RESEARCH ARTICLE
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3D Terahertz Confocal Imaging with Chromatic Metasurface
Xiaolong You,* Rajour Tanyi Ako, Sharath Sriram, and Withawat Withayachumnankul
objects that have thicknesses of multiple wavelengths. This limitation has been
overcome by terahertz confocal imaging that incorporates the confocal microscopy principle.[6,7] It mechanically
sweeps the focal spot across the object
in 3D, and images collected at different depths are stacked together to form
3D reconstruction of the object. Nonetheless, the existing approaches typically involve bulky optical components and experience time-consuming 3D mechanical scans for thick objects. As an alternative, 3D terahertz confocal imaging can
be realized by employing metasurfaces.
A metasurface is typically a planar
and periodic array composed of subwavelength dielectric[8–11] or metallic[12–15]
resonators. The resonators locally interact with incident waves and collectively alter the magnitude, phase, and
polarization of output waves.[16–21] By
means of phase control, focusing metasurfaces with tunable focal lengths are
of relevance to 3D confocal imaging. In
the microwave range, a water-filled focusing metasurface[22] was capable of
migrating its focal spot over a distance of 3.3𝜆0 at 5 GHz, by changing
the filling height of water in its unit cells. Its design concept
is similar to that of conventional liquid-based variable-focus
lenses.[23] That microwave design provided a limited measured
focusing efficiency of 11.7%. In the optics region, a focusing
metasurface[24] based on elastic substrate presented variable focal
lengths by mechanically altering its physical morphology. This
is a mimic of biological vision systems that varying its focal
lengths by involving shape changes. However, it suffered inconsistent focal spot sizes at different focal lengths. In addition,
mechanically tuning the focal distance limits the image acquisition speed. Alternatively, the chromatic aberration intrinsic to
metasurfaces can be exploited to reduce 3D mechanical scans to
2D raster scans. For instance, a tri-layer chromatic metalens[25]
was designed to achieving focused beams from 240 to 400 GHz
with a fractional bandwidth of 50%, while suffering a varied spot
size and a simulated focusing efficiency of less than 40%. In the
optics region, a chromatic dielectric metalens[26] relying on the
Pancharatnam–Berry (PB) phase worked for circular polarization
incidences and achieved an improved efficiency, but it exhibited
a relative bandwidth of 37.8% that limited its depth-of-field for
3D imaging. In short, chromatic designs are capable of improving 3D image acquisition speed with frequency-dependent focal
Terahertz confocal imaging allows 3D see-through of a non-metallic object
with high resolution. Conventional methods acquiring 3D images of thick
objects suffer from limited depth-of-field, constrained depth resolution,
and/or inconsistent spatial resolution at different depths. To address these
limitations, the intrinsic chromatic aberration of a typical focusing
metasurface is exploited to achieve frequency-dependent focal lengths. An
object located within this extended focal range can be readily 3D inspected by
performing 2D raster scans. A rigorous analysis reveals that the focal spot
maintains a constant waist diameter of 2.4 mm (equivalent to 2.2𝝀0 at 275
GHz) and migrates 68.1 mm (equivalent to 62.4𝝀0 , or 16.4 times of Rayleigh
length, or 1.4-fold of the designed focal length at 275 GHz) from 175 to 525
GHz, and thus achieving a consistent spatial resolution and a large
depth-of-field for 3D imaging. Importantly, this large depth-of-field is achieved
with a relatively high numerical aperture of around 0.42. Measurements
conducted between 220 and 330 GHz exhibit close agreement with the
calculation. To demonstrate its imaging functionality, two stacked papers with
different texts, a mobile phone, and earphones concealed in a charging case
are imaged, where a short-time Fourier transform is implemented in the
time-domain terahertz images to enhance image contrast. The presented
metasurface is technologically significant for imaging systems to rapidly
inspect objects in 3D with exceptional resolutions. Its potential applications
include in-situ defect detection and object identification in security screening.
1. Introduction
Conventional terahertz imaging employs non-ionizing electromagnetic waves from 0.1 to 10 THz to penetrate non-metallic
objects, and form high-resolution 2D images by collecting the
transmitted[1] or reflected[2] waves. It has found applications
in security screening,[3] in-situ inspection,[4] and biomedical
diagnosis.[5] However, with a focused beam of limited depth-offield, terahertz imaging struggles to provide 3D internals of those
X. You, W. Withayachumnankul
Terahertz Engineering Laboratory
The University of Adelaide
Adelaide, SA 5005, Australia
E-mail: xiaolong.you@adelaide.edu.au
R. T. Ako, S. Sriram
Functional Materials and Microsystems Research Group and The ARC
Centre of Excellence for Transformative Meta-Optical Systems
RMIT University
Melbourne VIC 3001, Australia
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/lpor.202401011
DOI: 10.1002/lpor.202401011
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phones hidden in a charging case. It is noted that the image
contrast at a specific depth is increased by applying a short-time
Fourier transform to suppress multiple out-of-focus reflections
at different frequencies.
2. Results
2.1. Analysis and Metasurface Design
Figure 1. Illustration of a chromatic focusing metasurface functionality. The metasurface converts incident collimated beams of different frequencies to focused beams at distinct focal lengths along the z-axis. The
waist diameters of the collimated and focused beams are denoted as d0′
and d0 , respectively.
lengths. However, many of them provided varied spot sizes
at different focal lengths, limited focusing efficiency, and constrained spot migration length. In addition, most of the existing
designs[22,24,25] indicate their potentials in imaging systems without experimental demonstrations.
Here, we propose a broadband and high-efficiency chromatic
focusing metasurface constructed by a typical tri-layer unit cell
design.[27] The novelty of the proposed metasurface lies in uncovering the untapped potential of a typical unit cell structure
through theoretical analysis and utilizing it to overcome the
aforementioned limitations. The theoretical analysis provides
clear guidance for designing passive focusing metasurfaces with
frequency-dependent focal lengths and frequency-independent
focal spot size. Importantly, the realized structure is the only
metasurface available to date that enables a high signal-to-noise
ratio, a depth-independent spatial resolution, an exceptional
depth resolution, and a large depth-of-field all together. Moreover,
this generic design concept is scalable to other frequency ranges.
As illustrated in Figure 1, the focusing metasurface exhibits
frequency-dependent focal length. Consequently, the internal
structure of an object that is placed within the metasurface extended focal range can be inspected by collecting the raster
scanned reflection image in the xy-plane. Specifically, the calculation reveals that the metasurface operating from 175 to 525 GHz
enables an exceptional depth resolution of 0.5𝜆0 at 275 GHz. Typically, a large numerical aperture accompanies a limited focal spot
migration length. However, the proposed metasurface achieves
a large focal length range from 29.1 to 97.2 mm. This extended
spot migration length of 68.1 mm or 62.4𝜆0 is achieved with a relatively large numerical aperture varying from 0.61 to 0.23 across
the operation bandwidth. Importantly, the focal spot beam waist
remains constant over the entire bandwidth. As such, the proposed metasurface is promising for rapidly inspecting an electromagnetically thick object, with a consistent spatial resolution
and refined depth resolution. It is worth mentioning that the
depth-independent spatial resolution ensures ease of measurement, simplifies object 3D reconstruction, and provides uniform
image quality. Its performance improvement over the existing
counterparts can be attributed to the nearly parallel transmission phase curves over a wide bandwidth provided by the employed resonators (see Figure S1e in Section A, Supporting Information). To investigate its capability in practical imaging, the
presented metasurface is applied to imaging two cascaded papers with different handwritten letters, a mobile phone, and ear-
Laser Photonics Rev. 2025, 2401011
In order to converge an incident collimated beam into a focused
beam, each resonator on the metasurface is expected to introduce an appropriate phase shift. The required phase distribution
across the metasurface for beam focusing can be calculated by[28]
𝜑(x, y) = kf
(√
)
F02 + (x2 + y2 ) − F0 ,
(1)
where kf is the wavenumber in free-space, (x, y) mark the location of a resonator in the Cartesian coordinates, and F0 is the focal length at the design frequency. Equation (1) indicates that the
focal lengths and spot sizes at other operation frequencies can
be roughly estimated by the phase distribution across the metasurface. It can be derived from Equation (1) that the frequencydependent focal length F can be expressed as
F=
(
)
kf x2 + y2
2𝜑(x, y)
−
𝜑(x, y)
.
2kf
(2)
As the value of kf is significantly larger than the phase profile
𝜑(x, y) at terahertz frequencies, the ratio of 𝜑(x, y)∕(2kf ) stays
nearly the same at different frequencies. As such, the focal length
F is linearly proportional to the operation frequency provided that
𝜑(x, y) is frequency-independent.
In addition, the focal spot waist diameter d0 depicted in
Figure 1 can be evaluated by[29]
d0′
d0 = √
,
1 + (z′R ∕F)2
(3)
where d0′ and z′R = 𝜋d0′2 ∕(4𝜆0 ) denote the beam waist diameters and Rayleigh ranges of the incident collimated beams,
respectively, while 𝜆0 represents the operation wavelength in
free-space. Given that horn antennas exhibit restricted output
beamwidth changes[30] across a relatively wide bandwidth, d0′ can
be approximated as a constant. Thus, the variation of the focal
spot diameter d0 is mainly determined by the ratio of z′R ∕F. In
accordance to Equation (2), the ratio of z′R ∕F can be written as
z′R
F
=
d0′2 𝜑(x, y)
4(x2 + y2 ) −
4𝜑2 (x, y)
.
(4)
k2f
For a frequency-independent phase profile 𝜑(x, y), the ratio of
4𝜑2 (x, y)∕k2f is the only variable in Equation (4) and it exhibits
subtle variations at terahertz frequencies. Hence, the spot diameter d0 in Equation (3) is expected to remain unchanged across
different frequencies.
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Figure 2. Manufactured focusing metasurface prototype. a) Photograph
of the fabricated sample. b) Magnified view of the middle layer containing 9 SRRs. The fabricated device has an aperture size of 45 × 45 mm2 ,
corresponding to 187 × 187 resonators.
It is found that a typical three-layer unit cell structure from the
literature[27] provides approximately parallel transmission phase
responses over a broad bandwidth (see Section A, Supporting Information). It is thus employed to form a focusing metasurface
that presents nearly frequency-independent phase profile from
175 to 525 GHz. Note that this typical unit cell has been implemented in some focusing metasurfaces.[28] However, none have
uncovered its potential in realizing frequency-dependent focal
lengths or demonstrated its feasibility in 3D imaging. As a proofof-concept, the metasurface focal length at the design frequency
of 275 GHz is determined as F0 = 50 mm.
2.2. Calculated and Measured Performance
The metasurface output field profile is calculated by using the
Huygens–Fresnel principle[31] and the simulated transmission
coefficients of the resonators. In the calculation, the collimated
beams incident on the metasurface are considered to have
equiphase fronts, making the approach valid for both coherent
and incoherent terahertz wave incidences. Moreover, as the
changes in resonator dimensions are sufficiently gradual across
the metasurface aperture, the resulting mutual coupling differences compared to an individual resonator simulation are nearly
negligible. Consequently, a local periodicity can be assumed in
the calculation. This practice is widely adopted and validated
by various non-uniform metasurface designs.[17,28] Images of
the fabricated metasurface prototype is shown in Figure 2,
while the manufacture techniques are reported in our previous publications.[32] Considering the facility accessible to
the authors, the manufactured metasurface is experimentally
evaluated from 220 to 330 GHz to confirm its focusing capability. The measurement setup is detailed in the Experiment
Section.
The calculated and measured field profiles given in Figure 3
demonstrate the focusing capability of the designed metasurface, where incident collimated beams of different frequencies
converge along the propagation axis to distinct focal points. Detailed processing of the measured data is presented in Section B
(Supporting Information). Due to the symmetry nature of a
Gaussian beam, the field profiles in the xz- and yz-planes are
similar. Specifically, Figure 3a–j reveals that the focal spot continuously migrates from 175 to 525 GHz with a total migration
Laser Photonics Rev. 2025, 2401011
length of 68.1 mm, or 62.4𝜆0 , or 1.4-fold of the focal length at
275 GHz. This relatively large migration length suggests that the
presented metasurface is able to inspect the internal structure
of an electromagnetically thick object by a single round of raster
scans. It is observed in Figure 3c,h that the electric field maximals at 275 GHz are located at z = 49.2 mm, which are slightly
deviated from the designed focal length of F0 = 50 mm. Further
calculations explain that this deviation is collectively contributed
by the incident beam truncation and phase discretization across
the metasurface. As depicted in Figure 4a, an excellent agreement between the calculated and measured focal lengths from
220 to 330 GHz is achieved, validating the local periodicity assumption in the calculation. In addition, the focal length varies
linearly with operation frequency as suggested by Equation (2).
The numerical aperture (NA) at the design frequency impacts
the focusing capability of metasurface, and a larger NA typically
results in a decreased focal spot migration length (see Section C,
Supporting Information). Figure 4b reveals that the calculated
and measured NA at 275 GHz are 0.42, and it varies in between
0.61 and 0.23 from 175 to 525 GHz. In accordance to the wave
diffraction theory, the spatial resolution and spot size are constrained by the NA. The Abbé diffraction limit is expressed as
d = 𝜆0 ∕(2NA). Based on this, Figure 4c specifies the frequencydependent diffraction limit of the spot size, and the focal spot
full width at half maximum (FWHM) at each frequency cannot
be smaller than its corresponding diffraction limit.
Figure 5 provides a comparison of the calculated and measured
focal spots at different frequencies. The calculation reveals that a
constant focal spot with FWHM of 1.5 mm is maintained from
175 to 525 GHz, corresponding to a waist diameter of d0 = 2.4
mm (2.2𝜆0 at 275 GHz). Note that this constant spot size across
the entire bandwidth is of importance for terahertz 3D imaging, while it compromises the spatial resolutions at higher frequencies. As compared in Figure 4c, the spot size approaches the
diffraction limit. Measured spot sizes well follow the calculation
with a maximum discrepancy of 0.1 mm. This deviation can be attributed to the inevitable fabrication tolerance and experimental
misalignment. Typically, the focal spot size is expected to be approximately proportional to the operation wavelength.[33] Its constant spot size is explained by Equations (3) and (4). In accordance
with the spot waist diameter, the depth-of-focus of a converged
beam can be evaluated, and it is illustrated in Figure 6. Due to
the shorter wavelength at higher frequencies, the slight discrepancy of 0.1 mm between the calculated and measured spot sizes
results in a depth-of-focus deviation up to 1.7 mm.
Two other important features of the metasurface are its focusing efficiency and bandwidth. In calculation, the focusing efficiency is evaluated by integrating the total power across the focal
plane with an area of 10 × 10mm2 , and normalizing it against
the total incident power delivered to an area of 45 × 45 mm2 in
the absence of the metasurface. The experimental evaluation of
efficiency is carried out in a simplified manner and details are
provided in the Experiment Section. The calculated focusing efficiency depicted in Figure 7 suggests that it reaches a maximum
value of 83.3% at 275 GHz, and remains higher than 50% from
175 to 525 GHz. The measured efficiency from 220 to 330 GHz
is higher than 71.5%. This relatively high focusing efficiency enables the metasurface to explore thick objects by providing a high
signal-to-noise ratio.
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Figure 3. Calculated and measured output field distributions of the metasurface along the propagation axis. Calculated field profiles in the a–e) xz-plane
and f–j) yz-plane. Measured field magnitude distributions in the k–m) xz-plane and n)-p) yz-plane. All plots are in linear scale and normalized to their
own maxima. The dashed boxes in (b–d) and (g–i) mark the corresponding measured areas. 2D interpolation with an interval of 0.1 mm along the x-,
y-, and z-axes is applied to the measured field profiles.
Figure 4. Calculated and measured focusing performance of the metasurface. a) Focal length, b) numerical aperture, and c) diffraction limit and focal
spot FWHM.
Figure 5. Calculated and measured focal spots. a–e) Calculated and f–h) measured focal spot field magnitude distributions at different frequencies. i–m)
Cross-sectional magnitude profiles of (a–h) along the y-axis. All plots are in linear scale and normalized against their respective maxima.
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Figure 6. Calculated and measured depth-of-focus of the converged
beams. The depth-of-focus is calculated by b = 𝜋d02 ∕(2𝜆0 ).
Figure 7. Calculated and measured focusing efficiency. The shaded area
marks the frequency range from 175 to 525 GHz with an efficiency above
50%.
As the presented metasurface targets 3D imaging, a consistent
spatial resolution across distinct depths and a sufficient signal-tonoise ratio are both desirable in imaging. As such, its bandwidth
is defined where the focal spot size remains constant and the focusing efficiency exceeds 50%. Accordingly, the bandwidth of the
presented metasurface spans from 175 to 525 GHz, equivalent to
a fractional bandwidth of 100%. The focal spot migrates 68.1 mm
across the entire bandwidth, corresponding to a large length of
62.4𝜆0 or 16.4zR at 275 GHz.
Spatial and depth resolutions are two critical metrics for evaluating the performance of the proposed metasurface in 3D
imaging. Its spatial resolution is experimentally characterized by
imaging a Siemens star at diverse focal lengths ranging from 38.1
to 59.1 mm in 3 mm increments. Measured results suggest that
the metasurface achieves a spatial resolution of ∼ 0.50 line pairs
per millimeter (lp/mm) (see Section D, Supporting Information).
The depth resolution of the proposed focusing metasurface can be estimated using the optical coherence tomography
theory[34,35] as Δz = 0.5 mm, equivalent to 0.5𝜆0 at 275 GHz and
it has been experimentally verified (see Section D, Supporting
Information). Note that the depth resolution pertains to timedomain reflections from an object, whereas the depth-of-focus of
a converged beam shown in Figure 6 is a concept related to wave
behaviour in the frequency-domain. As such, they are not directly
Laser Photonics Rev. 2025, 2401011
Figure 8. Optical photographs and terahertz images of two stacked papers. Photographs of a) two stacked papers attached to a 3D-printed
holder, b) front paper layer, and c) back paper layer. Frequency-domain
terahertz images of d) the front layer at 255 GHz and e) back layer at 315
GHz. Terahertz waves propagate along the z-axis. The two papers are separated by 13 mm. A 2D raster scan is performed in the xy-plane over an
area of 50 × 50 mm2 with a scanning step of 2 mm.
relevant. Instead, the depth resolution is determined by the operation bandwidth and center frequency of the metasurface.
Terahertz images created by frequency-domain reflection magnitudes of an object present all structures within the depth-offocus of a converged beam. This method is applicable to scenarios
such as extracting texts from warped papers or imaging systems
employing incoherent sources. Note that a frequency-selective receiver is needed for systems with incoherent sources, so as to
separate frequency components from the received signals. To acquire precise depth-specific features, an inverse Fourier transform is implemented to convert the complex-valued spectrum to
their time-domain counterparts. Subsequently, terahertz images
formed by time-domain reflection magnitudes are employed to
illustrate the object detailed structure at a particular depth. However, as this method requires frequency-domain reflection phase,
it is thus not effective when an incoherent source is used in the
imaging system.
2.3. 3D Terahertz Imaging
In order to verify the functionality of the proposed metasurface
in practical terahertz imaging, two stacked papers both containing handwritten letters, a mobile phone (Apple Inc., iPhone 12)
and earphones (Apple Inc., AirPods, first generation) concealed
in a charging case are imaged. The measurements are conducted
from 220 to 330 GHz and the experimental setups are detailed in
the Experiment Section. Constructed time-domain terahertz images contain both in-focus and out-of-focus reflections from the
object. The out-of-focus responses blur the resulting image. To
enhance image contrast, a short-time Fourier transform is employed to separate the frequency components at different time
intervals, and out-of-focus reflections are suppressed (see Section E, Supporting Information).
In Figure 8a–c, two papers each with different handwritten
texts are cascaded along the wave propagation direction, with
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Figure 9. Optical photographs and terahertz image of a mobile
phone. Photographs of the mobile phone a) with and b) without back
cover. c) Terahertz time-domain reflection image of the mobile phone
taken with the back cover in place. The terahertz image is in linear scale
and normalized to its maxima. The red solid box in (b) marks the scanning
area of 45 × 45 mm2 . 2D interpolation is applied to the reflection image
with an interval of 0.1 mm along the x- and y-axes. (a,b) are reprinted with
permission from The Dbrand.[36]
a separation of 13 mm. Terahertz waves can penetrate through
both thin ink layers and papers. However, due to their distinct
complex permittivities at terahertz frequencies, the ink layer
and paper exhibit different reflections to the incident terahertz
waves. This reflection contrast allows for the extraction of the
text contents on each paper, as shown in Figures 8d,e that are
formed by frequency-domain reflection magnitudes. Note that
the incomplete letters in Figure 8e are due to the non-uniform
spreading of ink pigments and dye particles.
The terahertz imaging a closed mobile phone is taken from its
back side as shown in Figure 9a. Focused terahertz waves penetrate the phone back cover that is made of glass-ceramic and other
non-metallic components within. Reflections from the internal
structures are then collected. Figure 9c clearly reveals the detailed features of the imaged mobile phone, including its camera
module and wireless charging coil. Moreover, thin slits and dents
surrounding different components are visible, demonstrating the
high spatial resolution of the proposed metasurface for terahertz
imaging. Due to the presence of highly reflective metallic structures that terahertz waves are unable to penetrate through, the
depth information of the phone cannot be acquired.
In the other experiment, earphones hidden in a charging case
and enclosed in a commercial package made of thick paper are
imaged to investigate its internal structures at different depths. It
can be inferred from Figure 10 that the focused terahertz waves
can penetrate through various non-conductive layers to find the
hidden earphones, including the thick package layer, multiple
plastics, PCB, and the charging case battery. Moreover, the metallic components inside of the charging case, such as the hinge
shown in Figure 10c and two chips of limited sizes in Figure 10d,
are also detectable by the focused terahertz beams. Therefore,
the 3D terahertz imaging capability of the proposed metasurface
is demonstrated.
3. Discussion
The metasurface with a designed focal length of F0 = 50 mm at
275 GHz is constructed, and its feasibility for 3D terahertz imaging is experimentally demonstrated. Further calculations of metasurfaces formed by the same set of resonators but with nominal
Laser Photonics Rev. 2025, 2401011
focal lengths of 25 and 100 mm have been conducted (see Section C, Supporting Information). The calculation suggests that a
smaller nominal focal length leads to a reduced focal spot size,
which enables an improved spatial resolution for terahertz imaging. However, it is accompanied by a decreased spot migration
length that limits the depth-of-field. As such, this trade-off needs
to be considered based on specific application scenarios.
Some lenses operating at a single wavelength with a relatively
large depth-of-focus have been employed for imaging by collecting the transmitted[38] or reflected[39] waves from objects. However, due to their nature of operating at a single wavelength,
they cannot be implemented in 3D imaging to acquire the depthspecific features of an optically thick object.
Several conventional optics such as Fresnel lenses and binary
lenses also possess some degree of focal spot migration. In order
to illustrate the advantages of the proposed planar metasurface,
the focusing capabilities of a phase-correcting Fresnel lens[42]
and positive-type binary lens[43] are investigated numerically (see
Section F, Supporting Information). As a comparison, key performance measures of the Fresnel lenses are listed in Table 1. Since
the binary lens provides a focusing efficiency of merely ∼25%
that limits its applications at terahertz frequencies, it is thus
excluded from the comparison table. Compared to the bulky
Fresnel lens, the relative bandwidth and focal spot migration
length of the proposed subwavelength metasurface are increased
by a factor of 2.9 and 3.4, respectively. Specifically, the performance of the diffractive Fresnel lens is limited by the presence of
a secondary focus that is due to the second-order diffraction[44]
(see Figure S9d in Section F, Supporting Information). This
secondary focus is located at a shorter distance from the lens aperture than the primary focus, and its field magnitude increases
at higher frequencies. As a result, it leads to a reduced focal
spot migration length and a decreased focusing efficiency at the
primary focus. Furthermore, its strong reflections add blurring
to the collected images, making the Fresnel lens less ideal for 3D
imaging.
In order to further illustrate the superiority of the presented
metasurface, its performance is compared to notable reported
planar structures from the literature as shown in Table 1. As
optical counterparts[45–49] experience significantly different fabrication processes and suffer increased metal losses, they are
thus excluded from Table 1 to make a fair comparison. Compared to other metasurfaces with mechanically tunable focal
lengths,[22,40] and chromatic aberration,[25,41] the presented metasurface exhibits advantageous focusing efficiency, and remarkably increased focal spot migration length by a factor of at least
4.8. Most importantly, it is the only planar structure to date that
is capable of maintaining a constant spot size with a high focusing efficiency across a wide bandwidth. The achieved broadband
chromatic aberration leverages the exceptionally large fractional
bandwidth of the terahertz spectrum to extend the depth-of-field
of the metasurface, thereby effectively harnessing the potential
of terahertz waves for practical applications. These achieved features are highly desirable for 3D imaging systems but are rarely
available using passive devices. As such, the presented work effectively advances the focusing metasurface designs by offering theoretical insights into realizing frequency-tunable performance using passive metasurfaces. Furthermore, the presented
work significantly improves the performance of terahertz 3D
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Figure 10. Optical photographs and terahertz images of the earphones concealed in a charging case and both enclosed in a commercial paper package. a) The closed package for 3D terahertz imaging. It contains a charging case, where earphones are concealed inside. b) Schematic representation
of the charging case with diverse internal layers. Photographs of c) the charging case sits in the package, d) printed circuit board (PCB), e) battery,
and f) earphones concealed in the charging case. g–j) Time-domain reflection images correspond to (c)–(f), scanned through packaging from the PCB
side. The red solid boxes in (c)–(f) indicate the scanning area of 45 × 45 mm2 . The terahertz reflection images are in linear scale and normalized against
their respective maxima. (c)–(f) are reprinted with permission from The ElectronicsNmore.[37]
Table 1. Performance comparison of the presented metasurface with conventional optics and representative varifocal structures from the literature. The
focal spot migration lengths are normalized to their own free-space design wavelength. All results presented in the Table without marks are experimentally
confirmed.
Structure
Design
Thickness
Relative
Min.
Migr.
Unvaried
Tunability
BW
eff.
length
spot size
method
[𝜆0 ]
[%]
[%]
[𝜆0 ]
freq.
[GHz]
Water pillars[ 22]
5
1.1
−a)
11.7b)
3.3b)
No
Mech.
Cut-wires[ 40]
18.5
1.8
−a)
−c)
13.0
No
Mech.
Circular SRRs[41]
11
0.1
−a)
50
2.6
No
Freq.
Metal strips[ 25]
300
0.3
−a)
15
6.4
No
Freq.
Fresnel lens
275
2.8
34.1b)
74.5b)
18.3b)
Yes
Freq.
This work
275
0.2
100
50
62.4
Yes
Freq.
a) The structure cannot provide a fixed spot size and an efficiency above 50% simultaneously; b) This quantity is determined based on calculated or simulated results; c) This
quantity is not available from the literature.
imaging systems and further drives the development of terahertz
technology toward real-world implementation.
4. Conclusion
This paper presents a focusing metasurface exploiting its broadband intrinsic chromatic aberration for 3D confocal terahertz
imaging. The analysis suggests that it maintains a constant
focal spot waist diameter of 2.4 mm (equivalent to 2.2𝜆0 at
Laser Photonics Rev. 2025, 2401011
275 GHz) from 175 to 525 GHz with a focusing efficiency
above 50%. Moreover, the focal spot migrates 68.1 mm over
the operation bandwidth, corresponding to 62.4𝜆0 or 16.4zR at
275 GHz. Experiments carried out between 220 and 330 GHz
closely agree with the calculation. Compared to the Fresnel
lens and existing planar structures, the proposed metasurface
provides a remarkably increased relative bandwidth and focal
spot migration length. To explore its capability in imaging, a
Siemens star is employed as an imaging target and the proposed
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Figure 11. Experimental characterization of the fabricated chromatic focusing metasurface from 220 to 330 GHz. a) Photograph of the measurement
setup in transmission mode, and b) its schematic illustration.
Figure 12. Terahertz 3D imaging system using the proposed metasurface. a) Photograph of the confocal imaging system, and b) its schematic view.
metasurface achieves a measured spatial resolution of
∼0.50 lp/mm. An exceptional depth resolution of 0.5 mm
that equivalent to 0.5𝜆0 at 275 GHz is achieved to resolve object
depth features. Further, its application in 3D terahertz imaging
is experimentally examined by finding the earphones hidden
in a charging case that are optically invisible. A short-time
Fourier transform is employed to enhance image contrast of
the time-domain reflection images. The proposed metasurface
can be potentially applied in agriculture, art conservation, and
medical imaging. Future works include developing a terahertz
coherence tomography technique[50] to acquire depth-specific
features of an object without involving its reflection phase.
5. Experiment Section
Metasurface experimental characterization: A Keysight PNA Series Network Analyer N5222B connected to VDI extension modules (VNAX WR3.4) was employed to probe output field distributions of the fabricated
metasurface. As illustrated in Figure 11, the PNA sends microwave signals
to the VDI extension module, where they were up-converted into terahertz
signals ranging from 220 to 330 GHz. The terahertz signals were subsequently transmitted into free-space by a y-polarized horn antenna. A 3Dprinted plano-convex lens with an aperture diameter of 48.4 mm and a focal length of 120 mm was implemented to transform the divergent beams
into collimated beams. The collimated beam has a measured beam diameter of 34.0 mm at 275 GHz. The metasurface output beams were detected
by a near-field probe that was connected to a waveguide twist accounting
for metasurface polarization rotation. This receiving section was mounted
on a three-axis motorized stage and scan a space of 10 × 10 × 49mm3 ,
with a sampling interval of 0.15, 0.15, and 1.00 mm along the x-, y-, and
z-axes, respectively. Absorbers were implemented to shield the VDI extension modules to suppress spurious reflections from their metallic cases.
Laser Photonics Rev. 2025, 2401011
In order to evaluate the focusing efficiency, a gold-coated mirror was
placed within the focal range of the metasurface and moves along the zaxis. A focused beam leads to a maximum reflection at each mirror position and its corresponding frequency was recorded. A reference measurement was taken by replacing the metasurface with the mirror.
3D Terahertz Imaging: The terahertz 3D imaging system using the proposed focusing metasurface is shown in Figure 12. The objects here include a Siemens star, two stacked slabs with different patterns, two cascaded papers with distinct texts, mobile phone, or earphones concealed in
the charging case. The object was placed in between z = 38.1 mm and 59.3
mm away from the metasurface, which were the measured focal lengths
of the metasurface at 220 and 330 GHz, respectively. The object reflects
multiple focused beams at different depths, and the reflected beams were
collected by the transceiver. In order to acquire the internal structure of the
object, it was mounted on the three-axis motorized platform and raster
scans in the xy-plane.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
The authors thank Harrison Lees and Bryce Chung from The University
of Adelaide for technical assistance on the experimental setup and fruitful discussions. The authors are grateful to Alex Paquet from Institut National d’Optique (INO, Canada) for valuable insights. This research was
supported by Australian Research Council (Grant Nos. ARC DP170101922
and CE200100010). This work was performed in part at the Micro Nano Research Facility at RMIT University, and the Melbourne Center for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).
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Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Keywords
chromatic aberration, confocal imaging, metasurface, terahertz imaging
Received: July 2, 2024
Revised: December 17, 2024
Published online:
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