Photon-Driven Broadband Emission and Frequency Comb RF Injection Locking in THz Quantum Cascade Lasers

Abstract

We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultra-broadband operation exceeding 1.83THz (∆f /f = 50%) with high phase noise, exhibiting self-sustained, periodic voltage oscillations. The low CW threshold (115 A· cm −2 ) and broadband comb operation (∆f /f = 25%) make these sources extremely appealing for on-chip frequency comb applications.arXiv:1912.00890v1 [physics.optics]

RF Injection of THz QCL Combs at 80 K Emitting over 700 GHz Spectral Bandwidth

“…We observe that, in fact, the regions of current where only one well defined beatnote is present, indicated by the pink boxes on the left side of each beatnote map, are more extended as the temperature is increased. Such behaviour is significantly different from what is observed with devices from the same epilayer processed as Au-Au ridges [23].…”

“…We showed the impacts of low-loss Cu-Cu waveguides on laser performance, allowing pulsed operation above 135 K and CW above 95 K, more than 30 K resp 10 K higher than the respective Au-Au counterparts [23]. Simultaneously, output power of roughly 8 mW in pulsed and CW up to 70 K and 5.8 mW in CW at 80 K for a 4 mm long and 85 µm wide device are found.…”

“…At 80 K, we still measure 5.6 mW in CW at rollover. The output power of the Cu-Cu lasers is also considerably increased with respect to the Au-Au waveguides (data not shown) [23] at temperatures above 70 K.…”

“…J thresh = 157 Acm −2 in CW at 20 K resp. 80 K), a wide gain bandwidth and a strong photon driven transport, indication of a long upper state lifetime [23]. The presented active region is fabricated into double-metal ridge waveguides, defined by Inductively Coupled Plasma (ICP) dry-etching, cleaved to the desired cavity length and indium soldered to a copper submount.…”

Homogeneous quantum cascade lasers operating as terahertz frequency combs over their entire operational regime

“…However, to achieve a stable comb regime, the total chromatic dispersion has to be minimized over the laser operational bandwidth [8] so that the phase mismatch from the material, the laser waveguide, and the active region gain approaches zero in the QCL cavity. This has been achieved either by integrating a dispersion compensator in a homogeneous QCL active region [8] providing a negative group velocity dispersion (GVD) to cancel the positive cavity dispersion or by gain medium engineering of heterogeneous multistacked active regions (ARs) [10,17,18] or of individual homogeneous ARs [19], in both cases designed to have a relatively flat gain top, within which the intrinsic dispersion is small enough to preserve comb formation. Heterogeneous designs are ideal for achieving a broad spectral coverage, but they also come with some inherent disadvantages, such as the design-related difficulties associated with a proper matching of the threshold currents between the different AR modules, which can prevent the simultaneous mode proliferation over the full operation range of the QCL.…”