Stable mode-locked pulses from mid-infrared semiconductor lasers

Stable trains of ultrashort light pulses with large instantaneous intensities from mode-locked lasers are key elements for many important applications such as generate short pulses in the mid-infrared (3.5-20 µm) molecular " fingerprint " region relies on the down-conversion of short-wavelength mode-locked lasers through nonlinear processes, such as optical parametric generation [9-11] and four-wave mixing [12]. These systems are usually bulky, expensive and typically require a complicated optical arrangement. Here we report the unequivocal demonstration of mid-infrared mode-locked pulses from a semiconductor laser. anticipate our results to be a significant step toward a compact, electrically-pumped source generating ultrashort light pulses in the mid-infrared and terahertz spectral ranges. Quantum cascade lasers (QCLs) [13], since their invention in 1994, have become the most prominent coherent light sources in the mid-infrared. One of the most striking differences between these unipolar devices and diode lasers is that their emission wavelength, gain spectrum [14], carrier transport characteristics, and optical dispersion can be engineered. This remarkable design freedom makes QCLs a unique candidate to serve as a semiconductor source of ultra-short pulses in the mid-infrared. There is, however, an obstacle of fundamental origin that has so far prevented achieving ultrashort pulse generation in QCLs [15, 16]. In intersubband transitions, the carrier relaxation is extremely fast because of optical phonon scattering. As a result, the gain recovery time in QCLs is typically on the order of a few picoseconds [17, 18] which is an order of magnitude smaller than the cavity roundtrip time of 40-60 ps for a typical 2-3mm-long laser cavity. According to conventional mode-locking theory, this situation prevents the occurrence of stable passive mode-locking and impedes the formation of high-intensity pulses through active mode-locking [19]. 3 In this work, we achieve stable mode-locking by designing a QCL structure with longer gain recovery time than conventional QCL designs and actively modulating the pumping current of a small section at one end of the laser cavity to provide net gain to the pulse. To our knowledge, this is also the first stable mode-locking of a laser with fast gain recovery time. Our QCL structure, shown in Figure 1a, is based on a " diagonal transition " in real space [20], i.e. the laser transition takes place between levels confined in two adjacent wells separated by a thick barrier. The reduced wavefunction overlap between the upper and lower laser states results in a phonon-limited upper state lifetime of …