Generating ultrafast pulses of light from quantum cascade lasers

Abstract

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.

Engineered far-fields of metal-metal terahertz quantum cascade lasers with integrated planar horn structures

“…Figure 4(b) shows the resulting spectrum using an injection seeding technique [16] which shows that emission extends from 2.4 THz to 3 THz. Although there appears to be a stronger distribution in intensities of the Fabry-Perot modes when compared to the as cleaved device [20], a similar bandwidth is obtained illustrating the broad spectral response of the PHS. As can be observed from figure 3, although larger output powers can be achieved, a large ridge with a PHS results in a double lobed far-field, which is not ideal for applications.…”

“…To illustrate the broadband spectral nature of the PHS, the spectrum of a QCL operating with a similar active region but operating around 2.7 THz with a bandwidth of ~ 600 GHz [20] was investigated. The sample ridge width here was 60 µm with a cavity length of 3mm and the PHS length was 150 µm.…”

Short Terahertz Pulse Generation from a Dispersion Compensated Modelocked Semiconductor Laser

“…Active mode locking, where the device is electrically modulated at its’ roundtrip, has been extensively applied but the pulses generated so far have been limited to the range of 10ps to 20ps, despite several years of research effort . Although THz QCLs with extremely large gain bandwidths have been realized leading to impressive developments in frequency comb generation, this has not translated directly into the formation of stable short pulses in the THz range (In general the pulse width is inversely proportional to the spectral bandwidth) …”

Short THz pulse generation from a dispersion compensated modelocked quantum cascade laser

“…Active mode locking, where the device is electrically modulated at its' roundtrip, has been extensively applied but the pulses generated so far have been limited to the range of 10ps to 20ps, despite several years of research effort. [2,3] Although THz QCLs with extremely large gain bandwidths have been realized leading to impressive developments in frequency comb generation, [4,5] this has not translated directly into the formation of stable short pulses in the THz range, although single pulses of 2.5ps have been shown [6].…”