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Second-harmonic generation using -quasi-phasematching in a GaAs whispering-gallery-mode microcavity

Figure and caption information from paper reprinted with permission of Authors: Input light at the fundamental wavelength (represented by red light) is converted inside the micro-disk to second harmonic light (represented by blue light). The inset shows a scanning electron micrograph of a fabricated device. Credit: T. Thomay/JQI

Can scientists generate any color of light? The answer is not really, but the invention of the laser in 1960 opened new doors for this endeavor. An early experiment injected high-power laser light through quartz and out popped a different color. This sparked the field of nonlinear optics and with it, a new method of color generation became possible: frequency conversion.

Not all crystals can perform this trick and only through careful fabrication of certain materials is frequency conversion possible. PFC-supported researchers have developed a new microstructure that does what’s called second harmonic generation (SHG), where the output light has twice the frequency as the input. This new device is a factor of 1000 smaller than previous frequency converters.

In the new design, gallium arsenide (GaAs) is fabricated into a micrometer-sized disk ‘cavity. Notably, GaAs has one of the largest second-harmonic frequency conversion constants measured. Previously, scientists have harnessed its extremely nonlinear properties for frequency conversion, leading to device sizes in the centimeter range. This new device is 1000 times smaller.

In the experiment, light is injected into the cavity. When light travels in a loop with the proper orientation, as opposed to a linear geometry, color conversion is achieved. The device can be so small because the light can interact many times with the medium by circulating around the disk.

In terms of future quantum information applications, this device could be used in reverse to generate entangled photon pairs. Gallium arsenide (GaAs) is a common semiconductor and has added benefits such as transmitting and emitting in the infrared (IR) and near IR light, respectively. IR-colored light has applications that include telecommunications and chemical sensing. 

P. Kuo, J. Bravo-Abad, G.S. Solomon
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