Terahertz (THz) frequency quantum cascade lasers emitting peak powers of >1 W from a single facet in the pulsed mode are demonstrated. The active region is based on a bound-to-continuum transition with a one-well injector, and is embedded into a surface-plasmon waveguide. The lasers emit at a frequency of ∼3.4 THz and have a maximum operating temperature of 123 K. The maximum measured emitted powers are ∼1.01 W at 10 K and ∼420 mW at 77 K, with no correction made to allow for the optical collection efficiency of the apparatus.
The generation of ultrashort pulses from quantum cascade lasers (QCLs) has proved to be challenging. It has been suggested that the ultrafast electron dynamics of these devices is the limiting factor for modelocking and hence pulse formation. Even so, clear modelocking of terahertz (THz) QCLs has been recently demonstrated but the exact mechanism for pulse generation is not fully understood. Here we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic modulation and the generated THz pulse in the QCL. By using phase resolved detection of the electric field in QCLs embedded in metal-metal waveguides, we demonstrate that active modelocking requires the phase velocity of the microwave round trip modulation to equal the group velocity of the THz pulse. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting short pulse generation. Modelocking was performed on QCLs employing phonon depopulation active regions, permitting coherent detection of large gain bandwidths (500 GHz), and the generation of 11 ps pulses centered around 2.6 THz when the above 'phase-matching' condition is satisfied. This work brings an enhanced understanding of QCL modelocking and will permit new concepts to be explored to generate shorter and more intense pulses from mid-infrared, as well as THz, QCLs.
Multi-Watt high-power terahertz (THz) frequency quantum cascade lasers are demonstrated, based on a single, epitaxially grown, 24-μm-thick active region embedded into a surface-plasmon waveguide. The devices emit in pulsed mode at a frequency of ∼4.4 THz and have a maximum operating temperature of 132 K. The maximum measurable emitted powers from a single facet are ∼2.4 W at 10 K and ∼1.8 W at 77 K, with no correction being made for the optical collection efficiency of the apparatus, or absorption by the cryostat polyethylene window.Introduction: Terahertz (THz) frequency radiation has many potential applications, ranging from imaging, bio-and chemical-sensing, and non-destructive testing, through to security scanning, industrial process monitoring, and telecommunications [1,2]. However, one of the principal challenges is to develop compact, low-cost, efficient THz sources. In this respect, the development of the THz quantum cascade laser (QCL) provides a potential solid-state solution [3]. Nevertheless, for many remote sensing and imaging applications, for example, realtime measurement using a THz camera, high optical powers are desirable [4]. In addition, a high-power THz source is attractive for the investigation of non-linear physics at THz frequencies.In general, increased output powers can be obtained, in both conventional interband semiconductor lasers and mid-infrared QCLs, by using broader area cavities [5]. Relying on this strategy, we previously demonstrated 1.01 W peak output powers (P peak ) from a broad-area THz QCL [6]. However, scaling the device area to an even larger value leads to difficulties in managing the significant Joule heating and random filamentation [5]. As an alternative, the power can be increased by increasing the active region thickness, i.e. the number of cascade periods [7]. Indeed, THz QCLs with P peak of up to 470 mW per facet at 5 K have been demonstrated, using a direct wafer-bonding technique to stack two separate 10-μm-thick THz QCLs together, thereby increasing the active region thickness [8]. This approach, however, requires the QCL to have a symmetric active region, limiting widespread applicability of the technique. In this Letter, we demonstrate multi-Watt high-power THz QCLs with a 24-μm-thick active region, grown in a single epitaxial growth. The devices operate in pulsed mode with emission at a frequency of ∼4.4 THz and deliver P peak up to ∼2.4 W at 10 K and ∼1.8 W at 77 K.
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