Short mode-locked pulses from planarized Y-coupled THz lasers

TL;DR

The paper addresses the challenge of generating short, high-bandwidth THz pulses with on-chip sources. It introduces inverse-designed planarized Y-coupled THz QCLs and demonstrates active mode-locking that yields pulse trains with durations as short as 2.3 ps and spectral bandwidths of 500–700 GHz around a center frequency of 2.9 THz. Symmetric devices show phase-locked, high-contrast far-field emission with broad but dispersion-limited comb-like behavior, while asymmetric devices reveal a dense, quasi-continuum mode structure that supports coherent mode-locking under microwave modulation, achieving 2.3–3.6 ps pulses as measured by SWIFT spectroscopy. This work provides a compact, tunable THz pulsed-source platform with potential applications in THz spectroscopy, imaging, and nonlinear THz studies, and suggests avenues for beam steering via controlled phase relations in the splitter arms.

Abstract

Short THz pulses are highly attractive both for fundamental research and practical applications in this underdeveloped region of the electromagnetic spectrum. Typically, THz pulses are generated using nonlinear optical methods, starting from a high-power visible or near-IR mode-locked source, but usually suffer from low conversion efficiencies. Here, we present a direct on-chip THz source of coherent short pulse trains based on active mode-locking of an inverse-designed planarized Y-coupled THz quantum cascade laser. By employing quasi-resonant microwave modulation of the asymmetric laser cavity, mode-locked pulses as short as 2.3 ps are generated, with emission bandwidths spanning 500 and 700 GHz at a central frequency of 2.9 THz.

Figures (5)

• Figure 1: Optical microscope images of two generations of planarized Y-coupled waveguides, where the splitter region was obtained using inverse design (zoomed-in region shown in the top right insets). The original symmetric design is shown in (a), while the second-generation splitter design with asymmetric arms is displayed in (b). The bottom right insets show the simulated electric field intensity at 3 THz, which is split equally into both Y-splitter arms with minimal reflections or scattering losses.
• Figure 2: Experimental emission spectrum (a-c) and corresponding far-field pattern measurements (d-f) of a symmetric Y-coupled device extracted from the side with two arms. The characteristic high-contrast symmetric interference pattern along the horizontal direction is an indication that the two arms emit equal intensities and are in phase throughout the spectrum for different laser bias points.
• Figure 3: (a) RF beatnote map of an asymmetric Y-split device measured in CW at 25 K. A seemingly chaotic RF signal is observed through the majority of the bias range. (b) Measured THz emission spectrum of an asymmetric Y-split device (bottom panel), which has a very irregular mode spacing. This is also evident in the extracted mode spacing (top panel) as well as in the RF spectrum, which features many lines (blue dashed line in (a)).
• Figure 4: Measured FTIR spectra and SWIFT spectroscopy results of actively mode-locked asymmetric Y-coupled devices. (a-c) A relatively flat spectrum over a bandwidth of 500 GHz with zero intermodal phase differences produces the shortest pulses of 2.3 ps. (d-f) An even broader emission spectrum of 700 GHz, similar in shape to a soliton spectral envelope, produces very clean pulses with a duration of 3.6 ps. The actual pulse duration is most likely even shorter (2.7 ps), as only the central part of the spectrum could be measured with SWIFTS.
• Figure 5: Experimental sweep of the RF modulation frequency with (a) the corresponding THz emission spectra and (b) extracted mode spacing.