Yes. Fs laser can induced plasma in air. In this plasma, the refractive index is less than 1.

Thanks for all .

The refractive index has values lower then one in the anomalous dispersion area around absorption line (look at Kramer-Kronig relation).

The negative value of the refractive index is caused because otherwise the absorption would generate a reflection before the absorption occurs which is a violation of the causality principle. Nevertheless, this negative refractive index cannot yield light propagation at a speed higher than the speed of light in vacuum. The reason is that n is a function of the whole absorption spectrum. So my answer is yes but...

Thank you very much Oded Kafri .

Thank you very much Shahin Firuzi

Refractive index of medium is inversely proportional to speed of light in the medium. As refractive index of vacuum is 1, speed of light is always greater in vacuum as compared to transparent medium. Hence, refractive index of transparent medium is always greater than one.

https://www.toppr.com/ask/question/why-is-refractive-index-in-a-transparent-medium-greater-than-one/

Air has the lowest refractive index, but it isn't mechanically stable. And the lowest refractive index found in solid, naturally occurring materials is 1.39. But now researchers have developed a film made of aluminum oxide that has a refractive index as low as 1.025 but that is mechanically stiff.

https://news.ncsu.edu/2015/10/chang-refractive-2015/

Dear Nedhal Sadie . No it should be always greater than one because velocity of light in any medium is always less than that in vacuum. The refractive index is defined as the ratio of the velocity of light in a vacuum to its velocity in a specified medium. Refractive index = velocity of light in vacuum/ velocity of light in medium. Therefore the numerator is always greater than the denominator, so the result is always greater than one. Thank you.

is always greater than 1

It must be greater than 1.

Thanks!

No, must be greater than 1