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We demonstrate multi-cycle terahertz (MC-THz) generation in a 15.5 mm long periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) crystal with a poling period of 300 µm. By cryogenically cooling the crystal to 77 K, up to 0.72 µJ terahertz energy is obtained at a frequency of 0.5 THz with a 3 GHz bandwidth. A maximum internal optical-to-terahertz conversion efficiency of 0.16% is achieved, which is comparable with results achieved using periodically poled lithium niobate crystal. Neither photorefractive effects nor damage was observed with up to 900 m J / c m 2 , showing the great potential of Rb:PPKTP for multi-millijoule-level MC-THz generation.

Highly-efficient optical generation of narrowband terahertz radiation enables unexplored technologies and sciences from compact electron acceleration to charge manipulation in solids. State-of-the-art conversion efficiencies are currently achieved using difference-frequency generation driven by temporal beating of chirped pulses but remain, however, far lower than desired or predicted. Here we show that high-order spectral phase fundamentally limits the efficiency of narrowband difference-frequency generation using chirped-pulse beating and resolve this limitation by introducing a novel technique based on tuning the relative spectral phase of the pulses. For optical terahertz generation, we demonstrate a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz pulses, yielding a record energy of 0.6 mJ and exceeding previous optically-generated energies by over an order of magnitude. Our results prove the feasibility of millijoule-scale applications like terahertz-based electron accelerators and light sources and solve the long-standing problem of temporal irregularities in the pulse trains generated by interfering chirped pulses., Optical generation of terahertz radiation is needed for many applications, but gaining high efficiency is still a challenge. The authors report a method to overcome dispersion effects in interfering chirp pulses used for THz pulse production by tuning their relative spectral phase, enabling 0.6 mJ of THz energy output.

The potential of a linear THz accelerator technology is discussed. Theoretical and first experimental results on THz-driven guns and accelerators are presented with a focus on laser based THz generation to drive these devices.

40th International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2015, Hong Kong, China, 23 Aug 2015 - 28 Aug 2015 ; (2015). doi:10.1109/IRMMW-THz.2015.7327586, Highly efficient generation of strong-field terahertz (THz) pulses by using very short laser pulses ~30 fs has some challenges due to chromatic aberrations. Here, we demonstrate an optical-to-THz conversion efficiency of 0.2% using a 3 mJ pump pulse energy. This result paves the way for strong-field applications of THz radiation. Commercial Ti:sapphire laser systems delivering more than 20 mJ output pulse energy at the central wavelength of 800 nm with 150 fs pulse width is appropriate for table-top, compact terahertz (THz) sources [1]. The expected maximum THz output pulse energy can be scaled up to ~100 μJ, when employing optical rectification using tilted-pulse-fronts (TPF) and cryogenic cooling to mitigate THz absorption in the lithium niobate crystal. We have already demonstrated 0.2% optical-to-THz energy efficiency by using 150 fs Ti:sapphire laser pulses.In order to further scale up the THz output energy from the μJ to mJ-level, customized Ti:sapphire systems delivering J-level output pulse energy are promising. However, this kind of laser system has an extremely broadband infrared spectrum. When these ultrashort laser pulses (30 fs) are used for THz generation with the conventional TPF technique, there are several limitations [2]: (i) Effective interaction length for efficient THz generation will be shorter than longer pulses (150 fs). (ii) The diffracted optical beam from the grating will be expanded to an unmanageably large size due to the large bandwidth. (iii) Different spectral components will be imaged into different spatial volumes in the crystal. We systematically investigatedifferent imaging schemes including one concave mirror (f=-100-mm), two concave mirrors (f1=-200 mm, f2=-100 mm) and one bi-convex lens (f=60 mm) for THz generation using TPF in lithium niobate driven by 30 fs Ti:sapphire laser pulses. The best results of 6 μJ THz output energy, 0.2% optical-to-THz conversion efficiency with 20 MV/m electric field in lithium niobate at room temperature pumped at 3 mJ is achieved from the simplest scheme with one bi-convex lens as the imaging element, shown in Fig. 1 (a). As exhibited in Fig. 1 (b) and (c), the single-cycle THz pulse holds a peak frequency at 0.32 THz. The maintenance of 0.2% optical-to-THz efficiency from 150 fs to 30 fs is helpful for scaling up THz output energy from μJ to mJ-level when employing J-level ultrashort Ti:sapphiure laser pulses. Future work will be focused on cryogenic cooling of the generationcrystal, improving out-coupling of the THz pulse at the interface of lithium niobate and air, newly designing the generation lithium niobate crystals, trying contact-grating method to make a linear generation geometry, and finally scaling up the output THz energy by impinging the generation crystal with J-level laser pulses for THz generation.

Optics letters 42(11), 2118 - 2121 (2017). doi:10.1364/OL.42.002118, We generate narrowband terahertz (THz) radiation in periodically poled lithium niobate (PPLN) crystals using two chirped-and-delayed driver pulses from a high-energy Ti:sapphire laser. The generated frequency is determined by the phase-matching condition in the PPLN and influences the temporal delay of the two pulses for efficient terahertz generation. We achieve internal conversion efficiencies up to 0.13% as well as a record multicycle THz energy of 40 μJ at 0.544 THz in a cryogenically cooled PPLN., Published by OSA, Washington, DC

We experimentally investigate the limits to 800 nm-to-terahertz (THz) energy conversion in lithium niobate at room temperature driven by amplified Ti:Sapphire laser pulses with tilted-pulse-front. The influence of the pump central wavelength, pulse duration, and fluence on THz generation is studied. We achieved a high peak efficiency of 0.12% using transform limited 150 fs pulses and observed saturation of the optical to THz conversion efficiency at a fluence of 15 mJ/cm2. We experimentally identify two main limitations for the scaling of optical-to-THz conversion efficiencies: (i) the large spectral broadening of the optical pump spectrum in combination with large angular dispersion of the tilted-pulse-front and (ii) free-carrier absorption of THz radiation due to multi-photon absorption of the 800 nm radiation., 4 pages, 6 figures

We investigate terahertz generation in lithium niobate via optical rectification using tilted-pulse-fronts in 800 nm wavelength. We obtain a 0.12% efficiency and verify the cascading effect with angular dispersion is the limitations to THz generation.

With the development of modem THz technology [1], which can provide electric fields with GV/m gradients, THz-based control and manipulation of the electron bunches has become possible. THz-driven electron acceleration, compression and streaking have attracted much attention recently [2, 3]. Here, we present a novel THz driven electron streak camera that provides sub-fs temporal resolution using a multilayer structure.