Nonlinear Photonic Applications Research Articles

Demonstration of DC Kerr effect induced high nonlinear susceptibility in silicon rich amorphous silicon carbide

In this study, we demonstrate the DC Kerr effect in plasma-enhanced chemical vapor deposition silicon rich amorphous silicon carbide (a-SiC). Using the resonance shift of the transmission spectra of a ring resonator, we experimentally extract the third order nonlinear susceptibility χ3 to be 6.90×10−19 m2/V2, which is estimated to be more than six times higher than previous reported values in stoichiometric a-SiC. The corresponding induced second order nonlinear susceptibility χ2 of 44.9 pm/V is also three times higher than the reported value in silicon and silicon rich nitride utilizing the DC Kerr effect. The high nonlinearity makes silicon rich a-SiC a good materials candidate for nonlinear photonic applications.

Tailoring of embedded dielectric alumina film in AlGaAs epilayer by selective thermal oxidation

Vertical optical confinement is a critical requirement for a wide range of III-V photonic devices where Al2O3 material is the typical oxide used due to its low refractive index. This oxide layer can be formed from the oxidation of AlAs in an epitaxial GaAs/AlAs/GaAs or AlGaAs/AlAs/GaAs stack, with the advantage that the top layer remains single-crystalline. The thick film oxidation of AlAs is required for photonic applications. In this article, we report the oxidation processes developed to fully convert AlAs to AlOx films by analyzing both thin (100 nm) and thick (500 nm) AlAs films on GaAs (001) and 2° miscut GaAs (111)A substrate. Systematic microscopic characterization is performed to demonstrate the absence of any delamination at the oxide interfaces and the evolution of the diffusive oxidation process microscopically characterized is compared with an optical (µ-Raman) characterization. We demonstrate the selectivity of the AlAs oxidation process with respect to the active Al0.18Ga0.82As layer and the GaAs substrate. Finally, the proposed method is adopted to create a high refractive index contrast between the active optical material and the environment in the specific case of a III-V photonic crystal device, highlighting the potential of this approach for non-linear photonic applications.

Silicon-integrated nonlinear III-V photonics

Mainstream silicon photonic integrated circuits are based on compact and low-loss silicon-on-insulator (SOI) waveguide platforms. However, monolithic SOI-based photonics provides only a limited number of functional device types. Here, to extend the on-chip capabilities, we propose a general heterogeneous integration approach to embed highly nonlinear III-V (AlGaAs) photonics into the SOI platform. We develop low-loss AlGaAs-on-SOI photonic circuits with integrated Si waveguides and showcase sub-milliwatt-threshold ( ∼ 0.25 mW ) Kerr frequency comb generation in ultrahigh- Q AlGaAs microrings ( Q over 10 6 ) at the telecom bands. Our demonstration complements existing mature Si photonics technology with efficient nonlinear functionalities provided by III-V and propels conventional Si photonics into emerging nonlinear photonic applications towards fully chip-based nonlinear engines.

Chiral Hybrid Copper(I) Halides for High Efficiency Second Harmonic Generation with a Broadband Transparency Window

AbstractChiral hybrid organic–inorganic metal halides (HOMHs) with intrinsic noncentrosymmetry have shown great promise for applications in second‐order nonlinear optics (NLO). However, established chiral HOMHs often suffer from their relatively small band gaps, which lead to negative impacts on transparent window and laser‐induced damage thresholds (LDT). Here, we have synthesized two chiral HOMHs based on CuI halides, namely (R‐/S‐MBA)CuBr2, which feature well‐balanced NLO performances with a highly efficient SHG response, outstanding optical transparency, and high LDT. The effective second‐order NLO coefficient of (R‐MBA)CuBr2 has been determined to be ≈24.7 pm V−1, which is two orders of magnitude higher than that of their CuII counterparts. This work shows the promising potential of CuI‐based chiral HOMHs for nonlinear photonic applications in wide wavelength regions.

Chiral Hybrid Copper(I) Halides for High Efficiency Second Harmonic Generation with a Broadband Transparency Window.

Chiral hybrid organic-inorganic metal halides (HOMHs) with intrinsic noncentrosymmetry have shown great promise for applications in second-order nonlinear optics (NLO). However, established chiral HOMHs often suffer from their relatively small band gaps, which lead to negative impacts on transparent window and laser-induced damage thresholds (LDT). Here, we have synthesized two chiral HOMHs based on CuI halides, namely (R-/S-MBA)CuBr2 , which feature well-balanced NLO performances with a highly efficient SHG response, outstanding optical transparency, and high LDT. The effective second-order NLO coefficient of (R-MBA)CuBr2 has been determined to be ≈24.7 pm V-1 , which is two orders of magnitude higher than that of their CuII counterparts. This work shows the promising potential of CuI -based chiral HOMHs for nonlinear photonic applications in wide wavelength regions.

Synthesis and the spectral, electrochemical, and nonlinear optical properties of β-dicyanovinyl-appended 'push-pull' porphyrins.

A series of "push-pull" porphyrins, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII), having triphenylamine (TPA) and dicyanovinyl (DCN) groups at antipodal positions were synthesized and characterised by UV-Vis, fluorescence and NMR spectroscopic techniques, MALDI-TOF mass spectrometry, cyclic voltammetry, DFT, and elemental analysis, which were then further utilized for third-order nonlinear optical measurements under mild conditions using femtosecond laser pulses. Remarkably, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII) exhibited 21-48 nm and 38-80 nm bathochromic shifts in B and Qx(0,0) bands as compared to the corresponding MTPPs (M = 2H, CuII, NiII, and ZnII); the results are consistent with the effect of enhanced resonance due to TPA and -I effect of DCN moieties. In cyclic voltammetry, the push-pull porphyrins exhibited a cathodic shift (0.13-0.51 V) in their first oxidation potential as compared to the precursor owing to the presence of electron-donating TPA groups. The third-order nonlinear optical responses were recorded using a single-beam femtosecond Z-scan technique to retrieve information about the nonlinear absorption and nonlinear refraction of the samples. The two-photon absorption coefficients (β) are in the range of 0.87 × 10-13 to 4.28 × 10-13 m W-1 and the nonlinear refractive index (n2) in the range of 1.21 × 10-19 to 7.36 × 10-19 m2 W-1. The ultrafast absorption dynamics of the ground-state bleaching (GSB) and photo-induced absorption (PIA) are monitored by femtosecond broadband transient absorption studies. The strong nonlinearity of these push-pull porphyrins makes them potential candidates for nonlinear optical and photonic device applications.

Nonlinear optical properties of two-dimensional palladium ditelluride (PdTe2) and its application as aerosol jet printed saturable absorbers for broadband ultrafast photonics

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have recently gathered substantial interest in the advanced photonic field due to their strong broadband absorption, tunable bandgap, and excellent nonlinear optical (NLO) properties. Palladium ditelluride (PdTe2) is a novel TMD material with high carrier mobility, great stability, and narrow bandgap, allowing it to be used in a wide range of applications such as superconductors, phototransistors, and millimeter-wave receivers. Here, the NLO properties of PdTe2 are investigated by Z-scan technology. The highest nonlinear absorption coefficient of PdTe2 has been determined as -9.25 × 103 cm/GW (1 µm) and -0.55 × 103 cm/GW (1.5 µm), implying its admirable NLO characteristics. Subsequently, for the first time, broadband PdTe2-based saturable absorbers (SAs) are prepared by utilizing the aerosol jet printing (AJP) method. By employing this PdTe2 based SA, the generation of broadband ultrafast laser pulses from the near-to-mid-infrared regime with pulse durations in the ps to fs range (170 ps, 570 fs, and 1.59 ps in 1 μm, 1.5 μm, and 2 μm laser sources, respectively) further signify the capability of PdTe2 as an ideal NLO material for ultrafast photonics devices. Moreover, Q-switched and high-repetition-rate (0.7 GHz) based 58th harmonic mode-locked pulses in the telecommunication band are also obtained with this SA. These findings indicate that 2D-PdTe2 is a promising material for nonlinear optics and broadband ultrafast photonics applications.

Synthesis, characterization, and femtosecond third-order optical nonlinearity of Au@Ag core–shell nanoparticles

In this work, the Au@Ag bimetallic core–shell nanostructures were synthesized by a seed-mediated growth. The crystal structure, morphology, elemental composition, atomic concentration, and absorption spectrum of the as-synthesized nanoparticles were characterized by means of X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible linear absorption spectrum, respectively. The femtosecond third-order optical nonlinearities of nanoparticle dispersions were investigated by carrying out the femtosecond-pulsed [Formula: see text]-scan measurements at 800[Formula: see text]nm. The experimental results indicate that Au@Ag core–shell nanoparticles exhibit the positive refractive nonlinearity and negative absorptive nonlinearity. The third-order nonlinear refraction indexes of Au and Au@Ag nanoparticles are measured to be [Formula: see text] and [Formula: see text][Formula: see text]cm2/GW, respectively. The results show that the bimetallic nanoparticle has potential possibility in nonlinear photonic applications.

Efficient and tunable blue light generation using lithium niobate nonlinear photonics

Thin-film lithium niobate (LN) has recently emerged as a playground for chip-scale nonlinear optics and leads to efficient frequency conversions from near-infrared to near-visible bands. For many nonlinear and quantum photonics applications, it is desirable to operate deep into the visible band within LN's transparency window. However, the strong material dispersion at short wavelengths makes phase-matching difficult, necessitating sub-micrometer scale control of domain structures for efficient phase-matching. Here, we report the operation of thin film LN in the blue wavelength and high fidelity poling of the thin-film LN waveguide to this regime. As a result, quasi-phase matching is realized between IR (871 nm) and blue (435.5 nm) wavelengths in a straight waveguide and prompts strong blue light generation with a conversion efficiency (1040% ± 140%/W). This blue second harmonic generator exhibits stable temperature tunability, which is important for applications that require precise frequency alignment, such as atomic clocks.

Single-mode, single-polarization and dispersion-flattened waveguides based on silicon carbide and diamond

Fundamental properties of an optical waveguide are mainly related to its guided modes and associated loss, dispersion, polarization and nonlinearity. Single-mode and single-polarization waveguiding has long been of interest, i.e., so-called truly single mode. We propose SiC- and diamond-based waveguides, which are truly single-mode and simultaneously dispersion-flattened. This is preferred for broadband nonlinear applications, such as octave-spanning supercontinuum and frequency comb generation. An ultra-low and flat dispersion profile with four zero-dispersion wavelengths (ZDWs) can be obtained over an octave-spanning bandwidth from 1.8 to 3.6 μm. Meanwhile, only TE00 mode is supported in the proposed waveguide over the low-dispersion bandwidth. Scattering loss of the waveguide is greatly reduced by 8.6 times, compared to a standard strip waveguide. Fabrication of the proposed waveguide is easier than previous dispersion-tailored waveguides, without dry etching SiC and diamond. It is expected that the proposed waveguides can find applications in nonlinear photonics, optomechanics and quantum optics.

Dispersion engineering and measurement in crystalline microresonators using a fiber ring etalon

Dispersion engineering and measurement are significant for nonlinear photonic applications using whispering gallery mode microresonators. Specifically, the Kerr microresonator frequency comb as an important example has attracted a great amount of interest in research fields due to the potential capability of full integration on a chip. A simple and cost-efficient way for dispersion measurements is thereby in high demand for designing such a microcomb device. Here, we report a dispersion measurement approach using a fiber ring etalon reference. The free spectral range of the etalon is first measured through sideband modulation, and the dispersion of the etalon is inferred by binary function fitting during the dispersion measurement. This method is demonstrated on two MgF 2 disk resonators. Experimental results show good agreement with numerical simulations using the finite element method. Dispersion engineering on such resonators is also numerically investigated.

Nonlinear optical characteristics of non-covalently functionalised graphene-pyrene-conjugated phthalocyanine hybrids

Improving the non-linear absorption properties of graphene materials is valuable for optical limiting applications. We report the non-linear optical characteristics of new hybrid materials formed by the non-covalent functionalisation of graphene nanoplatelets (GNs) with pyrene-conjugated Zn(II) phthalocyanines and pyrene-conjugated Zn(II) azaphthalocyanines. The GN–pyrene-conjugated Zn(II) phthalocyanine hybrid (GPc) shows a promising change from the two-photon absorption behaviour of GNs to three-photon absorption, the first such example in a graphene based hybrid system under nanosecond pulse excitation. Fluorescence and transient absorption spectroscopies are employed to describe the non-linear optical phenomena of the prepared hybrid materials. The GPc hybrid exhibits a low optical limiting threshold induced by three-photon absorption and a favourable linear transmittance (above 90%). The GN–pyrene-conjugated azaphthalocyanine hybrid (GAzaPc) also shows promising non-linear optical coefficients. A unique optical limiting performance (80% reduction in intensity) is achieved throughout the experimental range of excitation intensities. Our research offers a way to design hybrid material structures with a large effective three-photon absorption coefficient and nearly constant optical limitation across a wide intensity range with 92% linear transmittance, which would be highly useful in non-linear photonic applications.

Multiphoton absorption and dispersive nonlinear refraction of ZnO in VIS-NIR bands

Zinc oxide (ZnO), as one of the third-generation semiconductor materials, has attracted extensive research interest, but its nonlinear optical properties in near-infrared (NIR) band are rarely involved. Herein, the beam propagation diffraction model has been extended to handle up to the 9th-order optical nonlinearity, and still has excellent performance of fitting experimental data. Multiphoton absorption effects were experimentally investigated by z-scan measurement in visible (VIS)-NIR bands. In an N-photon absorption (N-PA) spectral regime, the respective N-PA effect dominates nonlinear absorption, while all other orders are negligible under the experimental laser intensities. In VIS-NIR bands, only the third-order nonlinear refraction is significant for Kerr nonlinear refractive index, while the higher orders can be ignored. The nonlinear figure of merit is low due to the strong 2PA effect in VIS band, but increases by an order of magnitude, up to a maximum value of 5.8, in the spectral region where the 2PA effect diminishes. These results indicate that ZnO is a valuable candidate for nonlinear photonics applications in a broad NIR band accommodating telecom wavelength.

Tin selenide: A promising black-phosphorus-analogue nonlinear optical material and its application as all-optical switcher and all-optical logic gate

All-optical technique is an effective method to breakthrough the speed limitation of electric devices. Due to the strong nonlinear optical properties, two-dimensional materials have been concerned. As a potential nonlinear two-dimensional material, SnSe has been applied in the field of ultrafast laser generation. The measurement of nonlinear refractive index of SnSe and its nonlinear application is rare. In this paper, SnSe nanosheets are fabricated by a facial Li intercalation-assisted liquid exfoliation method. The nonlinear optical parameters are measured by the Z-scan technology. The SnSe-coated microfiber acted as a nonlinear device to achieve an all-optical switch and an all-optical AND gate. The wavelength-tuning and long-term stability results confirm that SnSe materials are excellent nonlinear optical medium. It is anticipated that SnSe has potential applications in nonlinear photonics devices toward high-performance all-optical signal processing.

Broadband Plasmon-Enhanced Four-Wave Mixing in Monolayer MoS2.

Two-dimensional transition-metal dichalcogenide monolayers have remarkably large optical nonlinearity. However, the nonlinear optical conversion efficiency in monolayer transition-metal dichalcogenides is typically low due to small light–matter interaction length at the atomic thickness, which significantly obstructs their applications. Here, for the first time, we report broadband (up to ∼150 nm) enhancement of optical nonlinearity in monolayer MoS2 with plasmonic structures. Substantial enhancement of four-wave mixing is demonstrated with the enhancement factor up to three orders of magnitude for broadband frequency conversion, covering the major visible spectral region. The equivalent third-order nonlinearity of the hybrid MoS2-plasmonic structure is in the order of 10–17 m2/V2, far superior (∼10–100-times larger) to the widely used conventional bulk materials (e.g., LiNbO3, BBO) and nanomaterials (e.g., gold nanofilms). Such a considerable and broadband enhancement arises from the strongly confined electric field in the plasmonic structure, promising for numerous nonlinear photonic applications of two-dimensional materials.

Anomalously Strong Second‐Harmonic Generation in GaAs Nanowires via Crystal‐Structure Engineering

AbstractGaAs‐based semiconductors are highly attractive for diverse nonlinear photonic applications, owing to their non‐centrosymmetric crystal structure and huge nonlinear optical coefficients. Nanostructured semiconductors, for example, nanowires (NWs), offer rich possibilities to tailor nonlinear optical properties and further enhance photonic device performance. In this study, it is demonstrated highly efficient second‐harmonic generation in subwavelength wurtzite (WZ) GaAs NWs, reaching 2.5 × 10−5 W−1, which is about seven times higher than their zincblende counterpart. This enhancement is shown to be predominantly caused by an axial built‐in electric field induced by spontaneous polarization in the WZ lattice via electric field‐induced second‐order nonlinear susceptibility and can be controlled optically and potentially electrically. The findings, therefore, provide an effective strategy for enhancing and manipulating the nonlinear optical response in subwavelength NWs by utilizing lattice engineering.

Germanium Nanosheets with Dirac Characteristics as a Saturable Absorber for Ultrafast Pulse Generation.

Bulk germanium as a group-IV photonic material has been widely studied due to its relatively large refractive index and broadband and low propagation loss from near-infrared to mid-infrared. Inspired by the research of graphene, the 2D counterpart of bulk germanium, germanene, has been discovered and the characteristics of Dirac electrons have been observed. However, the optical properties of germanene still remain elusive. In this work, several layers of germanene are prepared with Dirac electronic characteristics and its morphology, band structure, carrier dynamics, and nonlinear optical properties are systematically investigated. It is surprisingly found that germanene has a fast carrier-relaxation time comparable to that of graphene and a relatively large nonlinear absorption coefficient, which is an order of magnitude higher than that of graphene in the near-infrared wavelength range. Based on these findings, germanene is applied as a new saturable absorber to construct an ultrafast mode-locked laser, and sub-picosecond pulse generation in the telecommunication band is realized. The results suggest that germanene can be used as a new type of group-IV material for various nonlinear optics and photonic applications.

Optical properties of Pb0.52Zr0.48TiO3 nanorod arrays: second harmonic generation and multiphoton carrier dynamics

Nonlinear optical properties of poled and unpoled, lead zirconate-titanate (Pb0.52Zr0.48TiO3) nanorod arrays, grown on Pt-coated Si with ∼200 nm diameter and ∼600 nm height, were investigated. Clear signatures of second harmonic generations (SHG), from 490–525 nm (2.38–2.53 eV) at room temperature, were observed. Furthermore, time resolved differential reflectivity measurements were performed to study dynamical properties of photoexcited carriers in the range of 690–1000 nm where multiphoton processes were responsible for the photo-excitations. We compared this excitation scheme, which is sensitive mainly to the surface states, to when the photoexcited energy (∼3.1 eV) was close to the band gap of the nanorods. Our results offer promises for employing these nanostructures in nonlinear photonic applications.

Ultrafast carrier dynamics of Cu2O thin film induced by two-photon excitation* *Project supported by the National Natural Science Foundation of China (Grant No. 61505178).

Cuprous oxide (Cu2O) has attracted plenty of attention for potential nonlinear photonic applications due to its superior third-order nonlinear optical property such as two-photon absorption. In this paper, we investigated the two-photon excitation induced carrier dynamics of a Cu2O thin film prepared by radio-frequency magnetron sputtering, using the femtosecond transient absorption experiments. Biexponential dynamics including an ultrafast carrier scattering (< 1 ps) followed by a carrier recombination (> 50 ps) were observed. The time constant of carrier scattering under two-photon excitation is larger than that under one-photon excitation, due to the different transition selection rules and smaller absorption coefficient of the two-photon excitation.

Racetrack microresonator based electro-optic phase shifters on a 3C silicon-carbide-on-insulator platform.

We report, to the best of our knowledge, the first demonstration of integrated electro-optic (EO) phase shifters based on racetrack microresonators on a 3C silicon-carbide-on-insulator (SiCOI) platform working at near-infrared wavelengths. By applying DC voltage in the crystalline axis perpendicular to the waveguide plane, we have observed optical phase shifts from the racetrack microresonators whose loaded quality ($ Q $) factors are $\sim\! {30,\!000}$. We show voltage-length product (${{V}_{\pi}} \cdot {{L}_{ \pi}}$) of ${118}\;{{\rm V}\cdot{\rm cm}}$, which corresponds to an EO coefficient ${{r}_{41}}$ of 2.6 pm/V. The SiCOI platform can be used to realize tunable silicon carbide integrated photonic devices that are desirable for applications in nonlinear and quantum photonics over a wide bandwidth that covers visible and infrared wavelengths.