On the other hand there are also companies, notably Elon Musk's Tesla, that prefer not to use LiDAR. The technology has a number of drawbacks, such as that every single LiDAR is very expensive and that they are prone to breaking often due to having many moving parts. The LiDARs from Velodyne simply have narrow-beam lasers that rotate 360 degrees, other companies use mirrors and microelectromechanical systems to scan laser beams over a smaller field of view, but they all seem to need moving parts, which tend to wear out quickly.
In March this year, however, the American company Lumotive claimed to have come up with a way to dispense with the moving parts completely. According to IEEE Spectrum, they accomplish this by using a liquid crystal metamaterial to slow down selected parts of the laser beam, thereby shifting its phase relative to the other parts. This means that the peaks and troughs of the electromagnetic wave in different parts of the beam will occur in different places, reinforcing each other in some directions while cancelling each other out in others. By controlling how much which parts of the beam are slowed down it is possible to control the direction of the beam (incidentally, this is what I tried to describe in my post about radar and the spinning thing, except now it's with infrared light instead of microwaves).
If this turns out to work well it would be extremely useful, but what exactly is a liquid crystal metamaterial? We know that a metamaterial is an artificial material that is constructed out of small bits of other materials, usually in a way that gives it very exotic properties. So apparently, we are dealing with an artificial material made with liquid crystals.
Liquid crystals are substances, usually consisting of very long molecules, that in some ways behave like liquids and in other ways like crystals. (This is "crystal" in the scientific sense, meaning that atoms are arranged in a regular three-dimensional grid.) For example, liquid crystals will often flow and change shape like liquids but the molecules will be arranged in a regular, crystal-like structure. Both Lumotive's LiDAR and the more well known application of liquid crystals, namely liquid crystal displays (LCD), make use of the ease with which the molecular orientation in the liquid crystal can be changed (because it is a liquid) in combination with the special optical properties that arise from the crystalline structure.
Since liquid crystals consist of long molecules they tend to be very anisotropic, meaning that the properties of the material are different depending on if you look at it along the length axis of the molecules, perpendicular to it, or from some other angle. When it comes to optical properties this means that the speed of light propagation in the liquid crystal, and therefore the refractive index, is different if the light is propagating along the molecules or across them. The effect also depends on the relation between the molecule orientation and the polarization of the light, in a way that makes liquid crystals able to change the polarization of light that passes through them.
Liquid crystals are substances, usually consisting of very long molecules, that in some ways behave like liquids and in other ways like crystals. (This is "crystal" in the scientific sense, meaning that atoms are arranged in a regular three-dimensional grid.) For example, liquid crystals will often flow and change shape like liquids but the molecules will be arranged in a regular, crystal-like structure. Both Lumotive's LiDAR and the more well known application of liquid crystals, namely liquid crystal displays (LCD), make use of the ease with which the molecular orientation in the liquid crystal can be changed (because it is a liquid) in combination with the special optical properties that arise from the crystalline structure.
Since liquid crystals consist of long molecules they tend to be very anisotropic, meaning that the properties of the material are different depending on if you look at it along the length axis of the molecules, perpendicular to it, or from some other angle. When it comes to optical properties this means that the speed of light propagation in the liquid crystal, and therefore the refractive index, is different if the light is propagating along the molecules or across them. The effect also depends on the relation between the molecule orientation and the polarization of the light, in a way that makes liquid crystals able to change the polarization of light that passes through them.
This ability to change the polarization of light is what is used in LCDs. The effects on polarization is dependent on the orientation of the long molecules of the liquid crystal, which can be changed by applying an electric field. By sandwiching a liquid crystal between polarization filters and manipulating the molecular orientation it is possible to turn transmission of light through the structure on, by ensuring that the polarization of the light is aligned with both polarization filters, or off, by ensuring that the polarization of the light is perpendicular to one of the filters. This is the basics of LCD display pixels.
The metamaterials in the Lumotive LiDARs, on the other hand, appear to make use of the anisotropy of the refractive index of liquid crystals*. Just like in LCDs the orientation of the molecules of the liquid crystal is controlled through application of an electric field, but in order to tune the refractive index to a specific value instead of achieving a shift in polarization. This can be seen as choosing if the light will propagate along the molecules, perpendicular to the molecules or at some angle in between, and making use of the difference in propagation speed for the different cases.
The other parts of the metamaterial appear to be dielectric resonators consisting of silicon elements with the liquid crystal material sandwiched in between. When the refractive index of the liquid crystal is changed, this changes the properties of the dielectric resonator as a whole. If a laser beam is reflected from a surface full of these dielectric resonators, the phase of each part of the reflected beam will depend on the refractive index of the liquid crystal in the resonators. By changing the refractive index according to some pattern, the direction of the reflected beam can be controlled.
If this idea turns out to work well in practice it could potentially make LiDARs much more affordable, and therefore probably much more common in vehicles. How much further along the path towards autonomous driving it can take us of course remains to be seen.
The metamaterials in the Lumotive LiDARs, on the other hand, appear to make use of the anisotropy of the refractive index of liquid crystals*. Just like in LCDs the orientation of the molecules of the liquid crystal is controlled through application of an electric field, but in order to tune the refractive index to a specific value instead of achieving a shift in polarization. This can be seen as choosing if the light will propagate along the molecules, perpendicular to the molecules or at some angle in between, and making use of the difference in propagation speed for the different cases.
The other parts of the metamaterial appear to be dielectric resonators consisting of silicon elements with the liquid crystal material sandwiched in between. When the refractive index of the liquid crystal is changed, this changes the properties of the dielectric resonator as a whole. If a laser beam is reflected from a surface full of these dielectric resonators, the phase of each part of the reflected beam will depend on the refractive index of the liquid crystal in the resonators. By changing the refractive index according to some pattern, the direction of the reflected beam can be controlled.
If this idea turns out to work well in practice it could potentially make LiDARs much more affordable, and therefore probably much more common in vehicles. How much further along the path towards autonomous driving it can take us of course remains to be seen.
* I must admit that I am guessing a bit here, since there are many recent patent applications for similar ideas and I have not been able to find exactly which ones belong or are licensed to Lumotive. The one that seems most likely to form the basis of Lumotive's work is WO2018156688A1, which has the Lumotive CTO listed as an inventor.
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