Observation of image pair creation and annihilation from superluminal scattering sources

The invariance of the speed of light is one of the foundational pillars of our current understanding of the universe. It implies a series of consequences related to our perception of simultaneity and, ultimately, of time itself. Whereas these consequences are experimentally well studied in the case of subluminal motion, the kinematics of superluminal motion lack direct evidence or even a clear experimental approach. We investigate kinematic effects associated with the superluminal motion of a light source. By using high-temporal-resolution imaging techniques, we directly demonstrate that if the source approaches an observer at superluminal speeds, the temporal ordering of events is inverted and its image appears to propagate backward. Moreover, for a source changing its speed and crossing the interface between subluminal and superluminal propagation regions, we observe image pair annihilation and creation, depending on the crossing direction. These results are very general and show that, regardless of the emitter speed, it is not possible to unambiguously determine the kinematics of an event from imaging and time-resolved measurements alone. This has implications not only for light, but also, for example, for sound and other wave phenomena

Action recognition using single-pixel time-of-flight detection

Action recognition is a challenging task that plays an important role in many robotic systems, which highly depend on visual input feeds. However, due to privacy concerns, it is important to find a method which can recognise actions without using visual feed. In this paper, we propose a concept for detecting actions while preserving the test subject's privacy. Our proposed method relies only on recording the temporal evolution of light pulses scattered back from the scene. Such data trace to record one action contains a sequence of one-dimensional arrays of voltage values acquired by a single-pixel detector at 1 GHz repetition rate. Information about both the distance to the object and its shape are embedded in the traces. We apply machine learning in the form of recurrent neural networks for data analysis and demonstrate successful action recognition. The experimental results show that our proposed method could achieve on average 96.47 % accuracy on the actions walking forward, walking backwards, sitting down, standing up and waving hand, using recurrent neural network

Superluminaalselt levivad lokaliseeritud valgusimpulsid

Käesolev uurimistöö kuulub füüsikalise optika valdkonda ja tegeleb ülilühikeste optiliste ehk valgusimpulsside levi uurimisega. 1983. aastal avaldas ameerika teadlane James Neill Brittingham uurimistöö, mille tihedasse ja keerukasse valemirägasse oli sügavale peidetud info, et elektromagnetlained võivad moodustada ülilühikesi impulsse (ehk ‘lainepakette’), mis on võimelised läbima pikki vahemaid ilma kuju muutmata ehk levi-invariantselt ning mille intensiivsusmaksimumi ruumilised mõõtmed on samas suurusjärgus lainepikkusega, ehk kümme korda väiksemad juuksekarva ristlõikest. See on märkimisväärne eriti seetõttu, et üldreeglina kõik lainepaketid, nagu ka näiteks laseri fokuseeritud kiirtekimp, valguvad levides laiali seda rohkem, mida väiksemad olid nende algsed mõõtmed. Levi-invariantsed laineväljad ehk lokaliseeritud lained leiavad rakendust näiteks mikroosakestega ning -organismidega manipuleerimise metoodikais, fluorestsentsmikroskoopias, mittelineaarses ja ülikiires optikas, plasmakanali juhtimiseks mööda kindlat trajektoori aga ka aatomite optiliseks lõksustamiseks ning nii optilise kui ultraheli kujutise edasikandmiseks. Märkimisväärne ja kirjanduses diskussiooni põhjustanud on ka asjaolu, et teatavat tüüpi lokaliseeritud lainete rühmakiirus on suurem valguse kiirusest (samas keskkonnas) ehk superluminaalne. Nii nagu levi-invariantsus, ei ole superluminaalne rühmakiirus vastuolus fundamentaalsete füüsikaseadustega, vaid vastupidi – sügavamal analüüsil ikkagi kooskõlas nendega. Doktoritöös on uuritud ülevalgusekiirusega levivaid lokaliseeritud laineid, välja on töötatud meetod nende eksperimentaalseks moodustamiseks ning läbi viidud nn Bessel-X impulsi ja selle kiirenevalt ja aeglustuvalt levivate analoogide kõrgrlahutusega ajalis-ruumilised mõõtmised. Muuhulgas on otseste katsete ja mõõtmistega demonstreeritud, et ülilühikese valgusimpulsi difrageerumisel ümmarguselt avalt, kettalt või pilult moodustub tõkke taga superluminaalselt, aeglustuvalt liikuv Besseli-tüüpi lainepakett, mis on klassikalises optikas tuntud Arago-Poissoni täpi impulss-versioon.The thesis belongs to the field of physical optics. In the thesis the propagation of ultrashort optical pulses has been studied. In year 1983 an American scientist James Neill Brittingham published a very theoretical paper announcing three-dimensional classical electromagnetic pulses, which propagate invariantly without any distortion or spread over long distances. The intensity maximum of the novel pulses was tightly localized to a “bulletlike” core which dimensions are in order of a wavelength, hence remain smaller than a cross-section of a human hair. The result was startling because in general all wave packets, among those also a focused laser beam, spread out in the course of propagation. The smaller the initial spatial dimensions of the wave packet, the larger the spread will be. The propagation-invariant pulses or the localized waves have applications in optical trapping and particle micromanipulation, in fluorescence microscopy, in nonlinear and ultrafast optics, for guiding plasma channel, as optical atom guide and for transfer of optical or ultrasound images. The fact that the group velocity of localized waves of certain type is superluminal, has started debates among scientific community. Similar to the propagation invariance, the superluminal group velocity is not in contradiction with fundamental laws of physics. Instead, the deeper analysis proves otherwise. In the thesis the superluminally propagating localized waves have been studied. A method for their experimental implementation has been proposed and analyzed. A localized wave called Bessel-X pulse and accelerating and decelerating Bessel-type waves have been generated experimentally and measured spatiotemporally with high resolution. Moreover, formation of superluminally propagating and decelerating Bessel-type pulses in the diffraction process was explicitly demonstrated by propagating ultrashort pulses through circular apertures, disks and slits. These Bessel-type pulses are in that case pulsed versions of the well-known Arago-Poisson spot

Graphologues: image sociale et statut professionnel. La graphologie

The social image and occupational status of the graphologist are investigated by examining published data. Three occupational groups are cited to show that image building is complex. These groups are detectives, psychologists and graphoanalysts. Such examples illustrate how status can be modified by unexpected people (e.g. writers of fiction and politicians). They also show that the occupation itself can help to shape reputation. There are obstacles to measuring image in graphology, but an attempt is made to identify countries where graphology is likely to be most well known. The calculation is based on the assumption that the level of awareness is related to the number of practitioners operating in a given country. Important countries are identified as Switzerland, France, Israel, Italy and the Netherlands. It is concluded that the image of the graphologist is fragmented and inconsistent. Part of the population is unaware of its nature or its existence. When it is known it is associated with such diverse groups as psychologists, questioned document examiners and "occult" practitioners. These facts do not imply that graphology has a good image. This implication should be a major concern to the graphological community. Without a good image, clients will be hard to find and new students will also think carefully before committing themselves to a training programme. In this situation a major image-building exercise is necessary

Realization of laterally nondispersing ultrabroadband Airy pulses

We present the measurements of the spatiotemporal impulse response of a system creating nondispersing Airy pulses, i.e., ultrabroadband Airy beams whose main lobe size remains constant over propagation. A custom refractive element with a continuous surface profile was used to impose the cubic phase on the input beam. The impulse response of the Airy pulse generator was spatiotemporally characterized by applying a white-light spatial-spectral interferometry setup based on the SEA TADPOLE technique. The results were compared with the theoretical model and previously spatiotemporally characterized Airy pulses generated by a spatial light modulator. (C) 2014 Optical Society of Americ