Terahertz waves have received more research attention recently than any other time. This is in view of the many potential applications, for example, in wireless communication and nondestructive imaging. To realize these special applications, compact terahertz-wave sources as well as sensitive detectors operating at room temperatures are extremely important.
Resonant tunneling diode is an important extremely compact terahertz wave source. At around 1 Terahertz region, the resonant tunneling diode devices can generate continuous wave and monochromatic terahertz wave output in the order of microwatts at room temperature.
Frequency up conversion in the nonlinear optical crystals appears to be a promising approach for detection of terahertz wave radiation at room temperature. In this process, terahertz-wave radiation is normally mixed with an optical pump wave in a nonlinear crystal to yield an optical up-converted wave. Then the up-converted wave can be separated from the residual pump wave and detected implementing a commercial sensitive photodetector designed with nanosecond time resolution.
Yuma Takida, Kouji Nawata and Hiroaki Minamide at RIKEN and in collaboration with Safumi Suzuki and Masahiro Asada at Tokyo Institute of Technology demonstrated a nonlinear optical detection of terahertz-wave radiation from continuous-wave resonant tunneling diode gadgets with frequencies of 0.58, 0.78, and 1.14 terahertz at room temperature. They demonstrated that the terahertz-wave radiation from resonant tunneling diode gadgets is up-converted to near-infrared waves in a nonlinear optical magnesium-oxide-doped lithium niobate crystal. They achieved a detection limit of the continuous-wave terahertz-wave power of 5nW at 1.14 terahertz, which corresponded to 2-aJ energy and about 2.7×103 photons. Their research work is now published in Optics Express.
The authors collimated the continuous wave terahertz-wave radiation from the resonant tunneling device and focused the wave on a magnesium-oxide-doped lithium niobate. Magnesium-oxide-doped lithium niobate crystals were used as the nonlinear medium for this study. A normal incidence configuration implementing a trapezoidal crystal was applied to effectively couple the waves into the nonlinear crystal.
In view of the nonlinear phase-matching condition in the crystal, the difference-frequency up-converted wave was produced, and then amplified during propagation through the crystal. The authors implemented a spatial filter to separate the up-converted wave from the residual pump wave. In the experiment, no phase-locking between the terahertz and pump waves was necessary since the difference-frequency up-conversion process was sensitive to the intensity of the two only.
The research team was able to achieve a minimum detection limit of as low as 5 nW at 1.14 terahertz. This was corresponding to 2-aJ energy and 2.7×103 photons. The results of their experiment indicated the the input frequency as well as the power of the resonant tunneling devices could be calibrated by determining, respectively, the output wavelength and energy of the up-converted waves.
Comparing the detection performance of the continuous wave resonant tunneling diode gadgets and a pulsed injection-seeded THz-wave parametric generator helped the authors observe that the method was not only applicable to resonant tunneling diodes, but also other terahertz wave sources.
The proposed detection technique can be implemented in real-time spectroscopic detection with multi-frequency terahertz-wave sources since, in the experiment; the non-collinear phase matching achieved simultaneous up-conversion from superposed terahertz waves. Therefore, this optical detection for the compact terahertz wave sources will be helpful in opening new opportunities for terahertz wave applications.
Yuma Takida, Kouji Nawata, Safumi Suzuki, Masahiro Asada, and Hiroaki Minamide. Nonlinear optical detection of terahertz-wave radiation from resonant tunneling diodes. Vol. 25, No. 5 | 6 Mar 2017 | OPTICS EXPRESS 5390.Go To Optics Express