Surface and bulk contributions to the second-harmonic generation in Bi2Se3

Abstract

Second harmonic generation (SHG) from three-dimensional topological insulators originates from both surface and bulk, which does not allow probing of surface states unless the measurement can separate the two contributions. In this study, we used combined measurements of transmitted and reflected SHG from epitaxially grown Bi 2 Se 3 thin films of different thickness on BaF 2 , and a bulk Bi 2 Se 3 crystal, to deduce surface and bulk nonlinear susceptibilities of Bi 2 Se 3 separately. We found that the surface * E-mail: yrshen@berkeley.edu, wtliu@fudan.edu.cn 2 contribution to SHG was comparable to that from the bulk of the crystal, but becomes dominant in ultrathin films. In the latter case, contributions from both air/Bi 2 Se 3 and Bi 2 Se 3 /BaF 2 interfaces were significant, and exhibited a strong out-of-plane polar ordering. The bulk contribution came mainly from the space charge region (SCR), which was formed by Se vacancies aggregated at the air/Bi 2 Se 3 interface; its magnitude can provide an estimate on the field strength in the SCR. Clarification of surface and bulk contributions to SHG can help nonlinear optical techniques be used as a versatile in situ probe for topological insulators.3

Surface versus Bulk Doping Effects on Second Harmonic Generation from Bi2Se3

“…For the surface doping, the SHG signal grows with the election concentration, whereas, bulk doping leads to a reverse effect. As we showed previously, the doping of the surface energy band into the non‐parabolic region can cause the surface SHG to increase. Meanwhile, for the topological insulator Bi 2 Se 3 , the bulk will contribute to the SHG signal by the space charge region (SCR) near the interface .…”

“…As we showed previously, the doping of the surface energy band into the non‐parabolic region can cause the surface SHG to increase. Meanwhile, for the topological insulator Bi 2 Se 3 , the bulk will contribute to the SHG signal by the space charge region (SCR) near the interface . The bulk response should be differently reflected by the two cases through the evolution of SCR.…”

“…The SHG setup is as that described in ref. . The nonlinear optical experiments were performed using 800 nm, 35 fs laser pulses from a Ti:Sapphire oscillator (Newport, MaiTai SP) with a 80 MHz repetition rate.…”

“…For SSS (S for the 2 beam, S for ) and SPP (S for the 2 beam, P for ) polarization combinations, and ; for PSS (P for 2, S for ) and PPP (P for the 2 beam, P for ), . Details of SH responses arising from surface and bulk of Bi 2 Se 3 were described in our previous work …”

Matrix formalism for radiating polarization sheets in multilayer structures of arbitrary composition

“…Experimentally, the optical signal depends on both the interfacial polarizations and local electric fields [13,14] . Yet, due to the coexistence of multiple interfaces and the interference between multiply reflected beams [15] , it is often challenging to perform quantitative analysis [16,17] .…”