The first reference defines reflection and transmission as momentum conserving processes for light coming in and light coming out. Here's figure 1 of
Patrick Back, Sina Zeytinoglu, Aroosa Ijaz, Martin Kroner, and Atac Imamoğlu
Phys. Rev. Lett. 120, 037401 – Published 18 January 2018
(a) Micrograph of the measured heterostructure: the MoSe2 monolayer is encapsulated in between 33 nm (top) and 7 nm (bottom) thick h-BN layers, which are indicated by blue and green dashed lines. The heterostructure is capped by a graphene bilayer and is placed on top of a transparent 500 μm thick fused-silica substrate. The graphene and the MoSe2 are electrically contacted by titanium and gold electrodes. The white line and colored stars indicate the position of optical measurements on and off the MoSe2 and the h-BN layers. (b) Interaction of an incident field with a MoSe2 monolayer. Optical fields can be characterized as consisting of right propagating input Einr and right and left propagating output modes Eoutr, Eoutl respectively. (c) The schematic of the experimental setup. The sample is mounted in a helium flow cryostat in confocal configuration. The sample can be moved in situ by piezostepper motors. A collimated excitation beam is focused onto the sample by the first lens. The reflected light is collimated again by the same lens ( NA=0.68). The transmitted light is collimated by the second lens ( NA=0.55). By reducing the diameter of the collection beam, we can reduce the effective NA of the detection optics.
The text states that by keeping the NA small, they minimize the lateral momentum so that it is dominated by specular reflection (rather than re-emission).
Here's a quote:
"To the extent that the MoSe2 monolayer and its environment is homogeneous, in-plane momentum k is a good quantum number for both exciton and radiation field modes. We emphasize that in-plane translational invariance is an essential property of an ideal mirror. The quality of a realistic mirror can in turn be assessed by the ratio of momentum conserving specular reflection to light that is absorbed (by inelastic processes) or scattered (by disorder) into other momentum states. The latter is given by 1−(R+T) where R and T are the specular reflection and transmission coefficients. In our experiments, this ratio is R/(1−R−T)=0.8. Unless otherwise stated, the terms “reflection” and “transmission” are used for momentum conserving processes only."
