Ultrafast response of harmonic modelocked THz lasers

Abstract

The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. However, unlike classic laser diodes, no demonstrations of harmonic modelocking, active or passive, have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity's fundamental round-trip frequency. Here, using time-resolved THz techniques, we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. We demonstrate the generation of the harmonic electrical beatnote within a QCL, its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach. Finally, we show indications of self-starting harmonic emission, i.e., without external modulation, where the QCL operates exclusively on a harmonic (up to its 15th harmonic) of the round-trip frequency. This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic, as well as photonic, nature of QCLs. These results open up the prospect of passive harmonic modelocking and THz pulse generation, as well as the generation of low-noise microwave generation in the hundreds of GHz region.

Chip‐Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching

“…At higher I B (between 270 and 320 mA), the lower-frequency AR module crosses threshold. In this range, the QCL operates in an harmonic state, [17,[32][33][34] where the spacing between adjacent modes is 2 f rt for the higher frequency AR module, whereas it is equal to 6 f rt for the lower frequency module (Figure 6c). This regime is driven by the interplay of the third-order population pulsation (PP) nonlinearity and the population grating (PG) induced in the cavity by the primary mode of the lower frequency AR, [32] the intense mode at 2.73 THz in Figure 6c.…”

Millimeter wave photonics with terahertz semiconductor lasers

“…The efficiency of QCL 1 , defined here as the ratio between the integrated mmWave and the THz spectrum, was found to be ~1.7 × 10 −4 . Previously work has indicated the possibility of phase matching in measurements of the microwave transmission 48 and pulse generation in THz QCLs 43 , 49 . Here, using the time resolved nature of our experimental technique, we show directly the process is phase matched between the THz QCL emission and the nonlinear mmWave generation.…”

Observation of Self Mode-Locking in Terahertz Quantum Cascade Lasers through Intra-Cavity Self-Detection

“…A first striking feature is that while WF-R4 shows one pulse per roundtrip, i.e. a repetition rate of 2.43GHz, corresponding to the cavity FSR, WF-R21 shows two pulses per roundtrip, with a repetition rate at ~4.83GHz, at twice the FRS [21,36]. Red and blue dots in Fig.…”