Adiabatic frequency shifting in epsilon-near-zero materials: the role of group velocity

TLDR

Nonlinear optics has long exploited photon frequency conversion, and adiabatic frequency shift can be induced by slow temporal changes in a material’s refractive index, offering potential for ultrafast signal processing. This study investigates adiabatic frequency shifting in epsilon‑near‑zero materials, demonstrating that operating near εr = 0 enables the shift over a shorter propagation length. The authors validate their theoretical predictions experimentally by measuring the adiabatic frequency shift of optical pulses transmitted through aluminum zinc oxide thin films at the ENZ wavelength. They predict that an intense optical pulse induces a more efficient frequency shift at the ENZ wavelength, attribute the effect to the slow group velocity there, and conclude that transparent metal oxides near the ENZ point are promising candidates for future frequency‑conversion devices.

Abstract

The conversion of a photon’s frequency has long been a key application area of nonlinear optics. It has been discussed how a slow temporal variation of a material’s refractive index can lead to the adiabatic frequency shift (AFS) of a pulse spectrum. Such a rigid spectral change has relevant technological implications, for example, in ultrafast signal processing. Here, we investigate the AFS process in epsilon-near-zero (ENZ) materials and show that the frequency shift can be achieved in a shorter length if operating in the vicinity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">R</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> <mml:mo fence="false" stretchy="false">{</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mi>ε</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:mrow> <mml:mo fence="false" stretchy="false">}</mml:mo> <mml:mspace width="thickmathspace"/> <mml:mo>=</mml:mo> <mml:mspace width="thickmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0</mml:mn> </mml:mrow> </mml:math> . We also predict that, if the refractive index is induced by an intense optical pulse, the frequency shift is more efficient for a pump at the ENZ wavelength. Remarkably, we show that these effects are a consequence of the slow propagation speed of pulses at the ENZ wavelength. Our theoretical predictions are validated by experiments obtained for the AFS of optical pulses incident upon aluminum zinc oxide thin films at ENZ. Our results indicate that transparent metal oxides operating near the ENZ point are good candidates for future frequency conversion schemes.