Abstract:We explore the outcomes of detailed microscopic models by calculating second-and thirdharmonic generation from thin film surfaces with discontinuous free-electron densities. These circumstances can occur in structures consisting of a simple metal mirror, or arrangements composed of either different metals or a metal and a free electron system like a conducting oxide.Using a hydrodynamic approach we highlight the case of a gold mirror, and that of a two-layer system containing indium tin oxide (ITO) and gold. W… Show more
“…We attribute the origin of the THz generation from the ultrathin ITO film to the surface second-order optical nonlinearity. Similar observations on the surface second-order optical nonlinearity of ITO have been made in previous literature studying the second-harmonic generation of ITO films around their ENZ wavelength 39 , 41 , 42 . Fig.…”
Broadband light sources emitting in the terahertz spectral range are highly desired for applications such as noninvasive imaging and spectroscopy. Conventionally, THz pulses are generated by optical rectification in bulk nonlinear crystals with millimetre thickness, with the bandwidth limited by the phase-matching condition. Here we demonstrate broadband THz emission via surface optical rectification from a simple, commercially available 19 nm-thick indium tin oxide (ITO) thin film. We show an enhancement of the generated THz signal when the pump laser is tuned around the epsilon-near-zero (ENZ) region of ITO due to the pump laser field enhancement associated with the ENZ effect. The bandwidth of the THz signal generated from the ITO film can be over 3 THz, unrestricted by the phase-matching condition. This work offers a new possibility for broadband THz generation in a subwavelength thin film made of an ENZ material, with emerging physics not found in existing nonlinear crystals.
“…We attribute the origin of the THz generation from the ultrathin ITO film to the surface second-order optical nonlinearity. Similar observations on the surface second-order optical nonlinearity of ITO have been made in previous literature studying the second-harmonic generation of ITO films around their ENZ wavelength 39 , 41 , 42 . Fig.…”
Broadband light sources emitting in the terahertz spectral range are highly desired for applications such as noninvasive imaging and spectroscopy. Conventionally, THz pulses are generated by optical rectification in bulk nonlinear crystals with millimetre thickness, with the bandwidth limited by the phase-matching condition. Here we demonstrate broadband THz emission via surface optical rectification from a simple, commercially available 19 nm-thick indium tin oxide (ITO) thin film. We show an enhancement of the generated THz signal when the pump laser is tuned around the epsilon-near-zero (ENZ) region of ITO due to the pump laser field enhancement associated with the ENZ effect. The bandwidth of the THz signal generated from the ITO film can be over 3 THz, unrestricted by the phase-matching condition. This work offers a new possibility for broadband THz generation in a subwavelength thin film made of an ENZ material, with emerging physics not found in existing nonlinear crystals.
“…This kind of multiresonant enhancement has been discussed previously for a generic ENZ material [13], and for a free-electron cloud that behaves as an ENZ patina that covers bulk metal layers [14,15]. The effect can be explained along broad lines by noting that SHG efficiency is generally proportional to 1/(ε ω √ ε 2ω ) [13,16]. Finally, the last maximum located near 5000 nm may be related to a maximum in the local curve that occurs near 8000 nm (not shown).…”
We study electromagnetic pulse propagation in an indium tin oxide nanolayer in the linear and nonlinear regimes. We use the constitutive relations to reconstruct the effective dielectric constant of the medium, and show that nonlocal effects induce additional absorption resonances and anisotropic dielectric response: longitudinal and transverse effective dielectric functions are modulated differently along the propagation direction, and display different epsilon-near-zero crossing points with a discrepancy that increases with increasing intensity. We predict that hot carriers induce a dynamic redshift of the plasma frequency and a corresponding translation of the effective nonlinear dispersion curves that can be used to predict and quantify nonlinear refractive index changes as a function of incident laser peak power density. Our results suggest that large, nonlinear refractive index changes can occur without the need for epsilon-near-zero modes to couple with plasmonic resonators. At sufficiently large laser-pulse intensities, we predict the onset of optical bistability, while the presence of additional pump absorption resonances that arise from longitudinal oscillations of the free electron gas give way to corresponding resonances in the second and third harmonic spectra. A realistic propagation model is key to unraveling the basic physical mechanisms that play a fundamental role in the dynamics.
“…From our discussion one may easily surmise that the reproduction of experimental conditions and results depends on a subtle balance between all these aforementioned factors, which are pivotal, are also at play in noble metals in similar fashion [19,21], but whose magnitudes cannot be known precisely. Nevertheless, the agreement that we find between predictions and experiment is quite extraordinary, and extends to the relative locations of transmitted and reflected SH peaks, and the ratio between transmitted and reflected SH maxima.…”
Section: Experimental Versus Theoretical Resultsmentioning
We report comparative experimental and theoretical studies of second and third harmonic generation from a 20nm-thick indium tin oxide layer in proximity of the epsilonnear-zero condition. Using a tunable OPA laser we record both spectral and angular dependence of the generated harmonic signals close to this particular point. In addition to the enhancement of the second harmonic efficiency close to the epsilon-near-zero wavelength, at oblique incidence third harmonic generation displays unusual behavior, predicted but not observed before. We implement a comprehensive, first-principles hydrodynamic approach able to simulate our experimental conditions. The model is unique, flexible, and able to capture all major physical mechanisms that drive the electrodynamic behavior of conductive oxide layers: nonlocal effects, which blueshift the epsilon-near-zero resonance by tens of nanometers; plasma frequency redshift due to variations of the effective mass of hot carriers; charge density distribution inside the layer, which determines nonlinear surface and magnetic interactions; and the nonlinearity of the background medium triggered by bound electrons. We show that by taking these contributions into account our theoretical predictions are in very good qualitative and quantitative agreement with our experimental results. We show that by taking these contributions into account our theoretical predictions are in very good qualitative and quantitative agreement with our experimental results. We expect that our results can be extended to other geometries where ENZ nonlinearity plays an important role.; in ε and out ε are the dielectric constants inside and outside the medium, respectively; z in E and z out E are the corresponding longitudinal components of the electric field amplitude, and require oblique incidence to excite the ENZ point. Therefore, if in ε decreases, then z in E increases and nonlinear optical phenomena are enhanced, including nonlinear index of refraction [1], harmonic generation, optical bistability, and soliton excitation [2-13].While ENZ materials can be made artificially, all natural bulk materials that display a Lorentz-like response also exhibit a real part of the dielectric permittivity that crosses zero, in
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