Experimental observation of non-Hermitian phase transitions using laser-induced thermoacoustics

Non-Hermitian physics in open systems has garnered significant attention for its exotic phenomena, particularly surrounding exceptional points that offer transformative potential for multifunctional devices. Central to this field are parity-time (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>P</mml:mi><mml:mi>T</mml:mi></mml:math>) symmetry-defined by balanced gain and loss-and its counterpart, anti-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>P</mml:mi><mml:mi>T</mml:mi></mml:math> symmetry. However, integrating these divergent concepts into a unified acoustic platform remains an unattainable challenge. In this study, we employ laser-induced thermoacoustics (LIT) to integrate a tunable amplifying component into a non-Hermitian system. By exciting an ultrathin carbon nanotube (CNT) film through laser irradiation, we experimentally observe the phase transitions between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>P</mml:mi><mml:mi>T</mml:mi></mml:math> and anti-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>P</mml:mi><mml:mi>T</mml:mi></mml:math> symmetries. Furthermore, our findings demonstrate the creation of selectable scattering states and the generation of acoustic vortex beams (VBs), facilitating both <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>P</mml:mi><mml:mi>T</mml:mi></mml:math>-symmetric scattering and the conversion of topological charges. This acoustically transparent strategy bypasses traditional, path-blocking compensation schemes, offering a versatile framework for controlled non-Hermitian phase transitions in next-generation integrated devices.