We haven’t seen any projects from serial experimenter [Les Wright] for quite a while, and honestly, we were getting a little worried about that. Turns out we needn’t have fretted, as [Les] was deep into this exploration of the Pockels Effect, with pretty cool results.
If you’ll recall, [Les]’s last appearance on these pages concerned the automated creation of huge, perfect crystals of KDP, or potassium dihydrogen phosphate. KDP crystals have many interesting properties, but the focus here is on their ability to modulate light when an electrical charge is applied to the crystal. That’s the Pockels Effect, and while there are commercially available Pockels cells available for use mainly as optical switches, where’s the sport in buying when you can build?
As with most of [Les]’s projects, there are hacks galore here, but the hackiest is probably the homemade diamond wire saw. The fragile KDP crystals need to be cut before use, and rather than risk his beauties to a bandsaw or angle grinder, [Les] threw together a rig using a stepper motor and some cheap diamond-encrusted wire. The motor moves the diamond wire up and down while a weight forces the crystal against it on a moving sled. Brilliant!
The cut crystals are then polished before being mounted between conductive ITO glass and connected to a high-voltage supply. The video below shows the beautiful polarization changes induced by the electric field, as well as demonstrating how well the Pockels cell acts as an optical switch. It’s kind of neat to see a clear crystal completely block a laser just by flipping a switch.
Nice work, [Les], and great to have you back.
Suppose you wanted to make a neuron measuring device, you pass polarized light through a fiber optic strand, through a tiny crystal, and then back. Will the electric field in the medium near the neuron cause the Pockel’s effect, and would you be able to detect a difference in polarization from such a small electric field change?
Such a system would be a completely passive way of measuring neuron activity.
Commercial fiber-tip electric field sensors have been available for a while, like from SPEAG and Agiltron. The sensitivity is quite low, however. In our (admitedly very different) application, it took kilowatts of power into a few dozen cubic centimeters to get measureable effects. I doubt the elctric field generated in biological tissue would produce anything measureable, but I’d love to see it work.
The optical effects probably need much higher voltage, but more promising way would be to place a tiny integrated circuit, composed of a field effect transistor and a resonant circuit with the gate next to the neuron. This would be read out by RF retro reflection. I talked to some biologists and they said that there are ways of introducing small biologically inert coated pieces of silicon into the tissues, so this might actually work in practice.
Something similar was available a decade or more ago: the NeuroWare from Triangle BioSystems: Up to 256 channels, operating at 3.4 GHz, but they closed up shop a few years ago. I’m sure there’s a more modern product replacing it.
I wouldn’t be so sure of it being completely passive. The crystal would be coupling into the electromagnetic field of the neuron and would have some influence on it, wouldn’t it? The Pockel cell seems to behave like a capacitor (well, imo it IS a capacitor) it has fast rise and slow fall. My intuition says that if the neuron’s electric signal is supposed to go off, the coupling and the slow fall would be coupled back into the neuron, possibly slightly lengthening the signal and so might cause glitches. As the brain is a fully real-time system, those glitches might have the potential to cause it to crash. :)
I saw the first three words of the title and immediately thought “Must be Les, yeah!”
Is this like opticaly clear piezoelectic transducer that changes refractive index when it stretches/contracts?
More or less! Ammonium dihydrogen phosphate was used in WW2 as piezo transducers in submarines!
Concerning the “proper polishing techniqes” mentioned at the very end…
In one of the first texts shown I saw a mention of a diamond flycutter being used. That would be a way to get the roughly sawn bandsaw surface (A bandsaw is never a very accurate instrument) flat and quite smooth quickly. (Industrial) diamonds and diamond tools are not that expensive anymore, and this may be worth exploring.
For the rest, I did see the whole video to the end but it’s not really an area for which I have any expertise. Still fun to see though.
Oh, I forgot: For gently clamping “weird shaped” crystals, rocks etc, you can make temporary vice jaws from polycaprolactone (a.k.a. “polymorph”). It’s a plastic that can be melted in hot water, and after taking it out of the water, heat transfer to your fingers is low enough that you can handle it with your bare hands and kneed it like clay. After it cools to around 35c, its properties are very much like nylon, but it does have quite a lot of (slow) creepage as it’s normally used quite close to it’s melting point.
Water-soluble mounting wax is also great for this: heat and set the part for workholding, it sets very stiff and hard with minimal shrinkage. You can carve or melt it away to recover and re-use it, and wash the rest away. Great for carving and polishing hard-to-hold pieces. Maybe not so great for holding water-soluble crystals though. Available from Freeman Supply (and probably a lot of other places).