Multimodal Optically Nonlinear Nanoparticles Exhibiting Simultaneous Higher Harmonics Generation and Upconversion Luminescence for Anticounterfeiting and 8‐bit Optical Coding

Nonlinear optical materials are essential in areas such as nanophotonics, optical information processing, and biomedical imaging. However, nanomaterials employed for these diverse applications to date are efficient only for one type of nonlinear optical activity. Herein, the first multimodal nonlinear optically active class of nanomaterials based on lanthanide‐doped lithium niobate nanoparticles, which simultaneously exhibit unprecedentedly efficient second and third harmonic generation, as well as up‐conversion photoluminescence, is reported. These dielectric nanoparticles retain their high nonlinear optical conversion efficiency both as powder and as aqueous colloidal solution. The high stability also allows for the fabrication of optically active biocompatible micron‐sized fibers and polymer‐based 3D‐printable objects, as well as for fingerprint detection. Finally, the first 8‐bit coding platform purely based on multimodal nonlinear optical activity originating from different parametric and nonparametric processes is demonstrated, showcasing the technological potential of these materials for both anti‐counterfeiting and advanced optical information processing.

The NLO activity of single-crystalline bulk materials is frequently associated with multi-photon, polarization-dependent parametric processes such as higher harmonics generation, including second harmonic generation (SHG) and third harmonic generation (THG), which are commonly used in laser technologies. [12,13]SHG and THG processes are based on the conversion of either two or three photons of frequency , into one photon of doubled (2) or tripled frequency (3), respectively.Their efficiencies are mainly related to the realization of the phase-matching conditions, spatial orientation of the NLO crystals, and their crystalline quality.
][28][29][30][31][32][33][34][35][36][37][38][39][40] Here, spatial phasematching is not required, as these ensemble materials consist of a large amount of randomly oriented crystallites, allowing for the observation of averaged SHG signals. [18,25][27][28][29][30] Here, we report on the first trimodal NLO active nanomaterials based on lanthanide-doped NPs, which show at the same time highly efficient and bright SHG (clearly visible to the naked eye), THG and UC-PL.The developed NPs act as efficient broad-range optical nano-converters (covering thenear-IR to UV) and are cheap to produce.Importantly, they sustain their NLO activity both as highly stable powders and as colloids, based on water as a green solvent.Combining these multimodal NLO NPs with various host materials, we here reported the first SHG and UC active biocompatible cellulose fibers, anti-counterfeiting NLO inks, as well as NLO polymer-based 3D-printable objects.With the latter, we also 3D-printed the first fully NLO binary codes of information based on parametric and nonparametric phenomena, and demonstrate 8-bit ASCII decoding based on continuous wave (CW) or pulsed optical read-out, show-casing the broadness of potential applications of these materials for next-generation biomedical, cryptographic, and optical information technologies.

Results and Discussion
It is generally known that, unlike UC-PL, for the generation of higher harmonics such as SHG and THG, the orientation of the nonlinear crystal axes with respect to the excitation beam direc-tion is related to the efficiency of the NLO processes, i.e., the phase-matching condition. [1]However, the spatial phase matching is crucial only for single-crystals.This is because polycrystalline, micro-and nanosized materials are composed of a large number of small crystallites with random orientations, which allows for the detection of averaged NLO signal such as SHG. [1,18,25,56]Hence, precise phase-matching for each crystal is no longer required.Therefore, we chose polycrystalline LiNbO 3 nanoparticles (NPs) as the phase for designing our NLO material platform (Figure 1a). [1,25]irst, it is worth mentioning that bulk lithium niobate (LiNbO 3 ) is a well-established dielectric material, crystallizing in a non-centrosymmetric rhombohedral lattice, being transparent in the UV-vis-NIR spectral ranges, with a high-energy band gap of about 4 eV and ferroelectric properties. [37]Most importantly, it has good NLO performance (high nonlinear susceptibilities), exhibiting such nonlinear phenomena as piezoelectricity, electrooptical effect, and higher harmonics generation. [37,38]For these reasons LiNbO 3 is widely used in various photonic applications such as waveguides, optical modulators, energy converters, laser technology or integrated tuneable Bragg gratings. [37,57]e synthesized NPs based on highly NLO active LiNbO 3 with an average particle size of ca.250 nm (Figure 1b; Figure S3, Supporting Information).Moreover, we dope our NPs with lanthanide ions (Yb 3+ /Er 3+ ) to endow them with UC-PL.The experimental powder X-ray diffraction (XRD) data for the pure and Lndoped LiNbO 3 compounds, synthesized under optimized conditions (Li/Nb precursors molar ratio of 70%; calcined at 850 °C) are shown in Figure S1 of the Supporting Information.All of the reflexes have good agreement with reference pattern (No. #96-210-1176) of hexagonal LiNbO 3 (trigonal system with rhombohedral lattice), belonging to R3c space group, indicating formation of the desired phase. [58]In this crystal structure, Li and Nb ions are both coordinated octahedrally by six oxygen ions.Due to the significant difference between the ionic radii of niobium (R Nb5+ = 0.64 Å) and Ln ions (R Er3+ = 0.89 Å; R Yb3+ = 0.87 Å), the Er 3+ and Yb 3+ ions, with higher probability, occupy the much larger lithium sites (R Li+ = 0.76 Å), with the ≈15% mismatch between the host and dopant ions.The performed Rietveld refinements (Figure S2, Supporting Information) reveal the calculated diffraction profiles correspond well with the recorded XRD patterns, showing slight expansion of the unit cell volume (from ≈317 to 320 Å 3 ) upon doping the niobate crystals with larger Ln ions, which substituted smaller cations of the host (the resulting fitting parameters and lattice constants are listed in Table S1, Supporting Information).
Importantly, the synthesized LiNbO 3 NPs show a spatially highly uniform SHG signal, as confirmed by orientation-and polarization-dependent measurements (Figure 1c).For comparison, an NLO active KH 2 PO 4 (KDP) single crystal, selected as a reference, shows a strongly orientation-dependent SHG response, limiting its application space (see the Supporting Information for details).The mechanisms governing UC-PL, SHG, and THG processes are indicated in energy level diagrams in Figure 1d, emphasizing the nonparametric character of UC-PL, occurring via light absorption, in contrast to the parametric nature of SHG/THG (via virtual states), not entailing real absorption, thus limiting laser heating and photobleaching, which is beneficial for bioapplications.Other important differences between the SHG and UC-PL processes are the instantaneous nature of SHG versus time-delayed UC-PL, as well as the occurrence of SHG only in non-centrosymmetric systems. [1,2,41]][43][44] The detailed discussion and characterization of the obtained pure and Ln-doped LiNbO 3 NPs, including the mechanisms of SHG/THG and UC-PL, are given in the Supporting Information (Figures S1-S6, Supporting Information).

Effect of Synthesis Conditions on Structure, Morphology, and SHG Performance
We also note that the precise synthesis conditions can be used to significantly change structure, morphology and thus NLO performance of the resulting NPs.An appropriate ratio of the raw materials during the LiNbO 3 NPs synthesis ensures the formation of the pure, non-centrosymmetric (SHG-active) LiNbO 3 phase.Figure 2a shows structural data, i.e., powder XRD patterns of the synthesized LiNbO 3 nanomaterials, as a function of relative molar concentration of lithium (vs niobate) used during the synthesis process.It can be clearly seen that the optimal molar concentration of Li + ions is ≈70%, where the desired pure phase structure of the non-centrosymmetric (SHG-active), rhombohedral LiNbO 3 is formed (in other words, an excess of ammonium niobate (V) oxalate is required during the synthesis process).Instead, the use of larger amounts of lithium source (Li 2 CO 3 ) for the synthesis, compared to the content of niobate (V) ions, results in the apparent formation of the centrosymmetric (SHG-inactive) cubic Li 3 NbO 4 phase as an additional component.The right side of Figure 2a is a 3D representation of the concentrationdependent phase transition between both phases of lithium niobates.The collected structural data agree well with measured nonlinear activities of the synthesized compounds, as presented in Figure 2b, which shows the corresponding emission spectra recorded at  ex = 1000 nm.As expected, the highest SHG signal is observed for the sample synthesized at a lithium/niobium molar ratio of 70%.Additionally, the photographs of the corresponding samples under NIR ns-pulsed laser irradiation at  ex = 1000 nm, visualizing their optical activity, i.e., cyan emission originating from the SHG light, are presented in Figure 2c.
It is clear that the synthesis conditions have an important impact on the resulting crystal structures and the related spectroscopic features.Hence, we have also investigated the influence of temperature on the material properties.Figures S7 and S8 of the Supporting Information show the XRD data, SHG signal intensities (emission spectra), and scanning electron microscopy (SEM) images for the niobate materials synthesized at different temperature values from 700 to 900 °C.As expected, alike the SEM images and XRD data (slight reflexes broadening at lower temperature) confirm that with increasing calcination temperature, the average crystallite sizes increase from ≈150 to 500 nm, and the SHG signal intensity is enhanced.This is due to the fact that, in larger nonlinear crystals, with better crystallinity (less defects), the conversion of the fundamental beam to higher harmonics is more efficient.It is worth noting, that all samples obtained in this work are in the form of white powders, except for the sample calcined at 700 °C, which was black (probably due to the incomplete thermal decomposition of the organic components of the starting materials) and did not exhibit detectable SHG signal, which is caused by its strong absorption of the incident and emitted photons, as well as by the small particle size and low crystallinity of the sample.On the other hand, above 900 °C, the resulting compounds start to melt, forming a hard, "glassy" mass.It is also worth noting, that the Ln doping negligibly affects structure, particle size and morphology of the final NPs, as presented in Figure S9 of the Supporting Information.

Wavelength-Dependent SHG and THG of the Nanoparticles as Powder and Colloidal Solution
We next investigate the detailed nonlinear optical properties of the synthesized NPs.A scheme of the experimental setup (Figure 3a) shows how the energy of photons is converted via SHG and THG processes.The wavelength-dependent relative intensities of the SHG and THG processes in the NP powder are presented in Figure 3b,c.The SHG and THG signal intensities are highest in the visible range, ranging from ≈400-600 nm, corresponding to excitation in the ranges of 800-1200 nm (SHG) and 1200-1800 nm (THG), respectively.Due to the very high optical transparency of the LiNbO 3 NPs in the near-IR, visible, and UV (see Figure S4, Supporting Information), we were able to record decent SHG signals in a very broad spectral range, from 1000 down to 330 nm.Importantly, the efficient generation of SHG in a UV range, i.e., in situ generated UV light from lowenergy radiation, allows for application of the LiNbO 3 NPs in various chemical and biomedical applications, such as photocatalysis, photodynamic therapy or cancer-cell damage with 2 UV light. [59]The excitation-density-dependent SHG/THG signal intensities confirm the respectively assigned nonlinear optical process (Figure S10, Supporting Information).
To quantitatively evaluate the SHG performance of our NPs, we compare the signal intensity to that of KDP powders, commonly employed as reference (Figure 3d; Figure S11, Supporting Information). [14,17,18,60]The SHG signal from the LiNbO 3 NPs is significantly higher (up to 80-fold) compared to the reference KDP powders, and it is hardly influenced by the Ln doping (Figure S11, Supporting Information).A similar excitationwavelength dependence of the SHG intensities is observed for our LiNbO 3 NPs and KDP powders (Figure 3d; Figure S12, Supporting Information).While, expectedly, the recorded THG intensities are lower than those from SHG, the THG signals from our NPs were strong enough to be easily recorded, in contrast to the KDP reference powders.
Moreover, our NLO NPs sustain their high optical efficiency even when dispersed in an aqueous colloidal solution (Figure 3e).Their efficient SHG in water can be clearly observed by the naked eye.The recorded emission spectra of the colloidal LiNbO 3 NPs (Figure 3f) display appreciable SHG signals almost in the whole visible range (up to ≈650 nm).Despite the strong light scattering in the UV range, we could also detect SHG at ≈375 and 355 nm, which is important for biomedical and industrial applications.Figure 3g shows photographs of the bright, multicolor SHG-emission from the powder LiNbO 3 NPs.

Simultaneous UC-PL and SHG of NPs
Next, we characterize the additional UC-PL of the Ln-doped NPs (Figure 4), while preserving the existing SHG properties of the undoped NPs. Figure 4a presents UC-PL spectra of the Yb 3+ /Er 3+ -doped NPs excited with different CW laser wavelengths ( ex = 808, 975, and 1532 nm).Shape of the UC-PL spectra changes with  ex used, resulting in different emission colors.This phenomenon is already well-established in the literature, and it is related to different excitation mechanisms of UC-PL. [61]The insets depict photographs of the samples irradiated with the respective CW lasers, confirming their bright lanthanide-based UC-PL, which can be tuned to cover a broad spectral range from green to yellow and red.Introducing the sensitizing Yb 3+ ions into the crystal lattice, the UC-PL intensity increases almost three times (Figure S13, Supporting Information).The expected square-dependence of the UC-PL intensity on the excitation density confirms its mechanistic origin (see Figure S14, Supporting Information, for details).
Switching from CW to ns-pulsed excitation results in the combined observation of UC-PL, SHG, and even THG from the doped NPs (Figure 4b).Nevertheless, for the first glance, the optical features of the lanthanide-doped samples are not different from the pure LiNbO 3 NPs.However, after careful evaluation and magnification, one can notice the low-intensity UC-PL of Er 3+ , and THG line for the 1532 nm excitation.The observed bright emission colors (insets) correspond to the SHG signals for the 808 and 975 nm excitations, whereas for the  ex = 1532 nm, the SHG line (766 nm) is hardly visible for the naked eye, as it is located very close to the NIR region, so the resulting emission color is pale green, being a combination of the UC-PL and THG.The observed UC-PL intensity is ≈3-5 orders of magnitude lower than the SHG signal, due to the low frequency of the ns-pulsed excitation (10 Hz).
By using a mode-tunable Ti:sapphire laser, operating either in fs-pulsed or CW mode, we effectively generate UC-PL upon excitation in both laser modes, while SHG signal exclusively arises for the fs-pulsed excitation (Figure 4c).In contrast to the nspulsed excitation, utilizing fs-pulsed laser excitation, it is possible to generate alike SHG and UC-PL of comparable intensities (Figure S15, Supporting Information).In this case, the observed intense UC-PL under pulsed excitation is a result of the high repetition rate (80 MHz) of the laser.Switching the lasing mode from CW to pulse leads to the sudden emission color change from green to blue, which provides additional opportunities for advanced anticounterfeiting of materials and optical coding.Changing the excitation density allows for yet another way of tuning the emission color in our system by altering the relative intensities between the SHG and UC-PL signals (see Figure 4d; Figure S15, Supporting Information).This is because UC-PL has in this case a lower saturation threshold than the SHG, exhibiting deviations from the initial nonlinearity, as evidenced by the change in initial slope, i.e., photon number, which for SHG is decreasing from ≈1.9 to 1.3, and for UC-PL from 1.8 to 0.4 (Figure S16, Supporting Information).Except for the apparently lower saturation threshold, the observed much higher relative intensity decrease for the UC-PL may be also related to the local sample heating, and the resulting quenching of UC-PL at extreme power densities.

Applications as Biocompatible Fibers and 3D-Prints, for Anticounterfeiting and 8-bit Optical Coding
To showcase the potential applications of our newly developed NLO nanomaterials, we demonstrate their use in various media as anti-counterfeiting and fully optical coding platform (Figure 5).To demonstrate latent fingerprint detection, we first show latent fingerprints on glass using the synthesized powder of the pure LiNbO 3 NPs.The fine, white powder was uniformly distributed with a brush on the mentioned fingerprints, revealing them in daylight (Figure 5a).Applying various excitation wavelengths of the ns-pulsed laser, we can then observe the bright various SHG colors in the fingerprints.Such a strategy can be an alternative to latent fingerprint detection/imaging with conventional, optically active luminescent materials. [62,63]A similar effect can also be achieved by drop-casting an aqueous colloid of the LiNbO 3 NPs on the microstructured template instead (Figure S17, Supporting Information).In another experiment, we homogenously mixed a standard blue-pen ink with our NP powder, and then applied it on a rubber stamp with a detailed inscription and logo.The optically active ink was then used to make stamp imprints on white paper.Irradiating it with the same pulsed excitation at 950 nm, bright blue SHG light can be observed across the whole inscription (Figure 5b).Due to the strong absorption of the blue ink in the green-red spectral regions, [64] the use of  ex = 950 nm resulted in the most intense SHG light.
these fibers, which are now imbued with strong NLO activity.Owing to the in situ synthesis process (see the Supporting Information for details), the optically active fibers have evenly distributed NPs throughout their whole volume (Figure S18, Supporting Information), ensuring uniform light propagation and emission.Upon the CW laser excitation, these fibers then also exhibit optical activity in the form of green UC-PL (Figure 5d, left), demonstrating their multimodal capabilities for biomedical imaging.
Moreover, we were able to produce 3D-printable polymer resins containing our NLO NPs, and these flexible polymers show five clearly distinguishable points (Figure 5d, right) due to alternating arrangements of the UC-active/inactive polymer units.For the ns-pulsed laser excitation, the observed multicolor SHG emission has the same brightness throughout the whole imprinted pattern, owing to the homogenous mixing of the resin with the fine NPs powder before the 3D-printing process, and due to the same SHG intensities of pure and Ln-doped NPs (Figure 5e).Using a mode-tunable Ti:sapphire excitation laser, operating either in fs-pulsed or CW mode, the fibers can be switched in their emission to exhibit either blue SHG or green UC-PL (Figure 5f), and a similar effect can be observed for the 3D-printed patterns (Figure S19, Supporting Information).
Lastly, we employ our NLO active NPs to demonstrate advanced, multimodal and multidimensional optical information storage, using high-resolution (≈50 μm) 3D-printing to fabricate multidimensional patterns (Figure 5g-i).First, we fabricate letters using three batches of polymer resins, i.e., i) a white resin without the NLO-active particles, ii) a resin containing the SHG active LiNbO 3 NPs, and iii) a resin mixed with the LiNbO 3 :Yb 3+ /Er 3+ doped NPs (SHG and UC-PL active), corresponding to the grey, blue, and green blocks in the simulation models shown, respectively.Due to the different optical response of the undoped NPs compared to the Ln-doped ones, it is possible to reveal different patterns upon near-IR irradiation, which can be changed and controlled by switching between CW and pulsed mode.This can be clearly seen in Figure 5g,h, where the "L" and "ULL" letters revealed under CW irradiation (yellowish UC-PL) are transformed into "EI" and "OCE" words when exposed to the near-IR pulsed excitation (multi-color SHG).
In a similar way, we 3D-printed an 8 × 8 matrix composed of optically inactive and different NLO active polymer blocks (each of 250 × 250 μm size), with encoded optical information in the form of two different binary codes (Figure 5i).The pre-stored information can be revealed by transforming NLO signals (UC-PL and SHG) into different 8-bit ASCII codes, using the different excitation modes.In both reading modes, the "0" values are represented by the optically inactive polymer units.For CW excitation, the observed UC-PL from the polymer units containing the doped NPs results in "1" values, and the remaining units with undoped NPs represent extra "0" values.This, after signal decoding, allowed us to reveal the hidden (secret) information "@PILJAB".Under pulsed excitation, the "1" values are generated from both NLO active polymer units, which can generate SHG, leading to the decryption of another optical information stored, i.e., "HARMONIC".Such multimodal and selective NLO activity allows to decode two different optical data sets from a single encrypted pattern.This system represents the first example of a purely NLO 8-bit coding platform, based simultaneously on different parametric and nonparametric phenomena for data storage and readout.Using more advanced 3D or conventional (2D) printing technologies with ultrahigh resolution, this NLO coding platform could significantly enhance information loading (i.e., storage capacity per unit volume) and read-out speed (as optical), which is in high demand for modern optical information technologies.
It is worth noting, that we just showed here a proof-of-concept of NLO dual-mode coding based on SHG and UC-PL active NPs, which has some inherent coding limitations in the applied configuration.In other words, the use of NPs which are simultaneously UC-PL and SHG active (like LiNbO 3 :Yb 3+ /Er 3+ ) for optical coding in combination with SHG active NPs (pure LiNbO 3 ) imposes that the units which are UC-PL active must be always SHG active upon pulsed laser irradiation ("1" value in 8-bit ASCII codes).However, the number of possible configurations, i.e., amount of the information storage ready to be independently read out using a dual-mode decryption system (i.e., CW laser for UC-PL and pulsed laser for SHG) could be greatly enhanced by adding some conventional UC-PL NPs being SHG inactive.Admixing them (e.g., NaYF 4 :Yb 3+ -Er 3+ ) separately or together with developed SHG active NPs in a desired polymer unit would result in the possibility of fabrication of dual-mode NLO optical coding systems without the mentioned limitations, but with improved functionality and information loading capacity.

Conclusions
In summary, we have developed a new nanomaterial platform based on powders and aqueous colloidal solutions of undoped and lanthanide-doped dielectric NPs with strong NLO activity.These NPs represent the first example of a nanomaterial displaying simultaneously strong SHG (clearly visible to the naked eye; up to 80 times brighter than reference KDP standard), THG and UC-PL, opening new horizons for multimodal NLO detection schemes.Depending on synthesis conditions, composition, and excitation/read-out scheme, strongly wavelengthtunable and energy-density dependent NLO activity is reached, and it can be tailored for the desired application.To demonstrate the technological impact of our material platform, we employ it in water-and organic-based inks, as dactyloscopic powder, micron-sized biocompatible cellulose fibers, and polymerbased 3D-printable objects of arbitrary shape.With these, we demonstrated high-resolution anti-counterfeiting and latent fingerprint detection, and a new optical coding platform.The latter presents the first example of a fully functional 8-bit ASCII encoding and decoding system based on binary code that can be programmed and read out purely based on different parametric and nonparametric NLO phenomena, i.e., using the multimodal photoresponse of the NPs depending on excitation mode.The possibility of efficient in situ generation of UV light from incident lower-energy photons makes these NPs also promising materials for photocatalysis and various bioapplications, where the generated UV light can be used to induce the desired in vitro or in vivo cell/tissue damage, as an alternative to conventional photodynamic therapy.In summary, this newly developed NLO nanomaterial platform with its multimodal optical addressability will open up new avenues in technologies ranging from biomedical applications to advanced anti-counterfeiting and next-generation optical information processing.

Experimental Section
Synthesis of Lithium Niobate NPs: The following protocol refers to the optimal synthesis conditions of the pure and lanthanide-doped LiNbO 3 NPs discussed thorough the article, where lithium to niobium molar ratio used for the starting batch is 0.7/1.For the entire sequence of undoped lithium niobate materials, molar ratios ranging from 0.5/1 to 3/1 were used.LiNbO 3 nanomaterials were synthesized via the modified solgel Pechini method.The appropriate amounts of solid Li 2 CO 3 (0.0437 g, 0.000592 mol; Sigma-Aldrich, ACS reagent, ≥99.0%) were mixed with citric acid (5 g; Stanlab, p.a.) and ethylene glycol (0.5 mL; Chempur, pure p.a.) in deionized H 2 O (20 mL) under mixing and heating in porcelain crucible to dissolve Li 2 CO 3 .Whereas to obtain the LiNbO 3 NPs doped with 0.1 mol% of Yb 3+ and 0.1 mol% Er 3+ , the solid Li 2 CO 3 (0.04366 g, 0.0005908 mol) was initially mixed with stock solutions of the ErCl 3 (0.024 mL of 0.05 m) and the YbCl 3 (0.024 mL of 0.05 m).Note that the stock solutions of lanthanide chlorides were prepared by dissolving the corresponding rareearth oxides, i.e., Er 2 O 3 and Yb 2 O 3 (99.99%,Stanford Materials, USA) in hydrochloric acid (HCL; 37%, ultrapure, POCh.S.A., Poland).Next, appropriate amount of powdered ammonium niobate (V) oxalate hydrate (0.5123 g, 0.001691 mol; Sigma-Aldrich, 99.99%) was added to the system and mixed until dissolved.After this, the prepared sols were dried at 80 °C for 24 h, and the porous xerogels were formed.The formed resin was annealed at 850 °C for 5 h, and the synthesized product was grounded in an agate mortar for further analysis.
Spinning of Cellulose Fibers Doped with the LiNbO 3 :Yb 3+ /Er 3+ NPs: The appropriate amounts of the cellulose pulp (DP = 1250, -cellulose content 96.8% water content ≈5.2%), 50% aqueous solution of Nmethylmorpholine N-oxide (NMMO) (Huntsman Holland BV), propyl ester of gallic acid-Tenox PG (Sigma-Aldrich), and the LiNbO 3 :Yb 3+ /Er 3+ NPs working as a modifier were placed in a kneader equipped with stirring and heating system to prepare high-quality spinning dope for further process.The 1% colloidal solution of the LiNbO 3 :Yb 3+ /Er 3+ NPs was added at the beginning of the cellulose dissolution process to reach a concentration ≈1.25 wt% of the modifier in the resulting cellulose fibers.Cellulose was dissolved under low pressure (≈20 hPa) at a proper temperature not exceeding 115 °C.The excess of water was removed during the process until ≈14 wt% content in the system was reached.A viscous, transparent spinning dope was obtained.The spinning dope was placed into a preheated cylinder (temperature ≈115 °C).The molten solution was pressed through the spinneret holes and the air gap, and then, it was immersed in an aqueous bath at 20 °C up to solidification.The fibers were spun with take-up speed 10 m min −1 , then washed and dried at room temperature.The details of the mechanical properties of the modified fibers, compared to the unmodified ones, can be found in the next paragraphs and in Table S2 of the Supporting Information.
Characterization: XRD patterns were measured with a Bruker AXS D8 Advance diffractometer (6°-60°2 range) using Cu K 1 radiation ( = 1.5406Å).Absorption spectra were acquired in diffused-reflection mode, using a JASCO V-770 UV-vis/NIR spectrophotometer, equipped with an ILN-925 spherical integrator (150 mm in diameter).Raman spectra were recorded in backscattering geometry with a Renishaw InVia confocal micro-Raman system, using a grating with 1200 grooves mm −1 and a power-controlled 100 mW 532 nm laser diode.The laser beam was focused using an Olympus ×20 SLMPlan N long working distance objective.Transmission electron microscopy (TEM) images were takes with a Hitachi HT7700 microscope, applying an acceleration voltage of 100 kV.SEM images were recorded with an FEI Quanta 250 FEG equipped with an EDAX detector.Particle size distribution and zeta potentials were measured by the use of a Malvern Zetasizer Nano-ZS, applying dynamic light scattering and electrophoretic light scattering methods, respectively.High-efficiency laboratory-scale IKAVISC kneader type MKD 0.6-H60 was used to prepare the cellulose solution.The fibers were spun with the use of dry-wet spinning method on a laboratory-scale piston-spinning device with 18 orifices spinneret equipped with a hole diameter of 0.4 mm.Mechanical properties of the fibers were measured using a Zwick Z2.5/TN1S tensile testing machine, according to the PN-EN ISO 5079:1999 standard.The linear density of the fibers was measured in accordance with the PN-EN ISO 1973:1995 standard.
The emission spectra, i.e., SHG, THG signals, and UC-PL of the samples were measured using an Andor Shamrock 500 spectrometer coupled to the silicon camera, i.e., Peltier-cooled Andor Indus, charge-coupled device.All recorded emission spectra were corrected for the apparatus response (wavelength-dependent detection sensitivity).The main excitation source for the SHG experiments was a tunable ns-pulsed laser EK-SPLA/NT342/3/UVE (optical parametric oscillator -OPO), working at 10 Hz repetition rate, pulse duration of ≈7-8 ns, and energy of ≈1 mJ.The additional excitation source for some SHG and UC-PL measurements was a mode-and wavelength-tunable fs-pulsed Ti:sapphire laser, working at 80 MHz repetition rate and pulse duration of ≈130 fs.The laser power was adjusted to ≈800 mW, and the wavelength was centered at ≈805 nm.The main excitation sources for the UC-PL measurements were solid-state diode-pumped CW NIR lasers, emitting at 808, 975/980, and 1532 nm (output power of ≈2 W).

Figure 1 .
Figure 1.Basic structural and optical properties of the nonlinearly optactive nanoparticles.a) Schematic representation of the origin of SHG in the randomly distributed nonlinear NPs.b) TEM image of the obtained LiNbO 3 NPs, scale bar is 200 nm.c) SHG polar plots as function of excitation laser polarization ( ex = 1064 nm), recorded for the LiNbO 3 NPs (red spheres), and for a bulk single crystal KDP (blue rhomboids) for reference; the NP SHG shows no dependence on excitation polarization.d) Energy level diagrams showing UC-PL (for the Er 3+ -doped samples), THG and SHG processes in the lithium niobate NPs exhibiting NLO activity; VS = virtual state, GS = ground state, ET = energy transfer, GSA = ground-state absorption, ESA = excited-state absorption, NR = nonradiative relaxation.

Figure 2 .
Figure 2. Tunability of structure and resulting NLO properties.a) Powder XRD patterns of the synthesized samples as a function of Li + content (50-300 mol%) versus molar concentration of niobate ions (left), 3D representations of the crystal structures of non-centrosymmetric LiNbO 3 (R3c; top middle) and centrosymmetric Li 3 NbO 4 (I 43m; bottom middle).b) Emission spectra showing SHG in LiNbO 3 materials as a function of Li + content (mol%) in starting batch.c) SHG photographs of the corresponding samples under NIR ns-pulsed laser excitation at 1000 nm (fluence = ≈50 mJ cm −2 ; flux = ≈0.5 W cm −2 ).

Figure 3 .
Figure 3. Efficient second and third harmonic generation in nonlinearly active nanoparticles as powders and in solution.a) Experimental setup used for the SHG and THG measurements.b) SHG and c) THG intensity spectra of the powder LiNbO 3 NPs as a function of excitation wavelength under ns-pulsed laser irradiation (excitation fluence ≈100 mJ cm −2 ; flux ≈1 W cm −2 ), respectively.d) Integrated SHG intensities of the LiNbO 3 NPs, compared to the KDP references, as a function of  ex .e) SHG photographs of aqueous colloidal LiNbO 3 NP solutions (c = 1 g L −1 ) under ns-pulsed laser irradiation at various excitation wavelengths.f) Corresponding SHG intensity spectra of colloidal NPs as a function of excitation wavelength (fluence ≈300 mJ cm −2 ; flux 3 W cm −2 ).g) Photographs of the powder LiNbO 3 NPs irradiated with an ns-pulsed laser at various excitation wavelengths, showing bright SHG of various colors (excitation fluence ≈50 mJ cm −2 ; flux 0.5 W cm −2 ).