Raman enhancement on a flat nonmetallic surface has attracted increasing attention, ever since the discovery of graphene enhanced Raman scattering. Recently, diverse two-dimensional layered materials have been applied as a flat surface for the Raman enhancement, attributed to different mechanisms. Looking beyond these isolated materials, atomic layers can be reassembled to design a heterostructure stacked layer by layer with an arbitrary chosen sequence, which allows the flow of charge carriers between neighboring layers and offers novel functionalities. Here, we demonstrate the heterostructure as a novel Raman enhancement platform. The WSe2 (W) monolayer and graphene (G) were stacked together to form a heterostructure with an area of 10 mm × 10 mm. Heterostructures with different stacked structuress are used as platforms for the enhanced Raman scattering, including G/W, W/G, G/W/G/W, and W/G/G/W. On the surface of the heterostructure, the intensity of the Raman scattering is much stronger compared with isolated layers, using the copper phthalocyanine (CuPc) molecule as a probe. It is found that the Raman enhancement effect on heterostructures depends on stacked methods. Phonon modes of CuPc have the strongest enhancement on G/W. W/G and W/G/G/W have a stronger enhancement than that on the isolated WSe2 monolayer, while lower than the graphene monolayer. The G/W/G/W/substrate demonstrated a comparable Raman enhancement effect than the G/W/substrate. These differences are due to the different interlayer couplings in heterostructures related to electron transition probability rates, which are further proved by first-principle calculations and probe–pump measurements.Keywords: Heterostructure; surface-enhanced Raman scattering;

We report on the enhancement and modulation of nonlinear optical response in an Nd:Y3Al5O12 (Nd:YAG) laser crystal through embedded silver nanoparticles (NPs) fabricated by Ag+ ion implantation. The linear absorption spectrum of the sample clearly reveals a localized surface plasmon resonance (SPR) band from 350 to 700 nm correlated to the Ag NPs. By using the Z-scan technique with femtosecond pulses at a wavelength of 515 nm, which is considered as an optical excitation within the SPR band, the nonlinear refraction index reaches values as high as ∼10–12 cm2/W, enhanced by ∼4 orders of magnitude in comparison to that of unimplanted Nd:YAG (without Ag NPs). In addition, it has been shown that embedded Ag NPs in the Nd:YAG host reveal saturable absorption signifying the nonlinear responses. We have also observed that the nonlinear absorption coefficients depend significantly on the excitation energy and can be modulated by varying the fluence of Ag+ ions.Topics: Materials science; Mechanical properties; Nanofabrication; Nanoparticles; Spectra;

•First fabrication of depressed cladding waveguides in β-BBO crystal by femtosecond laser writing.•The lattice microstructure and physical properties in the waveguide region are well preserved.•Good waveguiding performances with low propagation losses were achieved at wavelengths of 400 and 800 nm.We report on the fabrication and characterization of β-BBO depressed cladding waveguides fabricated by femtosecond-laser writing with no significant changes in the waveguide lattice microstructure. The waveguiding properties and the propagation losses of the cladding structures are investigated, showing good transmission properties at wavelengths of 400 and 800 nm along TM polarization. The minimum propagation losses are measured to be as low as 0.19 dB/cm at wavelength of 800 nm. The well-preserved waveguide lattice microstructure and good guiding performances with low propagation losses suggest the potential applications of the cladding waveguides in β-BBO crystal as novel integrated photonic devices.

•First fabrication of waveguides in Yb:SBN laser crystal.•Fluorescence spectra as evidence of ion beam modification of crystal.•Enhanced PL and Raman intensities in waveguides.We report on the fabrication of optical planar waveguides supporting both the TE and TM confinements in Yb:SBN crystal by swift C3+ ions irradiation. A combination of the micro-photoluminescence and micro-Raman investigations have evidenced the presence of lattice distortion, damage and disordering of the SBN network along the ion irradiation path, with these effects being at the basis of the refractive index modification. The enhanced micro-photoluminescence and micro-Raman intensity in the waveguide volumes show the potential application of the obtained waveguides as active laser gain media.

•The opposite index changes for ne and no are realized in LiTaO3 by femtosecond laser writing.•The structure contains both ne and no waveguide cores separately.•Low-loss and non-distorted polarization sensitive beam splitter function has been realized.A hybrid waveguiding structure has been fabricated in a z-cut lithium tantalate (LiTaO3) crystal wafer by direct femtosecond laser writing. Due to the laser-induced anisotropic modifications of the extraordinary and ordinary refractive indices (ne and no) of LiTaO3 crystal, the structure exhibits polarization-sensitive guiding features along vertical and horizontal orientations. Based on this feature, circularly-polarized light beam can be converted to vertically-/horizontally-polarized ones (i.e., TE and TM), with approximately 1:1 power splitting ratio. The well-guided performance of the polarization-sensitive structure shows the potential for integration with existing light signals to realize all-optical information processing.

•Dual-line and cladding MgO:LiNbO3 waveguides fabricated by fs-laser writing.•Sensitivity of polarization guidance of MgO:LiNbO3 waveguides reported.•Good optical damage resistance of waveguide obtained.We report on the fabrication of the dual-line waveguides and cladding waveguide in z-cut MgO:LiNbO3 crystal by femtosecond laser inscription. Due to the diverse modification of refractive index along TE/TM polarization induced by femtosecond laser pulses, the two geometries exhibit different guiding performances: the dual-line waveguides only support extraordinary index polarization, whilst the depressed cladding waveguide supports guidance along both extraordinary and ordinary index polarizations. The measured optical damage of these waveguides at the wavelength of 532 nm is higher than that of the previously reported ion-implanted waveguides in Zr-doped LiNbO3. The propagation loss of depressed cladding waveguide is measured as low as 0.94 dB/cm at 632.8 nm wavelength. It is found that the optical damage threshold (∼105 W/cm2) of the dual-line waveguide is one order of magnitude higher than that of the cladding waveguide (∼104 W/cm2).

•First fabrication of Nd:YCOB waveguides by ion irradiation and diamond dicing.•Enhanced SHG in ridge waveguides.•Well confined guiding modes observed in waveguides.We report on the fabrication of Nd:YCa4O(BO3)3 (Nd:YCOB) ridge waveguides by combining carbon ion irradiation and precise diamond blade dicing. The guiding and spectroscopic properties of the planar as well as ridge waveguides are investigated. The second harmonic generation (SHG) at 532 nm has been realized through the waveguide structures. With nearly the same input power, the maximum average output powers are 0.56 mW and 0.62 mW, and the maximum conversion efficiencies reach 0.3%W−1 and 0.5%W−1, for planar and ridge waveguides, respectively.

•First fabrication of ridge waveguides in phosphate glass by ion irradiation and laser ablation.•Guided-wave upconversion of red and green emissions observed.•Well confined waveguide modes in visible and near infrared wavelength bands.This work reports on the fabrication of ridge waveguides in Er3+/Yb3+ co-doped phosphate glass by the combination of femtosecond laser ablation and following swift carbon ion irradiation. The guiding properties of waveguides have been investigated at 633 and 1064 nm through end face coupling arrangement. The refractive index profile on the cross section of the waveguide has been constructed. The propagation losses can be reduced considerably after annealing treatment. Under the optical pump laser at 980 nm, the upconversion emission of both green and red fluorescence has been realized through the ridge waveguide structures.

•MIR ridge waveguide was fabricated in MgO:LiNbO3 crystal.•Annealing treatments enable acceptable propagation losses of MgO:LiNbO3 waveguide.•No significant microstructural changes were induced in waveguide region.We report on the fabrication of ridge waveguide operating at mid-infrared wavelength in MgO:LiNbO3 crystal by using O5+ ion irradiation and precise diamond blade dicing. The waveguide shows good guiding properties at the wavelength of 4 μm along the TM polarization. Thermal annealing has been implemented to improve the waveguiding performances. The propagation loss of the ridge waveguide has been reduced to be 1.0 dB/cm at 4 μm after annealing at 310 °C. The micro-Raman spectra indicate that the microstructure of the MgO:LiNbO3 crystal has no significant change along the ion track after swift O5+ ion irradiation.

•Ridge waveguides were successfully fabricated in Nd:CNGG crystal.•Fluorescence properties were well preserved in the waveguide region.•Acceptable propagation losses were achieved after moderate annealing.Ridge waveguides have been produced in Nd:CNGG disorder laser crystal by using precise diamond blade dicing of carbon ion irradiated planar waveguide. The propagation loss of the ridge waveguide is measured to be ∼3.8 dB/cm at the wavelength of 632.8 nm. The micro-Raman spectrum indicates that the microstructure of the Nd:CNGG crystal has no significant change after the carbon ion irradiation. The microphotoluminescence feature has been found well preserved in the waveguide structure. The thermal stability of the waveguide has been investigated, showing relatively stable feature below 260 °C.

•New design of waveguiding microstructure containing both cladding and inner dual-line configurations in Nd:YAG crystal are firstly inscribed by femtosecond laser processing.•Fluorescence properties are well preserved in the waveguide region.•A maximum output power of 148 mW and a slope efficiency of 15.1% were achieved in the new “cladding + dual-line” waveguide laser system.•Two-dimensional guidance of laser oscillation along both TE and TM polarizations was firstly demonstrated in the dual-line waveguides in cubic crystal.We demonstrate a new design of waveguiding microstructure, which contains both cladding and inner dual-line configurations, in Nd:YAG laser crystals inscribed by femtosecond laser processing. Based on this prototype, a few “cladding + dual-line” hybrid structures with different parameters have been successfully manufactured in Nd:YAG. The waveguide lasing at 1.06 μm has been realized in the hybrid structures under 808-nm optical pump. Compared to the single dual-line waveguides, the dual-line core in hybrid configuration benefits from the large-area pump from the external cladding, reaching an enhancement of 26% on the maximum output power and of 24% on the slope efficiency of the waveguide lasing. In addition, the symmetric stress field of external cladding structure produces anisotropic stress field in the inner cores, supporting guidance along both TE and TM polarizations, thus differing significantly from the single dual-line waveguides.

•Ridge waveguides fabrication in Yb:YAG crystal by C ion irradiation and laser ablation.•Acceptable guiding properties of waveguides after thermal annealing.•Fluorescence properties of Yb3+ ions almost unchanged after ion irradiation.We report on the fabrication of optical ridge waveguides in ytterbium-doped yttrium aluminum garnet (Yb:YAG) single crystal by applying swift C5+ ion irradiation and the followed femtosecond laser ablation. The planar waveguide layer is first produced by C5+ ion irradiation and the laser ablation is used to microstructure the planar waveguide surface to construct ridge structures. The lowest propagation loss of the ridge waveguide has been determined to be ~2.1 dB/cm. From the confocal micro-fluorescence and micro-Raman spectra obtained from the waveguide regions, the intensities, positions and widths of the emission-line peaks had no obvious changes with respect to those from the bulks, which indicate that C5+ ion irradiation does not affect the bulk-related properties of the Yb:YAG crystal significantly in the waveguide regions. The results obtained in this work suggest potential applications of the Yb:YAG ridge waveguides as integrated laser sources.

•First fabrication of optical ridge waveguides Nd:SrLaGa3O7 and Nd:SrGdGa3O7 crystals by swift carbon ion irradiation and diamond blade dicing.•The guidance observed at broadband till 4 μm.•Fluorescence features preserved in the waveguide.Optical ridge waveguides have been manufactured in the crystals of Nd:SrLaGa3O7 and Nd:SrGdGa3O7 by combining techniques of swift carbon ion irradiation with precise diamond blade dicing. The guiding properties of the waveguides are investigated at broadband (at wavelength of 633 nm, 1064 nm, and 4 µm). After annealing treatment at 200 °C for 1 h, the propagation losses of ridge waveguides could be reduced to as low as 1 dB/cm. The confocal microfluorescence emission spectra confirm that the fluorescence properties of Nd3+ ions are almost unchanged after the ion irradiation processing, showing promising potentials as application of miniature light sources in integrated optics.

•Channel waveguides were fabricated in Nd:KGW crystal by femtosecond laser writing.•Fluorescence properties were well preserved in the waveguide core.•Waveguide lasing at 1.06 μm was realized with a high slope efficiency of 52.3%.We report on the buried channel waveguide laser at 1065 nm in Nd:KGW waveguides fabricated by femtosecond laser writing with dual-line approach. A relatively high scanning speed of 0.5 mm/s enables acceptable propagation loss less than 2 dB/cm. The fluorescence emission spectra of Nd3+ ions measured shows that the fluorescence properties were well preserved in the waveguide region. A stable continuous wave laser at 1065 nm has been obtained at room temperature in the buried channel waveguides by optical pumping at 808 nm. A maximum output power of 33 mW and a slope efficiency of 52.3% were achieved in the Nd:KGW waveguide laser system.

•Channel waveguides were fabricated in Nd:YAG ceramics by proton beam writing.•Channel waveguides were fabricated in Nd:YAG ceramics by helium beam writing.•Refractive index increase occurred at end of H+ and He+ ion trajectories.We report on the fabrication of channel waveguides in Nd:YAG ceramics, using either focused proton beam writing (PBW) or He beam writing (HeBW) techniques. Energies of ions used in the writing process were at 1 MeV and 2 MeV, respectively, with different writing fluence. High quality channel waveguides were produced in both H+ and He+ implanted regions. Characteristics of the waveguides were explored, and refractive index distribution of the waveguide was reconstructed.

•First Nd:YAG ceramic ridge waveguides by ion implantation and femtosecond laser ablation.•Enhancement of lasing performance in Nd:YAG ceramic ridge waveguides to planar ones.•Presentation of a new method to form ridge waveguides in laser ceramics.We report on the fabrication of ridge waveguides in Nd:YAG ceramic by using femtosecond laser micromachining of the surface of a He ion implanted planar waveguide. Under optical pump of 808 nm light, continuous wave waveguide lasers have been realized at 1.06 μm at room temperature in the Nd:YAG ceramic ridge waveguide system, reaching a maximum output power of 46 mW. The lasing threshold of ∼64.9 mW and the slope efficiency of 42.5% are obtained for the ridge waveguide system, which shows superior lasing performance to the Nd:YAG ceramic planar waveguide.

We report on the planar waveguide fabrication in SrTiO3 crystal by using carbon ion irradiation at energy of 15 MeV. The reconstructed refractive index profile of the waveguide shows the “barrier”-like distribution. The planar waveguide supports guidance along both of the TE and TM polarizations. The near-field mode distribution of the waveguide shows reasonable agreement with the simulated modal profile. After thermal annealing treatment at 260 °C in air, the propagation loss of waveguide is reduced to as low as 0.8 dB/cm.

A new fiber-like photonic crystal structure has been fabricated in Nd:YAG laser crystal by using focused ion beam milling and ion implantation techniques. This structure contains a hexagonal lattice array of submicrometric-holes and an artificial photonic defect in the middle region. The enhancement of transmitted light at wavelength of 532 nm as well as micro-photoluminescence (μ-PL) intensity of Nd3+ ions emission line (4F3/2 → 4I9/2) around 938 nm have been observed at photonic defect regime, which are both due to the photonic band gaps of the structure correlated to the defect. This work has shown that the combination of focused ion beam milling and ion implantation could be a powerful tool to develop low threshold integrated laser sources based on the photonic fiber-like structure.Highlights► We fabricated a new fiber-like photonic crystal structure in a Nd:YAG crystal. ► The photonic crystal was formed by ion implantation and focused ion beam milling. ► Enhancement of transmitted light at 532 nm was found from the defect region. ► Enhancement of luminescence intensity at photonic defect regime was observed.

We report on the fabrication of Nd:GdCOB ridge waveguides by using femtosecond laser micromachining of planar waveguides that were produced by carbon ion irradiation. The guiding properties of the Nd:GdCOB ridge waveguides are investigated. The second harmonic generation (SHG) at 532 nm green laser from ridges in a series of transverse widths is realized. The results show that the optical conversion efficiencies of SHG in the fabricated ridge waveguides are considerably enhanced with respect to the planar waveguide, and the maximum value reaches 11.4% under a pulsed 1064 nm laser pump.Highlights► First Nd:GdCOB ridge waveguides by ion irradiation and femtosecond laser ablation. ► Enhancement of second harmonics in Nd:GdCOB ridge waveguides to planar ones. ► Presentation of a new method to form ridge waveguides in nonlinear crystals.

We report on the fabrication of KTiOPO4 channel waveguides by using a new technique, i.e., focused He ion beam writing. With direct inscription of 2 MeV He+ beams of diameter of 0.5 μm, a local modified region with positive refractive index changes is created at the end of the He+ trajectory, constructing buried channel waveguides supporting both TE and TM polarizations. The calculated modal profiles of the waveguides are in good agreement with the experimental near-field intensity distributions. The propagation loss has been determined to be as low as ∼0.7 dB/cm, exhibiting good guiding properties for diverse photonic applications.Highlights► A new technique to fabricate KTiOPO4 channel waveguides was demonstrated. ► By using focused He ion beam writing, buried waveguides in KTiOPO4 were formed. ► The KTiOPO4 waveguides support both TE and TM polarizations.

Optical planar waveguides have been manufactured in Nd:GdVO4 crystal by swift N3+ ions irradiation at fluence of 1.5 × 1014 ions/cm2. A typical “barrier”-style refractive index profile was formed and the light can be well confined in the waveguide region. The modal distribution of the guided modes obtained from numerical calculation has a good agreement with the experimental modal distribution. The measured photoluminescence spectra revealed that the fluorescence properties of the Nd3+ ions have been modified to some extent in the waveguide’s volume. The propagation loss of the planar waveguide can decrease to lower than 1 dB/cm after adequate annealing.Highlights► Planar waveguide was fabricated in Nd:GdVO4 crystal by swift N3+ ions irradiation. ► A typical “barrier”-style refractive index profile was formed in the planar waveguide. ► The structural changes were investigated by exploring the luminescence properties of the Nd3+ ions.

We report on the two-wave mixing of light in photorefractive waveguides in H ion-implanted Fe-doped near-stoichiometric lithium niobate crystals. For pump light of 632.8 nm wavelength a gain coefficient as high as 15 cm−1 is found. A response time of the order of a few seconds is achieved for micro-watt input powers.Research highlights► H implantation forms optical waveguides in Fe doped stoichiometric LiNbO3 crystal. ► Two-wave mixing gain coefficient is 15 cm−1 at 632.8 nm. ► Significant shorter response time than the bulk at very low power is obtained. ► Very good photorefractive features are found in the waveguide.

As one of most efficient techniques for material-property modification, ion implantation has shown its unique ability for alteration of surface refractive index of a large number of optical materials, forming waveguide structures. The induced refractive index changes are materials related, and closely dependent on the species, energies and doses of the implanted ions. This paper reviews the results of recent research on the fabrication, investigation and applications of the ion-implanted optical waveguides in various optical materials, including crystals and non-crystalline glasses. As will be summarized, ion implantation offers a nearly universal solution for the waveguide fabrication in most existing optical materials to date, which opens up new possibilities of attractive optical applications.

We report a bi-layer optical waveguide in a Ce-doped strontium barium niobate (SBN) crystal formed by double boron ion implantation. The energies of the B+ and B3+ ions are 3 and 6 MeV, respectively, while the doses of the two boron implantation are both 2 × 1014 ions/cm2. A classic m-lines arrangement is performed to investigate the waveguide properties of the sample. The reconstructed refractive index profile of the waveguide shows a double-barrier confined shape, which suggests the formation of a bi-layer waveguide structure. The thickness of the two guide layers are 2.6 and 4.6 μm, respectively, which is in a good agreement with the mean projective range of the implanted boron ions of 3 and 6 MeV in the crystal. The nuclear energy loss distribution of the boron ions into SBN has a similar shape to that of the waveguide index, which means an inherent relationship between the waveguide formation and the energy deposition of the energetic ions.

We report a planar optical waveguide in AgGaS2 crystal that is fabricated by 3.0 MeV Si+ ion implantation at a dose of 1 × 1015 ions/cm2 at room temperature. A classic m-lines arrangement is performed to investigate the waveguide properties by measuring the dark modes. The reconstructed refractive index profiles show, at the near surface region, that the extraordinary index increases while the ordinary index decreases. Two TE modes and one TM mode are considered to be real propagation modes from a waveguide mode analysis. The nuclear energy loss distribution of the Si ions into AgGaS2 shows an inherent relationship between the waveguide formation and the energy deposition of the energetic ions.

The Si+ ion-implanted KTiOPO4 optical waveguide was reported. The 3.0 MeV Si+ ions at a dose of 2×1015 ions/cm2 were implanted into the KTiOPO4 crystal at room temperature to form a waveguide structure with a thickness of more than 2 μm. The prism-coupling method was carried out to measure the effective refractive indices of the waveguide dark modes. The refractive index profile was reconstructed using reflectivity calculation method (RCM). The TRIM 98 (transport of ions in matter) code was applied for simulating the process of the ion implantation, which was believed to be helpful to a better understanding of the waveguide formation.

A planar waveguide was formed in Er:NaY(WO4)2 crystal by the implantation of 3.0 MeV Si+ ions at a dose of 2×1015 ions/cm2 at room temperature. The prism-coupling method was used to observe the dark modes of the waveguide. Both TE- and TM-polarized light were carried out to measure the effective refractive index of the modes. Refractivity calculation method (RCM) was used to reconstruct the profiles of the refractive index in the waveguide. Moreover, the lattice disorder and the ionization damage of 3.0 MeV Si in Er:NaY(WO4)2 were obtained by using the simulation of TRIM'98 (transport of ions in matter) code.

The first MeV copper and nickel ion implanted waveguides in Nd:NaY(WO4)2 are reported. The planar optical waveguides were formed in this crystal by 2.8 MeV Cu+ or Ni+ ion implantation with doses of 1–3×1015 ions/cm2 at room temperature, respectively. The dark modes were observed by prism coupling method. The reflectivity calculation method was used to analyze the refractive index profiles of the waveguides.

We report on the optical planar waveguides in Nd:YLiF4 laser crystals fabricated by 6.0 MeV C3+ ion implantation at doses of 1 × 1015 or 2.5 × 1015 ions/cm2, respectively. The refractive index profiles, which are reconstructed according to the measured dark mode spectroscopy, show that the ordinary index had a positive change in the surface region, forming non-leaky waveguide structures. The extraordinary index is with a typical barrier-shaped distribution, which may be mainly due to the nuclear energy deposition of the incident ions into the substrate. In order to investigate the thermal stability of the waveguides, the samples are annealed at temperature of 200–300 °C in air. The results show that waveguide produced by higher-dose carbon implantation remains relatively stable with post-irradiation annealing treatment at 200 °C in air.

•Silver nanoparticles are fabricated in KTP crystal by ion implantation.•Shape, size, and spatial distribution of the silver nanoparticles are measured and analyzed.•SPR absorption band at ∼459 nm induced by silver nanoparticles is revealed by experiments and theoretical calculation.Embedded silver nanoparticles were produced in KTP single crystal by 200 keV Ag+ ion implantation at fluence of 5 × 1016 ions/cm2. The shape, size and spatial distribution of the formed silver nanoparticles were revealed by transmission electronic microscope (TEM) images. The embedded sphere silver nanoparticles with average diameters of 9.9 nm were along the projected range of Ag+ ions, located at the depth between 35 nm and 80 nm from the surface. Both the measured and calculated absorption spectra of silver nanoparticles dispersed in KTP crystal indicate that the surface plasmon resonance (SPR) absorption peak was at ∼459 nm.

•Superficial waveguides were fabricated in Nd: CaF2 by femtosecond laser writing.•The near fundamental modal distributions and low propagation loss.•The low sensitivity on polarization and well-preserved photoluminescence.We report on the superficial cladding waveguides fabricated by direct femtosecond laser writing in Nd: CaF2 crystal with three different groups of parameters. The lowest propagation loss of waveguides has been determined to be 0.7 dB/cm at wavelength of 632.8 nm along TE polarization. The near fundamental modal distributions have been imaged through the end-face coupling technique. The guidance of the waveguides is found to possess low sensitivity on polarization of the probe light. By using a confocal microscope system, the micro-photoluminescence mappings and micro-fluorescence spectra are also obtained, which indicates the photoluminescence features of the Nd3+ ions are well preserved in the waveguide cores after direct femtosecond laser writing.

We report on the green up-conversion emission from Er3+, MgO codoped nearly stoichiometric LiNbO3 planar waveguides fabricated by the swift C5+ ion irradiation. The characteristics of the waveguides are investigated by using a continuous wave laser at wavelength of 800 nm. The reconstructed distribution of the refractive index of the C ion irradiated waveguides shows a “barrier and well” profile. The up-conversion emissions of green light, corresponding to the transition of 4H11/2→4I15/24H11/2→4I15/2 and 4S3/2→4I15/24S3/2→4I15/2 bands, have been observed in the waveguides with intensity of 40% of that from the bulk.