Tunable Nonlinear Optical Response Research Articles

Compact terahertz harmonic generation in the Reststrahlenband using a graphene-embedded metallic split ring resonator array

Harmonic generation is a result of a strong non-linear interaction between light and matter. It is a key technology for optics, as it allows the conversion of optical signals to higher frequencies. Owing to its intrinsically large and electrically tunable non-linear optical response, graphene has been used for high harmonic generation but, until now, only at frequencies < 2 THz, and with high-power ultrafast table-top lasers or accelerator-based structures. Here, we demonstrate third harmonic generation at 9.63 THz by optically pumping single-layer graphene, coupled to a circular split ring resonator (CSRR) array, with a 3.21 THz frequency quantum cascade laser (QCL). Combined with the high graphene nonlinearity, the mode confinement provided by the optically-pumped CSRR enhances the pump power density as well as that at the third harmonic, permitting harmonic generation. This approach enables potential access to a frequency range (6-12 THz) where compact sources remain difficult to obtain, owing to the Reststrahlenband of typical III-V semiconductors.

Manipulation of nonlinear optical responses in layered ferroelectric niobium oxide dihalides

Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbOX2 (X = Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl2 can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl2 and a reversible ferroelectric-to-paraelectric phase transition in NbOI2 under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbOX2 for nonlinear optical device applications.

Tunable Nonlinear Optical Response of ITO Films with Au@Ag Bimetallic Nanoparticles.

The nonlinear optical (NLO) response of indium tin oxide films covered with Au@Ag colloid layer was characterized by a femtosecond single-beam open aperture (OA) Z-scan technique in this study. As the Au@Ag thickness increased, the transition from saturated absorption (SA) to reverse saturated absorption (RSA) was found in these ITO matrix composites. The nonlinear absorption coefficient for these composite materials can be regulated from -6.85 × 10-7 m/W to 26.06 × 10-7 m/W. In addition, this work also characterized the structure, morphology, and other optical properties of the specimen, and the finite-difference time-domain (FDTD) results were consistent with the experimental results. The NLO response of the ITO/Au@Ag composites can be attributed to the phase properties, synergistic competition effect, strong interaction based on the epsilon-near-zero (ENZ) mode, and localized surface plasmon resonance (LSPR) between the indium tin oxide films and Au@Ag.

Connecting effect on the charge transport and nonlinear optical properties of heteronanotubes

Carbon/boron nitride heteronanotubes are now at the centre of experimental and theoretical studies because of their tunable properties and diverse applications. Herein, we investigate the charge transport and optical properties of carbon/boron nitride hybrid single- and double-walled nanotubes on the basis of first-principles calculations. Our results reveal that the C/BN heterojunctions with radial junctions parallel (horizontal) to the tube axis have more effects than the perpendicular (vertical) junctions on the charge transport properties of heteronanotubes. Furthermore, we find that the nonlinear optical response can be meaningfully controlled by altering the C/BN content and junction arrangement. In double-walled coaxial heteronanotubes, the interwall interaction is also found to increase the electron and hole transport rates noticeably and to have a major effect on the first hyperpolarizabilities of C/BN heteronanotubes. In contrast with coaxial carbon nanotubes, where the inner constituent is totally shielded by the outer tube, in our studied C/BN coaxial heteronanotubes, the inner shell is only partially shielded by the outer constituent. This study suggests a strategy to high-efficient design heteronanotubes with high charge transport properties and tunable nonlinear optical responses. Moreover, the designed C/BN heteronanotubes can discrete electrons and holes, suggesting their application in solar cell materials.

Tunable nonlinear optical responses of few-layer graphene through lithium intercalation

Abstract Intercalation has been demonstrated to be a powerful tool for tuning the physical and chemical properties of two-dimensional (2D) materials, providing the highest possible doping level and an ideal system to study various electronic states. In this work, we demonstrate that the nonlinear absorption effect of few-layer graphene (about 6–8 layers) is changed from saturable absorption (SA) to reverse saturable absorption (RSA) after lithium intercalation. This is attributed to the increase of Fermi energy owing to the charge transfer from Li to graphene layers in intercalated compounds (LiC6). And the change of nonlinear absorption effect is revisable after deintercalation. In addition, the modulation depth of RSA in lithiated graphene is found to rise with the decrease of incident laser wavelength, different from that of pristine graphene. Besides, the dispersion relationships of degenerate and nondegenerate two-photon absorption are analyzed from the results of nonlinear absorption and transient dynamics of lithiated graphene, indicating the 1.91–2.21 eV upshift of the Fermi surface. Our findings of the intercalation-tunable nonlinear optical absorption effect pave the way for the construction of nonlinear optical devices based on 2D intercalation compounds.

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

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 ∼ 1.2 2 × 10 − 1 6 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.

Giant enhancement of third-harmonic generation in graphene-metal heterostructures.

Nonlinear nanophotonics leverages engineered nanostructures to funnel light into small volumes and intensify nonlinear optical processes with spectral and spatial control. Owing to its intrinsically large and electrically tunable nonlinear optical response, graphene is an especially promising nanomaterial for nonlinear optoelectronic applications. Here we report on exceptionally strong optical nonlinearities in graphene-insulator-metal heterostructures, which demonstrate an enhancement by three orders of magnitude in the third-harmonic signal compared with that of bare graphene. Furthermore, by increasing the graphene Fermi energy through an external gate voltage, we find that graphene plasmons mediate the optical nonlinearity and modify the third-harmonic signal. Our findings show that graphene-insulator-metal is a promising heterostructure for optically controlled and electrically tunable nano-optoelectronic components.

Tunable nonlinear optical responses based on host-guest MOF hybrid materials

The realization of tunable nonlinear optical (NLO) responses in a single nano-/micro-structure is extremely important. However, in lack of effective ways to integrate multiple performances, it still faces severe limitations during applications. Herein, we demonstrate a wavelength-dependent NLO micro-structure based on host-guest metal-organic framework (MOF) materials through encapsulating linear dye molecules into periodic one-dimensional (1D) channels. The confinement to non-centrosymmetric polar dye molecules enhances the second-/third-order NLO responses of the hybrid crystals, causing obvious two-photon luminescence (TPL), second harmonic generation (SHG) and third harmonic generation (THG) responses in the as-prepared composites. The highly ordered structures of MOFs impart spatial regulation on the linear dye molecules to realize orientation alignment, resulting in the polarized anisotropy emission. NIR-to-NIR (NIR, near-infrared region) two-photon pumped lasing is realized with the natural whispering gallery mode resonance cavities of MOFs under the excitation of a 1200-nm fs laser. Furthermore, tunable NLO properties such as TPL, SHG and THG are achieved through switching the incident excitation wavelength from 800 to 1500 nm. Such hybrid materials with tunable NLO responses may open a new avenue toward designing multifunctional NLO devices in the future.

Enhanced and tunable nonlinear optical responses of nitrogen-doped nickel oxide induced by femtosecond laser excitation

In this work, we report the impact of nitrogen (N) doping in nickel oxide films on nonlinear optical responses (NORs) utilizing 532 nm, 330 fs laser pulses with both linear and radial polarization states. The pristine and N-doped NiO films are grown through radio frequency magnetron sputtering technique and further characterized by AFM, UV–Vis transmittance spectra and XPS. The experimental results demonstrate that NORs, including reverse saturable absorption and optical Kerr refraction, of the as-prepared NiO films along with their optical power limiting (OPL) are intimately related to the N doping and the laser polarization patterns. More specifically, both the NORs and OPL increase scalingly with the doping concentration under the linearly polarized laser excitation. The strongest optical nonlinear absorption coefficient, optical nonlinear refraction index and OPL efficiency are in turn determined to be 4.55 × 10−7 m/W, 6.5 × 10−16 m2/W and 50% for the heaviest N-implanted NiO film. In a sharp contrast, for the raidally polarized beam impinging onto the pristine and doped NiO films, the NORs and OPL initially drop off and then strengthen together as the increases of N-implanted content. Such enhanced and tunable NORs are due to the fact that the involvement of doping state gives rise to promoted optical transitions originating from the narrowing of band gap, as well as the use of ultrafast vectorial laser induces anisotropic nonlinearity and intricate light-matter interplay. Our results will open a new perspective to develop optical power limiter and ultrafast photonic devices.

Tunable nonlinear optical responses and carrier dynamics of two-dimensional antimonene nanosheets.

Sb nanosheets, also known as antimonene, have received ever-growing consideration as a promising new type of two-dimensional (2D) material due to their many attractive properties. However, how their nonlinear optical (NLO) properties are affected by their nanosheet structure and measurement conditions remains unclear. Herein, we report a successful size-selective production method for Sb nanosheets, which is based on a combination of lithium ion intercalation, solvent exfoliation and size selection centrifugation. This high-yield and size-selective preparation method enables fundamental investigation on the relation of the intrinsic optical properties of Sb nanosheets. Nanosecond Z-scan measurements revealed a unique size-dependent broadband NLO response. When the average size is reduced from 3 micrometers to 50 nanometers, the Sb nanosheets exhibit a clear transition from saturable absorption to reversed saturable absorption. Ultrafast transient absorption spectroscopic investigation indicated that exciton cooling is significantly faster in a small nanosheet than in large ones, revealing that the different exciton relaxation dynamic plays key roles in the distinct size-tunable nonlinear optical response. This work paves new ways towards the mass production and practical application of antimonene.

Broadband tunable nonlinear optical response in plasmonic metamaterials -INVITED

Nanostructured materials with tunable nonlinear optical response are of great interest for different applications in nanophotonics. In this work we report the results of a comprehensive study on the nonlinear optical properties of two kinds of plasmonic metamaterials, i.e., honeycomb nanoprism arrays and multilayer hyperbolic metamaterials, which proved to have a very rich spectrum of parameters (as metamaterials morphology and composition, wavelength and polarization of an input beam) to exploit for controlling their nonlinear response over a broad spectral range.

One-pot synthesis of sub-10nm LiNbO3 nanocrystals exhibiting a tunable optical second harmonic response.

Nanophotonics, dealing with the properties of light interacting with nanometer scale materials and structures, has emerged as a sought after platform for sensing and imaging applications, and is impacting fields that include advanced information technology, signal processing circuits, and cryptography. Lithium niobate (LiNbO3) is a unique photonic material, often referred to as the “silicon of photonics” due to its excellent optical properties. In this article, we introduce a solution-phase method to prepare single-crystalline LiNbO3 nanoparticles with average diameters of 7 nm. This one-pot approach forms well-dispersed LiNbO3 nanocrystals without additional organic additives (e.g., surfactants) to control growth and aggregation of the nanoparticles. Formation of these LiNbO3 nanocrystals proceeds through a non-aqueous sol–gel reaction, in which lithium hydroxide and niobium hydroxide species were generated in situ. The reaction proceeded through both a condensation and crystallization of these reactants to form the solid nanoparticles. These nanocrystals of LiNbO3 were active for optical second harmonic generation (SHG) with a tunable response from 400 to 500 nm. These nanoparticles could enable further development of non-linear optical techniques such as SHG microscopy for bioimaging, which requires the dimensions of nanoparticles to be well below 100 nm.

Structural second-order nonlinearity in plasmonic metamaterials

Nonlinear processes are at the core of many optical technologies whose further development require optimized materials suitable for nanoscale integration. Here we demonstrate the emergence of a strong bulk second-order nonlinear response in a plasmonic nanorod composite comprised of centrosymmetric materials. We develop an effective-medium description of the underlying physics, compare its predictions to the experimental results, and analyze the limits of its applicability. We demonstrate strong tunable generation of the p-polarized second-harmonic light in response to either s- or p-polarized excitation. High second-harmonic enhancement is observed for fundamental frequencies in the epsilon-near-zero spectral range. The work demonstrates emergence of structurally tunable nonlinear optical response in plasmonic composites and presents a new nonlinear optical platform suitable for integrated nonlinear photonics.