Ultrafast laser-written 3D nano-antenna for flux extraction and vertical focusing above near-infrared waveguides

G. Zhang, G. Martin, G. Grosa, Alain Morand, G. Cheng, R. Stoian, C. D'amico

► To cite this version:

G. Zhang, G. Martin, G. Grosa, Alain Morand, G. Cheng, et al. Ultrafast laser-written 3D nano-antenna for flux extraction and vertical focusing above near-infrared waveguides. Optics Express, 2026, 34 (4), pp.7138. 10.1364/OE.582868. hal-05525877

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# Ultrafast laser-written 3D nano-antenna for flux extraction and vertical focusing above near-infrared waveguides

G. ZHANG,<fnref n="1" />,<fnref n="4" /> ID G. MARTIN,<fnref n="2" />,* G. GROSA,<fnref n="3" /> A. MORAND,<fnref n="3" /> ID G. CHENG,<fnref n="1" />,<fnref n="4" /> R. STOIAN,<fnref n="4" /> ID AND C. D'AMICO<fnref n="4" />

**Abstract:** Ultrafast laser photoinscription of transparent materials has a unique capacity to generate tunable positive and negative multiscale index structures in flexible 3D designs. Combined with the employment of engineered non-diffractive beams, this enables the generation at the same time of micrometric size waveguides for efficient transport of optical signals and high aspect ratio nanostructures capable of nonperturbatively sampling the optical field propagating nearby in the waveguides, enabling therefore the implementation of signal extraction or antenna functions. We present results for the development of 3D nanoscaled grating arrangements in nonplanar circular geometry working as antennas with focusing properties, capable therefore not only to extract a part of the flux from a waveguide but also to focalize it at a given image plane above the sample, as a function of wavelength. Different geometries were studied, all with equal focusing radius, but different spacing between the nanostructures, in order to understand the sampling and grating efficiency as a function of the number of emitters and their relative distance to the waveguides. The goal of this work is to demonstrate how the high flexibility and the 3D aspect of ultrafast laser photo-inscription of bulk transparent materials can be used for developing efficient optical antennas that could direct flux from a waveguide to a detector, without the need of any relay optics, by fabricating a grating-lens inside the chip, with potential applications in the field of integrated optics spectrometry.

Published by Optica Publishing Group under the terms of the [Creative Commons Attribution 4.0 License](https://creativecommons.org/licenses/by/4.0/). Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

## 1. Introduction

One major challenge in 3D integrated photonics, once the optical functions have been realized in the chip (beam combination, multiplexing/demultiplexing...), is to re-image the optical flux on a detector. This is classically realized by bulk optics and eventually using classical gratings to split the different spectral channels. Most of these classical approaches are based on a propagating system (i.e. optical elements positioned along the propagation axis of the optical flux), which can eventually be a drawback if one wants to avoid straylight. Different approaches are based on the perpendicular extraction (or injection) of the flux, by vertically implementing gratings [1–4], phased arrays [5], metasurfaces [6], metalenses [7] or microlenses [8–10] that will radiate flux perpendicular to the propagation direction. Ideally, the ultimate goal is to directly detect light on the chip itself thanks to the antenna [11]. In the present work, we follow this approach, providing a laser-based fabrication solution for signal extraction from an

<footnote n="1">¹School of Artificial Intelligence, Optics and Electronics (iOPEN), Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China</footnote>

<footnote n="2">²Univ. Grenoble Alpes/CNRS, IPAG, F-38000 Grenoble, France</footnote>

<footnote n="3">³Univ. Grenoble Alpes/CNRS/Grenoble INP, CROMA, F-38000 Grenoble, France</footnote>

<footnote n="4">⁴Laboratoire Hubert-Curien, UMR CNRS 5516, Université de Lyon, Université Jean Monnet, St. Etienne, F-42000, France</footnote>

<footnote>*guillermo.martin@univ-grenoble-alpes.fr</footnote>
optical waveguide, equally laser fabricated. The fabrication is based on ultrafast direct laser writing (UDLW), given its ability to provide tunable positive and negative index changes in a fused silica glass substrate and the possibility to access vertical dimension during fabrication. A monolithic integration of the waveguide and of the extractors is therefore achieved. A part of the waveguide's optical flux is extracted from the waveguides by using 3D laser-written gratings in the evanescent region, consisting of periodically spaced elongated void nanostructures, arranged in concave circular geometry. Such antenna-like arrangement simultaneously achieves sampling and focusing in the vertical direction, with an angular emission dependent on the wavelength (spectral response). Several demonstrations of vertical antennas have been already realized [6–8] but no demonstration of a full 3D laser-solution has yet been given. A first demonstration using a horizontal distribution of antennas for spectrometry applications has been shown [12]. The UDLW involves engineered beams to achieve both waveguides and the antennas by shaping the focal region and the propagation direction. The antenna structures consist on laser-written nanoscaled one-dimensional voids acting as diffractive elements [13,14] coupled to laser-written single-mode waveguides in the near IR. In this paper, we will demonstrate that a 3D grating concave antenna with focusing properties can be realized and tailored in order to focus the extracted flux, initially confined in a single mode near infrared-waveguide, at a given and controlled vertical position. We will also show the angular dependence of the extracted flux as a function of wavelength. The experimental results on spectral and angular selectivity (and therefore the spectral resolution) of the 3D antenna as a function of the separation between the nanostructures composing the antenna will be presented. The goal of this work is to demonstrate how the flexibility and the design capabilities of UDLW can be used to implement a 3D distribution of localized nano-structures forming an antenna able to extract, direct and focus optical information from a waveguide directly to a detector, without any other bulky relay optics. A 3D circular focusing-grating, as the one presented in this paper, is impossible or very difficult to implement by using 2D techniques based on lithography. Although 2D metasurfaces [6] and metalenses [7] can achieve vertical focusing, the lithographic fabrication steps are eventually complex and numerous. Here, 3D laser-writing becomes an optimal tool to reduce fabrication complexity and time. Note that in this work, we don't aim at having one single antenna extracting all the flux on the waveguide, but just sample a small amount of flux, leaving enough power to be able to test different antenna configurations along the same channel waveguide. In a future work, we will achieve a quantitative analysis of the extraction efficiency as a function of nano-voids size and distance to the waveguide. In this work, a first parameter is to study the focusing performances of different concave grating, all at the same distance to the waveguide and same radius, but with different grating periods (nano-voids spacing).

## 2. Optical design

The three-dimensional concave gratings with circular geometry are laser-written on top of single-mode waveguides in the near IR, in order to extract and focus light on a detector placed on top of the sample. In our case, the element composing the gratings is represented by Bessel void nanostructures distributed along the arc of a circle following the design shown in Fig. 1.

In the case of planar gratings [15], the optimal spacing a between the nanostructures, in order to produce constructive interferences in the vertical direction, follows the classical grating law: $a = m \cdot \lambda/n_{\text{eff}}$, and all the nanostructures have the same distance from the waveguide. In order to detect the diffracted order, a lens must be used to get a clear image on the focal plane, assuming the signal extracted from the grating is composed of parallel beams. In the case of circular gratings, the design is done such as all the sampling centers interfere in a constructive way at the position given by the radius R. In this circular grating, as all the nano-emitters are co-phased, the constructive interference is obtained when all the optical paths are similar, hence the focus point is expected at a distance R and not at R/2 as in the case of a classical spherical mirror. The
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Schematic drawing of the 3D focusing antenna using a periodic distribution of nanovoids in concave configuration.</figcaption></figure>

central sampling nanostructure is situated at a minimum vertical distance d from the waveguide and all the other nanostructures are positioned increasingly higher with respect to the waveguide. Indicating with a the periodic lateral spacing and p the vertical increase (see Fig. 1), the link between a, p and the curvature radius R of the antenna is given by: $R^2 = (k-a)^2 + (R-p)^2$, where k is an integer that counts the number of periods. Assuming $k \cdot a \ll R$, the vertical increase $p_k$ corresponding to the k-th nanostructure can be written as: $p_k \approx (k \cdot a)^2/2R$.

### 3. Fabrication using direct laser writing

In the present case (signal wavelength $\lambda = 1.55$ µm in a fused silica sample), the fundamental spacing is given by $\lambda/n_{eff} = 1.55\mu m/1.45 = 1.069\mu m$, which is a very small separation and could generate some distortions due to the interaction between neighboring Bessel nanostructures during fabrication [16]. We chose therefore higher multiples starting from $m = 3$ ($a = 3.2$ µm) in order to avoid fabrication problems. The waveguide being buried 400 µm under the sample
surface, the radius R, and therefore the focal point, was fixed to 400 µm, in order to have the maximum optical power on the sample surface. The minimum vertical distance to the waveguide, shown in Fig. 1 is d = 3µm, distance already validated in other measurements [15], allowing for enough flux sampling without introducing strong perturbation and losses on the mode propagating in the waveguide.

As the antenna illumination depends on the coupling efficiency between the evanescent field of the guided mode and the Bessel nanovoid placed at the distance p<sub>k</sub>, we chose to work with p-values smaller than 2.5 µm, as larger values do not give efficient light diffraction. Note that the exponential field outside the waveguide core is going to apodize also the diffracted signal at the focus plane.

For the measurement, five circular antennas with the same concavity and radius were fabricated, using different spacing between the Bessel nanovoids given by a<sub>k</sub> = k·a, with k = 2,3,5,7,10, each having a central Bessel nanovoid close to the waveguide (d = 3 µm), and 10 nanovoids by each side, with increasing vertical distance p relative to the central one. Three of the five fabricated focusing gratings (R1, R3, R5) are shown in Fig. 2.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Phase-contrast image of the waveguides and the circular focusing-gratings, implemented by direct laser writing in bulk fused silica.</figcaption></figure>

The waveguides and the 3D focusing antennas were fabricated in to a standard SiO₂ bulk glass chip (Corning 7980) of 20 × 10 × 3 mm³ size. A UDLW technique employing 515 nm, 130 fs ultrashort laser pulses was applied with spatially-engineered beams adapted to the writing process. The waveguides were realized using a transverse slit-shaping method [17] that scans a disk-shaped focus at 50 kHz repetition rate, with 0.3 mm/s scan speed (two passages) and 1.12 µJ energy per pulse. The soft interaction regime generates a positive index region with a refractive
index contrast in the range of 2-3 × 10⁻³ (measured at 632 nm wavelength) and an estimated waveguide core diameter of 9-11 µm. The laser written waveguides designed here are straight and optimized for single mode behavior in the near-IR wavelength range (1.55µm), with expected propagation losses below 1 dB/cm [18]. The same UDLW setup enables the generation of adapted beams for one-dimensional nanoscale structuring, which innovatively combines temporal pulse shaping and non-diffractive beam spatial engineering. Zero-order Bessel beams are used for extractor fabrication.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Top: Example of the images obtained on the imaging set-up (Left: antenna plane, Right: focus plane, for R1 antenna). Following graphs: Left, transverse intensity of the antenna depending on the voids separation and their distance to the waveguide. Right, far-field (focusing plane) transverse intensity given by the diffraction grating having an intensity distribution as seen on the left.</figcaption></figure>

In order to fabricate the nano-gratings, the laser-writing pulse is transformed into non-diffracting Bessel beams of 9.7° half cone angle, with 5.4 µJ, 700 fs single-shot laser pulses at 515 nm wavelength by the use of an axicon and demagnification optics. The use of non-diffractive Bessel beams offers increased nonlinear stability and robustness against aberrations. The procedure allows the generation with high precision, of embedded high aspect ratio void nanostructures, which can be used as very efficient scattering centers forming the circular antenna. Such void-like one dimensional voxels are the results of a nanocavitation process triggered by the ultrafast excitation in the volume [13,19]. The high aspect ratio Bessel void nanostructures have a cross-section diameter of about 150 nm and are photoinscribed transversely in the vicinity of the waveguides (see Fig. 1 and 2), orthogonal to the waveguide axis. Due to the small size
these nanostructures do not perturb the traveling light modes in the waveguide, but they are
able to intercept part of the guided mode evanescent field and transform it in a transverse-
scattered radiation, which then will be focused onto a detector. The experimental set-up for the
characterization of the extracted signal is described in detail in Ref. [20].

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Top Left: Top view image of 5-nanovoids antenna, having 3 µm separation (non resolved). Top right: Side view image of the planar antenna above the waveguide. Bottom: Evolution of the intensity as the object plane is set at increasing height from the antenna.</figcaption></figure>

**4. Experimental results**

**4.1. Imaging the antenna-grating and the focusing-plane**

Near IR images of the light scattered by the antennas are given in Fig. 3 for a guided wavelength of 1550 nm. A fixed lens-detector imaging camera is used to observe the signal. The image on the left shows the individual emission of the antennas of the concave gratings when they are set on the object plane of the imaging camera. On the right, the light pattern of the coherent contribution of the antennas is shown, when the height of the imaging camera is increased, so that the object plane of the imaging system matches the image plane of the focusing grating (about 400 µm higher from the waveguides). As expected from the gratings theory, the first antenna (R1), having smaller periodic spacing between nanovoids, $a_2 = 2a = 6.4$ µm, shows a diffracted pattern with a more intense central order, and reduced side Bragg orders. In comparison, the R5 antenna, having larger periodic spacing, $a_{10} = 10a = 32.1$ µm, shows a focused light pattern with more Bragg orders (and lower intensity). Note also that antennas with higher value of spacing between nanovoids have a lower number of them within a value of $0 < p < 2.5$ µm, and therefore a lower number of nanovoids coupling efficiently with the evanescent field. The result is a less efficient signal extraction, and a less contrasted focused intensity pattern, as can be observed for the antenna R5 in Fig. 3.
## 4.2. Following the light from antenna-grating plane to the focusing-plane

In order to show the focusing efficiency of the concave gratings, as compared in particular with a planar grating, the vertical evolution of the intensity distribution from the plane of the antennas is measured, to the expected focusing plane, 400 µm above. In Fig. 4 this evolution is shown (NOTE: in Fig. 4, when the focus is set on the antenna plane, we visualize the planar 5-nanovoids antenna with 3 µm separation, as in [15], but the amplification is not enough to resolve the individual nanovoids. On the contrary, as shown on top of Fig. 5, as the nanovoids separation is higher, their individual optical emission can be discriminated):

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Top: Image of the R5 concave antenna, containing several nanovoids with 32 µm separation (only 5 are close enough to the waveguide to extract light). Bottom: Evolution of the intensity as the object plane is set at increasing height from the R1 (a), R3 (b) and R5 (c) antenna.</figcaption></figure>
As shown in Fig. 4, for the planar grating, the signal diverges progressively and the peak intensity decreases. The only way to recover the peak of diffracted intensity is to adjust the camera lens to look at infinity. On the contrary, in Fig. 5(a), (b), and c after having the nanovoids of the antenna clearly on-focus (on the object plane of the imaging system), an intensity blur is observed while moving the object plane to increasing heights, until the object plane corresponds to the expected focal plane of the antenna (here, 400 µm above the waveguide), where the constructive interference is expected, producing a high intensity diffraction peak, as defined by the concave focusing grating. At this height, corresponding to the radius R, the concave grating with more density (R1) allows to obtain diffracted orders with larger Free Spectral Range and more intensity, whereas the less dense grating (R5) results in weaker and spectrally closer peaks. At this focus point, the FWHM of the central peak as observed in Fig. 3(e)) for R1, having the larger Free Spectral Range, is 12.3 µm for the vertical direction and 6.4 µm for the horizontal direction.

### 4.3. Wavelength dependence of the diffracted signal

Finally, in order to show the gratings effect at the focal plane of the antenna, we performed a wavelength scan of the light injected in the waveguide, for the particular case of antenna R1, by using a tunable wavelength source (SANTEC). We report the result in Fig. 6. As can be seen, the focused central order moves along the direction of the grating when the injected wavelength is changed. In particular, as the wavelength increases the central order moves towards the right sense of the x axis. We can also see that the +1 and -1 side Bragg orders follow the same behavior.

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Diffracted intensity on the detector, as a function of wavelength. As wavelength increases, the diffracted peak shifts to the right, allowing to identify a given pixel area to a given spectral band</figcaption></figure>

From the graph of Fig. 6, the spectral resolution can be estimated as 20 nm at 1560 nm, which is a good value considering that only 21 Bessel nanovoids are acting as emitters, in the R1 concave grating. This high density grating achieves an efficient extraction of light, allowing to obtain a high intensity central peak (Fig. 5(a)), whose focusing spot along the detector's transverse axis is
sensitive to wavelength (Fig. 6) and validates the design of our concave focusing grating as an
important building block for integrated spectrometers.

**5. Conclusion**

In conclusion, we have demonstrated a 3D ultrafast laser fabrication technique that allows for the precise and flexible realization of 3D concave circular gratings acting as wavelength-dependent focusing antennas, embedded in bulk transparent materials, extracting and measuring signals from optical waveguides. The demonstrated focusing antenna function works satisfactorily, giving at the same time signal sampling, spectral sensitivity and focusing capability. This type of 3D photonic structures cannot be easily realized by 2D lithography-based technologies, highlighting therefore the high potential and the importance of ultrafast laser writing in the domain of 3D embedded photonics and related applications. This technology can be applied in order to avoid any other bulky relay optics carrying the signal information from the waveguide until the detection point, and represents therefore a further step toward the realization of ultracompact 3D spectrometers integrated in bulk glass chips. In particular, it could also be tested on laser-written silicon waveguides [21–23], as this could open the access to large scale technological platforms. Preliminary results have shown the feasibility of IR Bessel beams and burst modes that can provide resolutions in the 100 nm range [24]. In a near future we will work on concave gratings with reduced spacing between nanovoids (with an ideal target at a = λ/n<sub>eff</sub>) and therefore increase the Free Spectral Range between diffracted orders and reduce the width of the diffracted peak, to improve the resolution.

**Funding.** Institut national des sciences de l'Univers (ASHRA 2024); Région Auvergne-Rhône-Alpes (Projet Hirespect); Agence Nationale de la Recherche (ANR-11-LABX-0013); Agence Nationale de la Recherche (ANR2011 BS09 026 01, ANR-21-CE08-0005); National Key Research and Development Program of China (2025YFE0151700); National Natural Science Foundation of China (U24A20125).

**Acknowledgments.** Authors acknowledge the funding from ASHRA (Action Specifique Haute Resolution Angu-
laire) from INSU-CNRS, Région AURA via project Hirespect, Labex FOCUS (ANR-11-LABX-0013), from the Agence
Nationale de la Recherche via the projects ANR2011 BS09 026 01, ANR-21-CE08-0005, from the National Key R&D
Program (2025YFE0151700), and from the National Natural Science Foundation of China (U24A20125).

**Disclosures.** The authors declare no conflicts of interest.

**Data availability.** Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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