A lot of the recent developments in microscopy have centered on visible light (400-650nm) or near-infrared light (700-2500nm). This is because detectors are most sensitive to visible and near-infrared light and most commercial lasers operate in this wavelength range. The problem is that nothing interesting happens in this wavelength range. Most objects are reasonably transparent to light over the visible and near-infrared ranges, so images are generally created by labeling a region of interest with florescent materials, which then glow in the presence of the laser light. Another problem with microscopy is the diffraction limit, which tells us what the smallest resolvable image is. In most cases, this is something like the wavelength of the light (around 400nm) and that is too big to be able to resolve individual proteins or DNA molecules. Microscopy, using mid-infrared light and optical antennas to beat the diffraction limit may enable high resolution microscopy that can also identify the chemical it is imaging. Here, we report on some recent progress in developing the tightly focused light source required for such a microscope.
As we have discussed in other articles, there are methods for defeating the diffraction limit. For example, light can be guided in some structure that is tapered to a tip whose dimension is much smaller than the wavelength of light (say 10nm). If the outside of the tip is conductive, the light excites the electrons, causing them to collectively vibrate down the guiding structure. At the end of the structure, the electrons release the energy as light, as if it had been conducted down the structure. However, the light is emitted in every direction and is only very intense right at the end of the tip. The intensity of the scattered light can be used to map a surface with a resolution about the same as the tip diameter.
Using this and similar techniques, scientists could, in principle, resolve an individual protein molecule. The difficulty is that the protein is transparent to the light used and if we use a florescent label, we are imaging the label not the protein. In other words, labels are very useful when looking at populations of proteins (or other molecules) but are of more limited use when studying individual molecules.
Enter the quantum cascade laser, which is a unique class of laser that emit in the mid-infrared (3-5 micrometers). These lasers use a very finely structured semiconductor to weakly confine electrons in very small boxes. The boxes give the electrons a set of well-defined energy levels to occupy. When a voltage is applied, the electrons travel from box to box in such a way that they must transition down an energy level with each move. For every transition, they release a photon of light and the presence of photons can stimulate electrons to make the transition, hence a laser is born. The difference is that the wavelength of these lasers are limited only by the physical dimensions of the boxes, meaning that we are no longer stuck with laser light colors given to us by nature. Quantum cascade lasers have found their niche in the mid-infrared and infrared (3-15 micrometers), where they make a lovely reliable source for people wanting to do spectroscopy.
The thing that makes this interesting is that almost every molecule in existence absorbs somewhere in the mid-infrared, making mid-infrared spectroscopy a key tool for identifying and understanding molecules. The problem is that the diffraction limit means that you can only resolve objects around three micrometers big. In principle, the quantum cascade laser could be used to detect the absorption from a single protein molecule, but it can only tell you where that molecule is to within three micrometers.
Now a group of researchers from Harvard, with support from Agilent, have combined the ideas used for high resolution imaging with quantum cascade lasers. To do this, they deposited a couple of metallic strips on the emitting face of the quantum cascade laser, forming an antenna. This metallic layer absorbed a lot of the light from the laser, causing the electrons to oscillate coherently. The light emitted from the gap between the strips is very intense because it gets most of the energy from the antenna. However, it also radiates in every direction, so the intensity is only very high near the gap. Imaging with such a laser will reveal features on the order of the gap size, which is about 100nm. This is still too big to reveal single proteins, but is certainly much smaller than most microscopes operating in the mid-infrared.
Now, there is a downside to this. Unlike normal laser diodes, quantum cascade lasers aren’t really that tunable. If you ask for a quantum cascade laser with a wavelength of five micrometers, that is what you will get. Unfortunately, spectroscopy really requires accessing a broad range of colors, all in the mid-infrared. This means that the light source will have to be different if this is to be employed as a generalized microscopy tool. However, there are plenty of applications where the ability to image the locations of a few key chemicals would be required to obtain useful information. There is certainly room for a specialized instrument utilizing this technique.
Applied Physics Letters, 2007, DOI: 10.1063/1.2801551