Nonlinear couplings between photons and electrons in new materials give rise to a wealth of interesting nonlinear phenomena [1]. This includes frequency mixing, optical rectification or nonlinear current generation, which are of particular interest for generating radiation in spectral regions that are difficult to access, such as the terahertz gap. Owing to its specific linear dispersion and high electron mobility at room temperature, graphene is particularly attractive for realizing strong nonlinear effects [2]. However, since graphene is a centrosymmetric material, second-order nonlinearities a priori cancel, which imposes to rely on less attractive third-order nonlinearities [3]. It was nevertheless recently demonstrated that dc-second-order nonlinear currents [4] as well as ultrafast ac-currents [5] can be generated in graphene under optical excitation. The asymmetry is introduced by the excitation at oblique incidence, resulting in the transfer of photon momentum to the electron system, known as the

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We implement a Maxwell-Bloch simulation for a two-level system within the finite-difference time-domain method to simulate the seeding of lasers by broadband pulse injection. The model does not make the slowly varying envelope approximation, and the full electromagnetic field is simulated so that we are able to obtain time-resolved seeding by few-cycle pulses. The model is compared to recent results on seeding of THz quantum cascade lasers to aid in the interpretation of their complex signals. The simulations are found to be in good agreement with the data when gain recovery times of 15 ps are used. Furthermore, we find that the emission from the laser depends only weakly on the seed used to initiate laser action. The model is readily applicable to any seeded laser system where few-cycle seed pulses are used.

Pulses from an actively modelocked terahertz quantum cascade laser are fully characterized using an optical sampling technique to detect the total instantaneous terahertz intensity. By triggering the quantum cascade laser electronics with a femtosecond laser, we are able to measure both the formation of modelocked pulses and the quasi-steady state. The dependence of the pulse width on the modulation power and drive current are investigated. At low drive currents, we measure transform-limited gaussian-shaped pulses with a FWHM of 19 ps.

We measure the electric field of a train of modelocked pulses from a quantum cascade laser in the time-domain by electro-optic sampling. The method relies on synchronizing the modelocked pulses to a reference laser and is applied to 15-ps pulses generated by a 2-THz quantum cascade laser. The pulses from the actively modelocked laser are completely characterized in field and in time with a sub-ps resolution, allowing us to determine the amplitude and phase of each cavity mode. The technique can also give access to the carrier-envelope phase of each pulse. quantum cascade laser by an automated design approach," Appl. Phys. Lett. 93, 191119 (2008).

Abstract:The emission of a quantum cascade laser can be synchronized to the repetition rate of a femtosecond laser through the use of coherent injection seeding. This synchronization defines a sampling coherence between the terahertz laser emission and the femtosecond laser which enables coherent field detection. In this letter the sampling coherence is measured in the time-domain through the use of coherent and incoherent detection. For large seed amplitudes the emission is synchronized, while for small seed amplitudes the emission is non-synchronized. For intermediate seed amplitudes the emission exhibits a partial sampling coherence that is time-dependent. 211-212 (1982). 4. A. Nahata, A. S. Weling, and T. F. Heinz, "A wideband coherent terahertz spectroscopy system using optical rectification and electro-optic sampling," Appl. Phys. Lett. 69(16), 2321-2323 (1996). 5. J. Dai, X. Xie, and X. C. Zhang, "Detection of broadband terahertz waves with a laser-induced plasma in gases," Phys. Rev. Lett. 97(10), 103903 (2006). 6. D. H. Auston and K. P. Cheung, "Coherent time-domain far-infrared spectroscopy," J. Opt. Soc. Am. B 2(4), 606-612 (1985). 7. C. A. Schmuttenmaer, "Exploring dynamics in the far-infrared with terahertz spectroscopy," Chem. Rev. 104(4), 1759-1780 (2004). ©2012 Optical Society of America 41(3),

A novel scheme to achieve mode-locking of a multimode laser is demonstrated. Traditional methods to produce ultrashort laser pulses are based on modulating the cavity gain or losses at the cavity roundtrip frequency, favoring the pulsed emission. Here, we rather directly act on the phases of the modes, resulting in constructive interference for the appropriated phase relationship. This was performed on a terahertz quantum cascade laser by multimode injection seeding with an external terahertz pulse, resulting in phase mode-locked terahertz laser pulses of 9 ps duration, characterized unambiguously in the time domain.

We study the effect of parylene antireflection coatings on the gain of a 2.8 THz quantum cascade laser using terahertz time-domain spectroscopy. With antireflection coatings the threshold current increases as the mirror losses are increased, and the gain clamps at 16 cm−1, compared to 10 cm−1 for an uncoated device. These values are consistent with a drop in reflectivity from 0.320 to 0.053 as a consequence of the coating deposition. Further improvements could reveal the bare cavity gain and permit the quantum cascade laser to be used as an efficient terahertz amplifier.