\section*{AUTHOR DECLARATIONS}

\subsection*{Acknowledgments}

The authors acknowledge the financial support via the projects C5, A2, and Z3 in the CRC 1375 “NOA–Nonlinear optics down to atomic scales” funded by the Deutsche Forschungsgemeinschaft (DFG) under project-ID 398816777.


\subsection*{Conflict of Interest}

The authors have no conflicts to disclose.


\subsection*{Author Contributions}

\textbf{Francesco Vitale}: conceptualization (equal); investigation – experimental (lead); writing – original draft (lead). \textbf{Daniel Repp}: formal analysis, software – simulations (lead); writing – review \& editing (equal). \textbf{Thomas Siefke}: resources –  sample fabrication (lead); writing – review \& editing (equal). \textbf{Uwe Zeitner}: writing – review \& editing (equal). \textbf{Ulf Peschel}: writing – review \& editing (equal). \textbf{Thomas Pertsch}: funding acquisition (equal); writing – review \& editing (equal). \textbf{Carsten Ronning}: funding acquisition (equal); conceptualization (equal); project administration (lead); supervision (lead); writing – review \& editing (lead).






\section{Dielectric/metal substrate fabrication and EBL patterning}
The dielectric/metal substrates were fabricated by first depositing aluminum films with a thickness of $t \approx$ 68 nm on a fused silica substrate via ion beam deposition (Ionfab 300LC+ by Oxford Instruments). High relative densities and good adhesion were achieved with this method. However, the high normalized energy flux and the generalized temperature during the film growth fostered diffusion processes and, hence, recrystallization \cite{Anders}. Therefore, materials with a low melting point, such as aluminum, tend to become polycrystalline (PC) with very large grains, leading to high surface roughness. Figure \ref{Fig.S1}(a) shows an AFM image of an as-deposited Al layer: the RMS surface roughness was found to be $R_q \approx$ 3.8 nm, calculated from measurements conducted in tapping mode over a $10 \upmu \textup{m} \times 10 \upmu \textup{m}$ area with a resolution of 40 nm. This resulted in a decreased propagation length and increased scattering for the surface plasmon polaritons at the spacer/nanowire interface \cite{Raether}. In order to reduce the roughness, 10 at\% silicon was co-deposited by switching between an aluminum and a silicon target. The deposition time was 25 s at an Ar-ion energy of 1.3 keV and a current of 250 mA for aluminum, and 10 s at an Ar-ion energy of 1.3 keV and a current of 100 mA for silicon respectively. The cycles were repeated 13 times and ended with a Si-free termination layer of thickness $t \approx$ 5-6 nm. On top of this termination layer, a native aluminum oxide layer formed with an estimated thickness of $t \approx$ 2-3 nm. 
The co-deposition with silicon led to the reduction of the surface roughness by about one order of magnitude down to $R_q \approx$ 0.2 nm: the corresponding AFM height profile is shown in Figure \ref{Fig.S1}(b).


\begin{figure} [ht!]
\centering
    \includegraphics[width=\textwidth]{figures/S1.png}
        \caption{AFM height profiles for (a) pristine and (b) 10\% Si co-deposited polycrystalline Al substrates. The analysis revealed an average RMS surface roughness $R_q  \approx$ 3.8 nm for the pristine Al layer and  $R_q  \approx$ 0.2 nm for the co-deposited one.
}
        \label{Fig.S1}
\end{figure}

 Figure \ref{Fig.S2} shows the respective fractions of lasing yield for a sample of 30 nanowires on top of Al substrates with and without the Si co-deposition. As a consequence of the smoother surface, the yield of nanowires that showed plasmonic-like lasing at room temperature, significantly increased. This can be ascribed to the larger propagation length and reduced scattering of the surface plasmon polaritons at the spacer/nanowire interface \cite{Raether}, together with an overall better thermal dissipation that increased the optical damage threshold.

\begin{figure} [ht!]
\centering
    \includegraphics[width=\textwidth]{figures/S2.png}
        \caption{Lasing yield for nanowires on top of (a) pristine and (b) Si co-deposited PC aluminum layers. The improvement of the surface roughness led to an increase in the yield of devices exhibiting plasmonic-like lasing action.
}
        \label{Fig.S2}
\end{figure}


Subsequently, the grating structures were realized by character projection electron beam lithography (Vistec SB350 OS) \cite{Kley}. The mask, consisting of a positive tone CAR FEP171 (Fujifilm) was transferred onto the aluminum film by ion beam etching (Ionfab 300LC+, Oxford Instruments). The resist was, then, stripped by reactive ion etching in an oxygen atmosphere (Sentech SI-591), and, ultimately, the samples were laser diced. 


\section{Sample preparation and single-nanowire nano-manipulation}

\begin{figure} [ht!]
\centering
    \includegraphics[width=\textwidth]{figures/S3.png}
        \caption{(a) Optical microscope image of a section of the Al$_2$O$_3$/Al substrate, exhibiting an exemplary EBL-patterned metallic grating. (b) Photograph of the nano-manipulation setup: the arrow points to the manipulator arm, equipped with the whisker mounting a W tip. (c) Representative optical image of a single nanowire and the W tip, acquired prior to the nano-manipulation. The same nanowire was successfully placed on top of the grating with a ridge (trench) orientation of $\theta \approx 50\degree$, as shown in (d). 
}
        \label{Fig.S3}
\end{figure}

The ZnO nanowires were dispersed onto the EBL-patterned Al$_2$O$_3$/Al substrates, shown in Figure \ref{Fig.S3}(a), via a dry imprint technique consisting in gently rubbing the growth sample surface against that of the target substrate, which was, in turn, imaged under an optical microscope (Zeiss Axiolab 5). The mechanical nano-manipulation was performed in air under the PL microscope, as shown in the photograph of \ref{Fig.S3}(b), in order to avoid the modification of the optical properties induced by the electron beam irradiation, as discussed in the main text of the manuscript. To this end, we made use of a remotely controlled nano-manipulator (Kleindiek MM3A-EM) featuring a W tip with a radius of curvature of about 1 micron installed in between the sample and the 50x NUV objective. The sample was imaged via a white light fiber illuminator (Thorlabs OSL1-EC) on a microscopy camera (PixeLink PL-B776U). After the tip was brought in contact with the substrate, finer displacement steps were achieved via a remotely controlled piezo-controller (Thorlabs BPC303) connected to the piezo-actuators of the sample stage. This allowed for an accurate “nudging” of the single nanowire toward the metallic grating and a successful placement on top of it, as shown in Figures \ref{Fig.S3}(c)(d), in order to avoid abrupt and uncontrolled movements of the W tip, eventually leading to the nanowire breakage.

\section{Further optical measurements of nanowires on gratings}

\begin{figure} [ht!]
\centering
    \includegraphics[width=\textwidth]{figures/S4.png}
        \caption{PL spectra of a ZnO nanowire with $L \approx$  13 $\upmu$m and $\langle d \rangle \approx$ ≈ 230 nm placed on a grating with $p$ = 300 nm and $FF$ = 0.5 at different configurations.
}
        \label{Fig.S4}
\end{figure}

Figure \ref{Fig.S4} shows the PL spectra from another nanowire featuring a length $L \approx$ 13 $\upmu$m and average diameter $\langle d \rangle \approx$ 230 nm, placed on a grating with pitch $p$ = 300 nm and fill factor $FF = 0.5$. Also in this case, we can clearly observe the emergence of additional peaks in the low-energy side of the gain envelope, for an orientation of the nanowire perpendicular to the metallic ridges. The additional peaks disappeared for a subsequent re-orientation of the wire at $\theta \approx 45\degree$ as well as for the planar situation. We attribute the slight shift ($\approx$ 1 nm) of the spectral location of the long-wavelength peak, compared to the other two nanowires discussed in the main text, to the variations related to the cavity imperfections (both nanowire- and grating-related).


\section{Active lasing simulations}

The active lasing simulations were accomplished by choosing the 4-level-2-electron material model provided by Lumerical for the nanowire material. A non-dispersive dielectric with an index of $n$ = 2.4 was chosen as a base material. The nanowire was illuminated by a Total-Field Scattered-Field (TFSF) source, modeling the experimental pump, for an excitation time $\tau_{\textup{exc}}$ = 500 fs at a wavelength $\lambda_{\textup{exc}}$ = 350 nm, in resonance with the pump transition in the material. The pump was, then, switched off, leading to the excitation of temporal Fourier components resonant with the lasing transition, thereby achieving a broad-band seeding of the lasing modes. This resulted in the appearance of a lasing pulse, which experienced gain. The lasing wavelength was identified via the observation of a maximum in the spectral power distribution from a monitor placed perpendicularly to the nanowire axis in a transversal cross-section. The field distribution was given by a second monitor plane, spanned by the nanowire axis and the surface normal to the substrate.

