Tunable ultrasound propagation in microscale metamaterials

in normal 2D lithography, you're typically only concerned with exposing area on a relatively flat film. This allows you to essentially 'block' the areas you don't want to expose and flood the areas you do want to expose. So you make your reticle with your pattern on it and you just shine the active wavelength (lets say 400 nm) through it and you get an exposed pattern and the areas not exposed.

in this volume lithography, you still have a relative thin flat film, but the pattern is no longer confined to a 2D surface, but in general is a 3D volume. So the reticle now needs to project a 3D pattern (essentially a hologram) instead of a 2D pattern. This is in general going to be quite difficult, and in some cases physically impossible if we're illuminating with the active wavelength.

eg. imagine a pattern like this where we are exposing from the top

---------------------

++++++++++++

--++----++--------

where --- is unexposed and ++ is exposed. There is basically no way we can get the active light to the 2nd layer without at least partially exposing the first layer.

The way the nanoscribe and similar systems goes about this is to use nonlinear effects. If I shine very intense 800 nm light onto a medium with a special optical property called a third order nonlinearity, I can sometimes get the medium to accept 2 800 nm photons as a substitute for a 400 nm photon (energy conservation is obeyed here since an 800 nm photon has half the energy of a 400 nm photon).

The crux here is that it needs to be very intense 800 nm light, so it needs to be concentrated. It's much harder to get intense light to expose an entire 3d hologram than it is to focus it into a single spot and raster it around.

Of course I'm not saying it will never be possible, but it is a significant effort.