CI json from controller

//controller
public ajax_method(){
    //fill with your data
    $data = array('success'=>true);
    if($this->input->is_ajax_request()){
        $this->output
                 ->set_content_type('application/json')
                 ->set_output(json_encode($data));
    }else{
        //for example show_404()
    }

}

//jq script
$.getJSON("url",function(data){
    console.log(data);
});