OBSERVATION OF TUNABLE FOUR-PHOTON PARAMETRIC NOISE IN CALCITE

We have observed tunable four‐photon optical parametric noise in calcite with a Q‐switched rubylaser pump. The observed radiation corresponds to the spontaneous emission from a phase‐matched parametric system satisfying the relations: (1a) ω(ruby) + ω(ruby) = ω₁ + ω₂ and (1b) $k^0(\mathrm{ruby}{\mathrm{)\; +\; k}}^0(\mathrm{ruby}{\mathrm{)\; =\; k}}_1^{\epsilon }{\mathrm{+\; k}}_2^0$ , where ω and k denote the angular frequency and wavevector, respectively. The superscripts o and ε denote ordinary and extraordinary rays, respectively. The optical signal was tuned from 4300 to 5860 Å by adjusting the orientation of the calcite; this signal was detected by an RCA 7265 photomultiplier. Although no direct detection of the infrared signal was attempted, we would expect it to be tuned over the range from 0.843 to 1.80 μ. From experimental data, the estimated noise power in the high‐frequency optical signal was about 0.5 μW.