Sensitive terahertz (THz)-wave sensor at room temperature is crucial for most applications such as 2-dimensional realtime
imaging and nonlinear phenomena in semiconductors caused by multi-photon absorption, light-induced ionization,
and saturated absorption. LiNbO3 is a promising material for frequency up- and down-conversion because of its high
nonlinearity and high resistance to optical damage. In this report, we propose a slant-stripe-type periodically poled Mg
doped LiNbO3 (PPMgLN) crystal for the construction of a practical THz detector. The PPMgLN solves compromised
optical design and low coupling efficiency between THz and infrared (IR) pump beam due to imperfect dichroic coupler.
The effective coupling of both pump beam and THz-wave into identical interaction region of up-conversion device
promotes the THz detector in practical use. The phase-matched-condition in slant-stripe-type PPMgLN was designed to
offer collinear propagation of two optical waves, the pump and up-conversion signal beams, because of efficient
frequency conversion. The phase-mached-condition was calculated and a slant-stripe-type PPMgLN with an angle (α) of
20° and a grating period (Λ) of 29.0 μm was used in this experiment. A minimum detectable energy of 0.3 pJ/pulse at the frequency of 1.6 THz was achieved with the pump energy of 1.8 mJ/pulse in room temperature. The dynamic range of the incident THz-wave energy of 60 dB was demonstrated. Further improving for the sensitivity using longer interaction
length in a PPMgLN crystal was also investigated.
Terahertz imaging has attracted a lot of interests for more than 10 years. But real time, high sensitive, low cost THz imaging in room temperature, which is widely needed by fields such as biology, biomedicine and homeland security, has not been fully developed yet. A lot of approaches have been reported on electro-optic (E-O) imaging and THz focal plane arrays with photoconductive antenna or micro-bolometer integrated. In this paper, we report high sensitive realtime THz image at 60 frames per second (fps) employing a commercial infrared camera, using nonlinear optical frequency up-conversion technology. In this system, a flash-lamp pumped nanosecond pulse green laser is used to pump two optical parametric oscillator systems with potassium titanyl phosphate crystals (KTP-OPO). One system with dual KTP crystals is used to generate infrared laser for the pumping of THz difference frequency generation (DFG) in a 4- Dimethylamino-N-Methyl-4-Stilbazolium Tosylate (DAST) crystal. The other one is for generation of pumping laser for THz frequency up-conversion in a second DAST crystal. The THz frequency can be tuned continuously from a few THz to less than 30 THz by controlling the angle of KTP crystals. The frequency up-converted image in infrared region is recorded by a commercial infrared camera working at 60 Hz. Images and videos are presented to show the feasibility of this technique and the real-time ability. Comparison with a general micro-bolometer THz camera shows the high sensitivity of this technique.