Non-locking Sleep/waitfor/delay Function For ASP.NET?

I am writing an ASP.NET class that interfaces with an external application. The flow of the transaction between the web server and this application is as follows:My object writes a file to a directory.The outside application detects this file and processes it. This can take between 1-5 seconds.The outside application writes a response file to the same directory.My object detects the response file and parses the results.The 1-5 seconds it can take for the external application to process my file is my problem. The most straightforward way to wait for the file seems to be something like this:

Do While Not File.Exists(f)
Thread.Sleep(500)
Loop

Of course, Thread.Sleep() completely locks up the rest of my website until the outside application processes the file. Clearly, this is not a workable solution.How can I effectively "wait" for my file to be processed without locking up the rest of my website?