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1. Nonlinear optics at epsilon near zero: From origins to new materials

   Dhruv Fomra Adam Ball Samprity Saha Jingwei Wu Md. Sojib Amit Agrawal Henri J. Lezec; Nathaniel Kinsey (March 2024, Applied Physics Reviews)

   In the continuously evolving realm of nonlinear optics, epsilon near zero (ENZ) materials have captured significant scientific interest, becoming a compelling focal point over the past decade. During this time, researchers have shown extraordinary demonstrations of nonlinear processes such as unity order index change via intensity dependent refractive index, enhanced second harmonic generation, saturable absorption in ultra-thin films and more recently, frequency shifting via time modulation of permittivity. More recently, remarkable strides have also been made in uncovering the intricacies of ENZ materials' nonlinear optical behavior. This review provides a comprehensive overview of the various types of nonlinearities commonly observed in these systems, with a focus on Drude based homogenous materials. By categorizing the enhancement into intrinsic and extrinsic factors, it provides a framework to compare the nonlinearity of ENZ media with other nonlinear media. The review emphasizes that while ENZ materials may not significantly surpass the nonlinear capabilities of traditional materials, either in terms of fast or slow nonlinearity, they do offer distinct advantages. These advantages encompass an optimal response time, inherent enhancement of slow light effects, and a broadband characteristic, all encapsulated in a thin film that can be purchased off-the shelf. The review further builds upon this framework and not only identifies key properties of transparent conducting oxides that have so far made them ideal test beds for ENZ nonlinearities, but also brings to light alternate material systems, such as perovskite oxides, that could potentially outperform them. We conclude by reviewing the upcoming concepts of time varying physics with ENZ media and outline key points the research community is working toward. 

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2. Atomic Layer Engineering of Epsilon‐Near‐Zero Ultrathin Films with Controllable Field Enhancement

   https://doi.org/10.1002/admi.202000844  

   Gurung, Sudip; Anopchenko, Aleksei; Bej, Subhajit; Joyner, Jay; Myers, Jason_D; Frantz, Jesse; Lee, Ho_Wai_Howard (June 2020, Advanced Materials Interfaces)

   Abstract Enhanced and controlled light absorption, as well as field confinement in optically thin materials, are pivotal for energy‐efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films can support the so‐called epsilon near zero (ENZ) modes in a frequency region of near‐zero permittivity, which can lead to the perfect light absorption and ultrastrong electric field intensity enhancement (FIE) within the films. To achieve full control over absorption and FIE, one must be able to tune the ENZ material properties as well as the film thickness. Here, engineered absorption and FIE are experimentally demonstrated in aluminum‐doped zinc oxide (AZO) thin films via control of their ENZ wavelengths, optical losses, and film thicknesses, tuned by adjusting the atomic layer deposition (ALD) parameters such as dopant ratio, deposition temperature, and the number of macrocycles. It is also demonstrated that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This study on engineering ENZ material properties by optimizing the ALD process will be beneficial for the design and development of next‐generation tailorable photonic devices based on flat, zero‐index optics. 

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3. On the bounds of index tuning in transparent conducting oxides: opinion

   https://doi.org/10.1364/OME.565988  

   Kinsey, Nathaniel; Khurgin, Jacob_B (May 2025, Optical Materials Express)

   For more than a decade, the linear and nonlinear optical responses of materials and composites exhibiting an epsilon-near-zero (ENZ) region have been of keen interest to the community. Among the variety of effects realized, achieving index modulation near unity on the picosecond (or less) timescale has generated the most significant impact. As a long-sought combination of strength and speed, ENZ nonlinearities have reignited interest in nonlinear processes that appear to go beyond the typical perturbative expansion (e.g., non-perturbative) as well as in time-varying nonlinear processes. Here, we aim to take a physical and intuitive look at the nonlinear index modulation in Drude-like ENZ films and highlight the physical limits of tuning. We will focus particularly on their connection (or lack thereof) with non-perturbative effects and time-varying processes and provide our opinions as to the strengths and weaknesses of ENZ films in these areas. 

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

   https://doi.org/10.1364/OPTICA.374788  

   Khurgin, Jacob_B; Clerici, Matteo; Bruno, Vincenzo; Caspani, Lucia; DeVault, Clayton; Kim, Jongbum; Shaltout, Amr; Boltasseva, Alexandra; Shalaev, Vladimir_M; Ferrera, Marcello; et al (March 2020, Optica)

   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 $\mathrm{R}\mathrm{e}\left\{{\mathrm{\epsilon <\#comment/>}}_r\right\}=0$. 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. 

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5. Atomic layer engineering of epsilon near zero ultrathin films with controllable zero index field enhancement

   Gurung, Sudip; Anopchenko, Aleksei; Bej, Subhajit; Joyner, Jay; Myers, Jason; Frantz, Jason; Lee, Ho Wai (January 2020, Advanced materials interfaces)

   Enhanced and controlled light absorption as well as field confinement in an optically thin material are pivotal for energy-efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films with near-zero permittivity can support ENZ modes in the so-called epsilon near zero (ENZ) frequency region, which can lead to perfect light absorption and ultra-strong electric field intensity enhancement (FIE) within the films. To achieve full control over absorption and FIE, one must be able to tune the ENZ material properties as well as the film thickness. Here, we experimentally demonstrate engineered absorption and FIE in aluminum doped zinc oxide (AZO) thin films via control of their ENZ wavelengths, optical losses, and film thicknesses, tuned by adjusting the atomic layer deposition (ALD) parameters such as dopant ratio, deposition temperature, and number of macro-cycles. We also demonstrate that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This study on engineering ENZ material properties by optimizing the ALD process will be beneficial for the design and development of next- generation tunable photonic devices based on flat, zero-index optics. 

   more » « less