Which mechanism is NOT available to secure information about a party's expert witnesses for trial?

The correct choice indicates a mechanism that is not available for securing information about a party's expert witnesses. Conducting a witness deposition after the trial has concluded is not a permissible method in the context of preparing for trial. The deposition process is conducted prior to trial to gather testimony and evidence that may be relevant to the case.

In contrast, serving interrogatories on the opposing party is a recognized discovery tool used to obtain information regarding expert witnesses, including their identities and opinions. Additionally, filing a contradictory motion for a written report from an expert is a valid mechanism that can compel an expert to provide a detailed account of their findings and opinions. Lastly, requesting disclosures regarding the expert's qualifications and compensation is also permissible and can assist in evaluating the credibility of the expert's testimony.

Therefore, the mechanism involving post-trial depositions does not align with the procedural rules governing pre-trial discovery and is why this choice is identified as the correct answer.