<a href="#" class="cp_btn">button</a>

a.cp_btn {
  position: relative;
  display: block;
  width: 160px;
  padding: 0.8em;
  text-align: center;
  text-decoration: none;
  color: #fff;
  border:1px solid #8BC34A;
  background: #8BC34A;
  overflow: hidden;
  z-index: 0;
}
a.cp_btn:after {
  position: absolute;
  top: 50%;
  left: 50%;
  content:'';
  height: 0;
  width: 100%;
  background : #fff;
  opacity: 0;
  transform: translate(-50%,-50%) rotate(45deg);
  transition: 0.3s;
  z-index: -1;
}
a.cp_btn:hover {
  color: #8BC34A;
}
a.cp_btn:hover:after {
  height: 250%;
  opacity: 1;
}
a.cp_btn:active:after {
  height: 350%;
  opacity: 1;
}