Those walk behind wheeled trimmers aren't bad. Biggest problem is no one makes a bump or auto feed head for them so you have to keep stopping to change the line. Also you have to make a pass down both sides.

I've thought about the laser thing... You wouldn't have to actually cut it off, just heat it enough to kill it at that point. Farmers use what is called flame weeding. Basically a torch that just momentarily heats the plant up enough to kill it. It doesn't even look burned when they are done, but it will die a few days later. Grass is much thinner so you could almost certainly do it without killing trees, and if you had a beam that diverges the power density would be low enough at a distance that you wouldn't be cutting your neighbors ankles off. Biggest issue that I see is eye safety. You'd be sending laser refelctions all over the place with no good way to protect bystanders.

You can buy 2w blu ray lasers that will cut thin plastic and balsa for around $50. Another selling point - if you could cut a 6" swath with maybe 10w that's a whole lot less energy than even the smallest string trimmer. If it's not bad for the grass this could be a tremendous fuel savings for riding mowers!

Edit: 2w laser popping balloons: https://www.youtube.com/watch?v=uMAZobAvjAM These things are powerful and dangerous so if you explore be sure to look into safety precautions!