Unfortunately, the previous answer posted here contained significant mistakes and inaccuracies. I was going to post this in direct response to that comment, but it seems to have been removed now.

There is a common misconception, in part based on decades of scholarship cherry picking and misinterpreting Herodotus' histories, that the Persians lacked heavy infantry. This is untrue. The Persian people are rarely, if ever, portrayed as heavily armored foot soldiers, but that does not mean that the Persian Empire didn't have them. Much to the contrary, they had many Anatolian, Greek, Levantine, and Egyptian subjects within their ranks that fought in Persian armies with a very similar style of warfare to mainland Greece. u/Iphikrates discusses many aspects of this in past answers on this sub:

• Regarding Xerxes' invasion

• Skewed portrayal in Herodotus and popular media

• Skewed views of Greek mercenary dependence

• On the Persian cavalry advantage

• How the Greeks viewed mercenaries

I've also written several relevant answers before, see here, especially the answer about early Persian encounters with heavy infantry

This is a result of bias in our sources, for several reasons. We rely on Greek literature for narrative histories, and of those Greek works, the ones that survived intact are largely focused on Greece rather than Asia. This creates a geographic bias as well. Greeks tended to be concerned and informed about events closest to Greece AKA the regions where Greek mercenaries were most available to supplement Persian armies. Sources that do reference Persian forces operating east of the Euphrates never mention Greek involvement unless they started in the west. That accounts for more than half of the Achaemenids' territory.

The Persians themselves did not keep the same style of literary records. That said, the scope of this gap is sometimes overstated. The Persepolis Archive Tablets, the Achaemenid Royal Inscriptions, later Mesopotamian chronicles, and other administrative documents from within the empire are all valuable sources, just not in the same style or subject matter as the Greek sources.

If you are interested in Achaemenid military history, I highly recommend Sean Manning's Armed Force in the Teispid-Achaemenid Empire.