Ultrafast large third-order optical nonlinearity in titanium carbide MXene- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>VS</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math> hybrids

The ultrafast nonlinear optical responses of materials are not just theoretical concepts, but they play a significant role in developing modern photonic and optoelectronic devices, including optical time crystals. However, conventional materials often display either weak optical nonlinearities or slow response in the nanosecond timescale, rendering them ineffective for single-shot femtosecond pulse. This limitation can be overcome by leveraging the ultrafast charge transfer that is less than or equal to the pulse width of the laser in a donor-acceptor material pair. However, the low charge-transfer rates limit their effectiveness in high-speed applications. Here, we synthesized titanium carbide MXene- <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msub> <a:mi>VS</a:mi> <a:mn>2</a:mn> </a:msub> </a:math> hybrids characterized by strong electronic coupling, resulting in a remarkable enhancement in the ultrafast third-order nonlinear optical responses, enabled by charge transfer in less than the pulse width. For instance, we obtained a third-order nonlinear optical absorption coefficient of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mrow> <b:mo>(</b:mo> <b:mn>150</b:mn> <b:mo>±</b:mo> <b:mn>16</b:mn> <b:mo>)</b:mo> </b:mrow> <b:mo>×</b:mo> <b:msup> <b:mn>10</b:mn> <b:mrow> <b:mo>−</b:mo> <b:mn>3</b:mn> </b:mrow> </b:msup> </b:mrow> </b:math> cm/GW and a self-focusing (positive) nonlinear refractive index of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mrow> <c:mo>(</c:mo> <c:mn>94</c:mn> <c:mo>±</c:mo> <c:mn>9</c:mn> <c:mo>)</c:mo> </c:mrow> <c:mo>×</c:mo> <c:msup> <c:mn>10</c:mn> <c:mrow> <c:mo>−</c:mo> <c:mn>6</c:mn> </c:mrow> </c:msup> <c:mspace width="0.16em"/> <c:msup> <c:mrow> <c:mi>cm</c:mi> </c:mrow> <c:mn>2</c:mn> </c:msup> <c:mo>/</c:mo> <c:mi>GW</c:mi> </c:mrow> </c:math> at 800 nm in the femtosecond domain, both exceeding control samples by more than an order of magnitude. These enhancements are attributed to efficient charge transfer within the hybrid structure, supported by first-principles calculations and Bader charge analysis. Our findings have led to the development of an ultrafast optical limiter, featuring an onset threshold as low as <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mn>0.60</e:mn> <e:mo>±</e:mo> <e:mn>0.05</e:mn> <e:mspace width="0.16em"/> <e:mi>mJ</e:mi> <e:mo>/</e:mo> <e:msup> <e:mrow> <e:mi>cm</e:mi> </e:mrow> <e:mn>2</e:mn> </e:msup> </e:mrow> </e:math> , with a differential limiting transmittance of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mn>0.46</g:mn> <g:mo>±</g:mo> <g:mn>0.02</g:mn> </g:mrow> </g:math> . These parameters are better than or comparable to the known benchmarks for high-speed nonlinear optical applications, demonstrating the potential for advanced photonic and optoelectronic devices.