Pass nested JSON data from client and deserialize on server?

Not sure if this was the best forum but I'm trying to Stringify my search criteria and pass it as a parameter to an UpdatePanel as follows:

 __doPostBack('ctl00$ContentPlaceHolder1$upSearchResults', JSON.stringify(data));

NOTE: the JSON data being passed above looks like json below when I call stringify(). The "data" object is a nested object with searchType and a list of company objects ( It contains the type of search and the companies to search for)

JSON.stringify(data) ...
"{"searchType":"Company","companyList":[{"CompanyId":"36492","CompanyName":"ABC INC","Ticker":ABC},{"CompanyId":"37497","CompanyName":"ACME & CO","Ticker":ACME}]}"

However, when I recieve it on the server it looks like:

string data = Request.Params.Get("__EVENTARGUMENT");

data...
"{\"searchType\":\"Company\",\"companyList\":[{\"CompanyId\":\"36492\",\"CompanyName\":\"CATERPILLAR INC\",\"Ticker\":null},{\"CompanyId\":\"37497\",\"CompanyName\":\"DEERE & CO\",\"Ticker\":null}]}"

I can't seem to deserialize it into a C# class...neither calls below to Deserialize work....

System.Web.Script.Serialization.JavaScriptSerializer json = new System.Web.Script.Serialization.JavaScriptSerializer();
List<SearchCriteria> searchCriteria= json.Deserialize<List<SearchCriteria>>(data)
SearchCriteria criteria = json.Deserialize<SearchCriteria>(data);

The classes are below...

[Serializable]
public class SearchCriteria {   
  string searchType { get; set; }        
  List<Company> companyList { get; set; }
}

[Serializable]
public class Company {        
  public string CompanyId { get; set; }        
  public string CompanyName { get; set; }        
  public string Ticker { get; set; }
}