Impact of terahertz short pulses on the oxygen defect state in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>

Oxygen-deficient titanium dioxide (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>TiO</a:mi><a:mrow><a:mn>2</a:mn><a:mo>−</a:mo><a:mi>x</a:mi></a:mrow></a:msub></a:math>) is a very attractive material for several applications ranging from photocatalysis to resistive switching. Oxygen vacancies turn insulating anatase titanium dioxide into a polaronic conductor, while creating a defect-state band below the ultraviolet semiconducting gap. Here we employ a combination of broadband infrared (IR) reflectivity and THz-pump/IR-probe measurements to investigate the relationship between localized defect states and delocalized conducting polaronic states. We show that the THz pump allows to convert deeply localized electrons into metastable polarons with a lifetime in the ns range. These long-lived metastable states may find application in novel optoelectronic applications exploiting the interplay of dc resistivity, with terahertz and infrared signals.