Frequency Doubling with a KDP crystal

Growing your own KDP Crystal

For a final project in Modern Optics Laboratory at MIT, I decided to grow my own KDP (Potassium Dihydrogen Phosphate) crystal. My goal was to observe frequency doubling in the crystal using a 500mW continuous beam Nd:YAG laser. I chose KDP because it is cheap (bought 500g for $20 at MIT's chemical depot: Avantor Performance materials, Potassium Phosphate, Monobasic, Crystal. CAS number 7778-77-0, 7100 Macron).

First I boiled 500ml deionized water on a hot plate and added 150g of KDP (saturation is 220g/L). I hung a string weighted with a paper clip from a pencil into the 600ml beaker. Large (nearly 1cm) crystals grew overnight. They grew too fast and their crystal structure was cloudy (see leftmost photo below).

The second attempt involved reheating the saturated solution from the first attempt and dissolving the many crystals in the bottom of the jar back into the solution. I took a small crystal from this original batch, tied it on a string and hung it in this modified solution. The result was a bunch of little clear crystals (approximately 1mm across) growing off of the cloudy seed crystal (see middle photo). The crystals were very pointy and lacked nice, square crystalline structure.

Third attempt: I saturated 450ml of deionized water with 120mg of KDP. I hung a string into the beaker with a glass slide for weight on the bottom. I was careful to transfer the solution to a new beaker each day to prevent crystals from growing on the side and bottom of the glass. The crystals grew very clearly, and 5 days later, the largest crystal was approximately 1cm across (rightmost photo).

Frequency Doubling
Frequency doubling (or second harmonic generation) is a non-linear optical phenomenon, which I will not explain here, but you can read about it in my full report (PDF). In the case of the Nd:YAG laser, frequency doubling will cause the conversion of 1064nm light (infrared and invisible to the human eye) to 532nm (visible green light). To successfully see the output of this nonlinear polarization effect, you must also adhere to phase-matching conditions, which dictate the angle the crystal must be placed at for frequency doubling. If you are interested in learning more about the theory behind frequency doubling, please read Nonlinear Optics from Robert W. Boyd.

KDP is a tetragonal, negative uniaxial crystal. The crystallographic c-axis, which in the case of KDP is also the optical axis, points along the length of the crystal (see photo below). For successful frequency doubling in a Nd:YAG laser, the optical axis of the crystal must be oriented at a ~41 degreee angle to the incident light. To do this phase-matching calculation yourself as I did, please consult the equations in Boyd's Nonlinear Optics Chpt. 2. The laser should be polarized orthogonal to the optical axis.

I did not successfully see frequency doubling with the 500mW continuous beam Nd:YAG laser. According to literature I have read (D. Eimerl (1987): Electro-optic, linear, and nonlinear optical properties of KDP and its isomorphs, Ferroelectrics, 72:1, 95-139), 500mW is about the lowest power at which you can see frequency doubling for this wavelength of light. Therefore, I believe that the imperfections in my crystal and low laser power made it diffucult to observe frequency doubling.

Not wanting to give up, I took these crystals to the laboratory where I do research on non-line-of-sight imaging and have access to a 800mW femtosecond pulsed titanium-sapphire laser (near infrared, 795nm, looks red to the naked eye at this intensity). To observe frequency doubling, I calculated that the crystal's optical axis must be angled roughly 45 degrees to the laser beam. I focused the laser on the KDP crystal with a lens (~5cm focal length), which broadened the range of incident angles and made it easier to find the precise direction for frequency doubling. I was able to successfully observe frequency doubling (near UV, 397.5nm, looks blue/purple to naked eye) with this laser within a matter of minutes (see images below).

increasing intensity of frequency doubled light in output beam

The unchanged 795nm beam is output in the same direction as the frequency doubled beam. To see the frequency doubled light only, you should place a prism after the crystal or wear safety goggles that block out infrared light. The leftmost picture shows the laser beam with almost no frequency doubling. The middle beam shows the beam when I first witnessed frequency doubling (it was much easier to see blue/purple beam wearing safety goggles than in this picture). Finally, the beam on the right clearly shows the output beam turning blue due to the large second harmonic generation.

Amy Fritz
last updated: 12/15/2011