I'm going to reference https://what-if.xkcd.com/145/ for my answer:
You can't use lenses and mirrors to make something hotter than the surface of the light source itself
In other words, in order to double the amount of light that a distant observer receives from a star, you need an optical component that's as big - from the observer's point of view - as the star itself. That's quite a daunting task - and doubling the Sun's output will achieve spectacularly little. You need a bigger lens. Also, if you also don't want to put the lens directly in front of the Sun, as that would block direct sunlight, you'll have to figure out the lens shape so that its optical axis is outside of the lens.
Maybe you could use gas to form the lens shape? But how do you keep it in place? You don't need much optical power, but even then I feel like 1 atom per cm^3 of hydrogen won't cut it. You need a physical thing. The size of a star. That isn't a star. Yay. Take #3?
With a bit of mass fiddling, you could put a black hole in the middle of the optical path and use gravitational lensing instead of refraction. However, the last time I checked, perfect-size black holes aren't exactly easy to haul into place.
Perhaps your best bet would be to use a swarm of optical devices? And, because planetary-scale lenses are hard to make, you should probably use mirrors instead. Flat mirrors will do.
So.. how many? Let's be generous and say that you can craft a light sail the size of Earth - and keep it in orbit around the Sun and keep it visible and correctly oriented most of the time. I've been taught the radius of Earth is 6378 km. For Sun, that's 695 500 km. Let's round that to 637 800 km. So... in order to double Sun's output, you'd need a touch over 10000 ginormous solar sails. And that's just to give you a nice tan on Mars. It won't make your plants grow half-way to Alpha Centauri.
You could use active components to bypass the law of conservation of étendue. But even then, optics are against you. A point-like source will emit in all directions equally. My EWAG says that a phased array of microwave emitters the size of Sun will do the job (but better is easier in this case). Powering them won't be hard, but establishing microsecond synchronization will be a fun challenge when the Sun is much brighter than any radio antenna you could put on Earth - and a heliocentric polar 1-AU orbit isn't too cheap either. Still better than the equivalent amount of Earth-sized mirrors, though.
Also, as L. Dutch says, don't even think about putting your contraption on Earth.
I'd rather pack a couple tons of plutonium and use a low-power brand of cryogenic chambers for the trip.