send input type file value with ajax [duplicate]

i have made a form that will be send a input type file, in my server side i want to get $_FILES value so i used print_r($_FILES), but in my ajax response i don't get anything value, here my code..

  <form id="my_form">
     <input type="file" id="image_file" name="image_file"/>
  </form>

  $('#my_form').submit(function() {
     var data   = $('#my_form').serialize();
     $.ajax({
        url: 'ajax.php',
        type: 'POST',
        data: data,
        enctype: 'multipart/form-data',
        success: function(response) {
            alert(response);
        }, 
    });
  return false;
});

and here my php code

<?php  
  $name = $_FILES['image_file']['name']; // get the name of the file
  $type = $_FILES['image_file']['type']; // get the type of the file
  $size = $_FILES['image_file']['size'];
  echo $name;
  //or
  print_r($_FILES);
?>

please help me ...

thanks..