StringRequest - Format of arraylist is not correct

Question

Trying to send arraylist to server but it seems the format does not correct.

My params in StringRequest

@Override
protected Map<String, String> getParams() {
    Map<String, String> params = new HashMap<>();
    params.put("source", "en");
    params.put("target", "zh");
    JSONArray jsonArray = new JSONArray();
    jsonArray.put("hello world");
    params.put("stringArray", jsonArray.toString());
    return params;
}

My server response

{"result":[],"source":"en","target":"zh","arrayString":"[\"hello world\"]"}

arrayString does not make sense to me because "[\"hello world\"]" is just a string, not a array of string. I expect the arrayString should be something like this "arrayString":[\"hello world\"]

Any suggestions?

EDIT

Tried to use JsonObjectRequest.

Map<String, Object> params = new HashMap();
params.put("source", "en");
params.put("target", "zh");
params.put("stringArray", Arrays.asList("hello world"));
JSONObject parameters = new JSONObject(params);
JsonObjectRequest jsonRequest = new JsonObjectRequest(Request.Method.POST, requestString, parameters, new Response.Listener<JSONObject>() {
    @Override
    public void onResponse(JSONObject response) {
        //TODO: handle success
        Log.d("successful", response.toString());
    }
}, new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {
       error.printStackTrace();
       //TODO: handle failure
    }
});
Volley.newRequestQueue(getActivity()).add(jsonRequest);

Server response

successful: {"result":null,"source":null,"target":null,"arrayString":null}

Php code

<?php
$source = $_POST['source'];
$target = $_POST['target'];
$arrayString = $_POST['stringArray'];
echo json_encode(array('result'=>$results,'source'=>$source,'target'=>$target,'arrayString'=>$arrayString));
?>

Edit 2

If I use JSONObject with its put method and JSONArray as well, I still get the null results

JSONObject parameters = new JSONObject();
try {
    parameters.put("source", "en");
    parameters.put("target", "zh");
    JSONArray jsonArray = new JSONArray();
    jsonArray.put("hello world");
    parameters.put("stringArray", jsonArray);
} catch (JSONException e) {
    e.printStackTrace();
}
Log.d("JSONParams", parameters.toString());
JsonObjectRequest jsonRequest = new JsonObjectRequest(Request.Method.POST, requestString, parameters, new Response.Listener<JSONObject>() {
        @Override
        public void onResponse(JSONObject response) {
            Log.d("successful", response.toString());
        }
}, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            error.printStackTrace();
        }
});

Print and Server Response

JSONParams: {"source":"en","target":"zh","stringArray":["hello world"]}
successful: {"result":null,"source":null,"target":null,"arrayString":null}