Here's another idea, which I took from this
app note. It should provide 700ma drive, but you would need to test it carefully before driving an expensive LED. I would buy a 6.8v, 10 watt zener to test it on, such as NTE5181A.
Read the app note carefully. It explains the purpose of the 10 volt zener and how the resistors, transistor, Schottky diode, and inductor were chosen. I may have made mistakes. I haven't tested it or simulated it.
Here are datasheets to the other parts:
**broken link removed**
Schottky diode
MOSFET
**broken link removed**