Planarized terahertz quantum cascade laser frequency combs for coherent integrated photonics

Urban Senica1, Sebastian Gloor1, Paolo Micheletti1, Alexander Dikopoltsev1, David Stark1, Andres Forrer1, Sara Cibella2, Guido Torrioli2, Mattias Beck1, Jérôme Faist1, and Giacomo Scalari1

1Institute for Quantum Electronics, ETH Zürich, Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland
2Istituto di Fotonica e Nanotecnologie, CNR, Via del Fosso del Cavaliere 100, 00133 Rome, Italy

Terahertz quantum cascade lasers (THz QCLs) [1, 2] are chip-scale sources of THz radiation, which can operate as broadband frequency combs [3] within the frequency region between 1-5 THz, useful for spectroscopy applications. Recently, we have developed a high-performance platform for integrated THz photonics based on planarized double metal waveguide THz QCLs [4]. Embedding the active region waveguide within a low-loss polymer (BCB) enables the fabrication of an extended top metallization and placing the bonding wires over the passive area, which improves the radio-frequency (RF), dispersion, and thermal properties of the laser devices. Moreover, it enables a monolithic integration of various active and passive components on the same photonic chip. Leveraging on this new platform, we have demonstrated several novel devices and functionalities. One major drawback of double metal waveguide THz QCLs has been their low output powers and divergent far-field patterns, a consequence of the optical mode confinement to subwavelength dimensions. We implemented an inverse design approach to optimize the shape of the front laser facet to controllably reduce the reflectivity [5]. The optical mode is then coupled via a passive waveguide to a broadband surface-emitting antenna, as illustrated in Fig. 1. Compared to a reference device with a cleaved end facet, the slope efficiency (output power) is increased by a factor of seven, while the far-field produces a narrow beam with a measured full-width half-maximum (FWHM) beam divergence of (17.0° x 18.5°).

Figure 1: Illustration of a planarized waveguide (top left), where an active double metal waveguide is embedded in a low-loss polymer (BCB). In this specific device, the usual cleaved front facet is replaced by an inverse-designed facet reflector (the inset shows an SEM image of a fabricated facet designed for a reflectivity of R = 10%, before the planarization process step). The optical mode is then coupled via a passive waveguide into a broadband surface-emitting patch array antenna, significantly improving both the slope efficiency/output power (by a factor of seven) as well as the far-field pattern (the broadband simulation result is superimposed on top of the antenna).

To improve the frequency comb performance, we also developed several optimized active waveguide geometries. Using a tapered waveguide (consisting of wide and narrow sections with a width ratio of 4:1), a strong field enhancement results in effectively increased optical nonlinearities, generating frequency-modulated (FM) combs [6]. These feature flatter emission spectra (useful for spectroscopy) and quasi-constant output intensities with a linear frequency chirp (useful for external pulse compression). In Fig. 2, we compare the SWIFT spectroscopy measurement [3] and mean-field theory [7] results of such a device, showing excellent agreement. Additionally, we developed an integrated double-chirped reflector to compensate for the chromatic dispersion of the laser cavity and were able to broaden the measured comb emission spectrum bandwidths to more than 1 THz. Finally, with high-power RF modulation of the laser waveguide, the formation of mode-locked short pulses and of several novel exotic states were observed, potentially useful for applications.

Figure 2: Measurement (SWIFT spectroscopy) and simulation (mean-field theory) results of a tapered active waveguide cavity. Due to strong field enhancement effects, self-starting frequency-modulated (FM) combs are generated. These feature flatter intensity spectra and a quasi-constant output intensity with a linear frequency chirp.

[1] J. Faist, et al. “Quantum cascade laser,” Science 264 (5158), 553-556 (1994)
[2] R. Koehler, et al. “Terahertz semiconductor-heterostructure laser,” Nature 417 (6885), 156-159 (2002)
[3] D. Burghoff, et al. “Terahertz laser frequency combs,” Nat. Photonics 8 (6), 426–467 (2014)
[4] U. Senica, et al. “Planarized THz quantum cascade lasers for broadband coherent photonics,” Light Sci. Appl. 11, 347 (2022)
[5] U. Senica, et al. “Broadband surface-emitting THz laser frequency combs with inverse-designed reflectors,” APL Photonics 8 (9), 096101 (2023)
[6] U. Senica, et al. “Frequency-modulated combs via field-enhancing tapered waveguides,” Laser & Photonics Reviews 17 (12), 2300472 (2023)
[7] D. Burghoff. “Unraveling the origin of frequency modulated combs using active cavity mean-field theory,” Optica 7 (12), 1781-1787 (2020)

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