This video details the creation of a homemade Pockels cell using potassium dihydrogen phosphate (KDP) crystals grown in a previous episode. The Pockels effect, which allows for linear modulation of a material's refractive index with an applied electric field, is explained as a mechanism for changing light polarization. The creator addresses the challenges of working with fragile KDP crystals, demonstrating a custom-built reciprocating diamond wire saw for precise cutting. A key innovation is the use of index-matching mineral oil to circumvent the difficulty of achieving optical-grade polishing on the soft crystal surfaces. The constructed Pockels cell is then successfully demonstrated as an optical shutter, modulating a laser beam by applying high voltage. While a permanently bonded version showed optical imperfections, the oil-filled cell proved highly effective, laying groundwork for future experiments in nonlinear optics.
Summarizing the Pockels Effect and Building a Pockels Cell
0:02 The Pockels Effect: The video introduces the Pockels effect, where the refractive index of certain transparent materials (like potassium dihydrogen phosphate, KDP) changes linearly with an applied electric field, altering light polarization. This effect is stronger than the Kerr effect and occurs in crystals lacking inversion symmetry.
1:14 DIY Motivation & Inspiration: Commercial Pockels cells are very expensive, prompting a DIY approach. The creator is inspired by a 1962 Scientific American article describing a homemade cell using ammonium dihydrogen phosphate (ADP), a material historically used in piezoelectric transducers.
2:44 Challenge of Cutting KDP: KDP crystals are very fragile, making traditional hand tools unsuitable for cutting. Industrial band saws are too large and complex for home use.
3:08 Custom Diamond Wire Saw: A custom-built reciprocating diamond wire sectioning machine is introduced. Made from off-the-shelf parts and scrap aluminum, it precisely cuts KDP using two stepper motors, a vise with rubber pads, and a tensioning system.
5:17 Wet Cutting Process: The saw requires wetting the diamond wire with ordinary water using a Q-tip to prevent fouling and dissolve KDP dust, enabling efficient cutting (approx. 20 minutes per cut).
6:12 Polishing Challenges & Index Matching Solution: KDP crystal surfaces are soft and difficult to polish to optical flatness. A key technique is using mineral oil between the crystal and conductive indium tin oxide (ITO) glass plates. This "index matching" fills surface imperfections and makes reflections disappear, effectively negating the need for a perfect polish.
8:08 Index Matching Demonstration: The concept of index matching is visually demonstrated by making a small glass beaker "disappear" inside a larger one filled with mineral oil.
9:05 Optional Polishing Method: For those who still wish to polish, a modified method using a selvet cloth, 8000 grit diamond paste, and isopropyl alcohol is shown, resulting in a surface similar to window glass.
9:39 KDP Optical Properties (Birefringence): When placed between cross-polarizers, the KDP crystal displays a "gorgeous cross" (iso gy) and rainbow bands (isochromes) due to its uniaxial nature and how it splits light into rays travelling at different speeds.
10:57 Pockels Cell Construction: The cell is assembled by sandwiching the cut KDP crystal between two pieces of ITO glass (conductive side facing the crystal), with mineral oil for index matching, all held together by a laser-cut Perspex clamp and nylon bolts to insulate against high voltage.
12:10 High-Voltage Application & Effect: A homemade high-voltage power supply (8,000 to 35,000 volts) is used to drive the cell. Applying voltage subtly distorts the crystal lattice, making it temporarily behave as if it has two optical axes (biaxial), which changes the light polarization.
12:40 Visible Polarization Change: This change is visible in the interference pattern: the central "melotope" opens up and shifts from dark to light, indicating a change in polarization.
13:17 Optical Shutter Demonstration: The Pockels cell is successfully demonstrated as an optical shutter, effectively turning a laser beam on and off by modulating the applied high voltage.
14:07 Challenges with Permanent Bonding: An experimental permanently bonded Pockels cell (using UV adhesive and silicone) was functional but exhibited poor optical quality, likely due to stress or uneven curing. Commercial cells often use oil for this reason.
14:30 Future Work: Future plans include investigating proper polishing methods, trying a nanosecond driver, and experimenting with second and third harmonic generation using KDP.