Did you try:
PS> [System.IO.Ports.SerialPort]::getportnames()
COM1
PS> $port= new-Object System.IO.Ports.SerialPort COM1,9600,None,8,one
PS> $port.open()
PS> $port.WriteLine("Hello world")
PS> $port.Close()
And reading from a Serial Port
PS> $port= new-Object System.IO.Ports.SerialPort COM1,9600,None,8,one
PS> $port.Open()
PS> $port.ReadLine()