Observation of excitonic Floquet states in a one-dimensional organic Mott insulator using mid-infrared pump near-infrared probe reflection spectroscopy

When an electric field of light with a frequency of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi mathvariant="normal">Ω</a:mi></a:math> is applied to a solid, Floquet states, consisting of sidebands with an interval of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>ℏ</c:mi><c:mi mathvariant="normal">Ω</c:mi></c:mrow></c:math> around an electronic state, are expected to be formed. However, only a few studies have experimentally detected such sidebands. Here, we apply mid-infrared pump near-infrared reflection probe spectroscopy to a one-dimensional Mott insulator, bis(ethylenedithio)tetrathiafulvalene-difluorotetracyanoquinodimethane <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mo>(</e:mo><e:mrow><e:mrow><e:mi>ET</e:mi><e:mtext>−</e:mtext></e:mrow><e:msub><e:mi mathvariant="normal">F</e:mi><e:mn>2</e:mn></e:msub><e:mi>TCNQ</e:mi></e:mrow><e:mo>)</e:mo></e:mrow></e:math>, to detect the transient change in reflectivity <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mi>R</g:mi></g:math>, <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:mi mathvariant="normal">Δ</h:mi><h:mi>R</h:mi><h:mo>/</h:mo><h:mi>R</h:mi></h:mrow></h:math>, due to the formation of excitonic Floquet states. Analyses, considering both odd- and even-parity excitons, demonstrate that the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mrow><j:mi mathvariant="normal">Δ</j:mi><j:mi>R</j:mi><j:mo>/</j:mo><j:mi>R</j:mi></j:mrow></j:math> spectrum reflects the formation of the first-order Floquet sidebands of excitons, and its spectral shape strongly depends on the widths of excitonic states. The experimental and analytical approach reported here is effective in demonstrating excitonic Floquet states in various solids.