Strong light-matter coupling manipulation in SiGe quantum wells at terahertz frequencies
Description
The strong-coupling regime between an electronic transition and the photonic mode of an optical cavity manifests itself in the lifting of their degeneracy, giving rise to two polariton states with mixed optical and electronic character. The energy separation between these states, equal to twice the Rabi energy, serves as a fundamental measure of the coupling strength. In this context, recent interest in intersubband (ISB) transitions in semiconductor quantum wells has emerged due to their unique tunability: the Rabi energy can be controlled via charge density, while ISB transition energy can be adjusted in the mid-infrared and terahertz ranges through quantum well design. This flexibility allows for a transition from weak to (ultra-)strong coupling, unveiling fundamental vacuum fluctuation effects and enabling the development of efficient quantum devices such as emitters, photodetectors, and modulators.
ISB polariton studies have so far focused on III-V semiconductors, which are costly and not compatible with large-scale silicon-based fabrication. The aims of the project is to extend the ISB polariton research to the Ge/SiGe material platform which offers a viable and scalable alternative for future integration of ISBT polariton based devices into commercial photonic and electronic devices.
This research will establish fundamental physics insights and key parameters for terahertz free-space modulators on silicon wafers using foundry-compatible processes with minimal post-processing. Potential CMOS compatibility could enable large-area THz metamaterial modulators. In the long term, the project may contribute to integrating a polariton laser on silicon by exploring ISB polariton interactions with THz beams and the SiGe environment.
Aims
The aim of this project is to demonstrate the formation of ISB polaritons in the silicon-foundry-compatible group IV Ge/SiGe material system and to explore the modulation of ISB polariton spectra in the THz range—an essential advancement for future wideband wireless communications.
The innovative use of Ge/SiGe in ISB polariton physics provides a unique opportunity to study the effects of non-polar electron-phonon interactions in group-IV crystals. Unlike III-V materials, the Ge/SiGe lattice lacks a reststrahlen band in the many-THz range, avoiding the absorption and reflection issues caused by polar optical phonons, which hinder photonic experiments and applications. Additionally, the suppression of electron–longitudinal optical phonon scattering facilitates the use of superconducting (SC) cavities, enabling strong coupling at room temperature and simplifying the observation of absorption saturation effects.
To demonstrate strong/ultra-strong regime, we will use parabolic quantum wells representing the most promising design for room-temperature ISB polaritons, as they feature equidistant energy levels and uniform absorption resonance frequencies across all ISBTs. This ensures that electrons in all subbands contribute to the Rabi frequency ΩR, promoting strong and ultra-strong coupling regimes at room temperature, where multiple subbands are occupied.
Instead of using single subwavelength cavities, this project will implement optical cavities arranged in a metamaterial configuration, where each meta-atom functions as a THz microcavity. This extended-area design allows for Fourier-transform infrared (FTIR) spectroscopy investigations in the THz range. The ground plane will consist of heavily doped epitaxial SiGe, whose optical properties in the few-THz range are comparable to those of metals, thus greatly simplifying the fabrication process.
The coupling strength will be modulated by tuning the ISBT energy and applying an electric field to induce quantum tunneling between asymmetric coupled quantum wells, thereby altering the electron density in the ISB transition resonant with the cavity mode. Additionally, the possibility of coupling modulation using high-Tc superconducting metamaterial cavities will be explored.
Expected Results
- Deposition and optical characterization of n-doped compositional graded Ge/SiGe parabolic quantum well on silicon substrate having temperature independent ISBT spectra, with an absorption energy in the 5-6 THz spectral range (completed)
- Development of square patch resonators with a Metal-Insulator-Metal structure having the ground plane made of n++doped semiconductor structure and resonance energy in the 5-6 THz spectral range (completed)
- the first demonstration of ISB polaritons in the silicon-foundry compatible group IV Ge/SiGe material system; (completed)
- manipulation of the ISB polaritons via an electric field to change the electron sheet density or via switching of the losses Γ of high-Tc superconducting (SC) metamaterials
State of the art
Polaritons have been demonstrated using various approaches, including ultracold atoms in optical cavities, Cooper-pair boxes in microwave resonators, excitonic transitions in semiconductor microcavities, and surface-plasmon resonators.
In semiconductor systems, the Rabi frequency can reach extremely high values by leveraging the giant electric dipole moment of intersubband transitions between confined states in quantum wells. Moreover, ΩR can be externally controlled by adjusting the carrier population (N) in the quantum wells, scaling as √N. Additionally, for a given semiconductor material, the ISBT energy E21 can be tuned by modifying the QW thickness. These properties have paved the way for the ultra-strong coupling regime, where ΩR becomes comparable to the ISBT frequency (ω21) and exceeds the cavity decay rate (ΩR > Γc)
ISB polaritons, observed in the mid-infrared and THz ranges in III-V semiconductors material system, are studied via infrared spectroscopy, including near-field techniques. Large microcavity arrays, fabricated through lithography, enable far-field optical analysis and energy dispersion measurements by tuning cavity resonance.
Beyond fundamental physics—such as vacuum-induced correlated photon pair emission—ISB polaritons have applications in MIR/THz emitters, lasers, and QW photodetectors. “Polariton switching” experiments demonstrate rapid electron population control via laser or voltage, dynamically opening and closing the polariton gap. Strong coupling also reduces the intensity required for absorption saturation, enabling integration with quantum cascade lasers for ultrafast modulators and bistable systems, achieving speeds of hundreds of GHz.
Riferimento: PRIN 2022 – Codice progetto: 2022ZAZFSZ – CUP: F53D23001190006
Investimento totale del progetto: 234.840 euro
Partner/proponente: Sapienza Università di Roma, IFN – CNR Roma
Coordinatore dell’UdR Università degli Studi Roma Tre e PI del progetto: Monica De Seta.
