Hi try below....
In ur code behind: (write below code)
string url = "./Default1.aspx?EmployeeUID=" + strEmpUID.ToString();
string winFeatures = "toolbar=no,status=no,menubar=no,location=center,scrollbars=yes,resizable=no,height=650,width=950";
ClientScript.RegisterStartupScript(this.GetType(), "newWindow", string.Format("<script type="text/javascript">var popup=window.open('{0}', 'yourWin', '{1}'); popup.focus();</script>", url, winFeatures));