Tunable room-temperature spin-selective optical Stark effect in solution-processed layered halide perovskites

Abstract

Ultrafast spin manipulation for opto-spin logic applications requires material systems that have strong spin-selective light-matter interaction. Conventional inorganic semiconductor nanostructures [for example, epitaxial II to VI quantum dots and III to V multiple quantum wells (MQWs)] are considered forerunners but encounter challenges such as lattice matching and cryogenic cooling requirements. Two-dimensional halide perovskite semiconductors, combining intrinsic tunable MQW structures and large oscillator strengths with facile solution processability, can offer breakthroughs in this area. We demonstrate novel room-temperature, strong ultrafast spin-selective optical Stark effect in solution-processed (C6H4FC2H4NH3)2PbI4 perovskite thin films. Exciton spin states are selectively tuned by ~6.3 meV using circularly polarized optical pulses without any external photonic cavity (that is, corresponding to a Rabi energy of ~55 meV and equivalent to applying a 70 T magnetic field), which is much larger than any conventional system. The facile halide and organic replacement in these perovskites affords control of the dielectric confinement and thus presents a straightforward strategy for tuning light-matter coupling strength.

Keywords: Layered Halide Perovskites; Non-linear; Optical Stark Effect; Rabi-splitting; Solution Processed; Spin-selective; Ultrafast; light-matter interactions; opto-spin-logic; room temperature.

Fig. 1. OSE in PEPI.

(A) Structure of PEPI with alternating organic and inorganic layers, forming multiple natural type I QW structures, with the barrier (well) being the organic (inorganic) layer (18). CB, conduction band; VB, valence band. (B) Illustration of OSE in a two-level system represented by the equilibrium states (black line) and the pump-induced Floquet quasi-states (green line) and the corresponding linear absorption and TA spectra. (C) The energy separation Δ between the excitonic absorption peak E₀ of PEPI (red) and the excitation pump ħω (blue). OD, optical density. (D) TA spectrum of PEPI following a linearly polarized pump-probe at Δt = 0 ps. Inset: Ultrafast kinetics of OSE showing a fast process comparable to the pulse duration. a.u., arbitrary units.

Fig. 2. Spin-selective OSE.

(A) Optical selection rule for the lowest singlet exciton in PEPI. Both the electron and the hole have a total angular momentum quantum number $\mathit{J}=\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ and a magnetic quantum number ${\mathit{m}}_{\mathit{J}}=\pm \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$. (B) Schematic of the spin-selective OSE mechanism in PEPI, showing only the m_J in ket notation. The red (blue) arrow illustrates the interaction between the σ⁺ (σ⁻) photon that forms the Floquet quasi-states (green line). The hybridization of the equilibrium states (red or blue lines) with the Floquet quasi-states results in the shift in energy levels. The dashed (solid) lines represent the energy levels before (after) the repulsion. Repulsion only occurs between the equilibrium states and the Floquet states with the same m_J. (C) Co-circularly and counter-circularly polarized pump and probe TA spectra at various Δt. (D) The corresponding kinetics at the negative ΔA peak (2.37 eV). mOD, milli-optical density.

Fig. 3. Fluence dependence of OSE.

(A) Pump fluence–dependent TA spectra for co-circular (red) and counter-circular (blue) polarization pump-probe at Δt = 0 ps. (B) Resultant spectra from the difference between the co-circular TA spectra and the counter-circular TA spectra at the same pump fluence at a probe delay of 0 ps. The vertical black dashed line indicates the position of the exciton absorption peak. SWT, spectral weight transfer. (C) Estimated Stark shift as a function of pump fluence (green, left axis) and two-photon–excited exciton population (blue, right axis) and as a function of pump fluence (blue, right axis). The Stark shift exhibits a linear relation, whereas the two-photon-excited process exhibits a quadratic relation with the pump fluence.

Fig. 4. Correlation between the Rabi energy and the oscillator strength or dielectric contrast.

Measurement of Rabi energy via OSE on various lead-based 2D perovskite systems (that is, PEPB, PEPI, and FPEPI). There is a clear increasing relation between $\mathrm{\hslash }{\mathrm{\Omega }}_{\mathrm{R}}/\sqrt{\mathit{I}}$ (red) and the dielectric contrast. Meanwhile, no clear correlation is observed between the oscillator strength (blue) and $\mathrm{\hslash }{\mathrm{\Omega }}_{\mathrm{R}}/\sqrt{\mathit{I}}$.