450um terahertz waves at room temperature: one solid laser
To achieve this breakthrough, the company has developed a long-wavelength mid-infrared quantum cascade laser, in which it designed the laser structure based on research and analysis results of the terahertz wave generation principle. Terahertz waves are electromagnetic waves near 300 μm that correspond to a frequency of 1 THz. The shorter-wavelength terahertz waves overlap with far-infrared light and the longer-wavelength waves overlap with millimeter waves. Terahertz waves have intermediate characteristics between those of light and radio waves, and the bandwidth at wavelengths longer than 300 μm and frequencies lower than 1 THz is called the sub-terahertz range.
Last year, Hamamatsu Photonics developed a “terahertz nonlinear quantum cascade laser (QCL) that uses a unique anti-crossed dual-upper-state design (AnticrossDAU). This QCL produces two mid-infrared rays at different wavelengths within a range from 6 to 11μm from a single semiconductor device and induces a nonlinear optical effect inside the device. The QCL in this way serves as a single compact semiconductor laser that operates at room temperature and generates terahertz waves up to a wavelength of 150μm. To generate electromagnetic waves at even longer wavelengths in the sub-terahertz range, it is essential to produce and output two mid-infrared light rays at longer wavelength but this has been extremely difficult to achieve since the longer wavelength light is likely to be absorbed within the device.
Through this research, the company investigated the characteristics of many QCLs to clarify wavelength conversion process in a terahertz nonlinear QCL and found that it could apply the theory of a nonlinear optical effect, called coherent optical rectification, which had not been evaluated before. By applying this theory to the wavelength conversion mechanism using the nonlinear optical effect, the researchers optimized the anti-crossed dual-upper-state design to suppress the unwanted absorption of light inside the device, thus allowing output of the two mid-infrared light rays at longer wavelengths up to 13 to 14μm while increasing the wavelength conversion efficiency. In this way, they succeeded in outputting terahertz waves at 450μm in the terahertz range, believed to be the world’s longest wavelength available from a single semiconductor laser operating at room temperature.
Results from this research will be useful in applications such as quality testing and non-destructive inspection of drugs and foods containing components that absorb electromagnetic waves in the sub-terahertz range as well as submillimeter astronomy and high-speed and high-capacity communication over short distances.
Hamamatsu Photonics – www.hamamatsu.com
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