tag:theconversation.com,2011:/id/topics/nature-photonics-10533/articlesNature Photonics – The Conversation2015-12-07T18:28:48Ztag:theconversation.com,2011:article/519482015-12-07T18:28:48Z2015-12-07T18:28:48ZThe amazing camera that can see around corners<figure><img src="https://images.theconversation.com/files/104716/original/image-20151207-3133-79fjh0.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">It's just your average corner, but a far from average camera.</span> <span class="attribution"><span class="source">Gariepy et al./Heriott-Watt</span></span></figcaption></figure><p>How can a person see around a blind corner? One answer is to develop X-ray vision. A more mundane approach is to use a mirror. But if neither are an option, a group of scientists led by Genevieve Gariepy have developed a state-of-the-art detector which, with some clever data processing techniques, can turn walls and floors into a “virtual mirror”, giving the power to locate and track moving objects out of direct line of sight. </p>
<p>The shiny surface of a mirror works by reflecting scattered light from an object at a well-defined angle towards your eye. Because light scattered from different points on the object is reflected at the same angle, your eye sees a clear image of the object. In contrast, a non-reflective surface scatters light randomly in all directions, and creates no clear image. </p>
<p>However, as the researchers at Heriot-Watt University and the University of Edinburgh recognised, there is a way to tease out information on the object even from apparently random scattered light. Their method, <a href="http://www.nature.com/articles/doi:10.1038/nphoton.2015.234">published in Nature Photonics</a>, relies on laser range-finding technology, which measures the distance to an object based on the time it takes a pulse of light to travel to the object, scatter, and travel back to a detector.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/104700/original/image-20151207-3125-1ikdij2.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/104700/original/image-20151207-3125-1ikdij2.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/104700/original/image-20151207-3125-1ikdij2.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=415&fit=crop&dpr=1 600w, https://images.theconversation.com/files/104700/original/image-20151207-3125-1ikdij2.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=415&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/104700/original/image-20151207-3125-1ikdij2.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=415&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/104700/original/image-20151207-3125-1ikdij2.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=521&fit=crop&dpr=1 754w, https://images.theconversation.com/files/104700/original/image-20151207-3125-1ikdij2.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=521&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/104700/original/image-20151207-3125-1ikdij2.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=521&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Laser light fired at the floor is scattered in a spherical wave. It reflects from the hidden object and makes its way back to the SPAD detector, where the noisy data is processed to reveal the hidden object.</span>
<span class="attribution"><a class="source" href="http://nature.com/articles/doi:10.1038/nphoton.2015.234">Gariepy et al./Nature Photonics</a></span>
</figcaption>
</figure>
<p>In principle, the measurement is quite simple. A laser pulse is bounced off the floor and scatters in all directions. A small fraction of the laser light strikes the object, and the back-scattered light is recorded on a patch of floor – the “virtual mirror” – next to the spot the laser strikes. Because the speed of light is known and constant, by measuring the time interval between the start of the laser pulse and the scattered light reaching the patch of floor, the position of the object can be triangulated. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Pi7iCUSXctY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>However, the devil is in the detail. The timing measurement needs to be accurate to within around 50 thousand billionths of a second (5x10<sup>-11</sup>, or 50 picoseconds), and the light levels that must be detected are extremely low. Overcoming both of these obstacles requires some serious laser and detector technology. The laser pulses used for the timing measurement are just ten femtoseconds (10 million-billionths of a second, or 10<sup>-15</sup>) long, and each pixel in the ultra-sensitive “camera” (known as a single-pixel avalanche diode array, or <a href="http://www.micro-photon-devices.com/Products/Photon-Counters">SPAD</a>) used to image the patch of floor is essentially an ultrafast stopwatch that records the arrival time of the scattered light pulse to within a few hundred-billionths of a second.</p>
<p>The complications do not end there. Light scattered from the object of interest reaches the virtual mirror of the floor, but so does light scattered from every other object in the vicinity. The success of this technique requires that the two be separated, the “signal” of the hidden object from the background noise of everything else. </p>
<p>This is achieved by using the fact that the hidden object the device is trying to detect is moving, while other nearby objects are not. Because the moving object generates a signal in the virtual mirror that changes with time, it can be filtered from the constant background signal produced by the stationary objects of the surroundings.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/104705/original/image-20151207-3108-131s1o6.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/104705/original/image-20151207-3108-131s1o6.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=407&fit=crop&dpr=1 600w, https://images.theconversation.com/files/104705/original/image-20151207-3108-131s1o6.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=407&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/104705/original/image-20151207-3108-131s1o6.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=407&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/104705/original/image-20151207-3108-131s1o6.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=511&fit=crop&dpr=1 754w, https://images.theconversation.com/files/104705/original/image-20151207-3108-131s1o6.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=511&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/104705/original/image-20151207-3108-131s1o6.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=511&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A histogram is created of photon arrival times for every pixel, which must then be processed to remove background objects from the target object.</span>
<span class="attribution"><a class="source" href="http://nature.com/articles/doi:10.1038/nphoton.2015.234">Gariepy et al./Nature Photonics</a></span>
</figcaption>
</figure>
<p>The final complication is that the timing measurement for scattered light arriving at a single point on the virtual mirror and recorded by a single pixel in the detector unfortunately doesn’t locate the object to a single unique position. A similar time delay could result from objects located at any number of different positions located an appropriate distance from the virtual mirror. </p>
<p>While the timing data from a single pixel only locates the object to a range of positions, the range is different for each pixel. However, it turns out that there is only a single position at which the timing condition is satisfied simultaneously for all pixels, and this allows the object to be unambiguously identified from the background signals.</p>
<p>The prototype camera system allows the object’s position behind the wall to be localised to within a centimetre or two, and by making measurements every few seconds the camera can also detect the speed of a moving object. In contrast to previous methods, which required long data processing times, the new method can track moving objects in real time. At present it’s limited to locating objects up to 60cm away from the virtual mirror on the floor, but this should improve to around ten metres, as well as to more closely detect the shapes of hidden objects as well as their positions.</p>
<p>So while it’s not quite as promising, or as convenient, as the science-fiction powers of X-ray vision, the study’s authors note that the technology has interesting future applications in areas such as surveillance – to detect a moving person behind a wall, for example – or in car safety systems to detect incoming vehicles approaching around corners.</p><img src="https://counter.theconversation.com/content/51948/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Claire Vallance receives funding from EPSRC, STFC, ERC, Royal Society, University of Oxford, Leverhulme Trust.</span></em></p>To see around a corner, all you need is a camera that can detect light at 100,000 billionths of a second.Claire Vallance, Professor of Physical Chemistry, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/450722015-07-27T15:00:25Z2015-07-27T15:00:25ZWe transformed living cells into tiny lasers<figure><img src="https://images.theconversation.com/files/89538/original/image-20150723-22821-ernjsq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Green lasers glowing within cells.</span> <span class="attribution"><span class="source">Matjaž Humar and Seok Hyun Yun</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>In the last few decades, lasers have become an important part of our lives, with applications ranging from laser pointers and CD players to medical and research uses. Lasers typically have a very well-defined direction of propagation and very narrow and well-defined emission color. We usually imagine a laser as an electrical device we can hold in our hands or as a big box in the middle of a research laboratory.</p>
<p>Fluorescent dyes have also become commonplace, routinely used in research and diagnostics to identify specific cell and tissue types. Illuminating a fluorescent dye makes it emit light with a distinctive color. The color and intensity are used as a measure, for example, of concentrations of various chemical substances such as DNA and proteins, or to tag cells. The intrinsic disadvantage of fluorescent dyes is that only a few tens of different colors can be distinguished. </p>
<p>In a combination of the two technologies, researchers know that if a dye is placed in an optical cavity – a device that confines light, such as two mirrors, for example – they can create a laser.</p>
<p>Taking it all a step even further, our research, described in the journal Nature Photonics, shows we can create a miniature laser that can <a href="http://nature.com/articles/doi:10.1038/nphoton.2015.129">emit light inside a single live cell</a>.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/SHbXDlnLIYA?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<h2>Tiny, tiny lasers</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/89689/original/image-20150724-8478-c0ljzm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/89689/original/image-20150724-8478-c0ljzm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/89689/original/image-20150724-8478-c0ljzm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=583&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89689/original/image-20150724-8478-c0ljzm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=583&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89689/original/image-20150724-8478-c0ljzm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=583&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89689/original/image-20150724-8478-c0ljzm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=732&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89689/original/image-20150724-8478-c0ljzm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=732&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89689/original/image-20150724-8478-c0ljzm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=732&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Green laser bead in a cell.</span>
<span class="attribution"><span class="source">Matjaž Humar and Seok Hyun Yun</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>We made our lasers out of solid polystyrene beads ten times smaller than the diameter of a human hair. The beads contain a fluorescent dye and the surface of the bead confines light, creating an optical cavity. We fed these laser beads to live cells in culture, which eat the lasers within a few hours. After that, we can operate the lasers by illuminating them with external light without any harm to the cells.</p>
<p>Then we capture the light emitted from the cells via a spectrometer and analyze the spectrum. The lasers can act as very sensitive sensors, enabling us to better understand cellular processes. For example, we measured the change in the refractive index – the way light travels through the cell – while varying the concentration of salt in the medium surrounding the cells. The refractive index is directly related to the concentration of chemical constituents within the cells, such as DNA, proteins and lipids.</p>
<p>Further, lasers can be used for cell tagging. Each laser within a cell emits light with a slightly different fingerprint that can be easily detected and used as a bar code to tag the cell. Since a laser has a very narrow spectral emission, a huge number of unique bar codes can be produced, something that was impossible before. </p>
<p>With careful laser design, up to a trillion cells (1,000,000,000,000) could be uniquely tagged. That’s comparable to the total number of cells in the human body. So in principle, it could be possible to individually tag and track every single cell in the human body. This is a huge leap from cell-tagging methods demonstrated until now, which can tag at most a few hundred cells. So far we’ve tagged cells only in Petri dishes, but there’s no reason it shouldn’t also work for cells within a living body.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/89691/original/image-20150724-8451-xrye2l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/89691/original/image-20150724-8451-xrye2l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89691/original/image-20150724-8451-xrye2l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89691/original/image-20150724-8451-xrye2l.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89691/original/image-20150724-8451-xrye2l.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89691/original/image-20150724-8451-xrye2l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89691/original/image-20150724-8451-xrye2l.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89691/original/image-20150724-8451-xrye2l.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Green cells with their blue nuclei were injected with red oil droplets that act as deformable lasers.</span>
<span class="attribution"><span class="source">Matjaž Humar and Seok Hyun Yun</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Alternative materials for cellular lasers</h2>
<p>Instead of a solid bead, we also used an droplet of oil as a laser inside cells. Using a micro pipette, we injected a tiny drop of oil containing fluorescent dyes into a cell. In contrast to the solid bead, forces acting inside the cells can deform the droplets. By analyzing the light emitted by a droplet laser, we can measure that deformation and calculate the force acting on the droplet. It’s a way to get a very precise picture of the kinds of mechanical forces exerted within cells by processes such as cellular migration and division.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/89693/original/image-20150724-8457-1qsc6cy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/89693/original/image-20150724-8457-1qsc6cy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89693/original/image-20150724-8457-1qsc6cy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89693/original/image-20150724-8457-1qsc6cy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89693/original/image-20150724-8457-1qsc6cy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89693/original/image-20150724-8457-1qsc6cy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89693/original/image-20150724-8457-1qsc6cy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89693/original/image-20150724-8457-1qsc6cy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Yellow lipid cells within subcutaneous fat tissue, which can be used as natural lasers.</span>
<span class="attribution"><span class="source">Matjaž Humar and Seok Hyun Yun</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Finally, we realized that fat cells already contain lipid droplets that can work as natural lasers. They don’t need to eat or be injected with lasers, just supplied with a nontoxic fluorescent dye. That means each of us already has millions of lasers inside our fat tissue that are just waiting to be activated to produce laser light. Next time you’re thinking about trimming down, you could just reconceptualize your body fat as a huge number of tiny lasers.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/89696/original/image-20150724-8457-11ux4ju.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/89696/original/image-20150724-8457-11ux4ju.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89696/original/image-20150724-8457-11ux4ju.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=547&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89696/original/image-20150724-8457-11ux4ju.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=547&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89696/original/image-20150724-8457-11ux4ju.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=547&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89696/original/image-20150724-8457-11ux4ju.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=687&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89696/original/image-20150724-8457-11ux4ju.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=687&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89696/original/image-20150724-8457-11ux4ju.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=687&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Inserting an optical fibre into a piece of pig’s skin to excite and extract the laser light generated by subcutaneous fat cells.</span>
<span class="attribution"><span class="source">Matjaž Humar and Seok Hyun Yun</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Our new cell laser technology will help us understand cellular processes and improve medical diagnosis and therapies. They could eventually provide remote sensing inside the human body without the need for sample collection. A cell is a smart machine, equipped with a computer with “DNA Inside.” Specialized cells, such as immune cells, can find the disease and site of inflammation, carrying the laser to the target for laser-based diagnosis and therapies. Imagine rather than a biopsy for a lump that doctors suspect to be cancer, cell lasers helping determine what its made of. Cell lasers also hold promise as a way of deliver laser for therapies, for example, to activate a photosensitive drug at the target to kill microbes or cancerous cells.</p><img src="https://counter.theconversation.com/content/45072/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Matjaž Humar receives funding from Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme.</span></em></p><p class="fine-print"><em><span>Seok-Hyun Yun receives funding from National Science Foundation and National Institutes of Health.</span></em></p>Using fluorescent dye, researchers figured out how to turn cells into lasers – with applications for cell tagging and tracking as well as medical diagnoses and therapies.Matjaž Humar, Research Fellow in Dermatology, Harvard UniversitySeok-Hyun Yun, Associate Professor of Dermatology, Harvard UniversityLicensed as Creative Commons – attribution, no derivatives.