Ultrafast all-optical signal processing is highly required for the versatile applications in optical communication^[1], imaging^[2], quantum computing^[3], and data processing^[4]. Among the key enabling optical signal processing technologies, the ultrafast optical modulation^[5,6] and efficient wavelength conversion exploiting the nonlinear optical process play crucial roles. By manipulating the light-matter interaction, the ultrafast optical modulation and wavelength conversion can be realized by exploring nonlinear optical materials with ultrafast response and large optical nonlinearity^[7,8]. However, the weak nonlinearities and phase mismatch of nonlinear optical materials have constrained the signal processing applications.

With the emerging optical materials, epsilon-near-zero (ENZ) materials with vanishing permittivity have exhibited unprecedented optical properties due to their enhanced light-matter interaction^[9]. The transparent conducting oxides, including indium tin oxide (ITO), antimony tin oxide, and aluminum-doped zinc oxide (AZO), are preferred over other ENZ materials owing to their lower loss^[10] and straightforward fabrication with higher optical damage threshold^[11]. As the typical transparent conducting oxides, the large intensity-dependent refractive index^[12,13], harmonic generation^[14], time refraction^[15], and physical mechanisms^[16] in ITO have been investigated in the ENZ region. However, the search for stable, cost-effective transparent conducting oxides with ultrafast response and strong light-matter interaction is still ongoing. As an alternative to ITO, AZO has been proven to exhibit low cost, abundance, high thermal stability, and nontoxicity^[17]. In AZO, the Al doping in ZnO can increase the free carrier density from 1018 to 5×1020cm−3, which induces strong dispersion dominated by the Drude response in the near-infrared regime and unique ENZ characteristics^[18]. Besides, AZO is an ideal candidate for promising nonlinear optical materials owing to its tunable ENZ wavelength, which can be tuned across a broad range in the near-infrared regime by adjusting the process parameters^[19]. To date, the influences of the material parameters, such as Al3+ doping and thickness, on the nonlinearity of AZO have been investigated at separate wavelengths experimentally^[20–22]. Subsequently, the enhanced third-order nonlinear susceptibility and nonlinear Kerr coefficient at the ENZ wavelength of AZO have been demonstrated by the pump-probe technique^[23,24]. In addition, the high-harmonic generation and quasi-supercontinuum generation in AZO nanofilm have been investigated numerically, demonstrating the potential of AZO in wavelength conversion applications^[25]. Recently, the efficient high-order nonlinear frequency conversion has been investigated at the ENZ wavelength in the AZO film^[26]. However, the broadband wavelength and intensity-dependent nonlinear optical response and harmonic generation of AZO in the ENZ region have received less attention.

In this Letter, we demonstrated that the AZO thin film exhibited ultrafast response and efficient third-harmonic generation (THG) experimentally. The AZO thin film showed ultrafast carrier dynamics with sub-picosecond response and broadband nonlinear optical properties by time-resolved transient absorption spectroscopy and the Z-scan technique, respectively. In addition, the AZO thin film can produce efficient THG with an efficiency of 0.63×10−6 at the ENZ wavelength. The experimental results can offer a comprehensive understanding of the ultrafast broadband nonlinear optical properties and wavelength conversion performance in the AZO film, and may pave the way for efficient ultrafast optoelectronic device design and applications.

In the experiment, the AZO film was prepared by spin-coating a nano AZO solution on the quartz substrate. The nano AZO solution is the aqueous solution of AZO nanocrystals, obtained from Beijing Deke Daojin Science and Technology Co., Ltd. It has the composition of ZnO:Al2O3=99%:1%, a purity of 99.9%, a resistivity of 1.2×101Ω·cm, and a pH of 6–7. The morphology, composition, and linear optical absorption properties of the AZO film have been characterized, as presented in Fig. 1. Figure 1(a) reveals the scanning electron microscope (SEM) image of the AZO sample, which indicates the surface morphology of the sample. Figure S1 in the Supplement 1 is the atomic force microscope (AFM) image of the AZO film, revealing a thickness of approximately 203nm. Elemental composition analysis via an energy dispersive spectrometer (EDS) [Fig. 1(b)] indicates the expected presence of Zn, O, and Al, verifying successful aluminum doping. The Si and Au peaks originate from the quartz substrate and the sputtered gold layer, respectively. The linear absorption spectrum in Fig. 1(c) reveals broadband absorption extending into the near-infrared regime. The measured linear relative permittivity of the AZO film via the ellipsometer (RC2, J. A. Woollam) is displayed in Fig. 1(d). Specifically, we obtained the changes in the phase and amplitude of the light reflected by the sample. Subsequently, the complex dielectric constant was retrieved by fitting the ellipsometer data using a Drude model with a Lorentzian oscillator. The condition Re(ε)=0 is observed to appear approximately at λENZ=1550nm. The gray area emphasizes the ENZ spectral region (|Re(ε)|<1), which is from 1325 to 1755 nm.

Figure 2(a) presents the ultrafast carrier dynamics in the AZO film using a 350 nm pump (repetition rate: 6 kHz, pulse duration: 190 fs) and a broadband probe. We have extracted the carrier relaxation curves and time response parameters under various probe wavelengths from the transient absorption spectra. Specifically, the transient responses were fitted with double exponential functions^[27]: A(t)=A1exp(−t/τ1)+A2exp(−t/τ2),(1)where A1 and A2 represent the amplitudes, and τ1, τ2 represent the fast response and slow relaxation time, respectively.

As shown in Fig. 2(b), the fast decay constants (τ1) range from 336 fs to 5.6 ps, and slow components (τ2) span 3.94 to 65 ps, depending on probe wavelengths. The results reveal that AZO possesses the fastest response time at 707 nm probe wavelength with a rapid decay time τ1 of 336 fs and a slow component τ2 of 3.94 ps. The ultrafast response stems from the hot carrier cooling via electron-phonon interactions, whereas the slower relaxation dynamics arise from the free carrier recombination^[28]. Furthermore, the representative decay curves at 517 and 600 nm are presented, as shown in Figs. 2(c) and 2(d).

The nonlinear absorption and refraction properties in the AZO film at the ENZ wavelength were characterized by the Z-scan technique using a femtosecond laser with a tunable wavelength (pulse width of 35 fs, 1 kHz repetition rate), as shown in Fig. 3(a) for the open-aperture and Fig. 3(b) for the closed-aperture Z-scan results under different incident intensities. The results demonstrate that the AZO film exhibits reverse saturation absorption (RSA) characteristics and self-focusing behavior at λENZ=1550nm. In addition, we varied the pump intensity from 9.1 to 118.2GW/cm2 under 1550 nm excitation. The AZO film shows RSA behavior under variable intensity, and the valley of the normalized transmittance decreases with increasing excitation intensity. Moreover, we excluded the influence of the quartz substrate, demonstrating the reliability of the experimental results. We have plotted the ln(ΔT) as a function of ln(I) in Fig. 4, where ΔT and I represent the normalized transmittance change and incident laser intensity, respectively. The slope of the linear fit is 0.72, indicating that the RSA behavior of the AZO film is dominated by the two-photon absorption process^[29]. The enhanced RSA characteristics can benefit the design of AZO-based optical limiting devices in the optical communication band.

In addition, we have measured the nonlinear optical response of the AZO film from 1200 to 1800 nm and extracted the broadband nonlinear absorption coefficient β and nonlinear Kerr index n2 in the AZO film. The experimental results reveal a wavelength-dependent enhancement of the AZO’s nonlinear optical response. At λENZ=1550nm, the values of β and n2 attained 1.89×10−8 and 6.86×10−15m2/W, respectively, as displayed in Figs. 5(a) and 5(b). The peak nonlinearity at 1550 nm aligns with the ENZ condition, where field enhancement and the slow-light effect maximize the light-matter interaction^[18]. Despite the difference in pump laser parameters and film thickness compared with this experiment, AZO exhibits extraordinary nonlinear optical properties relative to other ENZ materials^[30–32].

To further investigate the third-order nonlinearity and validate the wavelength conversion efficiency in the AZO film, we performed THG measurement under identical laser conditions to the Z-scan experiments, as shown in Fig. 6. The THG intensity was maximized by moving the sample along the z-axis. The THG transmission spectral signal (green line) was captured by the spectrometer.

At the peak average excitation power of 6 mW, we varied the incident wavelength from 1200 to 1950 nm to investigate the wavelength-dependent THG intensity in the AZO film. In addition, the effect of a quartz substrate has been ruled out. The THG spectra concerning the various excitation wavelengths are illustrated in Fig. 7(a), which exhibits a maximum at the pump wavelength of 1550 nm. Next, we further explored the intensity dependence of the THG signal strength at λENZ=1550nm. Figure 7(b) confirms that THG signal strength and spectral broadening increase with the pump intensity.

As depicted in Fig. 7(c), a log-log plot of THG intensity versus pump intensity confirms a cubic dependence, validating the third-order nature of the process. The harmonic conversion efficiency (η=I3/I1) serves as a critical metric for characterizing the wavelength conversion performance in the AZO. Accordingly, we studied the wavelength dependence of THG efficiency in the AZO film. Figure 7(d) depicts the THG conversion efficiency versus pump wavelength, peaking at 0.63×10−6 for 1550 nm excitation, nearly an order of magnitude higher than the value at 1950 nm. This enhancement can result from the ENZ-induced field enhancement effect and reduced phase-matching condition^[33]. In addition, the third-order nonlinear susceptibility χ(3) was estimated to be 1.33×10−21m2/V2. Although a direct quantitative comparison is difficult due to differences in excitation conditions, material geometry, and detection schemes, AZO exhibits a competitive THG efficiency relative to other ENZ and nonlinear optical materials^[34–38], as shown in Table 1. The THG efficiency of AZO is one or even three orders of magnitude more efficient than graphene, silicon, and ITO nanolayers. The 200 nm gold film enhanced optical field confinement in the CdO layer, resulting in THG efficiency an order of magnitude higher than in AZO. Despite the inherent limitations of comparing results from different experimental conditions, we consider that the comparisons provide some evidence of the advantage of AZO film in terms of its wavelength conversion performance.

In conclusion, we have investigated the ultrafast nonlinear optical response and the wavelength conversion in the AZO thin film experimentally. The time-resolved transient absorption spectroscopy confirmed an ultrafast response time of 336 fs in the AZO film. The nonlinear absorption coefficient β (1.89×10−8m/W) and the nonlinear refraction index n2 (6.86×10−15m2/W) across a broad spectral range were obtained by the Z-scan method. Moreover, the harmonic conversion efficiency and third-order nonlinear susceptibility were calculated to be 0.63×10−6 and 1.33×10−21m2/V2, respectively. The experimental results revealed the great potential of AZO as a nonlinear optical material for ultrafast broadband photonics devices and efficient wavelength conversion applications.