[Paper Review] Topological Insulator Metamaterial with Giant Circular Photogalvanic Effect

Xinxing Sun, Giorgio Adamo+4 more|arXiv (Cornell University)|Aug 20, 2020

Topological Materials and Phenomena42 references4 citations

TL;DR

This paper demonstrates a topological insulator metamaterial based on Bi1.5Sb0.5Te1.8Se1.2 that achieves an 11-fold enhancement of the circular photogalvanic effect through nanostructured resonant field confinement. The design enables a record photocurrent dichroism of 0.87 at room temperature, leveraging spin-momentum locking and engineered electromagnetic hotspots to amplify helicity-dependent photocurrents for polarization-sensitive photodetection.

ABSTRACT

One of the most striking manifestations of electronic properties of topological insulators is the dependence of the photocurrent direction on the helicity of circularly polarized optical excitation. The helicity dependent photocurrents, underpinned by spin-momentum locking of surface Dirac electrons, are weak and easily overshadowed by bulk contributions. Here we show that the chiral response can be enhanced by nanostructuring. The tight confinement of electromagnetic fields in the resonant nanostructure enhances the photoexcitation of spin-polarized surface states of topological insulator Bi1.5Sb0.5Te1.8Se1.2, leading to an 11-fold increase of the circular photogalvanic effect and an unprecedented photocurrent dichroism ( {ho}circ=0.87) at room temperature. The control of spin-transport in topological materials by structural design is a previously unrecognised ability of metamaterials that bridges the gap between nanophotonics and spin-electronics, providing new opportunities for developing polarization sensitive photodetectors.

Motivation & Objective

• To overcome the weak and easily overwhelmed helicity-dependent photocurrents in conventional topological insulators.
• To engineer a metamaterial structure that enhances electromagnetic field confinement to amplify surface state photoexcitation.
• To achieve a strong, room-temperature circular photogalvanic effect for practical spintronic and photonic applications.
• To demonstrate that structural design in metamaterials can control spin-transport in topological materials.

Proposed method

• Nanostructuring of Bi1.5Sb0.5Te1.8Se1.2 into a periodic array of resonant subwavelength patterns to enhance local electromagnetic fields.
• Utilizing the topological surface states' spin-momentum locking to generate helicity-dependent photocurrents.
• Employing finite-element simulations and experimental characterization to map the spatial distribution and polarization dependence of photocurrents.
• Measuring photocurrent dichroism (ρcirc) under circularly polarized light excitation to quantify chiral response.
• Optimizing the geometry of the metamaterial to maximize field enhancement at the surface states.
• Validating the enhancement through comparison with bulk contributions and control samples.

Experimental results

Research questions

• RQ1 Can nanostructuring of a topological insulator enhance the circular photogalvanic effect beyond intrinsic limits?
• RQ2 To what extent can electromagnetic field confinement in a metamaterial amplify photoexcitation of spin-polarized surface states?
• RQ3 What is the maximum achievable photocurrent dichroism in a topological insulator metamaterial at room temperature?
• RQ4 How does the structural design of the metamaterial influence the spin-transport and chiral photocurrent response?
• RQ5 Can metamaterial engineering effectively decouple surface-state photocurrents from dominant bulk contributions?

Key findings

• The metamaterial design achieved an 11-fold enhancement of the circular photogalvanic effect compared to bulk topological insulator films.
• A record photocurrent dichroism of ρcirc = 0.87 was measured at room temperature, indicating strong helicity-dependent photocurrent generation.
• The enhanced response is attributed to tight electromagnetic field confinement in the resonant nanostructures, selectively exciting surface Dirac electrons.
• The contribution from bulk states was significantly suppressed, confirming the dominance of surface-state photocurrents in the engineered structure.
• The observed chiral photocurrent is robust at room temperature, enabling practical applications in polarization-sensitive photodetectors.
• The results demonstrate that metamaterial engineering can be used to control spin-transport in topological materials, bridging nanophotonics and spintronics.