The Apple chief executive paid a surprise visit to Eldim, a Normandy-based firm that develops optical technology used in the facial recognition system inside new top-of-the-line iPhone X smartphones due out next month. Prices start at $999.

Face ID, as the iPhone X feature is known, will replace the fingerprint sensor for unlocking the phone and features in Apple’s campaign to distinguish the phone from rival devices.

“Bravo for your work!,” Cook said on his official Twitter account in French. A picture of him chatting with Eldim employees was also posted. He also hailed the contributions of French start-ups on Twitter while on his way to a scheduled meeting with Macron on Monday afternoon.

The meeting between the French president and Apple CEO follows calls by Macron last week for Apple and other tech companies such as Amazon.com (AMZN) to pay more in taxes in Europe.

“Bravo to Europe for acting with determination to get tax rules and justice respected,” Macron said in a tweet last week praising European Union officials for taking member-state Ireland to court to reclaim 13 billion euros ($15.26 billion) in back taxes from Apple.

Eldim, located outside Caen, near the northern coast of France, had quietly worked with Apple on research projects for nearly a decade before gaining recognition for its optical work on behalf of Apple, the world’s most highly valued company.

“They (Eldim) certainly have a top-notch technology,” said industry analyst Thomas Husson of Forrester Research. “Facial recognition is one of iPhone X’s differentiating features.”

Apple (AAPL) is normally highly secretive about the suppliers it uses to develop new phones, computers, and other devices.

A spokeswoman for Apple declined to comment.

Franco-Italian chipmaker STMicroelectronics, one of Europe’s larger technology companies, is widely understood to be a supplier of sensors for the Apple iPhone 7 and forthcoming iPhone devices, but is barred by Apple from saying so.