Can a voltage make a non-chiral crystal respond to light's handedness?
Applying voltage pulses to a mirror-symmetric ferroelectric perovskite created mixtures of domains that made its frequency-doubled light depend strongly, and switchably, on whether the laser was left- or right-circularly polarized.
Source
Electrically switchable chiral nonlinear optics in an achiral ferroelectric 2D van der Waals halide perovskite
Study at a glance
- Design
- Other — Lab experiment: exfoliated (BA)2(EA)2Pb3I10 flakes imaged by polarization-resolved second-harmonic generation before and during sequential voltage-pulse poling on interdigitated gold electrodes, with symmetry-based tensor fits.
- N
- Not applicable; results come from a few exfoliated crystal flakes (a 340 nm as-prepared flake and a 540 nm poled flake shown in detail, with replication in other flakes reported in the supplement).
- Population
- Exfoliated flakes of the 2D Ruddlesden-Popper lead iodide perovskite (BA)2(EA)2Pb3I10, a room-temperature biaxial ferroelectric
- Outcome
- SHG circular dichroism, SHG rotational-anisotropy patterns, nonlinear tensor ratios and ferroelectric domain state versus applied voltage
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The researchers grew crystals of a layered lead-iodide perovskite that is ferroelectric along two perpendicular directions, peeled off thin flakes, and shone a 1253 nm infrared laser on them to generate frequency-doubled (second-harmonic) light. They measured how the second-harmonic signal depended on linear polarization angle and on left versus right circular polarization, first on untreated flakes and then on a flake lying across interdigitated gold electrodes while stepping voltage pulses up and down. They fitted the polarization patterns with symmetry-allowed nonlinear tensors to identify which ferroelectric domains were present.
What they found
Untreated flakes showed zero second-harmonic circular dichroism, consistent with their mirror-symmetric crystal structure. After poling, a circular dichroism signal appeared and switched sign with voltage in a hysteresis loop, reaching about -0.6 at -66 V and +0.8 at +66 V, larger than values reported for chiral halide perovskites. The analysis showed this arises when domains with perpendicular polarizations coexist below the optical resolution, breaking the in-plane glide mirror symmetry; regions containing just one polarization direction showed no circular dichroism.
The limits
What it doesn't show
The domains themselves are smaller than the roughly 900 nm imaging resolution, so the multidomain picture is inferred from symmetry fits rather than seen directly. The dichroism was spatially uneven across the flake, which the authors tentatively attribute to defects and internal charges but say needs further study, and the microscopic reason for the sign of the dichroism is only modelled. Detailed data come from a small number of flakes measured in vacuum at room temperature, and the effect did not appear when the initial polarization was perpendicular to the electrodes, so device orientation matters.
Key terms
- Second-harmonic generation (SHG)
- A nonlinear optical process in which two photons combine into one photon of twice the frequency; it requires a crystal without inversion symmetry.
- SHG circular dichroism
- The normalised difference in second-harmonic intensity for right versus left circularly polarized input light, a sensitive probe of chirality.
- Ferroelectric
- A material with a spontaneous electric polarization that can be reversed or reoriented by an applied electric field.
- Biaxial ferroelectric
- A ferroelectric whose polarization can point along several equivalent directions in more than one axis, allowing perpendicular domains.
- Planar chirality
- A two-dimensional arrangement that cannot be superimposed on its mirror image within the plane.
- Glide mirror plane
- A symmetry combining reflection across a plane with a translation; breaking it can make a structure chiral in the plane.
Flashcards
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Quiz yourself
Why can the unpoled crystal produce second-harmonic light at all?
Common questions
How can an achiral material show a chiral optical response?
The crystal itself is achiral, but a mixture of domains with perpendicular polarizations removes the in-plane mirror symmetry of the combined structure, producing planar chirality.
Why use SHG circular dichroism instead of ordinary circular dichroism?
The nonlinear signal is much more sensitive to chirality and less swamped by the material's linear anisotropy.
Why does the geometry of the electrodes matter?
The field must have components along both perpendicular polarization directions to create mixed domains; when the initial polarization was perpendicular to the electrodes no dichroism was induced.
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