Enabling high-power, broadband THz generation with 800-nm pump wavelength.

The organic terahertz (THz) generation crystal BNA has recently gained traction as a source for producing broadband THz pulses. When pumped with 100 fs pulses, the thin BNA crystals can produce relatively high electric fields with frequency components out to 5 THz. However, the THz output with 800-nm pump wavelength is limited by the damage threshold of the material, particularly when using a 1 kHz or higher repetition rate laser. Here, we report that the damage threshold of BNA THz generation crystals can be significantly improved by bonding BNA to a high-thermal conductivity sapphire window. When pumped with 800-nm light from an amplified Ti:sapphire laser system, this higher damage threshold enables generation of 2.5× higher electric field strengths compared to bare BNA crystals. We characterize the average damage threshold for bare BNA and BNA-sapphire, measure peak-to-peak electric field strengths and THz waveforms, and determine the nonlinear transmission in BNA. Pumping BNA bonded to sapphire with 3 mJ 800-nm pulses results in peak-to-peak electric fields exceeding 1 MV/cm, with broadband frequency components >3 THz. This high-field, broadband THz source is a promising alternative to tilted pulse front LiNbO3 THz sources, enabling many research groups without optical parametric amplifiers to perform high-field, broadband THz spectroscopy.

Heterogeneous layered structures for improved terahertz generation.

One of the most effective ways of generating terahertz (THz) radiation involves the conversion of short-pulsed IR or visible laser light into THz pulses at significantly lower frequencies. This conversion can be accomplished using organic crystals with nonlinear optical crystal (NLO) properties for IR to THz conversion through optical rectification. Due to the high refractive indices of organic crystals, pump laser light as well as generated THz radiation is lost from reflections at crystal surfaces. Here we report a structure composed of a layered series of materials with intermediate refractive indices designed to reduce reflective losses and improve the THz generation from organic crystals. This structure increases the transmission coefficients for both infrared pump input and THz output. We combine simple theoretical calculations with experimental data to show that a structure composed of materials with intermediate refractive indices can be used to increase generated THz intensity by nearly 50%.

Terahertz generation and optical characteristics of P-BI.

We present the structural and THz generation characteristics of the molecular salt crystal (E)-2-(4-(dimethylamino)styryl)-1,1,3-trimethyl-1H-benzo[e]indol-3-ium iodide (P-BI) using optical rectification with IR pump wavelengths. P-BI shows a peak-to-peak field ∼6 times greater than inorganic crystal GaP, and a broader THz spectrum. Data were obtained from 0-6 THz showing a significant dip in generation at 1.8 THz, similar to what has been observed with the THz generation crystal DAST at 1 THz. We characterized the power dependence of P-BI at different IR wavelengths, with optimal THz generation at the 1550-nm pump wavelength. To model THz generation as a function of P-BI crystal thickness, we measured the THz complex refractive index and the IR group index; modeling shows that imperfect phase matching leads to spectral narrowing centered at ∼2.5 THz as the crystal thickness is increased. P-BI could provide a useful alternative to inorganic THz generation crystals such a GaP.