Dynamic Dichroic Mirrors and Single-Shot Spectroscopy

光学光谱学涉及将光分解成其组成波长，就像玻璃棱镜将太阳光分解成彩虹(或光谱)颜色一样。虽然乍一看，这看起来很美观，但彩虹的颜色确实包含了一些关于它所发出的光的非常有用的信息。例如，如果仔细观察太阳光谱的橙色部分(大致在与旧的橙色路灯对应的颜色附近)，就会发现光线似乎缺失的一对暗线。这与钠的吸收相对应，钠的吸收告诉我们太阳的上层大气(或色层)中有钠；实际上，我们已经从9300万英里外的地方了解到了太阳的部分组成，只使用了一个玻璃棱镜。这就是光学光谱的力量。光学光谱还有许多其他用途；它可以调查法医样本的化学成分，帮助定位肿瘤，从远处识别化学武器，从轨道上监测森林砍伐，鉴定艺术品，等等。然而，如果我们试图像使用相机那样拍摄照片，就会出现问题；相机只能捕捉2D信息，如果我们在每个像素都有光谱，我们要么需要一次照亮一条线(并使用相机的另一个轴来测量光谱)，要么使用一系列滤光器一次一个波长地获取数据。事实上，如果我们知道我们正在寻找的频谱，过滤器方法要快得多，但这需要为您可能想要测量的每一件事情携带一叠过滤器。对于一些事情来说，这可能是可以的，但对于便携式或基于空间的应用，或者如果有很多潜在的分析物，这可能变得不可行。另一种选择是每次都用光折变聚合物制作一个新的滤光片。这个新系统可以制作用户可能想要的任何滤光片，将其写入类似于制作全息图的材料中。这与普通的可调谐滤光器不同，后者通常只能调谐单个传输频带的宽度和中心波长。这种新方法可以创建用户可能想要的任何过滤器配置文件，包括多个独立的频带和不同的频带形状。它的工作原理是使用两束激光在全息板上记录干涉图案。改变激光光束之间的角度，并记录另一干涉图案。这样做多次后，滤光片轮廓被记录在全息图中；整个过程只需不到一秒钟。一旦滤光片不再需要，它就可以被重写，并创建一个新的图案；可以搜索任何数量的分析物，包括系统以前从未见过的分析物；它们只需要由控制计算机编程即可。全息材料是一种新型的光折变聚合物。与普通全息图不同，这种材料是可重写的；它可以被擦除，并在其中写入新的图案。虽然这种材料以前曾被用来制造可重写全息显示器，但这是第一个将这种材料用于制造全息滤光片的例子。然而，合成它并不困难；它只需要从头开始制作两个组件，这两个都是简单的合成过程，而且产量很高。总体而言，这个项目提供了一种“光谱Instagram”；用户可以数字地应用任何可能需要的滤镜，包括在必要时从头开始编程，而不是局限于选择少量物理相机滤镜。这使得光谱成像过程更快、更高效，允许用户比以往任何时候都能在更大的区域和更高的精度上收集数据。

Optical spectroscopy involves splitting light up into its component wavelengths, exactly like how a glass prism splits sunlight up into a rainbow (or 'spectrum') of colours. While this may, at first, just appear to be aesthetically pleasing, the rainbow of colours does include some very useful information about the light it came from. For example, if one were to look very carefully at the orange part of the spectrum of the sun (roughly around the colour corresponding to old orange street lights), one would find a pair of dark lines where the light appears to be missing. This corresponds to absorption due to sodium, which tells us that there is sodium in the upper atmosphere (or 'chromosphere') of the sun; we have, in effect, learned part of what the sun is made of, from 93 million miles away, using nothing more than a glass prism. This is the power of optical spectroscopy.Optical spectroscopy has many other uses; it can investigate the chemical composition of forensic samples, help locate a tumour, identify chemical weapons from a distance, monitor deforestation from orbit, authenticate artwork, and many more besides. Nevertheless, if we try to take pictures like we would do with a camera, there's a problem; a camera can only capture 2D information, and if we have a spectrum at each pixel, we either need to illuminate one line at a time (and use the other axis of the camera to measure the spectrum) or use a sequence of filters to get the data one wavelength at a time. In fact, if we know what spectrum we're looking for, the filter approach is much faster, but that requires carrying a stack of filters for each thing you might want to measure. That might be OK for a few things, but for portable or space-based applications, or if there are a lot of potential analytes, that can become infeasible. The alternative is to make a new filter each time, using a photorefractive polymer.This new system can create any filter that the user might want, writing it into a material similar to that used to make holograms. This is unlike normal tunable filters, which are typically only capable of tuning a single transmission band's width and center wavelength. This new approach can create any filter profile the user might want, including multiple independent bands and different band shapes. It works by recording an interference pattern in a holographic plate, using two laser beams. The angle between the laser beams is changed, and another interference pattern is recorded. After doing this many times, the filter profile is recorded in the hologram; this entire process takes less than a second. Once the filter isn't needed any more, it can be rewritten, and a new pattern created; any number of analytes can be searched for, including those which the system has never seen before; they need only be programmed in by the control computer.The holographic material is a new type known as a photorefractive polymer. Unlike normal holograms, this material is rewritable; it can be erased, and a new pattern written into it. While this has been used to create rewritable holographic displays before, this is the first example where the material will be used to create a holographic filter. Nevertheless, synthesizing it is not difficult; it requires just two components to be made from scratch, and these are both easy synthetic procedures with high yields.Overall, this project offers to create a kind of 'Instagram for spectroscopy'; rather than being limited to a small selection of physical camera filters, a user can digitally apply any one that might be needed, including programming a new one from scratch if necessary. This makes the spectroscopic imaging process faster, and more efficient, allowing the user to gather data over larger areas, and with more precision, than ever before.