Frederike Ahr

Terahertz generation in lithium niobate driven by Ti:sapphire laser pulses and its limitations

We experimentally investigate the limits of 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/cm(2) for this pulse duration. 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.

Temperature dependent refractive index and absorption coefficient of congruent lithium niobate crystals in the terahertz range

Optical rectification with tilted pulse fronts in lithium niobate crystals is one of the most promising methods to generate terahertz (THz) radiation. In order to achieve higher optical-to-THz energy efficiency, it is necessary to cryogenically cool the crystal not only to decrease the linear phonon absorption for the generated THz wave but also to lengthen the effective interaction length between infrared pump pulses and THz waves. However, the refractive index of lithium niobate crystal at lower temperature is not the same as that at room temperature, resulting in the necessity to re-optimize or even re-build the tilted pulse front setup. Here, we performed a temperature dependent measurement of refractive index and absorption coefficient on a 6.0 mol% MgO-doped congruent lithium niobate wafer by using a THz time-domain spectrometer (THz-TDS). When the crystal temperature was decreased from 300 K to 50 K, the refractive index of the crystal in the extraordinary polarization decreased from 5.05 to 4.88 at 0.4 THz, resulting in ~1° change for the tilt angle inside the lithium niobate crystal. The angle of incidence on the grating for the tilted pulse front setup at 1030 nm with demagnification factor of -0.5 needs to be changed by 3°. The absorption coefficient decreased by 60% at 0.4 THz. These results are crucial for designing an optimum tilted pulse front setup based on lithium niobate crystals.

Cascaded interactions mediated by terahertz radiation

We investigate a regime of parametric amplification in which the pump and signal waves are spectrally separated by only a few hundreds of GHz frequency - therefore resulting in a sub-THz frequency idler wave. Operating in this regime we find an optical parametric amplifier (OPA) behavior which is highly dissimilar to conventional OPAs. In this regime, we observe multiple three-wave mixing processes occurring simultaneously which results in spectral cascading around the pump and signal wave. Via numerical simulations, we elucidate the processes at work and show that cascaded optical parametric amplification offers a pathway toward THz-wave generation beyond the Manly-Rowe limit and toward the generation of high-energy, sparse frequency-combs.

Frequency-shifted sources for Terahertz-driven linear electron acceleration

The generation of THz-frequency radiation via nonlinear parametric frequency down-conversion has long been driven by the spectroscopy and imaging communities. As a result, little efforts have been undertaken toward the generation of high energy THz-frequency pulses. THz-frequency radiation has however recently been identified has a promising driver for strong-field physics and an emerging generation of compact particle accelerators. These accelerators require THzfrequency pulses with energies in the multi-millijoule range therefore demanding orders of magnitude improvements from the current state-of-the-art. Much can be gained by improving the intrinsically low efficiency of the down-conversion process while still resorting to existing state-of-the-art lasers. However, the fundamental Manley-Rowe limit caps the efficiency of parametric downconversion from 1-μm wavelength lasers to sub-THz frequency to the sub-percent range. We present methods that promise boosting the THz radiation yield obtained via parametric down-conversion beyond the Manley-Rowe limit. Our method relies on cascaded nonlinear three-wave mixing between two spectrally neighboring laser pulses in periodically poled Lithium Niobate. Owing to favorable phase-matching, the down-conversion process avalanches, resulting in spectral broadening in the optical domain. This allows in-situ coherent multiplexing of multiple parametric down-conversion stages within a single device and boosting the efficiency of the process beyond the ManleyRowe limit. We experimentally demonstrated the concept using either broadband, spectrally chirped optical pulses from a Joule-class laser or using two narrowband lasers with neighboring wavelengths. Experimental results are backed by numerical simulations that predict conversion efficiencies from 1 μm to sub-THz radiation in the multi-percent range.

THz-driven electron streak camera based on a multilayer structure

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.

Narrowband THz generation via chirp-and-delay in PPLN

High-peak-field, narrowband terahertz (THz) sources are well suited for compact electron acceleration [1], however, these sources are still under development and yet to reach optimal energies. A high conversion efficiency via difference frequency generation is established by quasi-phase matching in periodically poled lithium niobáte (PPLN). Moreover, to achieve the required mJ-level THz pulses high energy input pulses are inevitable [2]. Chirp-and-delay pumping offers a method to increase the NIR pulse duration and to scale up the energy without damaging the crystal. Here, we present chirp-and-delay in PPLN of varying poling periods and study the effect of crystal length and temperature.

Terahertz Accelerator Technology

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.

Cascaded optical parametric amplifier in PPLN for efficient narrowband terahertz generation

We demonstrate experimentally the onset of cascaded optical parametric amplification (COPA) in periodically-poled lithium niobate. This technique permits narrowband terahertz wave generation beyond the Manley-Rowe limit.

On extracting the maximum terahertz conversion efficiency from optical rectification in lithium niobate

We report on a record 2% extracted optical-to-terahertz conversion efficiency in the mm-wavelength range through optical rectification in cryogenically-cooled lithium niobate by exploiting spatial and temporal shaping of the optical pump beam.

Terahertz Conversion Efficiency Scaling by Optical Rectification in the 800 nm Pump-Wavelength Range

We report on a record 800 nm-to-terahertz energy conversion efficiency of 0.13% at room temperature in LiNbO3 by tilting the pulse intensity front and experimentally studying optimal pumping conditions.