Highly efficient ultraviolet high-harmonic
generation from epsilon-near-zero indium tin
oxide films
WENDONG TIAN,
1
FEI LIANG,
1,2
DAZHI LU,
1
HAOHAI YU,
1,3
AND HUAIJIN ZHANG
1
1
State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, China
2
e-mail: liangfei@sdu.edu.cn
3
e-mail: haohaiyu@sdu.edu.cn
Received 12 November 2020; revised 21 December 2020; accepted 31 December 2020; posted 5 January 2021 (Doc. ID 414570);
published 11 February 2021
High-harmonic generation in the ultraviolet region is promising for wireless technology used for communications
and sensing. However, small high-order nonlinear coefficients prevent us from obtaining high conversion effi-
ciency and functional photonic devices. Here, we show highly efficient ultraviolet harmonic generation extending
to the fifth order directly from an epsilon-near-zero indium tin oxide (ITO) film. The real part of the annealed
ITO films was designed to reach zero around 1050 nm, matching with the central wavelength of an Yb-based fiber
laser, and the internal driving electric field was extremely enhanced. A high energy conversion efficiency of 10
−4
and 10
−6
for 257.5 nm (fourth-order) and 206 nm (fifth-order) ultraviolet harmonic generation was obtained,
which is at least 2 orders of magnitude higher than early reports. Our results demonstrate a new route for over-
coming the inefficiency problem and open up the possibilities of compact solid-state high-harmonic generation
sources at nanoscale.
© 2021 Chinese Laser Press
https://doi.org/10.1364/PRJ.414570
1. INTRODUCTION
Nonlinear optical harmonic generation is a typical strong-
field physical process, which requires strong electric field inten-
sity to trigger light–matter interaction [1–3]. For example,
second-order nonlinearity, including second-harmonic genera-
tion (SHG), optical parametric oscillation (OPO), and sum
frequency generation (SFG), extends the laser spectral range
from visible to mid-infrared, and even the terahertz (THz) re-
gion [4–6]. Furthermore, high-order harmonic generation
(HHG), producing coherent ultraviolet (UV) light, extreme-
ultraviolet (EUV) light, and soft X-ray sources [7], is gaining
increasing attention for space-to-space communication, laser
manufacturing, and attosecond physics [8–10]. The realization
of compact and reliable UV coherent sources at nanoscale is
now opening the door to industrial and scientific applications
with improved reliability, better material compatibility, and
greater resolution owing to their shorter wavelengths, such
as wafer scribing, photolithography, and flow cytome-
try [11,12].
Since the 1980s, HHG in the UV and EUV region has been
studied in atomic gases and bulk crystals for decades [13].
However, the conversion efficiency is very low (<10
−6
for
the fifth and higher orders) [14]. According to nonlinear optical
principles [3], the lowest-order induced polarization P
2
would
be comparable to the linear polarization P
1
when the ampli-
tude of the applied field E is of the order of the atomic electric
field strength E
at
(∼5.1 × 10
11
V∕m). For condensed matter,
first-order susceptibility χ
1
is of the order of unity; hence,
second-order susceptibility χ
2
would be expected on the order
of 1∕E
at
, that is, ∼10
−12
m∕V. For higher-order nonlinearity,
the χ
n
susceptibility would be reduced by scaling of 1∕E
at
n
,
and the response is too inefficient to be detectable. Therefore, it
remains a great challenge to obtain direct UV HH G with high
conversion efficie ncy, especially pumped by the commercial
light sources, e.g., the Ti:sapphire laser (λ ∼ 800 nm) and
Yb-based fiber laser (λ ∼ 1030 nm).
Clearly, a strong driving field (external and internal) is the
sufficient condition for improving the conversion efficiency. By
applying an external mid-infrared femtosecond laser with ultra-
high peak power, direct HHG has been realized in ZnO crystal
(pump source, 3.2–3.7 μ m, up to 27th order) [15] and MoS
2
monolayer (pump source, 4.1 μm, up to 13th order) [16].
Besides, another strategy to avoid the inefficiency problem is
the enhancement of the internal electric field. This could be
realized in some special nonlinear media with a refractive index
(permittivity epsilon) for the interacting wavelengths near zero
[17]. Based on Maxwell’s equations and boundary conditions
of the electric displacement vector, the epsilon-near-zero
(ENZ) effect can greatly boost the internal laser field, which
Research Article
Vol. 9, No. 3 / March 2021 / Photonics Research 317
2327-9125/21/030317-07 Journal © 2021 Chinese Laser Press