Second Harmonic Generation Process Research Articles

Twist-Angle-Controlled Nonlinear Circular Dichroism on Gold-Crystal Hybrid Metasurfaces.

Optical chirality, which plays important roles in liquid crystal display and biological and chemical detection, has been attracting scientists' attention due to its potential applications in optical information processing. Usually, the chiral optical response of natural molecules is very weak. However, the emergence of metasurfaces offers a promising solution to solve this issue. By judiciously designing the geometry of meta-atoms, we have realized strong optical circular dichroism (CD) in both linear and nonlinear optical regimes. However, tuning of the CD with a metasurface remains challenging. Here, we propose the twist-angle-controlled nonlinear CD effect by using the second-harmonic generation process on a gold-crystal hybrid metasurface. The CD effect of the second-harmonic waves can be tuned well by controlling the twist angle between the two constituent materials. The proposed hybrid metasurface may open new avenues for developing ultracompact and multifunctional nonlinear optical devices.

Analytical solution to second-harmonic generation of ultrashort laser pulse

We present an analytical method to evaluate the second-harmonic generation (SHG) process of ultrashort laser pulses in a nonlinear medium within the small-signal regime. Building upon the broadband nonlinear coupled wave theory, we derive an analytical solution that allows one to investigate the fundamental physics and detailed dynamics of the nonlinear interaction between ultrashort laser pulses. We find that SHG for ultrashort laser pulses in a nonlinear crystal arises from a series of self-SHG and self sum-frequency generation processes involving different frequency components. We have applied the analytical method to investigate specific pulse profiles with Gaussian and sech functions and found good agreement between analytical theory and numerical simulations, validating the accuracy of the analytical approach. The analytical method enables one to explore the evolution of the second-harmonic wave under various pulse profiles, pulse widths, and crystal thicknesses, and offers a more efficient and insightful approach to study the nonlinear optical dynamics of ultrashort laser pulses.

Degenerate spontaneous parametric down-conversion in nonlinear metasurfaces.

We propose a simple scheme of degenerate spontaneous parametric down-conversion (SPDC) in nonlinear metasurfaces or photonic crystal slabs with quasi-guided modes. It employs a band crossing between even- and odd-parity quasi-guided mode bands inside the light cone (above the light line) and a selection rule in the conversion efficiency of the SPDC. The efficiency can be evaluated fully classically via the inverse process of noncollinear second-harmonic generation (SHG). As a toy model, we study the SPDC and SHG in a monolayer of noncentrosymmetric spheres and confirm that the scenario works well to enhance the SPDC.

Growth and Temperature Properties of the Phase-Matching Angle of 23% Deuterated ADP Crystal.

Theoretical analysis indicated that partially deuterated ammonium dihydrogen phosphate (DADP) crystal with a deuterium content of 23% could realize spectral noncritical phase-matching (S-NCPM) for type-I frequency doubling of an Nd:glass laser. To explore the temperature dependence of the phase matching (PM) angle of the second harmonic generation (SHG) process and the output SHG bandwidth of DADP crystal at 1.053 μm, we used the point-seed rapid-growth method to grow targeted DADP crystal with 23% deuterium content. Experimental results indicated that the grown DADP crystal had high quality and large dimensions (7 × 6 × 6 cm3). Using a femtosecond OPO laser as a tunable light source, the temperature dependence of the PM angle of the SHG process in DADP crystal at 1.053 μm was investigated. The PM angle changed linearly with temperature, as predicted by the theoretical calculation. In addition, under the condition of higher temperature, broad bandwidths of the second harmonic of DADP crystal were still observed. These results provide excellent guidance and reference value for the application of wavelength insensitive phase-matched second harmonic generation in partially deuterated DADP.

Chromium iodate: the structure and origin of optical second harmonic generation.

Iodate crystal materials are some of the important candidates for mid-infrared nonlinear optical (NLO) crystals. Almost all the structures and NLO properties of monometallic iodates have been studied, except for a few difficult to synthesize crystalline phases. In this work, crystalline Cr(IO3)3 was synthesized and its optical properties were studied. Cr(IO3)3 has a strong powder second-harmonic generation (SHG) effect (1.3 × AgGaS2) and a large birefringence (0.24) at 2100 nm. Theoretical calculations indicate that the 3d orbital of Cr3+ is strongly involved in the SHG process, and the contribution of the metal cation centered unit CrO6 to the SHG response is much greater than that of the non-metal cation centered unit IO3. This study provides a new case among the few NLO materials in which the SHG response is dominated by metal cation centered groups.

Frequency-correlated photon-pairs tunable across the telecom spectrum via cascaded SHG ＋ SPDC in PPLN

We explore the cascaded second-order (χ(2)) nonlinear process of second-harmonic generation (SHG) + spontaneous parametric down-conversion (SPDC) at telecom wavelengths in a type-0 phase-matched Zn-diffused 5% MgO-doped periodically poled lithium niobate (PPLN) ridge waveguide. We simulate the waveguide modes using finite element method (FEM) to determine the signal and idler peak wavelengths produced via the cascading effect at a particular temperature and pump wavelength. We also derive an analytical expression for the effective χ(3) conversion efficiency (equivalently, the signal/idler photon flux) and experimentally verify that the cascaded process produces discrete temperature-tunable signal/idler sidebands (around a ∼1550-nm pump) that can be continuously tuned across the E-, S-, L-, and U-bands of the telecom spectrum. Such frequency-correlated photon pairs produced in a fiber-coupled integrated setup could have applications in quantum computing and communication.

Towards Double Resonant Cavity Enhanced Second Harmonic Generation in Monolayer MoS2

AbstractA characteristic property of monolayered transition metal dichalcogenides is their strong nonlinear response. While they display a high conversion efficiency per atomic layer, due to their low thickness, the absolute value of their nonlinear response remains low. Here enhancement of the second harmonic generation (SHG) of monolayer MoS2 through the design, fabrication, and characterization of a monolithic microcavity, which aims to be double resonant at a fundamental wavelength of λ = 800 nm as well as its second harmonic, is demonstrated. The MoS2 monolayers are embedded in such a cavity, with the aim to simultaneously enhance the light‐matter interaction at the excitation wavelength and the SHG from the monolayers. A resonance enhancement for the SHG process is achieved through the cavity.

Dual-Modal Nanoplasmonic Light Upconversion through Anti-Stokes Photoluminescence and Second-Harmonic Generation from Broadband Multiresonant Metal Nanocavities.

Metal nanocavities can generate plasmon-enhanced light upconversion signals under ultrashort pulse excitations through anti-Stokes photoluminescence (ASPL) or nonlinear harmonic generation processes, offering various applications in bioimaging, sensing, interfacial science, nanothermometry, and integrated photonics. However, achieving broadband multiresonant enhancement of both ASPL and harmonic generation processes within the same metal nanocavities remains challenging, impeding applications based on dual-modal or wavelength-multiplexed operations. Here, we report a combined experimental and theoretical study on dual-modal plasmon-enhanced light upconversion through both ASPL and second-harmonic generation (SHG) from broadband multiresonant metal nanocavities in two-tier Ag/SiO2/Ag nanolaminate plasmonic crystals (NLPCs) that can support multiple hybridized plasmons with high spatial mode overlaps. Our measurements reveal the distinctions and correlations between the plasmon-enhanced ASPL and SHG processes under different modal and ultrashort pulsed laser excitation conditions, including incident fluence, wavelength, and polarization. To analyze the observed effects of the excitation and modal conditions on the ASPL and SHG emissions, we developed a time-domain modeling framework that simultaneously captures the mode coupling-enhancement characteristics, quantum excitation-emission transitions, and hot carrier population statistical mechanics. Notably, ASPL and SHG from the same metal nanocavities exhibit distinct plasmon-enhanced emission behaviors due to the intrinsic differences between the incoherent hot carrier-mediated ASPL sources with temporally evolving energy and spatial distributions and instantaneous SHG emitters. Mechanistic understanding of ASPL and SHG emissions from broadband multiresonant plasmonic nanocavities marks a milestone toward creating multimodal or wavelength-multiplexed upconversion nanoplasmonic devices for bioimaging, sensing, interfacial monitoring, and integrated photonics applications.

Two-dimensional semiconducting SnP2Se6 with giant second-harmonic-generation for monolithic on-chip electronic-photonic integration

Two-dimensional (2D) layered semiconductors with nonlinear optical (NLO) properties hold great promise to address the growing demand of multifunction integration in electronic-photonic integrated circuits (EPICs). However, electronic-photonic co-design with 2D NLO semiconductors for on-chip telecommunication is limited by their essential shortcomings in terms of unsatisfactory optoelectronic properties, odd-even layer-dependent NLO activity and low NLO susceptibility in telecom band. Here we report the synthesis of 2D SnP2Se6, a van der Waals NLO semiconductor exhibiting strong odd-even layer-independent second harmonic generation (SHG) activity at 1550 nm and pronounced photosensitivity under visible light. The combination of 2D SnP2Se6 with a SiN photonic platform enables the chip-level multifunction integration for EPICs. The hybrid device not only features efficient on-chip SHG process for optical modulation, but also allows the telecom-band photodetection relying on the upconversion of wavelength from 1560 to 780 nm. Our finding offers alternative opportunities for the collaborative design of EPICs.

Transformation of spin and orbital angular momentum in second-harmonic generation process by the near-surface layer of isotropic nonlinear medium.

We investigate the conversion between orbital and spin components of the angular momentum of light beams in the second-harmonic generation process from the near-surface layer of the nonlinear isotropic medium without spatial dispersion at oblique incidence of the elliptically polarized fundamental beam. The conservation of the projection of both spin and orbital angular momenta onto the normal to the surface of the medium during the transformation of the incident wave into a reflected double frequency wave has been demonstrated.

Unconditional remote entanglement using second-harmonic generation and twin two-mode squeezed vacuum states

We propose a photonics-based, continuous-variable (CV) form of remote entanglement utilizing strictly second-order nonlinear optical interactions that does not require the implementation of a state-projective measurement (i.e. remote entanglement without conditioning). This scheme makes use of two separate down-converters, wherein the corresponding nonlinear crystals are driven by strong classical fields as prescribed by the parametric approximation, as well as a fully quantum mechanical model of nondegenerate second harmonic generation (SHG) whose evolution is described by the trilinear Hamiltonian of the form $\hat{H}_{\text{shg}} = i\hbar\kappa\big(\hat{a}\hat{b}\hat{c}^{\dagger} - \hat{a}^{\dagger}\hat{b}^{\dagger}\hat{c}\big)$. By driving the SHG process with the signal modes of the two down-converters, we show entanglement formation between the generated second-harmonic mode (SH-mode) and the non-interacting joint-idler subsystem without the need for any state-reductive measurements on the interacting modes.

C(NH2)2NHNO2][C(NH2)3](NO3)2: A Mixed Organic Cationic Hybrid Nitrate with an Unprecedented Nonlinear-Optical-Active Unit.

We herein report a mixed organic cationic hybrid nitrate, namely, [C(NH2)2NHNO2][C(NH2)3](NO3)2 (1). It was successfully achieved via combining three different planar groups: [(C(NH2)2NHNO2]+, C(NH2)3+, and NO3-. First-principles calculations confirm that the [(C(NH2)2NHNO2]+ group is an excellent cationic nonlinear-optical (NLO)-active unit. The title compound exhibits a moderate second-harmonic-generation (SHG) response (1.5 × KDP), a wide band gap (3.58 eV), and a suitable birefringence of 0.071 at 550 nm. Theoretical calculations indicate that the synergy effect between the [(C(NH2)2NHNO2]+ and C(NH2)3+ groups dominates the SHG process.

Nonlinear optical properties of superalkali@teetotum boron clusters with potential applications on the electro-optic modulator

The development of nonlinear optics has revolutionized the photonic and optoelectronic fields in recent years.In this paper, we have designed a new class of donor–acceptor frameworks with enhanced nonlinear optical (NLO)properties using a series of superalkalis including Li3O, Li2NaO, LiNa2O, Li2KO, LiK2O as donors and three sizes of teetotum boron clusters (B14, B18, and B22) as acceptors. A systematic first-principles study has been performed on the electronic, linear, and NLO properties of superalkali@Bnassemblies. Besides the static non-linearity, the frequency-dependent electro-optic Pockel's effect (EOPE), second harmonic generation (SHG), and nonlinear refractive index have also been evaluated. In the case of dynamic NLO, the SHG process has a stronger NLO response than EOPE. At the typical experimental frequency of the Nd:YAG laser (ω = 0.04282) a.u., the SHG coefficients are about 2–3 times larger than the corresponding EOPE at this wavelength. Also, the nonlinear refractive index shows a remarkable response atω = 0.04 a.u. for LiK2O@B14.

Van der Waals Nonlinear Photodetector with Quadratic Photoresponse.

With unique electronic and optical attributes and dangling-bond-free surface, two-dimensional (2D) materials have broadened the functionalities of photodetectors. Here, we report a quadratically nonlinear photodetector (QNPD) composed of a van der Waals (vdW) stacked GaSe/InSe heterostructure. Compared with the reported 2D material-based photodetectors, the extra second-harmonic generation (SHG) process in GaSe/InSe leads to the quadratically nonlinear function between photocurrent and optical intensity, extending the photodetection wavelength from 900 to 1750 nm. The QNPD is highly sensitive to the variation of optical intensity with improved spatial resolution. With the light-light interaction in SHG converted into electrical signal directly, we also demonstrate the QNPD as an autocorrelator for measuring ultrafast pulse widths and an optoelectronic mixer of two modulated pulses for signal processings. The simultaneous involvement of light-light interaction and photoelectric conversion in the vdW stacked QNPD promises its potential to simplify the optoelectronic systems.

Harmonics-assisted optical phase amplifier

The change in the relative phase between two light fields serves as a basic principle for the measurement of the physical quantity that guides this change. It would therefore be highly advantageous if the relative phase could be amplified to enhance the measurement resolution. One well-known method for phase amplification involves the use of the multi-photon number and path-entangled state known as the NOON state; however, a high-number NOON state is very difficult to prepare and is highly sensitive to optical losses. Here we propose and experimentally demonstrate in principle a phase amplifier scheme with the assistance of a harmonic generation process. The relative phase difference between two polarization modes in a polarized interferometer is amplified coherently four times with cascaded second-harmonic generation processes. We demonstrate that these amplification processes can be recycled and therefore have the potential to realize much higher numbers of multiple amplification steps. The phase amplification method presented here shows considerable advantages over the method based on NOON states and will be highly promising for use in precision optical measurements.

Radially Polarized Second-Harmonic Generation from a Single-Element Nanoantenna via Dark Plasmon Coupling of Nonlinear Polarization

Controlling the process of second-harmonic (SH) generation at the nanoscale is important in photonic applications but remains challenging in nanophotonics. Herein, based on theoretical and experimental studies, we found that a cross-shaped single-element plasmonic nanoantenna resonating with bright and dark modes at the fundamental (excitation) and SH wavelengths, respectively, generates radially polarized ring-shaped SH radiations with axial symmetry by exciting the bright dipole mode with linearly polarized light. Mode expansion analysis of the generated SH radiation revealed that the radial polarization arises from a dark breathing mode in which coherent dipoles are radially distributed along the arms of the cross-shaped antenna. This study reveals that surface SH polarization generated by the electric near-field of the bright mode at the fundamental wavelength efficiently is coupled to the dark modes rather than the bright mode because its spatial distribution has the same symmetry as the dark modes. By engineering bright and dark modes at the fundamental and SH wavelengths, respectively, novel plasmonic nanodevices that simultaneously perform polarization control and efficient wavelength conversion are expected.

Giant Enhancement of Second-Harmonic Generation in Hybrid Metasurface Coupled MoS2 with Fano-Resonance Effect

Plasmonic nanostructures have been regarded as potential candidates for boosting the nonlinear up-conversion rate at the nanoscale level due to their strong near-field enhancement and inherent high design freedom. Here, we design a hybrid metasurface to realize the moderate interaction of Fano resonance and create the dual-resonant mode-matching condition to facilitate the nonlinear process of second harmonic generation (SHG). The hybrid metasurface presents dipolar and octupolar plasmonic modes near the fundamental and doubled-frequency wavelengths, respectively, further utilized to enhance the SHG of low-dimensional MoS2 semiconductors. The maximum intensity of SHG in hybrid metasurface coupled MoS2 is more than ten thousand times larger than that of other structure-units coupled MoS2. The conversion efficiency is reported to be as high as 3.27 × 10−7. This work paves the way to optimize nonlinear light–matter interactions in low-dimensional structures coupled with semiconductors.

Measurement of the mercury (6s6p)P13 -state lifetime in the frequency domain from integrated absorbance data

Quantitative UV laser absorption spectroscopy is implemented to measure the strength of the $(6{s}^{2})\phantom{\rule{0.28em}{0ex}}^{1}S_{0}\ensuremath{\rightarrow}(6s6p)^{3}P_{1}$ intercombination transition of Hg atoms near the wavelength of 253.7 nm and to retrieve the lifetime of the excited state. Tunable, single-mode, coherent radiation in the deep UV is produced through a double-stage second-harmonic generation process, starting from the output radiation of an external-cavity diode laser at 1014.8 nm. This latter is referenced to an optical frequency comb synthesizer. The measurement strategy exploits the relationship between the strength of an optical transition and the spontaneous emission Einstein coefficient. The complete uncertainty budget leads to an overall relative uncertainty of 0.6%, with a statistical contribution of 0.28 ns, which is the lowest ever obtained for the $(6s6p)\phantom{\rule{0.28em}{0ex}}^{3}P_{1}$ lifetime. The new value, amounting to $125.12\ifmmode\pm\else\textpm\fi{}0.79\phantom{\rule{0.28em}{0ex}}\mathrm{ns}$, compares well with most of the past determinations within two times the quoted uncertainties. Despite this, it does not agree with the weighted average of the 12 prior values, thus suggesting that a critical reassessment of the uncertainty of some of the past measurements is probably necessary.

Energy-transfer controlled dual-wavelength visible laser generation based on parallel nonlinear frequency up-conversions

A compact frequency up-conversion scheme is demonstrated for dual-wavelength visible laser generation. The parallel sum-frequency generation (SFG) and second-harmonic generation (SHG) processes are simultaneously phase-matched and integrated in a well-designed periodically poled MgO:LiNbO3 crystal. Concurrent nonlinear up-conversions of the coupled SFG and SHG are thus realized. In the proof-of-principle experiment, two near-IR lasers at wavelengths of 800 nm and 1180 nm were used as independent fundamental beams. As a result, the generation of blue light at ∼477 nm by SFG (800 nm + 1180 nm → 477 nm) and the orange light at ∼590 nm by SHG of 1180 nm (1180 nm + 1180 nm → 590 nm) was simultaneously achieved. Finally, it is proved the relative strength of these two coupled nonlinear interactions and consequently the relative power of the blue and orange light could be freely controlled by changing the pump intensities of the fundamental beams, and a maximum photon conversion efficiency (∼25%) comparable to that of the separated SHG process was observed for this simultaneous dual-up-conversion device. This result provides a promising strategy for developing high-efficiency multi-wavelength visible laser systems. Combined with an engineered QPM crystal with different poling periods in sequence, more prospective nonlinear applications may be realized based a series of cascaded single- and dual-QPM nonlinear processes.

Simultaneous Second Harmonic Generation and multiphoton excited photoluminescence in anatase TiO2 nano powders

Second Harmonic Generation (SHG) and Multiphoton Excited Photoluminescence (MEPL) are observed in retro-reflection mode from a 22 nm diameter anatase TiO2 nanoparticle powder. Depth intensity profiles are obtained by scanning the fundamental beam focal point through the TiO2 powder surface into its volume for the energy of the incident photons tuned around the anatase TiO2 band-gap energy of 3.2 eV. Evidence for the appearance of both SHG and MEPL is presented, their relative contribution depending on the conditions of photo-excitation, in particular the fundamental wavelength and the focal point depth location. MEPL dominates at fundamental photon energies matching the band-gap energy whereas below and above the band-gap SHG is clearly seen. Depth intensity profiles exhibiting a rising edge determined by the fundamental laser beam focusing conditions and a decreasing edge determined by extinction and multiphoton scattering strongly differ between SHG and MEPL, SHG profiles being by far narrower than that of MEPL. Re-absorption of the emitted SHG and MEPL light occurs due to the spectral proximity with the TiO2 band gap energy but excitation with fundamental light below band-gap further suggests that the intrinsic difference between the two SHG and MEPL processes is responsible for the contrasted depth profiles. This combined study where both SHG and MEPL occur presents perspectives in the study of the photonic properties of powder materials.