Ultrafast terahertz saturable absorbers using tailored intersubband polaritons

Abstract

Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices. While nonlinearities play a central role for cutting-edge functionality, they require strong field amplitudes owing to the weak lightmatter coupling of electronic resonances of naturally occurring materials. Here, we ultrastrongly couple intersubband transitions of semiconductor quantum wells to the photonic mode of a metallic cavity in order to custom-tailor the population and polarization dynamics of intersubband cavity polaritons in the saturation regime. Two-dimensional THz spectroscopy reveals strong subcycle nonlinearities including six-wave mixing and a collapse of lightmatter coupling within 900 fs. This collapse bleaches the absorption, at a peak intensity one order of magnitude lower than previous all-integrated approaches and well achievable by state-of-the-art QCLs, as demonstrated by a saturation of the structure under cw-excitation. We complement our data by a quantitative theory. Our results highlight a path towards passively mode-locked QCLs based on polaritonic saturable absorbers in a monolithic singlechip design.

Chip‐Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching

“…By pumping the polaritonic mirror with a CW, 2.75 THz laser, we observe a spectral change of the reflectivity at intensities larger than 7.8 W cm −2 , reflected in the bleaching of the upper polaritonic mode. [ 26 ] This results in a visible reflectivity change in the 2–3 THz spectral window, as shown in Figure 1a, where three prototypical reflectivity curves, corresponding to driving currents of the incident QCL of 520, 580, and 632 mA, are shown.…”

“…[22] In addition to GDD compensation, the coupling with the nonlinear polaritonic mirror also induces non negligible effect in the QCL cavity with respect to designs relying solely on incoherent absorption. Strong-field excitation by single-cycle THz waveforms recently unveiled coherent nonlinear dynamics in the employed polaritonic grating, [26] including four-wave and six-wave mixing processes, which lead to a transient collapse of light-matter coupling on sub-cycle time scales. [26] The retrieved FWM and six-wave mixing nonlinearities might contribute to FC stabilization through the same mechanism as the fast saturable gain in the QCL AR, forcing the THz QCL into frequency-modulated operation.…”

“…Strong-field excitation by single-cycle THz waveforms recently unveiled coherent nonlinear dynamics in the employed polaritonic grating, [26] including four-wave and six-wave mixing processes, which lead to a transient collapse of light-matter coupling on sub-cycle time scales. [26] The retrieved FWM and six-wave mixing nonlinearities might contribute to FC stabilization through the same mechanism as the fast saturable gain in the QCL AR, forcing the THz QCL into frequency-modulated operation. [31] We then couple the polaritonic mirror with laser B.…”

“…The polaritonic mirror is based on a semiconductor multiquantum-well (MQW) heterostructure, resonant at the ISB transition frequency ISB = 2.7 THz [25,26] having a doping concentration of 1.0 × 10 10 cm −2 charge carriers per QW, carefully optimized to achieve an optically pumping-induced reflectivity change (polaritonic bleaching) at reasonably low pump intensities I p ∼ 11 W cm −2 . [26] The MQW stack is embedded in an ≈2 µm cavity, consisting of a back Au reflector and a top Au grating with period 16 µm. The latter provides the required optical coupling to the MQW.…”

“…This field distribution satisfies the ISB transition selection rule in the near-field via the nonvanishing component E z along the MQW growth direction. [26] The reflectance of the polaritonic mirror, measured in vacuum at low temperature (6 K) for normal incidence (Figure 1a), reveals a characteristic polaritonic doublet [27] separated by a Rabi frequency of 0.18 THz. We exploit the ultrastrong light-matter coupling of the designed ISB transition (black curve Figure 1a) to the near-field of the metallic grating; the strong field enhancement induced by the reduced mode volume significantly enhances the optical absorption when compared to the bare electronic transition, therefore leading to a reduced saturation power in the twostate polaritonic system.…”