Equity (401k) (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Matching contribution	$ 1.0	$ 0.6	$ 0.5

Defined Contribution Plan, Employer Matching Contribution

No definition available.

Name:	emkr_DefinedContributionPlanEmployerMatchingContribution
Namespace Prefix:	emkr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration