All-optical adaptive control of quantum cascade random lasers

Spectral fingerprints of molecules are mostly accessible in the terahertz (THz) and mid-infrared ranges, such that efficient molecular-detection technologies rely on broadband coherent light sources at such frequencies. If THz Quantum Cascade Lasers can achieve octave-spanning bandwidth, their tunability and wavelength selectivity are often constrained by the geometry of their cavity. Here we introduce an adaptive control scheme for the generation of THz light in Quantum Cascade Random Lasers, whose emission spectra are reshaped by applying an optical field that restructures the permittivity of the active medium. Using a spatial light modulator combined with an optimization procedure, a beam in the near infrared (NIR) is spatially patterned to transform an initially multi-mode THz random laser into a tunable single-mode source. Moreover, we show that local NIR illumination can be used to spatially sense complex near-field interactions amongst modes. Our approach provides access to new degrees of freedom that can be harnessed to create broadly-tunable sources with interesting potential for applications like self-referenced spectroscopy.

( , ) plane on the active region ( = 3.6, blue). Panel e shows an eigenstate of at freuuency ..3 Tz, spatially extended across the device, which emphasi,es that lasing modes are not locali,ed. Panel f displays the angular far-field emission associated with the mode in panel b. Remarkably, this mode is mainly vertically-emitting as discussed in our former work 1 . Supplementary Fig. 1 emphasi,es that lasing modes are mainly governed by the random scattering provided by the holes. As shown in Supplementary Fig.1e, the spatial confinement is reali,ed through the holes' disorder while the impact of the boundary is no longer the prevalent factor as in Supplementary Fig. 1b.

Supplementary Note 2: NIR Illumination of the QCRL
Throughout the main text, the Ti:Sapphire laser is used to illuminate the QCRL according to two distinct configurations: -In Figs. 1, 3 and 4, the NIR beam is focused with a diameter of .70 µm and scanned across the surface (Fig. 1a). The beam scans the surface in 1.x1. steps and at every position ( , ) a spectrum is recorded with the FTIR. A scheme of spots' positions across the device is shown in Supplementary Fig. .a.
-In Fig. ., the NIR beam is expended on a spatial light modulator to create an intensitymodulated pattern onto the device (

Supplementary Note 3: Permittivity Tuning by NIR Illumination
The NIR beam locally tunes the permittivity of the active region. In Supplementary Fig. 3, a NIR spot is focused in the center of the chip (configuration of Supplementary Fig. .a). The intensity of the Ti:Sapphire is maintained constant at 500 mW, while its wavelength is swept and the current through the QCRL is seen increasing for wavelength below the bandgap of GaAs (813 nm). When the NIR energy exceeds the GaAs bandgap, photons penetrate into the active region and produce electronhole pairs, which results in a current increase.

Supplementary Note 4: Thermal Heating
When absorbed by the device, the NIR illumination does not produce any substantial heat increase. Supplementary Fig. 4a displays FEM simulations, in which the QCRL is modeled by a block of GaAs that absorbs the NIR light from the top. The device sits on top of a copper heat sink and is surrounded by vacuum. The Ti:Sapphire laser can emit up to 500 mW, but due to losses in the optical setup (e.g., spatial light modulator) only a fraction of this light is received by the chip. zere we assume that 100 mW are absorbed, which results in a mean temperature increase of 0.1 K. Supplementary Fig. 4b shows spectra experimentally collected while increasing device's temperature through an external thermal controller. Significant modifications in the spectra reuuire temperature increases of more than 10 K and, thus, we deduce that the NIR-induced heating of panel a is negligible. Supplementary Fig. 4 Thermal heating. a NIR-induced heating (FEM simulations). The sample is modeled by a 1000 μm-wide and 13 μm-deep rectangle of GaAs (white rectangle), which sits on top of a copper sink (Cu) while being surrounded by vacuum (Vacuum). The NIR-heating is simulated by a heat source of thickness 1 μm (corresponding to GaAs absorption length at 813 ). The heat source mimics the absorption of an optical power of 100 and produces a mean temperature increase of the whole chip of roughly 0.1 K. b Spectra experimentally measured under an increasing bulk temperature. The QCRL is pumped under a bias voltage of 11.4 V and we observe substantial changes only for temperature increases on the order of tens of Kelvins.

Supplementary Note 6: Spectral Modifications Induced by Scanning the NIR Illumination
Supplementary Fig. 6 shows spectra measured while the system is scanned by the NIR beam (configuration of Supplementary Fig. .a). Panels a to c show the evolution of the Tz, emission for a spot in the middle of the chip, while its NIR power is increased. We observe that stronger NIR power induces larger modifications. Like in Supplementary Fig. 5, the NIR beam redistributes the gain differently amongst lasing modes, which induces a modulation (i.e., increase or decrease) of intensities. The modes that are observed emerging (respectively vanishing) are brought above (respectively below) lasing threshold by the new gain distribution imposed by the NIR spot. Panel d is a color plot, in which the power is kept to 500 mW and the spot is scanned across the whole chip. It confirms that the laser emits at specific freuuencies: if a mode can stop or start lasing for given positions, it always emits at the same freuuency. Supplementary Fig. 6 Spectral modifications under spot scanning. a Spectra of the unperturbed system (red) and under a NIR beam (30 mW) at position (5,8) (green). b Spectra of the unperturbed system (red) and under a NIR beam (240 mW) at position (5,8) (green). c Spectra of the unperturbed system (red) and under a NIR beam (500 mW) at position (5,8) (green). d Color plot of the spectra collected while scanning the spot across the whole sample (500 mW).