Abstract:Optical nonlinearities can be strongly enhanced by operating in the so-called near-zero-index (NZI) regime, where the real part of the refractive index of the system under investigation approaches zero. Here we experimentally demonstrate semi-degenerate four-wave mixing (FWM) in aluminum zinc oxide thin films generating radiation tunable in the visible spectral region, where the material is highly transparent. To this end, we employed an intense pump (787 nm) and a seed tunable in the NIR window (1100–1500 nm)… Show more
“…The direct comparison between these alternative flat systems reveals that NZI TCO films enable an unprecedented nonlinear conversion efficiency. It is worth pointing out that the maximum fluence used in our experiments was set at 28 mJ/cm 2 , which is well below the material damage threshold 38 , 40 , 43 and still quite below the nonlinear saturation point. Fig.…”
Section: Resultsmentioning
confidence: 96%
“…The specific TCO we focus our attention on is AZO, which does not make use of rare elements, is highly sustainable 34 , possesses a wide statically tuneable NZI window (1200–2000 nm) 35 , and can be fabricated at low temperatures by various CMOS compatible techniques on a huge variety of available substrates 36 , 37 . Most importantly, AZO is transparent in the visible spectral window, allowing for unattenuated generation of visible light via nonlinear processes 38 . Our FROG system employs a 270 nm film of AZO deposited on a fused silica substrate as the nonlinear element (See Fig.…”
Transparent conducting oxides exhibit giant optical nonlinearities in the near-infrared window where their linear index approaches zero. Despite the magnitude and speed of these nonlinearities, a “killer” optical application for these compounds has yet to be found. Because of the absorptive nature of the typically used intraband transitions, out-of-plane configurations with short optical paths should be considered. In this direction, we propose an alternative frequency-resolved optical gating scheme for the characterization of ultra-fast optical pulses that exploits near-zero-index aluminium zinc oxide thin films. Besides the technological advantages in terms of manufacturability and cost, our system outperforms commercial modules in key metrics, such as operational bandwidth, sensitivity, and robustness. The performance enhancement comes with the additional benefit of simultaneous self-phase-matched second and third harmonic generation. Because of the fundamental importance of novel methodologies to characterise ultra-fast events, our solution could be of fundamental use for numerous research labs and industries.
“…The direct comparison between these alternative flat systems reveals that NZI TCO films enable an unprecedented nonlinear conversion efficiency. It is worth pointing out that the maximum fluence used in our experiments was set at 28 mJ/cm 2 , which is well below the material damage threshold 38 , 40 , 43 and still quite below the nonlinear saturation point. Fig.…”
Section: Resultsmentioning
confidence: 96%
“…The specific TCO we focus our attention on is AZO, which does not make use of rare elements, is highly sustainable 34 , possesses a wide statically tuneable NZI window (1200–2000 nm) 35 , and can be fabricated at low temperatures by various CMOS compatible techniques on a huge variety of available substrates 36 , 37 . Most importantly, AZO is transparent in the visible spectral window, allowing for unattenuated generation of visible light via nonlinear processes 38 . Our FROG system employs a 270 nm film of AZO deposited on a fused silica substrate as the nonlinear element (See Fig.…”
Transparent conducting oxides exhibit giant optical nonlinearities in the near-infrared window where their linear index approaches zero. Despite the magnitude and speed of these nonlinearities, a “killer” optical application for these compounds has yet to be found. Because of the absorptive nature of the typically used intraband transitions, out-of-plane configurations with short optical paths should be considered. In this direction, we propose an alternative frequency-resolved optical gating scheme for the characterization of ultra-fast optical pulses that exploits near-zero-index aluminium zinc oxide thin films. Besides the technological advantages in terms of manufacturability and cost, our system outperforms commercial modules in key metrics, such as operational bandwidth, sensitivity, and robustness. The performance enhancement comes with the additional benefit of simultaneous self-phase-matched second and third harmonic generation. Because of the fundamental importance of novel methodologies to characterise ultra-fast events, our solution could be of fundamental use for numerous research labs and industries.
“…[1][2][3] This condition is associated with a slow light effect leading to giant nonlinearities, which manifest within the ultra-fast optical regime. [4,5] By using near-zero-index (NZI) thin films (thickness < 1 μm) in out-of-plane configurations, ground-breaking nonlinear experiments have been conducted demonstrating record high-frequency conversion in 𝜒 (3) processes, [6][7][8] large bandwidth shifts of the wavelength carrier of an ultra-fast pulse, [4,9] and unitary change of the refractive index, [10] just to mention a few. However, due to the absorptive nature of nonlinearities in TCOs, propagation distances of several microns are already prohibitive, thus rendering impossible the straightforward design of all-optical onchip devices.…”
Novel photonic nanowires are fabricated using low‐index materials and tested in the near‐infrared spectrum to assess their nonlinear optical properties. In this work, the need to redefine the standard nonlinear figure of merit in terms of nonlinear phase shift and optical transmission for a given propagation distance is argued. According to this new metric, the devices largely outperform all established platforms for optical modules with a linear footprint in the range of 50–500 µm, which is demonstrated to be an outstanding technological gap. For 85 fs pulses, with carrier wavelength at 1480 nm and sub‐µW power levels, a spectral broadening exceeding 80% of the initial bandwidth was recorded over a propagation length of just 50 µm. Leveraging on CMOS‐compatible processes and well‐established materials such as silicon, silica, and indium tin oxide, the devices bring great promise for developing alternative all‐optical devices with unparalleled nonlinear performances within the aforementioned range.
“…[13] Initial key demonstrations of TCO's enhanced nonlinearities include optically induced unity-index shift, [14,15] and highly efficient frequency conversion. [16] More recent application-oriented efforts such as an NZI frequency resolved optical gating (FROG) system [17] and many NZI-enhanced modulator designs [18][19][20] have also shown great promise. However, so far, a great body of studies on TCOs' nonlinearities have been focused on Kerr effects for direct modulation of the optical phase.…”
Transparent conducting oxides (TCOs) show unprecedented optical nonlinearities in the near infrared wavelength range, where the real part of their linear refractive index approaches zero. More specifically, the Kerr nonlinearities of these materials have sparked widespread attention due to their magnitude and speed. However, due to the absorptive nature of these nonlinear processes, it is of fundamental interest to further investigate the imaginary component of the nonlinear index. The present work studies the nonlinear optical absorption properties of aluminium‐doped zinc oxide (AZO) thin films in their near‐zero‐index (NZI) spectral window. It is found that the imaginary part of the refractive index is reduced under optical excitation such that the field penetration depth more than doubles. An optically induced shift of the NZI bandwidth of ≈120 nm for a pump intensity of 1.3 TW cm−2 is also demonstrated. Looking into the optically induced spectral redistribution of the probe signal, local net gain is recorded, which is ascribed to a nonlinear adiabatic energy transfer. The present study adds key information about the fundamental interplay between real and imaginary nonlinear indices in NZI media, while advancing parametric amplification as viable direction for loss compensation.
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