Patrick F. Tekavec

High power THz sources for nonlinear imaging

Many biological and chemical compounds have unique absorption features in the THz (0.1 - 10 THz) region, making the use of THz waves attractive for imaging in defense, security, biomedical imaging, and monitoring of industrial processes. Unlike optical radiation, THz frequencies can pass through many substances such as paper, clothing, ceramic, etc. with little attenuation. The use of currently available THz systems is limited by lack of highpower, sources as well as sensitive detectors and detector arrays operating at room temperature. Here we present a novel, high power THz source based on intracavity downconverison of optical pulses. The source delivers 6 ps pulses at 1.5 THz, with an average power of >300 μW and peak powers >450 mW. We propose an imaging method based on frequency upconverison that is ideally suited to use the narrow bandwidth and high peak powers produced by the source. By upconverting the THz image to the infrared, commercially available detectors can be used for real time imaging.

Intracavity terahertz generation from gallium arsenide in a fiber laser pumped type 0 doubly resonant optical parametric oscillator

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.

Video rate imaging at 1.5 THz via frequency upconversion to the near-IR

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.

Video rate imaging of narrow band THz radiation based on frequency upconversion

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.