Rainbow Stars—Multi-Color Laser Emission for Science and Art

December 28, 1987: Shuichi Kawasaki, one of my graduate students, shouted for me to: “Come to the laboratory, immediately!” When I arrived, to my great surprise, I saw numerous laser spots twinkling very strongly on the screen. “How fantastic! How could you generate such a colorful beam, today?” I asked him. “I really don’t know. I was trying to get better data for my thesis, and I maximized the output power of the laser which is introduced into pressurized hydrogen.” I thought this is a sort of nonlinear optical phenomenon, and that I might be able to enhance this colorful emission by increasing the output power of the dye laser. However, I immediately noticed that the dial indicating the voltage applied to the pump laser was already in the red zone, warning of possible, serious damage to the excimer laser. “Shuichi! We might be able to slightly increase the output power by adjusting the emitting wavelength to the gain maximum, which might enhance this curious phenomenon.” “That is not correct, although I am not sure why. These colorful spots only appear when the wavelength is adjusted 6–7 nm away from the maximum of the gain curve.” Shuichi said. “I cannot believe it. This phenomenon can be observed only when the output power is at a maximum, indicating that it is generated by a type of nonlinear optical effect.” He moved the laser wavelength to the maximum of the gain curve, and surprisingly all of the colorful spots disappeared. “Did you adjust the laser wavelength precisely at the center of the gain curve?” He motioned with his eyes to look at the meter, indicating the laser power. I realized that the power definitely increased when the laser wavelength was changed. “Unbelievable! Is this power meter working?” I asked. Without saying anything, Shuichi interrupted the laser beam path. The needle went down to zero, indicating that the power meter was functioning correctly. “Unbelievable! This cannot be possible! How can we explain the generation of this colorful beam? This appears through a nonlinear optical effect that requires a high level of power for more efficient generation. It appears, however, only when the laser power is decreased slightly from the maximum. This is logically inconsistent.” “Look! Totaro. I will change the laser wavelength and we can observe a continuous color change. This is so fantastic, and so beautiful.” All of the colorful spots, however, disappeared when the laser wavelength was moved far away from the optimum. “Shuichi! Move the wavelength back in the opposite direction.” “Oh! So fantastic!”