Nonlinear Photoluminescence from Patterned ITO Thin Films

. ITO is a transparent conductive material commonly used in everyday life and for its potential in material research. In nonlinear optics for instance, ITO has shown great capabilities for harmonic generation in its epsilon-near-zero configuration. In this article, we demonstrate a completely new nonlinear behaviour from ITO thin layers. After a Ga-Focused Ion Beam (FIB) milling of the thin film, we observe nonlinear photoluminescence produced for tightly focused femtosecond pulses. The signal shares strong similarities with that commonly detected from noble metals. We show that the arising of this nonlinear photoluminescence originates from the radiative decay of metal-like hot electron distribution in the material and is associated to a modification of the optical properties by the Ga ions.


Introduction
Nonlinear photoluminescence (NPL) is known as a property of noble metals to generate a broad up-converted signal under near-infrared excitation [1].This broad emission is spanning the whole visible region decaying towards the short wavelength range, see Figure 1.For low laser irradiance, NPL emission is attributed to a nonlinear absorption process, while at large power density, it is the radiative signature of an electron bath brought at temperatures exceeding thousands of degrees [2].
In the realm of nonlinear transparent conductive oxides, indium tin oxide (ITO) is a widely used material [3] sustaining a free-electron system with dispersive properties enabling controllable nonlinear properties notably in the context of epsilon-near-zero material [4].
Here we demonstrate the emergence of NPL emission from ITO-coated glass substrate [5].The emission is activated by the action of a focused Ga ion beam (Ga-FIB) on the surface, which modifies the electronic band structure of the film.An in-depth analysis of the NPL dependency on laser intensity unambiguously unveils the role of a heated electron gas in the emission process.

Sample preparation
10 nm thick ITO-coated glass coverslips are purchased from Diamond Coatings Ltd. (West Midlands, U.K.).The layer has a resistivity of approximately 1 kΩ/square and transmission of > 80% in the wavelength range of 450−700 nm as given by the supplier.The indium oxide In2O3 is doped with about 3 %at of Sn giving Sn2O2 compound as revealed by XPS analysis.The patterned area consists of 4 × 3 square arrays milled with a Ga-FIB on a FEI Helios 660i dual FIB-SEM microscope, as illustrated in Fig. 1.The parameters used for etching are a nominal probe current of 7 pA and an accelerating voltage of 30 kV with an incremental dose D ranging from 6.3 to 76.1 pC cm−2 with a constant step of 6.34 pC cm−2.Each square covers an area of 9 μm2, and the squares are spaced 2 μm from each other to avoid any proximity effects.

Dose dependant NPL emission
Figure 2 shows the NPL confocal response of the Gapatterned ITO layer for two different incident polarizations.The intensity of the NPL follows a nonmonotonous relationship with milling doses and can be related to the modification of the ITO electronic band structure by the implantation of Ga ions.This affirmation is supported by a thorough structural and chemical surface analysis correlated to the NPL variations with the milling dose.

Side effects and polarization
From a certain dose highlighted by a white star on panel (a), we also observed a strong nonlinear signal originating from the edges of the patterned areas.The presence of this enhanced signal is linked to the creation of a bump along patterned regions, which is an artefact of the FIB process.Unlike NPL coming from patterned regions, NPL from the edges is sensitive to the polarisation of the incident beam.Counterintuitively, a component of polarization must be aligned with an edge to produce an NPL response.The emitted light is also polarized; we determine a degree of polarization of about 80%.

Radiative decay of photogenerated hot carriers
The shape and the power dependence of the spectrum are systematically studied and share strong similarities with those of nonlinear photoluminescence arising from metals.The results are consistent with a nonlinear process originating from the radiative decay of photogenerated hot carriers.Indeed, electronic temperatures reaching several thousands of Kelvin have been extracted from Planck's law fitting of the spectra.Moreover, the dispersion of the exponent of the power law relating NPL intensity and laser power density is characteristic of a thermal emission process.Lastly, the thermal coefficient relating the carrier temperature to the laser intensity is determined as a function of the milling dose.

Fig. 1 .
Fig. 1.Scheme of NPL emission emerging from FIB-patterned thin ITO layer under NIR pulsed laser excitation.

Fig. 2 .
Fig. 2. NPL confocal response of the patterned ITO thin film.The polarization of the incident beam is (a) horizontal and (b) vertical (white arrows).The white star on (a) shows the limit dose from which the central homogeneous NPL response is replaced by a strong polarization-dependent NPL response coming from the edges.