"to explore further" is correct and usable in written English. You can use it when you want to express the idea of looking into something in more depth or detail. For example, "We need to explore further into the possible causes of the phenomenon.".

"explore further" is a perfectly acceptable phrase in written English. It is usually used to imply that more research or investigation needs to be done into a particular topic. It can also be used to suggest that one should go deeper into a subject.

The phrase 'to further explore' is correct and can be used in written English. For example, "We plan to further explore the impact of this change on our organization.".