透明导电氧化物：近红外等离子体中的ENZ特性与应用综述

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本文主要探讨了透明导电氧化物（Transparent Conductive Oxides, TCOs）在近红外（Near Infrared, NIR）等离子体领域的应用。TCOs作为一种关键的光电子器件组件，要求它们在可见光范围内具有高透光率的同时，还要具备良好的电子导电性。在近红外波段，高掺杂半导体氧化物表现出显著特性：其真实部分（即介电常数的实部）从正变为负，形成了所谓的"epsilon-near-zero" (ENZ) 频率区域，该频率位于NIR光谱范围内。 ENZ现象使得TCOs呈现出类似金属的介电损耗特性，这意味着它们有能力在近红外区域支持表面等离子体（Surface Plasmons, SPs）。作者Zhewei Wang、Chaonan Chen、Ke Wu、Haining Chong和Hui Ye在文中详尽总结了诸如碘化银（ITO）、氧化锌铝（AZO）和氟钛酸钠（FTO）等常见TCO的一些物理参数，这些参数包括载流子密度（carrier density）、载流子迁移率（carrier mobility）、等离子体频率（plasma frequency）以及ENZ波长。理解这些参数对于设计和优化NIR等离子体器件至关重要，因为它们直接影响了TCOs在光通信、传感器、纳米电子设备以及光热转换等应用中的性能。例如，载流子密度决定了材料的导电性能，高的载流子密度意味着更好的导电性；而迁移率则影响了TCOs响应光信号的速度。等离子体频率与材料激发等离子体的能力相关，是决定SPs质量的关键因素。ENZ波长则是衡量材料在NIR区域表现出ENZ效应的具体频率范围，这直接影响到材料在支撑高效SPs传播时的性能。通过深入研究这些参数，科研人员能够更好地利用TCOs在NIR等离子体中的特性，开发出高性能的光电子器件，如高效太阳能电池、生物传感器和隐身技术中的隐身材料等。此外，未来的研究可能会探索新型TCO材料的设计，以进一步提高其在NIR光谱区的性能和应用范围。这篇研究论文为我们提供了关于TCOs在近红外等离子体领域应用的宝贵参考，对推动相关领域的科技进步具有重要意义。

vacuum permittivity. Noble metals, such as silver and gold, have

relatively high carrier densities exceeding 10

3

, locating

their plasma frequencies in a range starts from the ultraviolet

(UV) region to the visible light. TCOs have a lower conductivity

and their carrier density can reach 10

–10

3

, an order

lower than metals, shifting their plasma frequency to near-

infrared range.

2.2. Epsilon Near Zero (ENZ) Regime

In normal dispersion, the real part of permittivity (e

) of material

decreases as wavelength increases. When e

decreased from

positive to negative, the wavelength is called as epsilon-near-zero

(ENZ) wavelength. The ENZ wavelength of TCOs, depending on

carrier density and preparation process, is always located in the

near-infrared (NIR) range which is convenient for optical

communication applications. For example, the ENZ wavelength

of a 900 nm thick AZO thin ﬁlm deposited by PLD is 1410 nm,

[8]

while for a kind of commercial ITO is 1528 nm.

[25]

In the ENZ regime, a strong nonlinear optical coefﬁcient has

been revealed in AZO thin ﬁlms.

[8]

Suppose n

as the nonlinear

refractive index then, the total refractive index can be written as

n ¼ n

þ δn

, where δn

¼ n

I. In the ENZ regime, if e

is small

enough and consequently the linear refractive n

would be tiny,

thus the nonlinear refractive could make a signiﬁcant effect.

Figure 1a shows more than a 0.4 variation of AZO ﬁlm refractive

index using pump-probe measurement system, which is

considered as a high change compared to the linear index of

0.09, around 500% of relative changes.

[8]

Another similar

experiment by Z-scan measurement has shown a 170% change

of the linear refractive index in ITO.

[25]

Nonlinearity results

convey that TCOs have a promising future in optical nonlinear

applications.

There are a few zero-index phenomena in TCOs. The phase

velocity v

ﬃﬃﬃﬃ

eμ

can exceed c when e  0 in ENZ regime

(μ ¼ 1), where c is the speed of light in vacuum, e and m are

permittivity and permeability, respectively. For instance, the light

with a wavelength range from 1.11 to 1.25 mm exhibits

superluminal in ITO.

[24]

But a large frequency dispersion exists,

there leads to a sub-c group velocity, where v

@ω

, indicating

that the speed of energy and information is also lower than light

in TCOs.

If imaginary part of permittivity (e

) approaches zero, the

model of permittivity e

 0 may occur. According to non-

resource Maxwell equation

[29]

r

H ¼ iωe

E ð3Þ

the interrelationship between electric and magnetic ﬁelds will no

longer exist with an e near zero. A gentle spatial variation of the

magnetic ﬁeld (left part of Equation 3) corresponds to a dramatic

temporal variation of the electric ﬁeld (a large

ω in the right part

of Equation 3), thereby space-time decoupling would occur,

[30,31]

which produces a quasi-static electromagnetic wave. Hence, an

electromagnetic wave can be guided through ENZ channel no

matter how the channel is bent or distorted. It is explained that

there is no phase change in the ENZ medium, and the length

from the start to end of the channel can be seen as a point, just as

Figure 1b shows.

[7]

TCOs’ optical losses (corresponding to the

imaginary part of permittivity) are considerably low in their

respective ENZ range. For example, the e

of AZO at ENZ regime

is around 0.2, making it a promising future for these “near zero”

applications.

2.3. Surface Plasmon (SP)

Surface plasmon (SP) is a kind of Transverse Magnetic (TM)

wave which is usually occurring at the interface between metal

and dielectric, enabling the material to focus the optical ﬁeld in a

narrow area around the interface.

[32,33]

By solving Maxwell

equation, we are able to obtain the plasmon wave-vector as

¼ k

ﬃﬃﬃﬃﬃﬃﬃﬃﬃ

þe

, where e

and e

represent for the permittivity of

metals and dielectric, respectively, k

is vacuum wavevector of

incident light, k

is SP wave-vector.

[34]

As k

is not equal to that

of incident light in the same dielectric, a prism or grating

coupling is necessary for exciting SP. Once the resonance

condition is satisﬁed, namely, e

þ e

¼ 0, the denominator will

approach 0, that means SP resonance will be excited. Due to the

ability to overcome diffraction limit, surface plasmon has the

potential to be the basis of ultra-compact photonic devices. These

Figure 1. Nonlinear and zero-index phenomena in ENZ regime. a) The change of real part of refractive index (δn

¼ n

I) versus wavelength. In around

1390 nm, the nonlinear refractive index change reaches maxium. Reproduced with permission.

[8]

schematic view of tunneling effect of ENZ waveguide, where the phase of electromagnetic wave remains unchanged and exhibits “quasi-static”.

Reproduced with permission.

[7]

status

solidi

physica

www.advancedsciencenews.com www.pss-a.com

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