EMB135 FLIGHT CREW EXPERIENCES A HARD LANDING AFTER ENCOUNTERING UNFORESEEN BLACKHAWK HELICOPTER'S WAKE TURBULENCE.

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WE WERE CLRED FOR THE VISUAL APCH. WHEN WE WERE ON A 2 MI FINAL; TWR TOLD US THAT A HELI WAS XING THE RWY AT ABOUT 1000 FT FROM THE THRESHOLD. WE COULD SEE A BLACKHAWK HELI AT ABOUT 100 FT AGL CROSS THE RWY OVER THE 1000 FT RWY AIMING POINT MARKING. NEITHER ME NOR MY FO GAVE THE HELI A SECOND THOUGHT. AS WE CROSSED THE RWY THRESHOLD AND PRIOR TO THE 1000 FT FIXED DISTANCE MARKER; THE AIRPLANE SUDDENLY DROPPED AND WE SLAMMED ONTO THE RWY. UP UNTIL THAT POINT WE WERE ON A STABILIZED APCH. I HAD NOT MADE ANY CONFIGN; PITCH; OR THRUST CHANGES UP TO THAT POINT. OUR AIRSPD WAS CONSTANT WITH A STEADY WIND THAT WAS RIGHT DOWN THE RWY AT ABOUT 15 KTS. I THEN REALIZED THAT WE HAD HIT THE WAKE OF THE BLACKHAWK HELI. I HAVE NEVER EXPERIENCED WAKE FROM A HELI BEFORE AND THIS CAUGHT ME COMPLETELY OFF GUARD. IT HAPPENED SO FAST I WAS NOT ABLE TO REACT IN TIME TO PREVENT THE ACFT FROM SLAMMING ONTO THE RWY. TO PREVENT THIS FROM HAPPENING IN THE FUTURE; ATC SHOULD NOT BE PERMITTED TO CROSS HELIS AT LOW ALTS OVER THE TOUCHDOWN ZONE OF THE LNDG RWY.