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sible to enhance the scintillation

yield and control the emitted light

spectrum. However, to design

nanophotonics-enhanced scintil-

lation requires a comprehensive

understanding of the complex

process involving both electron

and photon dynamics.

Roques-Carmes et al. provide

a complete model for the entire

scintillation process involving

the use of nanophotonic struc-

tures. They modeled the process

of electron excitation as a result

of the incoming high-energy par-

ticles, as well as the subsequent

excitation and spatial transporta-

tion of secondary electrons ( 6 ).

The model can produce the den-

sity distribution of the nonequi-

librium secondary electrons that

is responsible for the ultimate

scintillation. To model the radia-

tive decay of these electrons at

specific crystal defect sites where

scintillation occurs, the model

also computes the electronic lev-

els of these defect sites and mod-

els the radiative transition ( 7 ) be-

tween these electronic levels.

From these electronic simula-

tions, the scintillation source can

be described as a spatial distri-

bution of fluctuating dipoles that results

from the radiative transition. Although

the light emission from fluctuating dipoles

can be modeled, in principle, using a typi-

cal electromagnetic field solver that solves

Maxwell’s equations ( 8 ), this brute force

approach is computationally costly and

often impractical. Roques-Carmes et al.

simplify this computation process by ex-

ploiting electromagnetic reciprocity, which

states that the relationship between a fluc-

tuating dipole and the resulting electric

field remains the same if their positions

are interchanged ( 9 ). Based on this ex-

ploit, the emission enhancement can then

be computed by simulating the absorption

of an electromagnetic plane wave. Finally,

the scintillation can be described by com-

bining emission-enhancement distribution

obtained from photonic simulations with

the power of the scintillation source as ob-

tained from electronic simulations.

Based on their framework of theoreti-

cal modeling, Roques-Carmes et al. design

and demonstrate several nanophotonics-

enhanced scintillators (see the figure). In

one set of experiments, they investigate

the scintillation from a silicon-on-silica

device bombarded by an electron beam.

Scintillation in the device is emitted by

the defects in the silica layer ( 10 ), and the

silicon layer is designed to provide scintil-

lation enhancement. They compare results

from similar scintillators where the silicon

layer is uniform with the enhanced ver-

sion where the silicon layer has a lattice

of specially patterned cuts. The scintilla-

tion emission from the patterned samples

shows very strong and wavelength-depen-

dent enhancement. Overall, the patterned

sample shows up to sixfold enhancement

compared with the sample that has the

unstructured silicon layer, with the addi-

tional ability to tune the scintillation spec-

tra by changing the cut depth. Electronic

dynamics can also play a role in control-

ling the scintillation process. The authors

also measure the scintillation emission in

both red and green wavelength ranges. By

changing either the energy or the power

of the incident free electrons, the ratio of

green to red scintillation peak powers can

be controlled, which can be understood by

using the rate equations to describe the

microscopic transition dynamics.

In another set of experiments, Roques-

Carmes et al. demonstrate a roughly nine-

fold enhancement over the measured scin-

tillation spectrum in a patterned

cerium-doped yttrium aluminum

garnet scintillator under x-ray

compared with an unpatterned

sample. The higher efficiency is

beneficial for scintillation appli-

cations where x-ray exposure is of

concern. Moreover, by using the

inverse-design technique ( 11 ) to-

gether with electromagnetic reci-

procity, further enhancements

beyond what has been demon-

strated in the experiments may

be achievable. For both experi-

ments, their results show good

agreement with the back-to-back

theoretical modeling that incor-

porates both electronic and pho-

tonic processes.

The demonstrated nanopho-

tonic scintillators show that one

can engineer and enhance the per-

formance of a given scintillating

material through nanostructures.

The work of Roques-Carmes et

al. points to new possibilities for

applications ranging from high-

resolution x-ray detectors used

in medical imaging to efficient

ultraviolet-light sources. The de-

veloped comprehensive theoreti-

cal framework can be applied to

explore other light-emission phe-

nomena, such as harmonic generation and

surface-enhanced Raman spectroscopy,

where the understanding of both electron

and photon dynamics is important. Finally,

the present theoretical framework relies

upon the use of reciprocity. It is known

that nonreciprocal nanophotonic struc-

tures can exhibit unusual thermal emis-

sion ( 12 ) and cathodoluminescence ( 13 )

properties. It may therefore be of interest

to explore scintillation in nonreciprocal

nanophotonic structures. j

REFERENCES AND NOTES

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4. C. Roques-Carmes et al., Science 375 , eabm9293
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6. H. Demers et al., Scanning 33 , 135 (2011).


7. F. J. García de Abajo, Rev. Mod. Phys. 82 , 209 (2010).


8. C. Luo, A. Narayanaswamy, G. Chen, J. D. Joannopoulos,
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9. V. S. Asadchy, M. S. Mirmoosa, A. Diaz-Rubio, S. Fan, S. A.
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10. S. Girard et al., Rev. Phys. 4 , 100032 (2019).


11. S. Molesky et al., Nat. Photonics 12 , 659 (2018).


12. L. Zhu, S. Fan, Phys. Rev. B 90 , 220301 (2014).


13. R. Yu, A. Konečná, F. J. García de Abajo, Phys. Rev. Lett.
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    10.1126/science.abn8478

Ginzton Laboratory, Department of Electrical

Engineering, Stanford University, Stanford, CA, USA.

Email: [email protected]

X-ray Light

Electrons Light

Patterned silicon

Silica (scintillating)

Silicon substrate

Cerium-doped yttrium

aluminum garnet

25 FEBRUARY 2022 • VOL 375 ISSUE 6583 823

Improving scintillator capabilities with cutouts

By stacking patterned cutouts of silicon on a silica layer and creating

patterned dimples on cerium-doped yttrium aluminum garnet, researchers

improved the efficiency of, and offered wavelength-dependent tunability to,

the two scintillation devices.