Polarized　light　has　already　been　widely　used　for　photography　and　display　technologies.　Magneto-optical　Faraday　effect,　i.　e.,　the　light　polarization　rotates　in　the　magnetic　field　applied　to　the　material　in　the　direction　of　light　propagation,　plays　a　crucial　role　in　the　interaction　between　light　and　spin.　Faraday　effect　allow　us　to　understand　the　nature　of　magnetization　in　condensed　materials.　As　an　effect　opposite　to　the　Faraday　effect,　the　magnetization　can　be　induced　in　a　transparent　medium　exposed　to　a　circularly　polarized　electromagnetic　wave,　which　is　called　inverse　Faraday　effect.　Knowledge　of　the　mechanism　provides　the　opportunities　of　modulation　devices　in　photonics,　ultrafast　opto-magnetism　and　magnonics.　In　this　paper,　we　experimentally　demonstrate　a　proof-of-concept　ultrafast　polarization　modulation　by　employing　circularly　polarized　light　to　demonstrate　a　strengthened　terahertz　(THz)　frequency　Kerr　modulation　signal,　at　room　temperature.　By　using　the　transient　pumpprobe　spectroscopy　with　the　reflected　geometry,　we　are　able　to　demonstrate　the　feasibility　of　such　an　ultrafast　magneto-optical　polarization　modulation　at　0.19　THz　in　a　paramagnetic　Li:　NaTb　(WO4)2　crystal　with　a　thickness　of　3　mm.　The　time-resolved　modulation　signal　is　explained　by　the　interaction　between　two　counter-propagating　laser　pulses　(central　photon　energy　of　1.　55　eV)　within　the　crystal　via　the　optical　Kerr　effect.　We　find　that　the　amplitude　of　the　modulation　increases　with　the　pump　fluence　increasing,　while　the　modulation　frequency　is　dependent　neither　on　the　pump　fluence　nor　on　polarization　of　pump　beam.　However,　it　can　further　be　found　that　the　phase　and　amplitude　of　the　transient　Kerr　modulation　are　strongly　dependent　on　the　helicity　of　the　circularly　polarized　pump　pulses.　Indeed,　these　oscillating　signals　may　be　mistaken　for　spin　excitation　modes.　The　present　findings　allow　us　to　get　an　insight　into　the　transient　magneto-optical　dynamical　process　in　transparent　medium.　In　addition,　the　polarization　modulation　of　ultrashort　laser　pulse　on　a　picosecond　time　scale　will　facilitate　all-optical　data　processing,　as　well　as　the　polarization-dependent　ultrafast　dynamics　in　various　material　systems,　which　span　from　condensed　matter　to　molecular　spectroscopy.　In　this　regard,　our　experimental　results　provide　a　possibility　for　designing　novel　all-optical　(magneto-optical)　modulators　operating　at　THz　clock　frequencies.　The　magneto-optical　polarization　response　modulated　at　THz　frequencies　may　have　new　possibilities　for　designing　all-optical　devices,　such　as　ultrafast　modulators.　©　2018　Chinese　Physical　Society.