Search for a stochastic background of 100-MHz gravitational waves with   laser interferometers
  This letter reports the results of a search for a stochastic background of gravitational waves (GW) at 100 MHz by laser interferometry. We have developed a GW detector, which is a pair of 75-cm baseline synchronous recycling (resonant recycling) interferometers. Each interferometer has a strain sensitivity of ~ 10^{-16} Hz^{-1/2} at 100 MHz. By cross-correlating the outputs of the two interferometers within 1000 seconds, we found h_{100}^2 Omega_{gw} < 6 times 10^{25} to be an upper limit on the energy density spectrum of the GW background in a 2-kHz bandwidth around 100 MHz, where a flat spectrum is assumed. 
Tomotada Akutsu, Seiji Kawamura, Atsushi Nishizawa, Koji Arai,   Kazuhiro Yamamoto, Daisuke Tatsumi, Shigeo Nagano, Erina Ni...