External cavity terahertz quantum cascade laser with a metamaterial/graphene optoelectronic mirror

Abstract

Photonic engineering of the terahertz emission from a quantum cascade laser (QCL) is fundamental for the exploitation of this unique source in a myriad of applications where it can be implemented, such as spectroscopy, imaging, and sensing. Active control of the frequency, power, polarization, and beam profile has been achieved through a variety of approaches. In particular, the active control of the emitted frequency, which is difficult to determine a priori, has been achieved through the integration of a photonic structure and/or by using external cavity arrangements. In this work, an external cavity arrangement, which implements a metamaterial/graphene optoelectronic mirror as an external feedback element, is proposed and demonstrated. The reflectivity and dispersion properties of the external active mirror were tuned via electrostatically gating graphene. It was possible to electronically reproduce the mode-switch occurring in a QCL emitting ∼2.8 THz by mechanically changing the external cavity length formed by an Au mirror. The external cavity arrangement was investigated and described in the framework of the self-mixing theory. These results open a way for the all-electronic engineering of the QCL emission by the use of a fast reconfigurable external mirror. This approach can uniquely address both power and frequency control, with ∼100 MHz reconfiguration speeds, using an integrated external element. Furthermore, the metamaterial/graphene mirror's strong dispersive properties might be implemented for the active mode locking of THz QCLs. Finally, this approach offers a unique opportunity to study the laser dynamics and mode competition in THz QCLs in the self-mixing feedback regime.

All in One‐Chip, Electrolyte‐Gated Graphene Amplitude Modulator, Saturable Absorber Mirror and Metrological Frequency‐Tuner in the 2–5 THz Range

“…All measurements have been acquired in the weak feedback regime, i.e., with an Acket's feedback coefficient C < 1. [ 70–72 ]…”

“…All measurements have been acquired in the weak feedback regime, i.e., with an Acket's feedback coefficient C < 1. [70][71][72] The reflected intensity along the z-axis comprises the unsaturated and the saturated terms, the latter manifesting itself as a deviation from the unsaturated intensity, having the typical Lorentzian lineshape whose FWHM is the Rayleigh length. This corresponds to an experimental increase of the reflection coefficient in the focal plane, whose magnitude depends on the impinging beam fluence and on the sample saturation intensity threshold.…”

Self-Pulsations in Terahertz Quantum Cascade Lasers under Strong Optical Feedback: The Effect of Multiple Reflections in the External Cavity

“…It is worth noting that the results shown in Figure 5 and Figure 6 apply to general QCLs without any coatings on the laser facets where the reflection coefficient is determined only by the reflective index of the semiconductor material of the active region of the laser (3.3 in this case). However, for EC QCLs with an AR coating on the front facet of the laser chip, the reflection coefficient is reduced to lower than 2% [ 29 , 31 ] and the transmittance of the light beam coupled into the external cavity is significantly enhanced. In this case, the feedback coupling coefficient is greatly increased.…”

“…However, the influence of multiple ECRTs, which emerges in the presence of moderate and strong optical feedback, on the self-pulsations and the laser response to the optical feedback in general are still unexplored so far using a more rigorous model. The strong feedback condition with multiple ECRTs is automatically satisfied for external cavity (EC) QCLs with emission frequency tunabilities because the front laser facet is anti-reflection (AR)-coated with the reflection coefficient lower than 2% [ 29 , 30 , 31 ] to make the laser more sensitive to optical feedback and to suppress the laser cavity modes.…”

Wavelength Scaling of Widely-Tunable Terahertz Quantum-Cascade Metasurface Lasers

“…Such a device would be similar to swept-wavelength VCSELs and could have applications in time-resolved laser absorption spectroscopy and swept-source optical coherence tomography [47], [48]. Another promising approach would be metasurface output coupler mirrors with optoelectronic control of the reflection phase, due to their fast modulation speeds (>100 MHz) and lack of moving parts [49], [50]. To be suitable to use with the VECSEL, such a metasurface output coupler would need to be partially transmitting, and have sufficiently low loss so that reflectances >90% could be obtained across the tuning band.…”