纳米点光学性质研究：宽带近场光谱技术的应用

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"该研究通过宽带干涉式同相扫描近场光谱技术，对锑硫化物（Sb2S3）纳米结构的空间和光谱模态映射进行了深入探索，这是一种具有直接能隙的半导体材料，近年来在薄膜太阳能电池吸收层领域引起了广泛关注。通过纳米颗粒墨水溶液和二维、三维纳米结构化技术，已经实现了50nm尺度的图案制作。为了深入了解这些纳米结构的未知纳米级光学性质，这对于它们在纳米光子学中的未来应用至关重要。研究者采用了一种光谱宽带散射型近场光学光谱技术，以20nm的空间分辨率对单个Sb2S3纳米点进行研究，覆盖了700至900nm的光谱范围。" 正文: 这项研究的核心是利用宽带干涉式同相扫描近场光谱法（Spatial and spectral mode mapping）来探测锑硫化物（Sb2S3）纳米结构的光学特性。Sb2S3是一种直接带隙半导体，因其在薄膜太阳能电池领域的潜力而备受关注。最近的研究表明，通过纳米粒子墨水溶液，可以实现这种材料的精细结构，包括二维和三维的纳米结构，最小尺寸可达到50纳米。这些微小的纳米结构有可能带来新的光学性能和功能，因此对其纳米级的光学性质有深入理解是推动其应用的关键。宽带散射型近场光学光谱技术的应用，允许研究人员以高空间分辨率（20纳米）和宽光谱范围（700至900纳米）对单个Sb2S3纳米点进行详细分析。这种技术的创新之处在于，它能够揭示传统远场光学方法无法捕捉到的局部光场信息，从而提供关于纳米结构内部模式和光相互作用的深度见解。通过对Sb2S3纳米点的空间和光谱模态映射，研究者可以解析纳米结构如何影响光的传播、吸收和发射特性。例如，这种映射可能揭示纳米点的局域等离子体共振、量子限制效应以及表面散射等现象。这些发现对于优化纳米结构的设计，以提升太阳能电池的效率或开发新型光电子设备具有重要意义。此外，该研究还可能为其他半导体纳米材料的研究提供一种通用的方法，因为宽带干涉式同相扫描近场光谱技术可以应用于各种纳米尺度的光学研究。通过对不同材料和结构的纳米光谱分析，科研人员可以探索新材料的潜在应用，如增强光捕获、光催化、光电转换等。这项工作不仅深化了我们对Sb2S3纳米结构光学特性的理解，而且也为纳米光子学领域的未来发展提供了重要的实验工具和技术路线。通过这些先进的测量手段，科学家们将能够设计出更高效、更精确的纳米光子器件，进一步推动光电子学和能源技术的进步。

(MO, Beck Optronics Solutions, model 5003-000) with NA ¼

0.4 and with a working distance of 14.5 mm. Using a reflective

MO minimizes dispersion that would otherwise result in a

varying fringe spacing in the recorded spectral interferograms.

After the BS and the reflective MO, <1mWis incident on the

sample and tip. For these input powers, laser-induced tip- and/or

sample-heating effects were not observed. 80% of the incident

light is transmitted at the BS and serves as a reference field E

our measurements. For this, the power is adjusted by a variable

gray filter before it is reflected back to the BS by a mirror (M

)

that forms the reference arm of the Michelson interferometer.

The reference arm length is adjusted to be shorter than the sam-

ple arm by ∼100 μm in order to enable spectral interference

with a convenient fringe spacing and is then kept constant dur-

ing the measurement. To access the near-field region, a sharply

etched gold nanotaper is brought in close proximity to the

sample surface. Scattered light is collected in the backward di-

rection by the MO. The part that is transmitted through the BS is

overlapped with the reference field E

, and the resulting super-

position is collected for detection.

The near-field probe is a sharply etched single-crystalline

gold nanotip with an apex radius of curvature of ∼10 nm. The

tip is brought in close vicinity to the sample surface and the tip–

sample distance is modulated at a frequency f ≈ 26 kHz and

with an amplitude of 12 nm. The minimum tip–sample distance

is kept at 4 nm. Here, we set the light polarization to s, i.e., the

electric field vector is along the y axis such that it lies within the

sample plane and is perpendicular to the tip axis.

We employ two different detection methods. In the first case,

for quasimonochromatic measurements, the reference arm is

blocked and the signal backscattered from the focus region is

detected with an avalanche photodiode (APD, Hamamatsu,

model C5331-02). It is further processed by a lock-in amplifier

(Zurich Instruments HFLI), with the tip modulation frequency

as the reference signal. The signal demodulated at the funda-

mental tip modulation frequen cy as well as those demodulated

at its second, third, and fourth harmonics are obtained and

stored. We label these spectrally unresolved signals produced

by the lock-in amplifier with S

, S

, and S

. In this case,

with the reference arm blocked, the signal detected by the APD

is constituted of light scattered out of the near field by the sharp

tip, E

, and of the unwanted background field E

scattered

from the sample and from the tip shaft. Together, this results

in a voltage proportional to jE

þ E

. Demodulating the

signal at higher-order multiples of the tip modulation frequency

makes the signal sensitive to the near-field contribution by re-

cording an interference pattern between E

and E

In the second case, for spectrally resolved measurements,

the reference arm is unblocked and the output of the Michelson

interferometer is steered to the monochromator (Princeton

Instruments, IsoPlane-160) equipped with a fast line camera

(e2V AViiVA EM4 with 512 pixels). The spectra measured

now constitute a mixture of near field, background, and refer-

ence field:

SðλÞ ∝ jE

þ E

: (1)

In front of the monochromator, the power detected from the

sample arm alone amounts to 10 μW, and the power detected

from the reference arm alone is 46 μ W. The reference arm

length is adjusted such that spectral interference fringes emerge

with a spacing of 3 to 5 nm (see the inset at the lower right in

Fig. 1). Equation (1) can be written as

SðλÞ ∝ jE

þ E

þjE

þ 2Re½ðE

þ E

Þ · E



· cosðk · ΔLÞ; (2)

where the wavevector k ¼ 2πλ

−1

and the path length difference

between sample and reference arm is ΔL. By separating the

modulated part of the measured spectra, the term RefðE

Þ · E

g is isolated and the complexity of Eq. (2) is consider-

ably reduced.

The remaining problem of suppressing E

with respect

to E

is solved by the fast line camera behind the mono-

chromator.Thelinecamerahasamaximumreadoutrateof

210 kHz > 8 · f, and we typically record 60,000 consecutive

spectra with the maximum rate. In postprocessing, the recorded

values are considered separately, i.e., for each wavelength com-

ponent of the spectrum SðλÞ, the line camera records a time

series of 60,000 values. A Fourier series expansion is performed

around multiples of the tip modulation frequency. For each

pixel, the so-determ ined Fourier coefficients are quantities

analogous to the signals generated by the lock-in amplifier in

the monochromatic measurements. Assembling the respective

Fourier components for all camera pixels results in spectra

ðλÞ to S

ðλÞ, demodulated at the first to fourth harmonic

of the tip modulation frequency.

As we will show below, for

higher-order demodulation the background field E

is strongly

suppressed in comparison with the near-field E

, such that

these spectra produce RefE

We investigate individual antimony sulfide (Sb

) nanopar-

ticles created on top of an Sb

film by EBL.

Our samp les are

annealed at 300°C to form crystalline thin films and nanostruc-

tures. This procedure shifts the bandgap from 2.24 to 1.73 eV,

and it reduces the above-gap absorption coefficient from 1 .8 ×

−1

at 450 nm to 7.5 × 10

−1

at 550 nm.

Here, we

study their optical properties at wavelengths between 700

and 900 nm, well below the bandgap. In this spectral region,

has relatively low losses (absorption <2 × 10

−1

800 nm)

and a high index of refraction, with a positive real part

varying from 3.2 at 700 nm to 2.9 at 900 nm, which means that,

in our experiments, it can basically be considered as a lossless

dielectric.

Here, we cover a flat, 140-nm-thick film of crystalline Sb

with Sb

nanoparticles, aiming at a cylindrical structure that

should guide the incident light into the underlying thin film. The

nanoparticles are created by EBL

on top of the flat film in a

regular order, with a spacing of 2 μm [Fig. 2(a); more details

on the sample preparation can be found in the Supplemental

Materials]. The spacing is chosen sufficiently large to avoid

dipolar couplings between adjacent nanopar ticles such that

each nanodot may be considered as an isolated nanoparticle.

They are typi cally dome-shaped, with a round or slightly

elliptical cross section of ∼350 nm diameter and a height of

∼150 nm. Figure 2(a) shows a scanning electron microscope

(SEM) image of the nanoparticles. A higher resolution image

of an individual nanoparticle [Fig. 2(b)] reveals a slightly

elliptical shape and a somewhat rough surface. The center of

the particle seems to have collapsed to some extent after the

annealing proces ses following EBL. These features (ellipticity,

surface roughness, and indent ation) vary slightly from one

nanoparticle to the next.

Zhan et al.: Spatial and spectral mode mapping of a dielectric nanodot…

Advanced Photonics 046004-3 Jul∕Aug 2020

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