# Parabolic lens In the `examples/parabolic_lens_collimation.py` example, we create a collimated phonon flux by reflecting phonons from a parabolic surface. The figure below illustrates the concept:

Concept of collimation and focusing using parabolic surface.

We place the parabolic boundary at the bottom and move the hot spot that emits the phonon to *y* = 300 nm, as shown in the input file: ``` # Hot and cold sides: COLD_SIDE_POSITION_TOP = True # Phonon source: PHONON_SOURCES = [Source(y=300e-9, size_x=100e-9, size_y=100e-9, size_z=THICKNESS, angle_distribution="uniform")] # Parabolic mirror: HOLES = [ParabolaBottom(tip=0, focus=300e-9)] ``` Note that `angle_distribution="uniform"` because we need to emit phonons in all directions evenly. This model produces the following structure, which collimates phonons after reflection from the parabolic boundary:

Phonon trajectories show collimation of phonons reflected from the parabolic boundary.

Angular distribution show that many phonons have an angle of zero degrees at the top of the structure (blue), while initial distribution was uniform (red).

The parabolic lens can also be inverted and placed at the top as follows ``` # Phonon source: PHONON_SOURCES = [Source(x=0, y=0, z=0, size_x=WIDTH, size_y=0, size_z=THICKNESS, angle_distribution="directional")] # Parabolic boundary: HOLES = [ParabolaTop(tip=1000e-9, focus=100e-9)] ``` Here, the source is place in the bottom, while parabola at the top. ### Reference * Singh et al. [Applied Physics Letters, (2023)](https://aip.scitation.org/doi/10.1063/5.0137221)