We demonstrate enhanced light funneling through subwavelength features at optical frequencies using an epsilon near zero (ENZ) material layer. Transmission through a subwavelength slit filled with the epsilon near zero material is characterized.

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Funneling Light Through a Subwavelength Aperture Using Epsilon Near Zero Materials

With its unique and exclusive linear and nonlinear optical characteristics, epsilon-near-zero (ENZ) photonics has drawn a tremendous amount of attention in the recent decade in the fields of nanophotonics, nonlinear optics, plasmonics, light-matter interactions, material science, applied optical science, etc. The extraordinary optical properties, relatively high tuning flexibility, and CMOS compatibility of ENZ materials make them popular and competitive candidates for nanophotonic devices and on-chip integration in all-optical and electro-optical platforms. With exclusive features and high performance, ENZ photonics can play a big role in optical communications and optical data processing. In this review, we give a focused discussion on recent advances of the theoretical and experimental studies on ENZ photonics, especially in the regime of nonlinear ENZ nanophotonics and its applications. First, we overview the basics of the ENZ concepts, mechanisms, and nonlinear ENZ nanophotonics. Then the new advancements in theoretical and experimental optical physics are reviewed. For nanophotonic applications, the recent decades saw rapid developments in various kinds of different ENZ-based devices and systems, which are discussed and analyzed in detail. Finally, we give our perspectives on where future endeavors can be made.

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Epsilon-near-zero photonics: infinite potentials

Optical Kerr nonlinearity (n2) in n-type indium tin oxide (ITO) films coated on glass substrates has been measured using Z-scans with 200-fs laser pulses at wavelengths ranging from 720 to 780 nm. The magnitudes of the measured nonlinearity in the ITO films were found to be dependent on the carrier concentration with a maximum n2-value of 4.1 x 10-5 cm2/GW at 720-nm wavelength and an electron density of Nd = 5.8 x 1020 cm-3. The Kerr nonlinearity was also observed to be varied with the laser wavelength. By employing a femtosecond time-resolved optical Kerr effect (OKE) technique, the relaxation time of OKE in the ITO films is determined to be ~1 ps. These findings suggest that the Kerr nonlinearity in ITO can be tailored by controlling the carrier concentration, which should be highly desirable in optoelectronic devices for ultrafast all-optical switching.

Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films

With wide applicability and low-cost processing advantages, laser processing, as a mature and versatile tool, is forming an alternative to conventional processing technologies. In recent years, the global laser market ushered in a broad new demand since the gradual increase in the penetration rate of laser processing in consumer electronics, automotive processing, aerospace, medical and other fields. Ultrafast laser and continuous wave (CW) laser are the representatives of high-quality and high-efficiency in laser processing, respectively, but it is a huge challenge to achieve high-quality and high-efficiency laser processing at the same time in the true sense, which has been the goal of the joint efforts of scientific researchers in recent decades. In this context, combined pulse laser (CPL), one of the hybrid laser processing technologies, has proven to be a reliable tool for the high-quality and high-efficiency processing through the processing advantages of different types of lasers and controlling the laser-matter interaction. In this review, the basic principles and characteristics of the CPL processing method are systematically introduced. Also, how the CPL method is for precisely and efficiently ablating, drilling, welding and structuring and their recent advancements are analyzed. Finally, a summary and outlook of the CPL technology is highlighted. 

Combined pulse laser: Reliable tool for high-quality, high-efficiency material processing

Plasmon-enhanced ultrafast and tunable thermo-optic nonlinear optical properties of femtosecond laser ablated TiO2 and Silver-doped TiO2 nanoparticles

Indium tin oxide (ITO) films have recently emerged as a new class of functional materials for nonlinear optical (NLO) devices due to their exotic properties around epsilon-near-zero (ENZ) wavelength. Here, we experimentally investigated and tailored the NLO absorption properties of ITO films. The NLO absorption response of ITO films is investigated by using the femtosecond Z-scan measurement technique at two different wavelengths of 1030 nm (out of ENZ region) and 1440 nm (within ENZ region). Interestingly, we observed conversion behavior from saturable absorption (SA) to reverse saturable absorption (RSA) at 1030 nm with the increasing incident laser intensity, whereas only SA behavior was observed at 1440 nm. We demonstrate that SA behavior was ascribed to ground-state free electrons bleaching in the conduction band, and RSA was attributed to three-photon absorption. Moreover, results reveal that ITO film shows more excellent SA performance at 1440 nm with a nonlinear absorption coefficient of ∼−23.2  cm/GW and a figure of merit of ∼1.22×10−16  esu·cm. Furthermore, we tailored the SA and RSA behaviors of ITO films at 1030 and 1440 nm wavelengths via post-annealing treatment. The modulatable NLO absorption was ascribed to the changing of free-carrier concentration in ITO films via annealing treatment. The experimental findings offered an inroad for researchers to tailor its NLO absorption properties by changing the free-carrier concentration through chemical modification such as annealing, oxidation, or defect implantation. The superior and tunable nonlinear optical response suggests that ITO film might be employed as a new class material with potential applications in novel optical switches or optical limiters to realize the all-optical information process.

Principles to tailor the saturable and reverse saturable absorption of epsilon-near-zero material

Influences of nanosecond pulse pre-irradiation on femtosecond laser damage resistance of gold pulse compression grating

Refining multi-photon polymerization feature size by optimizing solvent content in SU-8 photoresist

In recent years, there has been increasing interest in optoelectronic applications of transparent conductive oxide (TCO) thin-film-based materials and devices fabricated using patterning techniques. Meanwhile, femtosecond laser processing is a convenient method that further improves the performance of TCO-based functional devices and expands their application prospects. In this study, we proposed a simple and effective strategy to determine the fluences required for laser processing TCOs. We investigated the modification of an indium tin oxide (ITO) film induced by a femtosecond laser (45/150 fs, 800 nm) at different pulse fluences. The results reveal that the laser modification of ITO films is highly dependent on the irradiated pulse fluences. Several distinct types of final micro/nanostructures were observed and may be attributed to superficial amorphization, spallation ablation, stress-assisted delamination, boiling evaporation, and phase explosion. The final micro/nanostructures were studied in detail using optical microscopy, scanning electron microscopy, transmission electron microscopy and a surface profiler. At a lower fluence above the melting but below the ablation threshold, a laterally parabolic amorphous layer profiled with maximum thicknesses of several tens of nanometers was quantitatively attained. At a higher fluence, stress-assisted delamination and superheated liquid-induced micro-honeycomb structures emerged. Furthermore, the electron and lattice temperature evolutions were also obtained using a two-temperature model to prove the ablation mechanism and ascertain the micro/nanostructure formation principle. The predicted surface temperatures confirmed film amorphization without ablation below 0.23 J/cm2. These results reveal the interaction mechanism between femtosecond laser pulse and ITO film including the competition between the free electron heating of intraband transition and the multiphoton absorption of the interband transition, which promotes the potential applications for femtosecond laser processing TCO films and other wide-band-gap semiconductors such as photodetectors, solar cells, UV-light-emitting diodes, and flat-panel displays.

Determining femtosecond laser fluence for surface engineering of transparent conductive thin films by single shot irradiation

Since negative photoresist SU-8 has become a common material for multi-photon micro-lithology, it is necessary to study laser conditions adopted in lithology process. Optical transmittance of SU-8 was tested. According to Urbach optical-absorption theory and Gaussian laser lateral spatial intensity envelope, relationship between theory and actual polymerization size of SU-8 was shown. Experimentally, we investigated multi-photon polymerization threshold and laser-induced damage of SU-8 under femtosecond laser irradiation with the pulse width of 45 fs at 800 nm by 1-on-1 tests. The polymerization and damage threshold at 45 fs are 2.7 and 8.9 TW/cm2, respectively. Polymerization and damage morphologies are shown with high contrast and polymerization sizes are measured under SEM. Theoretical polymerization sizes versus laser fluence are calculated by laser-induce multi-photon polymerization size analysis (LMPSA), including Urbach optical-absorption theory and Gaussian laser lateral spatial intensity distribution. The calculated results show that diffusion exists in the femtosecond laser-induced polymerization.

Yuchen Shao

Papers

Principles to tailor the saturable and reverse saturable absorption of epsilon-near-zero material

Influences of nanosecond pulse pre-irradiation on femtosecond laser damage resistance of gold pulse compression grating

Refining multi-photon polymerization feature size by optimizing solvent content in SU-8 photoresist

Determining femtosecond laser fluence for surface engineering of transparent conductive thin films by single shot irradiation

Efficient method for determination of laser conditions adopted in laser-induced micro-lithology based on laser polymerization size analysis