Sending data using ajax, blank response from server side PHP

I am using Ajax to send data to PHP. But the received result is blank.

client code

 <script>
     //on click of a button, Below jQuery Ajax code running.
      var prodStock= $("#product").html(); 
            $.ajax({
                data: prodStock,
                url: url,
                type: 'POST',
                async: false,
                success: function(response) {
                    if (response) {
                        alert(response);
                    }
                }
            });
    </script>

    <?php 
        foreach($data['Products'] as $val){
            $pro.= $val['product_id'].':'.$val['quantity'].',';
        }
    ?>  
    <div id='product'>{<?php echo rtrim($pro, ',');?>}</div>

The browser provides:

<div id="product">{18:1,10:1}</div>

server side code

public function update(){
    if ($this->RequestHandler->isAjax()) {
        print_r($this->request->data);
    }
}

test:

If I send statically data: {18:1,20:1},in ajax it works fine. Return an array.