Gian Lorenzo Paravicini-Bagliani

Enhanced current injection from a quantum well to a quantum dash in magnetic field

Resonant tunneling injection is a key ingredient in achieving population inversion in a putative quantum dot cascade laser. In a quantum dot based structure, such resonant current requires a matching of the wavefunction shape in k-space between the injector and the quantum dot. We show experimentally that the injection into an excited state of a dash structure can be enhanced tenfold by an in-plane magnetic field that shifts the injector distribution in k-space. These experiments, performed on resonant tunneling diode structures, show unambiguously resonant tunneling into an ensemble of InAs dashes grown between two AlInAs barrier layers. They also show that interface roughness scattering can enhance the tunneling current.

Superradiantly Limited Linewidth in Complementary THz Metamaterials on Si-Membranes

We study complementary double split ring THz resonators fabricated on a 10 mum thin Si-membrane. The linewidths of the fundamental LC-mode and dipolar mode are drastically narrowing with increased resonator spacing. The extracted decay rate of the LC-mode as a function of the resonator density shows a linear dependence, evidencing a collective superradiant effect of the resonator array. Furthermore, we show that a metamaterial can be designed for a low superradiant broadening of the resonance at high resonator densities, i.e. in the metamaterial condition. The use of a thin membrane as a substrate is crucial, since it shifts the THz surface plasmon polaritons modes to much higher frequencies, preventing them to couple to the LC mode and unveiling the superradiant broadening mechanism for a large range of lattice spacings. At higher frequencies, not interfering with the high Q LC-mode, other additional modes, which we ascribe to photonic crystal modes, form in the Si-membrane. We map the angle dependent band structure and show corresponding simulated electric field distributions.

Highly non-parabolic strained Ge quantum well for THz ultra-strong light-matter coupling

Ultra-strong light-matter interactions can be realized in various physical systems and has thus attracted many experimental and theoretical investigations [1-4]. One possible realization is to couple strongly subwavelength split ring resonators (SRR) to the Landau level transition of a two dimensional electron (or hole) gas [2, 3]. In a previous work on parabolic AlGaAs/GaAs QWs, we showed that very high values of the normalized vacuum Rabi frequency Ω/ω = 0.87 can be reached [5]. Strained Ge quantum wells, which are used for the present study, are very appealing as the material exhibits a strong non-parabolicity. The non-parabolicity can be directly observed in THz spectroscopy [6, 7]. The heavy-hole cyclotron resonance spin-splits at a magnetic field of B ∼ 4.5 T [6, 7]. A THz split ring resonator array is deposited on top of the sGe QW, as shown in a sample sketch in Fig. 1 d) with the resonances shown in Fig. 1 b). Coupling a LC-resonance at f = 0.4 THz to the single cyclotron resonance at B ∼ 1.5 T leads to an anti-crossing, as shown in Fig. 1 a). In Fig. 1 c) a zoom to the LC-resonance with fitted polaritons branches is shown and reveals a normalized coupling ratio of Ω/ω = 0.25. The polaritons branches are fitted with a Hopfield-like Hamiltonian [1, 2] and is in good agreement with the experiment. The dipole-like mode of the resonator couples to the spin-splitted heavy-hole cyclotron resonance at B = 4.5 T and f = 1.25 THz. Multiple polaritons can be observed [8], as shown in Fig. 1 a). A modeling of this multiple oscillator system is done by expanding the Hopfield-like Hamiltonian to include all involved oscillators.

THz surface plasmon polariton modes coupled to complementary metasurfaces tuned by inter meta-atom distance

Tailoring the electro-magnetic response of materials beyond naturally occurring properties is possible with the concept of meta-materials [1]. Subwavelength elements which are usually closely spaced can influence the electro-magnetic response and form a fundamental building block of modern optics. The influence of the spacing of the meta-atoms has been investigated for direct meta-materials [2] but only little for complementary metamaterials [3], which are of interest e.g. in ultra-strong coupling experiments at THz frequencies [4]. The effective medium condition is changing due to the presence of a metal sheet in between the meta-atoms which has a very high refractive index in the THz.

Ultra-strong coupling with spin-split heavyhole cyclotron resonances in sGe QWs (Conference Presentation)

We study the ultra-strong coupling (USC) of Landau level transitions in strained Germanium quantum wells (sGe QW) to THz metasurfaces. The spin-splitting of the heavy-hole cyclotron resonance in sGe QWs due to the Rashba spin-orbit interaction in magnetic field offers an excellent platform to investigate ultra-strong coupling to a non-parabolic system. THz split ring resonators can be tuned to coincide with the single cyclotron transition (around 0.4 THz and a magnetic field of 1.5 T) or the spin-resolved transitions of the sGe QWs (at 1.3 THz and 4.5 T). Coupling to the single cyclotron yields a normalized USC rate of 25%, resulting from fitting with a Hopfield-like Hamiltonian model. Coupling to two or three cyclotron resonances in sGe QWs lead to the observation of multiple polaritons branches, one polariton branch for each oscillator involved in the system. An adaption of the theory allows to also describe this multiple-oscillator system and to determine the coupling strengths. The different Rabi-splittings for the multiple cyclotrons coupling to the same resonator mode relate to the underlying differences in the material. Furthermore, the visibility of an additional transition, possibly a light hole transition with very low carrier density, is strongly enhanced due to the coupling to the LC-resonance with a normalized strong coupling ratio of 4.7%. Future perspectives include controlling spin-flip transitions in USC and studying the impact of non-parabolicity on the ultra-strong coupling physics.