We demonstrate frequency domain THz anisotropy signature detection for protein crystal models using newly developed compact tunable narrow band THz sources based on Orientation Patterned Gallium Phosphide for turn-key spectroscopic systems.
We demonstrate tunable narrowband THz generation by optical rectification of a femtosecond pulse in Orientation Patterned Gallium Phosphide. Center frequencies of 0.9 - 3.8 THz with average power up to 15 μW were achieved using a 1.064 µm fiber laser for the pump laser. Biomolecular characterization for an early application of this system is also shown in this work by anisotropic spectroscopic signature detection of molecular crystals in the THz region.
We demonstrate video rate THz imaging in both reflection and transmission by frequency upconverting the THz image to the near-IR. In reflection, the ability to resolve images generated at different depths is shown. By mixing the THz pulses with a portion of the fiber laser pump (1064 nm) in a quasi-phase matched gallium arsenide crystal, distinct sidebands are observed at 1058 nm and 1070 nm, corresponding to sum and difference frequency generation of the pump pulse with the THz pulse. By using a polarizer and long pass filter, the strong pump light can be removed, leaving a nearly background free signal at 1070 nm. We have obtained video rate images with spatial resolution of 1mm and field of view ca. 20 mm in diameter without any post processing of the data.
We demonstrate video rate THz imaging by detecting a frequency upconverted signal with a CMOS camera. A fiber laser pumped, double resonant optical parametric oscillator generates THz pulses via difference frequency generation in a quasi-phasematched gallium arsenide (QPM-GaAs) crystal located inside the OPO cavity. The output produced THz pulses centered at 1.5 THz, with an average power up to 1 mW, a linewidth of <100 GHz, and peak power of >2 W. By mixing the THz pulses with a portion of the fiber laser pump (1064 nm) in a second QPM-GaAs crystal, distinct sidebands are observed at 1058 nm and 1070 nm, corresponding to sum and difference frequency generation of the pump pule with the THz pulse. By using a polarizer and long pass filter, the strong pump light can be removed, leaving a nearly background free signal at 1070 nm. For imaging, a Fourier imaging geometry is used, with the object illuminated by the THz beam located one focal length from the GaAs crystal. The spatial Fourier transform is upconverted with a large diameter pump beam, after which a second lens inverse transforms the upconverted spatial components, and the image is detected with a CMOS camera. We have obtained video rate images with spatial resolution of 1mm and field of view ca. 20 mm in diameter without any post processing of the data.
sonant cavity enhancement results in substantial improvement in the efficiency of photonic THz-wave generation via
difference frequency generation (DFG). A nearly degenerate optical parametric oscillator (OPO) was pumped by 6 ps
pulses at 1064 nm, producing signal and idler pulses with average total power in excess of 80 W. By placing a sample of
quasi-phasematched gallium arsenide (QPM-GaAs) at a focus of a ring cavity OPO, multicycle, narrowband THz
radiation was produced, with average powers in excess of 100 μW and peak powers exceeding 150 mW. The
dependence of the THz power on pump power shows no signs of saturation, so with higher power pump lasers, mW
levels of average THz should be obtainable.
Resonant cavity enhancement results in substantial improvement in the efficiency of photonic THz-wave generation via
frequency down conversion. Efficient THz wave generation was demonstrated at 2.8 THz previously by difference
frequency mixing between resonating signal and idler waves of the linear-cavity type-II-phase-matched PPLN optical
parametric oscillator (OPO). A new, simplified approach to resonantly-enhanced THz-wave generation in periodic GaAs,
featuring (i) ring, instead of linear, OPO cavity with much higher finesse, (ii) type-0, instead of type-II-phase-matched
PPLN crystal as a gain medium, resulting in much lower OPO threshold, (iii) a compact picosecond 1064-nm fiber laser
as a pump source, and (iv) the use of a thin intracavity etalon with a free spectral range equal to the desired THz output
frequency is presented here. Intra-cavity THz generation was performed by 2.1 μm anti-reflection coated stacks of
optically contacted GaAs wafers (OC-GaAs) and diffusion bonded GaAs wafers (DB-GaAs) with periodic-inversion
placed in the second OPO focal plane. Using 6.6 W of average pump power, narrowband output in the range 1.4 - 3 THz
was produced with more than 130 microwatts of average power at 1.5 THz. By optimizing the OPO PPLN crystal length
and spectral characteristics of the fiber pump laser and OPO the demonstrated approach can be extended to generate 1-10
mW of THz output in a compact setup.
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