Lenses? You're thinking too small, my friend. A photospheric laser can provide so much energy you will be growing potatoes in the open air on the moon Triton in the orbit of Neptune
The trick is to utilize the solar photosphere as the lasing medium, allowing you to scale the laser to unimaginable power levels without all the fuss and muss of solar collectors, giant electron beams, wigglier magnets, diffraction gratings and so on. The basics of the idea can be found in an article entitled "Constructing Artificial Laser Stars
by John Talbot".
- Orbiting Mirrors
The mirror method would extract laser energy by creating a resonant cavity in which the radiation would make multiple passes dramatically increasing the effective gain. The path would ideally be perpendicular to any velocity gradient in order to reduce Doppler smearing and increase coherence and thus directivity. The laser beam is emitted tangential to the surface of the star.
Sherwood (1988) described a platoon of orbital mirrors around Mars or Venus to extract a coherent and directed CO2 laser beam from the gain medium. He foresees the use as means of transmitting/propagating our entire culture to a future nano-civilisation established in various other solar systems throughout our galactic neighbourhood.
Mirrors external to the stellar atmosphere would reduce the problems of erosion and orbital deflection by the violent and corrosive stellar winds. Although pairs of mirrors would be ideal to extract directed energy from the gain medium, there are power limitations due to the finite reflectivity of the mirrors (which absorb a small but damaging fraction of the radiation). To overcome this problem larger mirrors could be constructed to distribute the heat load, and reduce the Airy diffraction pattern (beam divergence). However large mirrors are extremely vulnerable to orbital debris such as an encounter with a handful of sand from an intersecting orbit etc...
Large mirrors will act like solar sails, this side-effect could be used to maintain a stationary satellite (statite) over the desired location near the solar limb. (see papers on 'statites' by R.L. Forward) The photon pressure from the laser photons and the photosphere could be adjusted to exactly compensate for the solar gravitational force and prevent the hovering statite from falling onto the star's surface. Conservation of momentum would impart a momentum of 2p every time a photon bounced off the mirror. For a multiple pass optical cavity the photon travels back and forth many times stimulating emissions at every pass. The force on the mirrors from this photon pressure in combination from the photon pressure from the star's photosphere acts like a solar sail and can be designed to exactly balance the inward gravitational force pulling the mirror onto the star. These three counterbalancing forces could be harnessed to maintain the mirror stationary against the gravitational pull of the star. ('statites' or stationary satellites by R.L.Forward). Very near the star there may be deviation from 1 over r squared law by dilution of radiation effect of finite angular width of solar disk.
If more than two mirrors are used a ring laser could be produced, the radiation would make numerous quasi-circular trips around the sun; in other words, a photon merry-go-round spinning at the speed of light!
The mirror scheme will produce an artificial laser star, which would appear as an extremely bright spot on the limb of the parent star. The disk of the star would be difficult to discern in the glare from this 'star-like' spot. If we take photographs of the sun and are careful to mask out the bright point, we would notice a faint disk, corresponding to the light from the underlying 'natural' sun. Although most stars are too far away to resolve their disk, amateur astronomers could easily observe the unusual spectra of laser stars.
In order to send the beam to where you want it to go, a series of huge mirrors or fresnel lenses in free space will be needed to steer the beam, this is similar to Robert L Forward's plans to use super lasers to power light sails to distant stars.
Robert Forward's lasersail system. The huge fresnel lens directs the beam
Modulating this amount of laser energy to power the Earth is a matter of calculating the amount of lasing plasma is needed for a particular power output, sizing the photosphere mirrors accordingly and if necessary, splitting the beam on its way to Earth and directing part of it to other cooperative targets. Modulating the beam to a frequency where there is an atmospheric "window" for passage to the ground is important, and then the beam is collected by high efficiency solar cells (which can work at a high level of efficiency since they will be tailored to the frequency of the laser).
Problems like heat rejection will be left as an exercise for the reader, but there will be no need for thermal, nuclear, hydro or other power plants if you are harnessing the Sun's energy on that scale.