You have a few options:
Examples:
PS C:\> explorer
PS C:\> explorer .
PS C:\> explorer /n
PS C:\> Invoke-Item c:\path\
PS C:\> ii c:\path\
PS C:\> Invoke-Item c:\windows\explorer.exe
PS C:\> ii c:\windows\explorer.exe
PS C:\> [diagnostics.process]::start("explorer.exe")