SHG in a centrosymmetric crystal – can it be possible?

Abstract

The present report lists selected publications on centrosymmetric compounds that manifest second harmonic generation responses in a laser, along with a few publications that dispute the laser outcomes. Two studies provide a plausible explanation for this apparent contradiction between second-order nonlinear susceptibility and inversion symmetry: the crystals are noncentrosymmetric and are twinned by inversion. If crystal structures of SHG-active compounds are presented in centrosymmetric settings, the authors of the publications may consider stipulating that the true space group is likely to be one of the noncentrosymmetric sub-space groups.

Growth and characterization of Na3R(BO3)2 (R = La–Gd) borates: crystal structure, high-temperature behavior, and optical properties

“…It should be noted that intense second harmonic generation in centrosymmetric crystals is known and typically associated with structural defects as vacancies, distortion of the structure, 79–82 or twins. 83 Fig.…”

“…69,77,78 The quenching in Na 3 Gd 1−x Sm x (BO 3 ) 2 is due to the non-radiative energy transfer from excited Sm 3+ ion to non-excited Sm 3+ ions or opening of cross-relaxation channels. 69,77,78 Second harmonic generation It should be noted that intense second harmonic generation in centrosymmetric crystals is known and typically associated with structural defects as vacancies, distortion of the structure, [79][80][81][82] or twins. 83 Fig.…”

HgBrI: a possible tecton for NLO molecular materials?

“…A recent study revealed that centrosymmetric crystals can generate the Second Harmonic, in the presence of twinning by inversion or local asymmetry. 52 The components of hyperpolarizability and the second order susceptibility matrix have been calculated for HgBrI ( 1 ) and [Hg(bpy)BrI] ( 2 ), according to Kleinman's symmetry. The assumptions for Kleinman's symmetry are always valid for SHG.…”

Bis(benzoxazol-2-yl)methanides in main group organometallic syntheses and enquiry-based education

“…Also difficult to understand is the increasing number of publications that describe the observation of second harmonic generation (SHG) responses upon irradiation with a laser for materials with a centrosymmetric crystal structure, [213] although it has already been shown that a centrosymmetric structure cannot exhibit such properties. [214,215] Measurement of SHG responses even became an experimental tool to prove the absence of centrosymmetry.…”