Negar Hekmat

Narrow-band injection seeding of a terahertz frequency quantum cascade laser: Selection and suppression of longitudinal modes

A periodically poled lithium niobate (PPLN) crystal with multiple poling periods is used to generate tunable narrow-bandwidth THz pulses for injection seeding a quantum cascade laser (QCL). We demonstrate that longitudinal modes of the quantum cascade laser close to the gain maximum can be selected or suppressed according to the seed spectrum. The QCL emission spectra obtained by electro-optic sampling from the quantum cascade laser, in the most favorable case, shows high selectivity and amplification of the longitudinal modes that overlap the frequency of the narrow-band seed. Proper selection of the narrow-band THz seed from the PPLN crystal discretely tunes the longitudinal mode emission of the quantum cascade laser. Moreover, the THz wave build-up within the laser cavity is studied as a function of the round-trip time. When the seed frequency is outside the maximum of the gain spectrum the laser emission shifts to the preferential longitudinal mode.

Broadband terahertz dispersion control in hybrid waveguides.

Dispersion control is a key objective in the field of photonics and spectroscopy, since it enhances non-linear effects by both enabling phase matching and offering slow light generation. In addition, it is essential for frequency comb generation, which requires a phase-lock mechanism that is provided by broadband compensation of group velocity dispersion (GVD). At optical frequencies, there are several well-established concepts for dispersion control such as prism or grating pairs. However, terahertz dispersion control is still a challenge, thus hindering further progress in the field of terahertz science and technology. In this work, we present a hybrid waveguide with both broadband, tuneable positive and more than octave-spanning negative terahertz GVD on the order of 10-22 s2/m, which is suitable for either intra- or extra cavity operation. This new terahertz device will enable ultra-short pulse compression, allow soliton propagation, improve frequency comb operation and foster the development of novel non-linear applications.

Spectral modification of the laser emission of a terahertz quantum cascade laser induced by broad-band double pulse injection seeding

We demonstrate by injection seeding that the spectral emission of a terahertz (THz) quantum cascade laser (QCL) can be modified with broad-band THz pulses whose bandwidths are greater than the QCL bandwidth. Two broad-band THz pulses delayed in time imprint a modulation on the single THz pulse spectrum. The resulting spectrum is used to injection seed the THz QCL. By varying the time delay between the THz pulses, the amplitude distribution of the QCL longitudinal modes is modified. By applying this approach, the QCL emission is reversibly switched from multi-mode to single mode emission.

Modelocked thin-disk lasers and their application for high-power THz generation

Progress in the performance of high-power ultrafast lasers continues to give momentum to many fields of science and technology. Nowadays, ultrafast laser systems delivering hundreds of watts of average power with pulse energies ranging from hundreds of microjoules to hundreds of millijoules start to be even commercially available. In particular, disk lasers have consistently been at the forefront of this progress: their geometry is particularly well-suited for power and energy scaling of ultrashort pulses. Among these laser systems based on the disk technology, one particular technology is particularly attractive as a potential path to achieve the desired level from a simple, one-box, multi-MHz repetition rate oscillator: mode-locked thin-disk oscillators can reach hundreds of watts of average power with femtosecond pulses at multi-MHz repetition rate. Exponential progress in the achievable levels is only an illustration of their enormous potential. So far, these oscillators reach up to 275 W average power, and pulse energies up to 80 μJ, both based on Yb:YAG thin-disk lasers. This talk will review latest progress achieved with this technology and next steps and challenges towards further scaling, as well as their prospect as compact driving sources for the generation of high-power THz radiation.

Two-dimensional coherent spectroscopy of a THz quantum cascade laser: observation of multiple harmonics

Two-dimensional spectroscopy is performed on a terahertz (THz) frequency quantum cascade laser (QCL) with two broadband THz pulses. Gain switching is used to amplify the first THz pulse and the second THz pulse is used to probe the system. Fourier transforms are taken with respect to the delay time between the two THz pulses and the sampling time of the THz probe pulse. The two-dimensional spectrum consists of three peaks at (ωτ = 0, ωt = ω0), (ωτ = ω0, ωt = ω0), and (ωτ = 2ω0, ωt = ω0) where ω0 denotes the lasing frequency. The peak at ωτ = 0 represents the response of the probe to the zero-frequency (rectified) component of the instantaneous intensity and can be used to measure the gain recovery.

Selection of longitudinal modes in a terahertz quantum cascade laser via narrow-band injection seeding

A terahertz quantum cascade laser is injection seeded with narrow-band seed pulses generated from a periodically poled lithium niobate crystal. The spectral emission of the quantum cascade laser is controlled by the seed spectra.