Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural “multiple quantum wells” that have strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to “Rashba splitting” close to the extrema in the electron bands. We have used a plethora of ultrafast transient, nonlinear optical spectroscopies and theoretical calculations to study the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D perovskite, namely, (C₆H₅C₂H₄NH₃)₂PbI₄. The density functional theory calculation shows the occurrence of Rashba splitting in the plane perpendicular to the 2D barrier. From the electroabsorption spectrum and photoinduced absorption spectra from excitons and free carriers, we obtain a giant Rashba splitting in this compound, with energy splitting of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·Å, which are among the highest Rashba splitting size parameters reported so far. This finding shows that 2D hybrid perovskites have great promise for potential applications in spintronics.

We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ and ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{1.1}{\mathrm{Br}}_{1.9}$ using polarized broadband pump-probe spectroscopy in the spectral range of 0.3–2.7 eV with 300 fs time resolution. For ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{1.1}{\mathrm{Br}}_{1.9}$. Surprisingly, we also discovered in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, but not in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{1.1}{\mathrm{Br}}_{1.9}$, transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, $D\approx 0.01{\mathrm{cm}}^2{\mathrm{s}}^{-1}$.

Partial circularly polarized photoluminescence with polarization degree of 2% from CH₃NH₃PbBr₃ film at 77 K is observed, excited using a circularly polarized 532 nm laser. Moreover, the Hanle effect is measured to study spin relaxation in CH₃NH₃PbBr₃ film, yielding a spin lifetime of ∼240 ps at 77 K.

We used a variety of nonlinear optical (NLO) spectroscopies to study the singlet excited-state order and primary photoexcitations in polyfluorene, an important blue-emitting $\pi$-conjugated polymer. The polarized NLO spectroscopies include ultrafast pump-probe photomodulation in a broad spectral range of $0.2–2.6\mathrm{eV}$, two-photon absorption in the range of $3.2–4.2\mathrm{eV}$, and electroabsorption covering the spectral range of $2.8–5.0\mathrm{eV}$. For completeness, we also measured the linear absorption and photoluminescence spectra. We found that the primary photoexcitations in polyfluorene are singlet excitons with $\sim 100\mathrm{ps}$ lifetime that have a characteristic photomodulation spectrum comprising of two photoinduced absorption (PA) bands, ${\mathrm{PA}}_1$ at $0.55\mathrm{eV}$ and ${\mathrm{PA}}_2$ at $1.65\mathrm{eV}$, and a strong stimulated emission band that peaked at $\sim 2.5\mathrm{eV}$. The two-photon absorption and electroabsorption spectra identify the exciton PA bands with optical transitions between the lowest-lying odd symmetry $1B_u$ exciton at $3.1\mathrm{eV}$ and two strongly coupled even symmetry states, namely, $m{A}_g$ at $3.7\mathrm{eV}$ and $k{A}_g$ at $4.7\mathrm{eV}$. The excited state manifold also contains a strongly coupled odd symmetry exciton $n{B}_u$ at $4.1\mathrm{eV}$, which identifies the continuum band threshold. A polarization memory of $\sim 2.1$, typical to films of $\pi$-conjugated polymers, characterizes all three NLO spectra, reflecting the highly anisotropic third-order NLO coefficient, ${\chi }^{\left(3\right)}$ of the polymer chains. The four essential states, namely, $1A_g$, $1B_u$, $m{A}_g$, and $n{B}_u$, were used to satisfactorily fit the ${\chi }^{\left(3\right)}$ optical spectra using the summation over states model. The combination of the three NLO spectra and the model fit conclusively show that the band model, typical in inorganic semiconductors, cannot properly describe the poly(9,9-dioctylfluorene) polymer. On the contrary, a strongly bound exciton with intrachain binding energy of $\sim 1\mathrm{eV}$ dominates the linear and NLO spectra of this polymer.