Integrated injection seeded terahertz source and amplifier for time-domain spectroscopy

Abstract

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

Measuring the sampling coherence of a terahertz quantum cascade laser

“…(Besides the primary seed pulse at ~0ps which occur at the round-trip time, secondary seed pulses arises from reflections in the photoconductive antenna substrate. Without the secondary seed pulses the QCL emission would only consist of the primary pulses separated by the round-trip time [21]. )…”

Detection of terahertz frequency radiation via the photothermoelastic response of zincblende crystals

“…(16). It should be noted that in the above analysis ΔB i and hence Δnr; x are both functions of x due to attenuation of the THz power as the beam propagates through the crystal, as well as functions of the transverse position y; z [or equivalently r; φ in cylindrical coordinates] by virtue of the spatial distribution of the stress field described by Eq.…”

“…(10), (11), and (24). These tensor components thereby enable calculation of the modulation of (14) and (16). The total phase delay can then be obtained from Eq.…”

“…In the latter case, synchronization of the optical probe with the QCL has been accomplished by phase-seeding the QCL emission with THz pulses generated both externally [15] and internally [16]t o the QCL cavity, as well as by electrical stabilization of the QCL using a phase-locked loop [17]. The use of these coherent EO sampling schemes has led to the study of active modelocking [18], gain clamping [15], and more generally, the sampling coherence [19] of THz QCLs.…”

Gain recovery time in a terahertz quantum cascade laser

“…This has revealed a complex time-domain pulse structure, even when the QCL is operated at a fixed current. 9 At the same time, active modelocking has also been developed. Coherent measurements have been reported using both an electrical feedback technique 10 and a technique based on THz pulse injection seeding of a QCL.…”