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),

We used a terahertz (THz) quantum cascade laser (QCL) as an integrated injection seeded source and amplifier for THz time-domain spectroscopy. A THz input pulse is generated inside a QCL by illuminating the laser facet with a near-IR pulse from a femtosecond laser and amplified using gain switching. The THz output from the QCL is found to saturate upon increasing the amplitude of the THz input power, which indicates that the QCL is operating in an injection seeded regime. © 2012 Optical Society of America OCIS codes: 300.6270, 300.6495, 140.5965.In the technique of terahertz (THz) time-domain spectroscopy (TDS), THz radiation is generated and detected using femtosecond (fs) near-IR laser pulses, enabling measurement of both the THz amplitude and phase [1]. In TDS systems, the THz radiation can be generated when the near-IR fs laser illuminates a GaAs photoconductive antenna [2], travels through an electro-optic crystal with a nonvanishing χ 2 [3], or is focused in air [4]. In all these cases, the THz field amplitude scales linearly with the power of the fs pulse, until saturation takes place. One strategy to increase THz field amplitudes is to use higher fs pulse energies, and TDS systems that utilize mJ pulse energies are becoming increasingly common [5]. Although this is a promising laboratory-based technique, it is not practical for applications outside the laboratory owing to the large cost and size of regenerative amplified laser systems.An alternative approach to generate higher THz fields is to amplify the THz waves directly after generation, and hence enable low-power fs laser pulses to generate high THz wave amplitudes. THz frequency quantum cascade lasers (QCLs) are natural candidates for THz amplifiers, and they have already been used as external THz amplifiers to measure the QCL gain [6][7][8]. In the steady state, external amplification is limited by gain clamping and depends on the facet reflectivity [9]. Gain switching with RF pulses can be used to unclamp the gain in the QCL and augment external pulse amplification [10]. Amplifying THz "seed' pulses in QCLs with sufficiently long RF pulses [11] can lead to coherent injection seeding, where the QCL emission is phase locked to the fs laser. This enables phase-resolved detection of the QCL emission in the time domain, and it permits the use of QCLs as intense THz sources for TDS. However, if the THz seed pulses are generated by external photoconductive antennas, the THz must be coupled from free space into the QCL cavity. This introduces large coupling losses that are associated impedance mismatch between the cavity and free space and can negate the effect of THz amplification.In this Letter, injection seeding of a QCL is achieved with THz seed pulses that are generated directly inside the QCL cavity. This results in a completely integrated

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A terahertz (THz) frequency quantum cascade laser (QCL) is used as an integrated injection seeded source and amplifier for use in THz time-domain spectroscopy. A THz input pulse is generated inside a QCL, by illuminating the laser facet with a near-infrared pulse from a femtosecond laser, and amplified using gain switching. The THz output from the QCL is found to saturate upon increasing the amplitude of the THz input power, which indicates that the QCL is operating in an injection seeded regime.