# Gallium Phosphide Nanowires in a Free-Standing, Flexible, and Semitransparent Membrane for Large-Scale Infrared-to-Visible Light Conversion

Vladimir Fedorov, Alexey Bolshakov, Olga Sergaeva, Vladimir Neplokh, Daria Markina, S. Bruyere, Grégoire Saerens, Mihail Petrov, Rachel Grange, Maria Timofeeva, et al.

► To cite this version:

Vladimir Fedorov, Alexey Bolshakov, Olga Sergaeva, Vladimir Neplokh, Daria Markina, et al. Gal-
lium Phosphide Nanowires in a Free-Standing, Flexible, and Semitransparent Membrane for Large-
Scale Infrared-to-Visible Light Conversion. ACS Nano, 2020, 14 (8), pp.10624-10632. 10.1021/ac-
snano.0c04872. http://onlinelibrary.wiley.com/doi/abs/10.1021/ac-
snano.0c04872

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# Gallium Phosphide Nanowires in a Free-Standing, Flexible, and Semitransparent Membrane for Large-Scale Infrared-to-Visible Light Conversion

Vladimir V. Fedorov<sup>1,2</sup>, Alexey Bolshakov<sup>1</sup>, Olga Sergaeva<sup>3</sup>, Vladimir Neplokh<sup>1</sup>, Daria Markina<sup>3</sup>, Stephanie Bruyere<sup>4</sup>, Grégoire Saerens<sup>5</sup>, Mihail I. Petrov<sup>3</sup>, Rachel Grange<sup>5</sup>, Maria Timofeeva<sup>5</sup>, Sergey V. Makarov<sup>3,*</sup> and Ivan S. Mukhin<sup>1,3</sup>

**ABSTRACT:** Engineering of nonlinear optical response in nanostructures is one of the key topics in nanophotonic as it allows for broad frequency conversion at the nanoscale.

Nevertheless, the application of the developed designs is limited by either high cost of their manufacturing or low conversion efficiencies. This paper reports on the efficient second harmonic generation in free-standing GaP nanowires array encapsulated in a polymer membrane. Light coupling with optical resonances and field confinement in the nanowires together with high-nonlinearity of GaP material yield a strong second harmonic signal and efficient near-infrared (800-1200 nm) to visible upconversion. The fabricated nanowire-based membranes demonstrate high flexibility and semi-transparency for the incident infrared radiation, allowing to utilize them for infrared imaging, which can be easily integrated into different optical schemes without disturbing the visualized beam.

**KEYWORDS:** nonlinear nanophotonics, second harmonics, infrared imaging, flexible devices, nanowires.

## Introduction

Second harmonic generation (SHG) from optically resonant nanostructures has become a powerful tool for nanoscale optical probing of internal materials properties,<sup>1–4</sup> sensing,<sup>5,6</sup> beam-shaping,<sup>7</sup> imaging,<sup>8,9</sup> and nonlinear holography,<sup>10</sup> as well as for creation of nanoscale color-tunable light sources *via* infrared (IR)-to-visible frequency conversion.<sup>11–13</sup> Nanostructures, demonstrating high surface to volume ratio and variety of optical resonances, can exhibit highly nonlinear response even with centrosymmetric materials, *e.g.* Si.<sup>4</sup> While truly nanoscale metal-based structures reach SHG conversion efficiencies up to the level of 10<sup>-12</sup> - 10<sup>-8</sup>,<sup>8,14,15</sup> the subwavelength dielectric and semiconductor nanoparticles demonstrate SHG efficiencies in the range of 10<sup>-6</sup> - 10<sup>-3</sup>.<sup>16–19</sup>

Further increase of the SHG conversion efficiency can be achieved with elongation of the nanoparticles in one direction forming nanowires (NWs).<sup>3,20–22</sup> In such designs, combination of Mie resonances and Fabry-Perot modes can lead to strong incident light localization and outstanding optical properties.<sup>23–26</sup> Moreover, NWs geometry and lithography-free methods of fabrication can provide high structural quality of NWs grown on lattice-mismatched substrates.<sup>27</sup> However, despite semiconductor (*e.g.*, III-

<footnote n="1"><sup>1</sup>Alferov University (former St. Petersburg Academic University), Khlopina 8/3, 194021, St. Petersburg, Russia</footnote>

<footnote n="2"><sup>2</sup>Peter the Great St. Petersburg Polytechnic University, Politekhnicheskaya 29, 195251, St. Petersburg, Russia</footnote>

<footnote n="3"><sup>3</sup>ITMO University, Kronverkskij 49, 197101, St. Petersburg, Russia</footnote>

<footnote n="4"><sup>4</sup>Institut Jean Lamour, CNRS, Université de Lorraine, 54011 Nancy, France</footnote>

<footnote n="5"><sup>5</sup>ETH Zurich, Optical Nanomaterial Group, Institute for Quantum Electronics, Department of Physics, Auguste-Piccard Hof 1, 8093 Zurich, Switzerland</footnote>
V) NWs have already found various advanced applications in modern optoelectronics<ref>28,29</ref> and nanophotonics,<ref>30,31</ref> their application for frequency "up-conversion" and infrared (IR) light visualization for large scale optical devices has not been demonstrated so far.

In this work, we develop an approach to create wafer-scale free-standing arrays of epitaxially grown gallium phosphide (GaP) NWs encapsulated in an optically transparent polymer matrix. We apply these flexible and semi-transparent structures for efficient IR visualization in a broad spectral range. We use indirect bandgap semiconductor GaP, because it exhibits a large value of the quadratic nonlinear susceptibility<sup>32</sup> ($\chi^{(2)}$ ~ 70 pm/V at a wavelength around 1 µm), a broad transparency range<sup>33</sup> (0.5-11 µm), and, thus, low losses of upconverted visible light emission. The embedding of NWs into a polymer matrix solves one of the main drawbacks impeding the applications of functional nanophotonic designs based on as-grown GaP NWs on Si wafer due to the low optical contrast limiting light confinement.<sup>34</sup> We demonstrate that the developed free-standing-NWs-based IR viewers possess a conversion efficiency comparable with commercial ones that are employing rare earth metals and operating on anti-Stokes upconversion luminescence mechanisms.<sup>35-37</sup> Due to the transparency, the fabricated device does not hinder IR laser beam propagation allowing for non-disturbing control in advanced optical schemes.

## RESULTS AND DISCUSSION

In order to create samples, epitaxial growth of GaP NW arrays was performed by molecular beam epitaxy on Si(111) substrates within the self-catalyzed vapor-liquid-solid (VLS) approach following the procedure reported previously.<sup>38-40</sup> The small lattice mismatch between GaP and Si (0.37%) promotes GaP epitaxial growth with high crystalline perfection on widely available Si substrates (for details, see Section S1, Supporting Information). Typical morphology of the as-grown vertical GaP NWs on Si wafer is shown in scanning electron microscopy (SEM) image in Figure 1a. High P<sub>2</sub>/Ga flux ratio and consequent catalytic droplet shrinkage result in the formation of tapered NWs<sup>38</sup> with a tapering angle (α) of 0.6±0.3°. Schematic representation of the NW geometry is shown in the inset of Figure 1a. According to the statistical analysis data, the NW tapering angle is found to be constant for various NW lengths.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) Cross-sectional colorized SEM image of GaP NW arrays on Si (111). NW geometry is schematically presented in the inset. (b) Bright-field TEM image of NW top region. Inset: electron micro-diffraction image showing a twinned ZB</figcaption></figure>
diffraction pattern with corresponding reflexes marked with yellow (ZB) and red
circles (ZB-twin). (c) SEM image of rolled PDMS/NWs membrane released from the
growth wafer. Lower inset: colorized SEM image of NWs bottoms protruding from the
PDMS membrane. Upper inset shows the statistical distribution of the NWs diameter
measured at the tip.

A typical transmission electron microscopy (TEM) bright-field image of a single NW is shown in Figure 1b. Both TEM and selected area electron micro-diffraction (inset of Figure 1b) studies show a twinned zinc-blende (ZB) structure of the NWs. The presented microdiffraction image demonstrates a streaky pattern with diffraction spots corresponding to the <110> zone axis of ZB GaP with a rotational twining along the <111> growth axis.<sup>38,41</sup> Numerous bright and dark stripes along the NW axis seen in the TEM image correspond to the twinning boundaries and stacking faults.<sup>38,39</sup> Thus, we conclude that strong ZB lattice twining with no dominant phase and formation of segments much smaller than the light wavelength are present. The effect of twinning on the SHG process<sup>3</sup> will be discussed in the next section.
One of the main drawbacks impeding the fabrication of integrated photonic circuits based on GaP/Si heterostructures is the low refractive index contrast (n(GaP)≈3.4, n(Si)≈3.9 @ λ = 600 nm) and, as a result, high optical losses due to leakage into the substrate.<sup>42,43</sup> Therefore, to achieve light confinement, the NWs should be grown on the buffer layer or transferred on the wafer with high refractive index contrast, suspended in air or low-refractive index polymer. Commonly, such processes involve costly and time-consuming selective etching of sacrificial layers and complex transfer procedures. In contrast to the planar heterostructure, NWs can be easily embedded in a transparent elastomer matrix and mechanically cleaved from the growth substrate due to the small NW footprint.

In order to create a free-standing NWs array, we perform wafer-scale NW silicone
rubber encapsulation with the use of the "G-coating" approach.<sup>44</sup> Namely, GaP NWs
on Si wafer were embedded in a homogeneous layer of polydimethylsiloxane
(PDMS) and, then, mechanically peeled off from the substrate (for details, see
Methods and
section S3, Supporting Information). As expected, surface density of the NWs in the
fabricated PDMS membrane is similar to the initially grown epitaxial array on Si,
demonstrating assured transfer of the NWs into the PDMS. This conclusion is also
supported by SEM imaging of the used epitaxial wafer after the membrane release
(see Figure S2, Supporting Information). It should be noted that the vertical
orientation of epitaxial NWs is preserved during the transfer. Moreover, GaP NWs
demonstrate outstanding mechanical properties due to their high aspect ratio shape
and can be bent with a curvature radius of several microns, that prevents NWs from
any damages during membrane releasing.<sup>45</sup> The thickness of the PDMS membrane
can be tuned in a controllable manner and in the present case was approximately
equal to the length of encapsulated NWs. Typical SEM image of the mechanically
peeled PDMS 3×3-cm<sup>2</sup> membrane with encapsulated GaP NWs is shown in Figure
1c, where high magnification SEM image of the membrane surface with the NWs
sticking out is also presented on the lower inset of Figure 1c. The NWs size
distribution as a function of the tip diameter is shown on the top inset of Figure 1c,
revealing the average diameter of the GaP NWs around 150 nm, which has high
potential to support various optical modes.<sup>20</sup> The combination of NWs dimensions
and high second-order susceptibility of GaP material provides the opportunity to exploit them for nonlinear nanophotonics in an optical and near-IR spectral range. An efficient SHG in the nanostructures can be achieved when the following conditions are satisfied: (i) SHG intensity increases along the length of the NW; (ii) overlap between the SHG and the pump dipolar eigenmodes of the first harmonic is high.46 Due to dispersion of NWs' length and diameter in the array of NWs embedded in matrix, the criteria of efficient SHG can be satisfied for a number of nanostructures that defines a high conversion efficiency from IR to the visible in a very broad spectral range.

Importantly, the studies on the nonlinear properties of GaP at nanoscale were previously carried out with structures obtained by the top-down approach allowing for fine tuning of their geometry.34,47–49 Sanatinia et. al.47,48 studied arrays of vertical GaP nanocolumns with a diameter of 100–200 nm, obtained by nanosphere lithography and dry etching of the GaP (001) surface. Authors demonstrated SHG at 420 nm with an efficiency of 2·10⁻⁶. The drawback of the fabrication technique is the poor crystallinity of the surface providing scattering and spontaneous outcoupling affecting the SHG efficiency compared to the epitaxial NW growth used in our approach.50,51 In our experiments, we firstly study basic nonlinear power and polarization dependent characteristics at fixed pump wavelength of the fabricated membrane containing NWs with an average length of 12 µm and a top diameter of 150 nm which morphology is presented in Figure 1. For this, we focus 150-fs 1048-nm laser pulses into a spot with a diameter of around 10 µm (for details, see Methods) providing SHG signal around 524 nm. We observe the conversion efficiency (i.e. the ratio between incident and generated power) up to 10⁻⁴ at the maximum pump powers. To prove the second order nonlinear nature of the signal, we measure its pump power dependence (Figure 2a) exhibiting almost perfect quadratic slope in log-log scales. The polarization dependence of SHG intensity shows nearly isotropic behaviour from the bunch of NWs located within laser spot-size and is presented in Figure S4 (Supporting Information).

To study the effect of the NW size on the SHG efficiency, we prepare a series of samples grown for different periods of time resulting in the formation of vertical NW arrays with average length ranging from 3 up to 25 µm according to the procedure described in detail in the Section S1 (Supporting Information). As the NWs surface density and diameter differ from one sample to another, to quantify the effect of the active medium on SHG we introduce an effective layer with equivalent thickness of GaP material. The equivalent thickness was calculated for each sample as the total volume of NWs per unit area. The experimentally obtained dependence of the SHG intensity on the equivalent thickness is plotted in Figure 2b. As expected and further confirmed by our simulations, the increase of active medium volume leads to the increase of SHG signal, owing to the more efficient conversion of fundamental frequency to the doubled one at Fabry-Perot resonances in NWs with larger length and diameter.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Experimental characterization of SHG from a membrane with NWs. (a) Experimental dependence of the SHG intensity on average pump power at wavelength 1048 nm and repetition rate of 80 MHz (dots); red line is the result of fitting procedure revealing almost perfect quadratic slope. (b) Experimental dependence of SHG intensity on the equivalent GaP thickness corresponding to different lengths and diameters of GaP NWs. Bars show the errors of equivalent thickness estimation related to the NWs' size dispersion and experimental accuracy of SHG intensity measurements. The arrow indicates the sample with 12 µm NW membrane. (c) Experimental spectral dependence of the SHG intensity (dots, left ordinate axis), χ(2)(ω,ω;2ω) spectral dependence for bulk GaP (adopted from<sup>52</sup>), and absorption coefficient α for bulk GaP (adopted from<sup>53</sup>).</figcaption></figure>

To estimate the SHG enhancement for NWs as compared with bulk GaP, we create a reference 200-nm thick GaP planar layer epitaxially grown on an Al<sub>2</sub>O<sub>3</sub> substrate (the growth details are given in Section S2, Supporting Information). Despite this layer demonstrates almost one order of magnitude higher signal collected in forward direction (in cone with solid angle of 0.128 sr), its absolute value of average SH power is 3.6 times lower as compared to a NW membrane with similar equivalent thickness when the power meter is placed just in front of the sample collecting almost all signal emitted to the forward semi-space. It means that the membranes with NWs give much more power of SH emission scattered to nonzero angles, which is useful for IR light visualization even by naked eye.

According to the data presented in Figure 2c, the spectral dependence of the SHG efficiency measured for 12 µm NW membrane follows the second-order susceptibility χ(2) of the bulk GaP,₅₂ which demonstrates smooth rise at around λ≈1100 nm corresponding to the matching of the second harmonic energy with the bandgap. Contrary to the bulk χ(2) spectral dependence, SHG efficiency of the NWs increases abruptly at λ≈1170 nm and diverges qualitatively from the bulk χ(2) behavior till λ≈850 nm. Noteworthy, absorption of the SH signal in GaP should have a noticeable effect on the SHG efficiency for pump wavelengths shorter than 890 nm when the SH energy exceeds the GaP direct interband transition energy (absorption coefficient α for the SHG wavelength plotted as a blue curve in Figure 2c).₅₄ These features point to a crucial role of optical resonances in each GaP NW, which can be excited in the
spectral range corresponding to high transparency (low $\alpha$) and determined by the geometrical parameters of the NWs.

The nonlinear response from GaP NWs membrane is governed by the resonant response of each individual NW, which was simulated in a commercial package COMSOL Multiphysics. At the first stage, we analyse the optical field distribution in NWs at the fundamental wavelength in a broad spectral range of $\lambda \approx 700-1300$ nm. Following the experimental data, NWs were modelled as tapered cylinders with a tapering angle of 0.6° (see scheme in Figure 3a) and varied bottom diameters $d_b$ and lengths $h$. The total linear scattering spectra for NW with fixed length of 3 µm contain pronounced resonances as it is shown in Figure 3b. Generally, the resonant scattering can be attributed to whispering gallery modes $\text{excitation}_{55,56}$ or Mie modes interference,$_{20}$ while the range of geometrical parameters of the NWs under study provides a resonance at the fundamental wavelength due to excitation of the Fabry-Perot $\text{HE}_{11}$ modes propagating along NW axis (Figure 3df). The near-field distributions at the points of maximal and minimal scattering (points 1 and 2 in Figure 3b, correspondingly) are shown in Figure 3d panel demonstrating constructive and destructive interference of the Fabry-Perot modes with the exciting field. It is worth noting that the excitation efficiency of higher-order modes decreases due to their weaker coupling with the plane wave.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Theoretical analysis of SHG from individual and multiple GaP NWs. (a) Schematic illustration of the SHG from a single GaP NW of a conical shape in PDMS matrix. (b) Calculated spectra of plane wave scattering efficiency for GaP NW of different bottom diameters $d_b$ and length of $h=3$ µm. (c) Calculated SHG efficiency to a solid angle of 45 degrees from a single NW with 200 nm base diameter and length of $h=3$ µm for the case of monocrystalline structure and with taking into account twinning structures. (d) Calculated near-field distributions of the electric field intensity of the fundamental mode at different wavelengths corresponding to maximal (1) and minimal (2) values in Figure 3b. (e) Calculated near-field distributions of electric field intensity of SH at the wavelengths corresponding to maximal (3) and minimal (4) values of SHG efficiency from (c). (f) Calculated near-field distributions of electric field intensity at the fundamental harmonic for different lengths $h$ of NW. The corresponding linear scattering spectra are shown in Figure S8 (Supporting Information).</figcaption></figure>
At the following stage, SHG is modelled in the non-depleted pump approximation.⁵⁷ In order to perform numerical simulations of SHG, the second order nonlinear susceptibility tensor for a zinc-blend crystalline lattice structure $\chi_{ijk(2)} = \chi(2)$ for $i=j=k$, and 0 otherwise in the coordinate system corresponding to [100], [010], [001] directions was used. As was mentioned previously, NWs are grown along the [111] direction and present a 180° crystal lattice rotation twinning about the growth direction. The twinning structure introduces a certain ambiguity in the form of the second order tensor changing from one NW twin segment to another, which also partially corresponds to the model of SH generation in disordered media.⁵⁸,⁵⁹ Based on that, and assuming that the coherence length of the SH field is comparable with the NW length, one can introduce the averaged second order susceptibility tensor $\chi(2)$. The nonlinear tensor is averaged over different rotations of the crystalline lattice allowed in NW, which results in the following form of the tensor:

$$ \chi(2)_{xxz} = \chi(2)_{yyz} = 2\chi(2)/\sqrt{3}; $$

$$ \chi(2)_{zxx} = \chi(2)_{zyy} = \chi(2)/(2\sqrt{3}); $$

$$ \chi(2)_{zzz} = -4\chi(2)/\sqrt{3} $$

(for details see Section S7, Supporting Information).

For comparison, a tensor corresponding to a monocrystalline ZB structure is also considered (see Fig.3c), which gives slightly higher SHG. In the simulations a finite collection aperture corresponding to the experimental conditions was used. One can see from Figure 3c that the SHG oscillates in accordance with the oscillations of the pumping field, but also it is modulated by the phase-matching conditions. The SH field distribution in the NWs is shown in Figure 3e at the wavelengths of maximal (point 3) and minimal (point 4) scattering, and the destructive phase matching conditions is achieved in the latter. The decrease of the SH intensity in the shortwavelength region is caused by the suppression of the field amplitude at the fundamental mode (see Figure 3b). One can notice that there is a particular discrepancy between the monocrystalline case and the twinning structure. However, the general SHG spectral behaviour is similar.

The performed simulations describe the SHG in the case of large coherence lengths. However, another limiting case, which is highly likely to be valid, is that the coherence length of the SH is much shorter than the NW length due to strong disorder of the crystalline structure. In the limit of uncorrelated disorder, the effect of phase matching will be suppressed and only resonant enhancement of the field at the fundamental and second harmonic will be important. For that, we develop and simplify a model, which takes into account the field amplification factors and also accounts for the distribution of the NW size (see Section S7, Supporting Information).

This model provides a smearing of the resonance and a relatively flat spectrum of SHG as can be seen in the experimental measurements. Moreover, the spectral dependence of the scattering cross section weakly depends on the length of the NW, and the corresponding near-field plots are shown in Figure 3f and Figure S8 (Supporting Information), because transversal modes govern the SHG process mostly. The observed efficient and broadband SHG in the NWs membranes can be applied for IR visualization. In Figure 4a, we demonstrate bright and spectrally tuneable SHG in a broadband range from the membrane with a NWs average length of 12 µm. The optical images of the SHG radiation are taken at different pump wavelengths in the range of $\lambda \approx 800-1300$ nm and an average power around 50 mW
with a conventional digital single lens reflex camera (Figure 4a). It is worth noting that the generated light at the doubled frequencies is clearly visible from the side with the naked eye with normal background lighting for the whole excitation range. This is partially attributed to the geometry of the NWs providing efficient light scattering in a forward and backward directions, useful for the SHG visualization, whereas smooth thin film of GaP (see SEM image in Figure S1a, Supporting Information) does not give observable signal of SH to the sides and emits light preferably in forward direction as we mentioned above.

In order to compare the functionality of our IR-to-visible converting membranes with a commercial analogous based on rare-earth upconverting elements, we first demonstrate a high transparency (up to 80%, see Figure S6, Supporting Information) of our samples both for visible and near-IR spectral range. In Figure 4b, the simultaneous visualization capability and transparency of our membrane are demonstrated, when the commercial IR viewer is placed behind the membrane. In this experiment, both our transparent and commercial non-transparent viewers confirm a good quality of the laser beam weakly focused (AC254-100-C-ML lens with focal length f = 100 mm) on the membrane, which is very convenient for non-disturbing and permanent control of the laser path in advanced optical schemes. Secondly, we show that the fabricated membranes with NWs are very flexible. As shown in Figure 4c, the SHG intensity stays almost the same at different bending conditions. The flexibility might be useful for creation of IR- viewers of different shapes and curvatures.

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
**Figure 4.** Infrared viewer made of a polymer membrane with GaP NWs. (a) Photographs of a PDMS membrane with 12 µm GaP NWs irradiated by fs-laser pulses at different wavelengths (from 800 nm to 1280 nm), generating light at halved wavelengths (from 400 nm to 640 nm). (b) A photography of SHG from the 12 µm GaP NWs membrane and commercial viewer behind it. The black and white arrows in Figure (a) and (b) indicate the propagation direction of the incident fs-laser pulses. (c) Photographs of the differently bent membrane generating second harmonic.

## CONCLUSION

We have demonstrated an approach to visualize IR laser pulses by means of semitransparent and flexible large-scale membranes consisting of free-standing GaP NWs. For this purpose, we have employed the strong SHG process in the resonant GaP NWs in a broad spectral range. Such GaP NWs structures enabled the development of thin 10-20 µm membranes with efficiently converting IR radiation into visible light. The proposed SHG based principle of IR visualization differs in both functional and operational characteristics from most commercial analogues made of special ceramic with anti-Stokes luminophores. In contrast to the anti-Stokes luminescence frequency conversion, the SHG process is less temperature dependent<sup>36</sup> and makes it possible to distinguish the IR pump wavelength by the colour of its second harmonic. Also, as compared to the ceramic based IR viewers suffering hygroscopicity problem, the GaP NW array can be easily fully encapsulated in silicone elastomer protecting them from the environment.<sup>60</sup>

Weak absorption of the pump radiation together with high thermal conductivity of GaP material<sup>61</sup> make the fabricated membranes suitable for high power density applications. It should be noted that the cost estimation of proposed NWs-based IR viewer gives a price level around 1 $/cm<sup>2</sup>, making them suitable for industry (more detailed comparison of our design with various commercial infrared-to-visible converters is presented in Table S1 in Section S8, Supporting Information). As a result, the proposed principle of IR visualization would be in demand for the advanced optical schemes where efficient, transparent, permanent, flexible and non-disturbing control of the laser beam is of high importance.

## METHODS

*Nanowire encapsulation technique*

In contrast to the conventional approaches of the NWs encapsulation based on the drop-casting and spin-coating of the polymer,<sup>62,63</sup> in this paper we fabricate the membrane using swinging bucket centrifuge, providing thinning force perpendicular to the sample surface ensuring uniform deposition of the PDMS compare to the spin-coating. The force in the bucket centrifuge can reach ten thousand G-force with standard swinging bucket rotor and up to one million G-force with ultracentrifuge rotor allowing fine control over the polymer layer thickness. For convenience and to underline similarity with the gravity force we propose to call the method "G-coating" by analogy to spin-coating. The other advantage of the G-coating for the NW polymer encapsulation is the high pressure applied to PDMS that ensures filling of the space in between the NWs with polymer. The procedure results in fabrication of highly flexible uniform flat films and assured transfer of the epitaxially oriented NWs into PDMS matrix. Details on the G-coating procedure are presented in Section S3 (Supporting Information).
It is worth noting, that PDMS is a widely used material for Fresnel lens in concentrating photovoltaics. In such systems the optical lenses must provide good performance upon intense illumination and high temperatures over the service life exceeding 30 years,⁶⁴,⁶⁵ thus we suggest that PDMS is sufficiently stable and durable for the infrared imaging membrane fabrication.

### Electron microscopy characterization

Morphology of the NWs array was studied with scanning electron microscopy (SEM) (Zeiss SUPRA 25). Crystal structure was studied with scanning transmission electron microscopy (STEM) using JEOL JEM-ARM200F Cold FEG TEM/STEM operating at 200 kV and equipped with a spherical aberration (Cs) probe corrector (point resolution 0.19 nm in TEM mode and 0.078 nm in STEM mode). For TEM studies the NWs were mechanically detached from the substrate and transferred to a carbon coated TEM grid.

### Nonlinear optical characterization

We perform non-linear optical characterization of the fabricated membranes in a transmission geometry with a custom nonlinear optical setup. Detailed schematic of the experiment is presented in Figure S3 (Supporting Information). Tuneable sample excitation in the range of 700 – 1080 nm was performed by a femtosecond Ti:Sapphire laser, whereas an optical parametric oscillator (OPO) system extending the wavelength range up to 1080 – 1600 nm. In all experiments, the membranes were placed on glass cover slips and aligned perpendicular to the laser propagation axis. The laser light was focused with a 10× objective on the glass sample. The signal was then collected with a 100× objective (NA = 0.8) and focused onto a scientific CMOS camera. The polarization was controlled *via* a half-wave plate placed on the excitation beam path. For the nonlinear optical characterization, we performed the measurements of SHG intensity for different excitation wavelengths as well as different polarizations of the incoming excitation. Study of the SHG efficiency from the GaP NWs and thin film of GaP excited with femtosecond Yb+3 laser (pulse duration 150 fs, central wavelength 1048 nm) was performed *via* analysis of the spectra and power of SH and measured with Horiba LabRAM HR spectrometer and power meter, respectively.

## AUTHOR INFORMATION

### Corresponding Author

* Address Correspondence to Sergey V. Makarov (e-mail: s.makarov@metalab.ifmo.ru) and Vladimir V. Fedorov (e-mail: vfedorov.fl@mail.ioffe.ru)

### Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## ACKNOWLEDGMENT

A.D.B. thanks for the support of the NWs characterization the Russian Science Foundation (Grant № 18-72-00231). V.V.F. thanks the Russian Foundation for Basic
Research (RFBR project № 19-32-60037) for the support of the membrane fabrication and MBE growth. Optical characterization was supported by the Russian Science Foundation (project № 17-73-20336). M. T., R. G., and G. S. thanks the Swiss National Science Foundation Grants 150609 and 163916, and Ambizione Grant No. 179966, by the European Union's Horizon 2020 research and innovation program from the European Research Council under the Grant Agreement No. 714837 (Chi2-nano-oxides). M.P. and O.S. thank the Grant of the President of the Russian Federation (MK-2360.2020.2) for the support of theoretical modeling.

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