Nonlinear silicon photonics on CMOS-compatible
tellurium oxide
NEETESH SINGH,
1,2,
*HAMIDU M. MBONDE,
3
HENRY C. FRANKIS,
3
ERICH IPPEN,
2
JONATHAN D. B. BRADLEY,
3,4
AND FRANZ X. KÄRTNER
1
1
Centre for Free Electron Laser Science (CFEL)-DESY and University of Hamburg, 22607 Hamburg, Germany
2
Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3
Department of Engineering Physics, McMaster University, Hamilton, Ontario L8S 4L7, Canada
4
e-mail: jbradley@mcmaster.ca
*Corresponding author: neeteshs@mit.edu
Received 23 June 2020; revised 29 September 2020; accepted 18 October 2020; posted 19 October 2020 (Doc. ID 400057);
published 30 November 2020
Silicon photonics is coming of age; however, it is still lacking a monolithic platform for optical sources and non-
linear functionalities prompting heterogeneous integration of different materials tailored to different applica-
tions. Here we demonstrate tellurium oxide as a complementary metal oxide semiconductor silicon photonics
platform for nonlinear functionalities, which is already becoming an established platform for sources and am-
plifiers. We show broadband supercontinuum generation covering the entire telecom window and show for the
first time to our knowledge third-harmonic generation in its integrated embodiment. Together with the now-
available lasers and amplifiers on integrated TeO
2
this work paves the way for a monolithic TeO
2
-based nonlinear
silicon photonics platform.
© 2020 Chinese Laser Press
https://doi.org/10.1364/PRJ.400057
1. INTRODUCTION
Silicon photonics is reaching its mat urity in terms of passive
and active optoelectronic functionalities. However, monolithic
integration of light sources and nonlinear optical devices is still
an ambitious goal. Silicon cannot be electrically pumped due to
its indirect band gap, and it cannot be optically pumped due to
its low solubility of rare-earth ions [1,2]. There has been sig-
nificant progress with electrically pumped III-V material as
sources on silicon photonics platforms and recently as nonlin-
ear optics platforms [3,4]; however, they tend to be expensive,
thermally instable, and have low yield [1,5,6]. On the other
hand, for nonlinear photonics, silicon has a very high Kerr fac-
tor, 200–400 times that of silica, leading to various demonstra-
tions [7–12]. However, it suffers from high two-photon
absorption in the telecom band due to being too close to
the band gap [13]. This compels researchers to operate either
at longer wavelengths, which is far away from the band gap, or
employ new material platforms having higher band-gap energy
[8,14–16]. One such material that has received significant at-
tention recently is silicon nitride, which has been used routinely
for generating over an octave spanning supercontinuum
[17–19]. The Kerr factor of the stoichiometric silicon nitride
(Si
3
N
4
) is only 10 times that of silica [20,21] and thus requires
high pump power, but such a limitation can be relaxed with
silicon rich silicon nitride, albeit with the increase in propaga-
tion losses and reduction of the band-gap energy, which thus
makes it susceptible to nonlinear absorption [21,22]. Most im-
portantly, however, silicon nitride has not been demonstrated as
a reliable host material for rare-earth gain to allow monolithic
integration of sources and optical nonlinear functionalities.
A rare-earth-doped medium is an excellent candidate for
monolithic integration of an optically pumped laser, and there
have been many demonstrations recently of high-power inte-
grated lasers [23–27]. However, the host material used in these
works tends to have low optical nonlinearity, which thus limits
its applicability as a nonlinear medium. Tellurium oxide
(TeO
2
) is an excellent candidate for monolithic integration
of optical sources and linear and nonlinear functionalities in
silicon photonics without incurring nonlinear absorption loss.
As a source it is establishing itself as a host medium for various
rare-earth ions, offering opportunities for amplifiers and lasers
at many wavelengths [28–30]. Furthermore, it has highly de-
sirable material properties, such as a high refractive index of 2.1
at 1550 nm and high nonlinearity—Kerr coefficients up to 30
to 50 times that of silica [30 –33]. Its Raman coefficient is 60
times that of silica and demonstrates extremely strong second-
order optical nonlinearity for a glass [34–37]. Due to these ex-
cellent optical properties it has been used to generate over 2.6
octaves of supercontinuum in sub-centimeter photonic crystal
fiber [38]. However, fabrication of integrated tellurium oxide
has been challenging thus far. In the first half of the last two
1904
Vol. 8, No. 12 / December 2020 / Photonics Research
Research Article
2327-9125/20/121904-06 Journal © 2020 Chinese Laser Press