48 sweeps was attained. In Experiment 2, these stopping criteria were applied on 10 normal hearing and 10 hearing-impaired adults to asses the efficiency. Results: The application of these stopping rules resulted in ASSR threshold values that were comparable to other multiple-ASSR research with normal hearing and hearing-impaired adults. Furthermore, in 80% of the cases, ASSR thresholds could be obtained within a time-frame of 1 hour. Investigating the significant response-amplitudes of the hearing-impaired adults through cumulative curves indicated that probably a higher noise-stop criterion than “≤ 4 nV” can be used. Conclusions: The proposed stopping rules can be used in adults to determine accurate ASSR thresholds within an acceptable time-frame of about 1 hour. However, additional research with infants and adults with varying degrees and configurations of hearing loss is needed to optimize these criteria

Statistical PT-symmetric lasing in an optical fiber network

PT-symmetry in optics is a condition whereby the real and imaginary parts of the refractive index across a photonic structure are deliberately balanced. This balance can lead to a host of novel optical phenomena, such as unidirectional invisibility, loss-induced lasing, single-mode lasing from multimode resonators, and non-reciprocal effects in conjunction with nonlinearities. Because PT-symmetry has been thought of as fragile, experimental realizations to date have been usually restricted to on-chip micro-devices. Here, we demonstrate that certain features of PT-symmetry are sufficiently robust to survive the statistical fluctuations associated with a macroscopic optical cavity. We construct optical-fiber-based coupled-cavities in excess of a kilometer in length (the free spectral range is less than 0.8 fm) with balanced gain and loss in two sub-cavities and examine the lasing dynamics. In such a macroscopic system, fluctuations can lead to a cavity-detuning exceeding the free spectral range. Nevertheless, by varying the gain-loss contrast, we observe that both the lasing threshold and the growth of the laser power follow the predicted behavior of a stable PT-symmetric structure. Furthermore, a statistical symmetry-breaking point is observed upon varying the cavity loss. These findings indicate that PT-symmetry is a more robust optical phenomenon than previously expected, and points to potential applications in optical fiber networks and fiber lasers.Comment: Submitted to Nature Communications, Pages 1-19: Main manuscript; Pages 20-38: Supplementary material

A Software-Defined Channel Sounder for Industrial Environments with Fast Time Variance

Novel industrial wireless applications require wideband, real-time channel characterization due to complex multipath propagation. Rapid machine motion leads to fast time variance of the channel's reflective behavior, which must be captured for radio channel characterization. Additionally, inhomogeneous radio channels demand highly flexible measurements. Existing approaches for radio channel measurements either lack flexibility or wide-band, real-time performance with fast time variance. In this paper, we propose a correlative channel sounding approach utilizing a software-defined architecture. The approach enables real-time, wide-band measurements with fast time variance immune to active interference. The desired performance is validated with a demanding industrial application example.Comment: Submitted to the 15th International Symposium on Wireless Communication Systems (ISWCS 2018