More Like this

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. 

   more » « less
2. On the bounds of index tuning in transparent conducting oxides: opinion

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

   Kinsey, Nathaniel; Khurgin, Jacob B (January 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. 

   more » « less
3. 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
4. Field Enhancement of Epsilon-near-Zero Modes in Realistic Ultra-Thin Absorbing Films

   Anopchenko, Aleksei; Gurung, Sudip; Bej, Subhajit; Lee, Ho Wai (January 2020, Physical review letters)

   Using basic considerations on the average power absorbed in ultra-thin conducting films, we derive a closed-form expression for the average electric- field intensity enhancement (FIE) due to epsilon-near-zero (ENZ) polariton modes. We show that FIE in ENZ media with realistic losses reaches a maximum value in the limit of ultra-small film thickness. The maximum value is reciprocal to the second power of ENZ losses. This is illustrated in an exemplary series of aluminum-doped zinc oxide nanolayers of varying thickness grown by atomic layer deposition technique. The limiting behavior of FIE is shown in exact cases of the perfect absorption, normal incidence, and in a case of ultra- thin lossless ENZ films. Only in the case of lossless ENZ films FIE is inversely proportional to the second power of film thickness as it was predicted by S. Campione, et al. [Phys. Rev. B 91, 121408(R) (2015)]. We also show that FIE could achieve values as high as 100,000 in ultra-thin polar semiconductor films, which have losses as small as 0.02 close to the longitudinal optic (LO) phonon frequency. 

   more » « less
5. Perspectives on epitaxial InGaP for quantum and nonlinear optics

   https://doi.org/10.1063/5.0241224  

   Akin, Joshua; Zhao, Yunlei; Haque, A_K_M Naziul; Fang, Kejie (December 2024, Applied Physics Letters)

   Nonlinear optical materials are essential for the development of both nonlinear and quantum optics and have advanced recently from bulk crystals to integrated material platforms. In this Perspective, we provide an overview of the emerging InGaP χ(2) nonlinear integrated photonics platform and its experimental achievements. With its exceptional χ(2) nonlinearity and low optical losses, the epitaxial InGaP platform significantly enhances a wide range of second-order nonlinear optical effects, from second-harmonic generation to entangled photon pair sources, achieving efficiencies several orders of magnitude beyond the current state of the art. Moreover, the InGaP platform enables quantum nonlinear optics at the few- and single-photon levels via passive nonlinearities, which has broad implications for quantum information processing and quantum networking. We also examine the current limitations of the InGaP platform and propose potential solutions to fully unlock its capabilities. 

   more » « less