• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
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    • 专利摘要:
    Multi-line solid state nanolaser system having a non-linear optical gain means from an optically active ferroelectric crystal, having walls of ferroelectric domains 14 and metal nanoparticle chains 12 on the surface of the domain walls. An optical resonator 20 where the non-linear optical gain means is inserted and a laser pumping device 40 emits a pumping laser light passing through a microscope objective 50 to focus the pumping laser light on the metallic nanoparticles and to generate radiation coherent multi-line with three frequencies, a laser frequency, a self-bending frequency and an autosum frequency, said multi-line radiation being confined at the nanoscale. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2665885A1
    申请号:ES201631391
    申请日:2016-10-31
    公开日:2018-04-30
    发明作者:Pablo MOLINA DE PABLO；Luisa Eugenia BAUSA LOPEZ；María DE LA O RAMÍREZ；David HERNÁNDEZ PINILLA；Carmen DE LAS HERAS MOLINOS
    申请人:Universidad Autonoma de Madrid；
    IPC主号:
    专利说明:

    5 Technical field of the invention
    The present invention belongs to the field of Photonics and relates to solid-state nanolysis devices with multi-line operation. That is, it refers to plasmonic lasers that operate on the submicron scale and also emit simultaneously at different wavelengths.
    State of the Art
    Due to their properties, lasers are the object of great interest. Their degree of coherence, spectral density of energy and possibility of manipulation and control, make them essential systems in many applications. In particular, the lasers of
    15 solid state are, for their physical and chemical stability and for their compactness, a fundamental tool in a wide variety of areas that include research, medicine, industry or telecommunications.
    Today there are a variety of solid-state laser devices that operate in extreme configurations and that include high power lasers for fusion.
    20 nuclear, ultra-fast lasers with pulses of the order of the femto-seconds for the study of chemical reactions and atomic processes or compact lasers with micrometric sizes that allow their large-scale integration into optical communications networks.
    However, new challenges in nanoscience and nanotechnology require sources of
    25 coherent light of even smaller sizes, in the submicron range. The limitations suffered by conventional optical systems in these very small size scales, mainly due to the phenomenon of optical diffraction, make the development of nano-lasers require physical concepts and designs radically different from those used in conventional volumetric lasers to produce
    30 confinement of laser oscillation modes in sizes smaller than the wavelength of the generated light itself.
    Similarly, there are lasers with multi-line operation that allow to increase the compactness of the system and are of great technological interest. Especially when


    They provide coherent radiation in the regions of the spectral range corresponding to green and blue visible light.
    However, currently, to achieve multiline lasers, it is based on conventional technological devices that attach additional non-linear crystals to the laser crystal that perform the frequency conversion function. As a result, they are bulky and not suitable for integration into nanometric devices.
    Another alternative way to achieve multi-line operation consists in the use of laser devices of solid-state frequency self-doubling lasers that simultaneously supply several emission wavelengths. However, 10 have been obtained only in volumetric configurations. Therefore, the laser oscillation modes do not have spatial confinement in dimensions smaller than the emission wavelength. In addition, the efficiency of frequency conversion phenomena is directly related to the size (length) of the device, so they are not valid in integrated devices with sizes
    15 nanometers
    Brief Description of the Invention
    In the light of the limitations identified in the state of the art, it would be desirable to have a laser system with submicron spatial confinement that generates radiation simultaneously at various wavelengths in a stable manner and preferably at room temperature.
    The object of the invention is a nano laser system with plasmonic effect and multiline operation that presents applications in diverse fields such as bio-detection with ultra-extreme sensitivity, the possibility of being incorporated into optical circuits.
    25 ultra-compact or multicolored display design in which images with extreme resolution would be achieved.
    In general, the multiline solid state nanollaser system according to the invention uses a non-linear optical gain means to generate laser radiation with ωL frequency which further includes: an optically active ferroelectric crystal with a plurality of 30 ferroelectric domain walls and a plurality of chains of metal nanoparticles arranged on the surface of domain walls. It also incorporates an optical resonator to engage the non-linear optical gain medium and confine the generated radiation. The system also incorporates a pumping laser device to emit a pumping laser light with a ωp frequency and a target of


    microscope coupled to the pumping laser device to focus the pumping laser light on the metal nanoparticles and to simultaneously generate multiline coherent radiation with three frequencies, a laser frequency ωL, a self-bending frequency ωSFD = 2ωL and a self-consumption frequency ωSFS = ωL + ωp, being said
    5 multiline radiation confined in the vicinity of metal nanoparticles. By immediate means the nanometric space region adjacent to the surface of the nanoparticles. Generally, in the order of several tens of nanometers around the nanoparticle.
    Preferably, the optical resonator includes several facing mirrors, where at least one of the mirrors partially allows the passage of the laser light.
    Preferably, the pumping laser device can emit a pumping light with a tunable pumping frequency ωp.
    Preferably, the pumping laser device can emit polarized pumping laser light with respect to the direction of the nanoparticle chains.
    Preferably, the polarization of the pumping laser light is parallel to the nanoparticle chains.
    Preferably, the optically active ferroelectric crystal can be one of the following: LiNbO3, LiTaO3, SrxBa1-x (NbO3) 2, RbTiOPO4, Ba2NaNbO15; and where the optically active ion can be one of the following: Nd3 +, Er3 +, Yb3 +, Tm3 +, Pr3 +.
    20 Preferably, the metal nanoparticle chains are silver.
    Preferably, the non-linear optical gain medium when it comprises LiNbO3 as an optically active ferroelectric crystal also contains MgO to avoid photorefractive damage.
    Preferably, the non-linear optical gain medium is LiNbO3: MgO: Nd3 + PPLN 25 with Y-cut.
    Preferably, the non-linear optical gain medium is obtained through the modified Czochralski crystal growth technique.
    Brief description of the figures
    30 FIG. 1: Example of an embodiment of the multi-line nanollaser system.


    FIG. 2A: PPLN crystal: MgO: Nd3 + in Y section with Ag metal nano-particles forming chains on the domain wall surfaces parallel to the crystallographic axis c.
    FIG. 2B: Scheme of the configuration used in the experiments of laser gain 5 and autoconversion of frequencies in the nanoscale.
    FIG. 3: Absorption spectra of the Nd3 + ion in the LiNbO3 crystal: MgO in the spectral region corresponding to the pumping used.
    FIG. 4A: Stimulated emission spectrum corresponding to the 4F3 / 2 transition
    → 4I11 / 2 of the Nd3 + ions in the immediate vicinity of the 10 Ag nanoparticle chains.
    FIG. 4B: Spontaneous emission spectra corresponding to the 4F3 / 2 transition
    → 4I11 / 2 of the Nd3 + ion in LiNO3: MgO in the π and σ configurations.
    FIG. 4C: Spatial distribution of the laser radiation obtained by integrating the stimulated emission signal on the surface of the crystal.
    15 FIG. 5: Laser gain curves of the PPLN crystal: MgO: Nd3 + in the vicinity of the Ag nanoparticle chains for different types of polarization of the pumping laser light beam: parallel (circles) and perpendicular (square) to the chains, and also for volume configuration devoid of metal nanostructures (triangles). The threshold power values are compared with those obtained in Z cut.
    20 FIG. 6A: Scheme of multiline laser operation in the nanoscale at different frequencies by means of stimulated emission ωL, self-doubling in frequency ωSFD and autosum ωSFS of the laser emission with the frequency of the pumping laser light (ωp).
    FIG. 6B: Spectral distribution of the generated radiation.
    FIG. 7A: Signal spectrum obtained by self-doubling SFD. The inner box
    25 shows the spatial distribution of intensity in the vicinity of Ag nanoparticles.
    FIG. 7B: Evolution of the radiation obtained in the immediate vicinity of the metal nanoparticles by means of an SFD self-doubling process depending on the pumping power for parallel polarization (circles) and perpendicular (square) to the
    30 chains of metal nanoparticles.
    FIG. 8: Tunability of the autosuma SFS signal obtained in the nanoscale. The wavelength of the laser light beam has been varied between 813 and 819 nm.


    Detailed description of the invention
    A solid-state nano-laser system with multi-line emission is described. This system uses a non-linear gain medium based on a solid-state ferroelectric laser crystal 5, assisted by surface localized plasmons, which presents laser operation in the nanoscale and which, in addition, through non-linear frequency conversion processes is capable of generating, simultaneously and in the same active medium, coherent radiation in emission wavelengths additional to that obtained by stimulated emission. Since the laser action generation has
    In the nanoscale, the radiation obtained through the frequency conversion processes is also generated in the nanoscale.
    The ferroelectric crystal has a structure of alternating polarity ferroelectric domains that are preferably periodically arranged (PPLN) in this embodiment. In general, the PPLN periodicity is not essential since, unlike volumetric systems where the generation of non-linear processes is a collective effect that involves periodic ordering of domains of alternating polarity, in the present invention frequency conversion occurs on the domain walls individually and with the presence of a single domain wall would suffice. The fact of having many walls is an added advantage 20 since it allows to obtain “displays” of hundreds / thousands of nanolores with multiline emission. The nanoscale frequency conversion processes are carried out at the interface between said domains, that is, on the domain wall. More specifically, the breaking of symmetry in the domain walls is exploited to provide nanometric centers for generating non-linear processes. The
    25 walls also favor the deposition of metal nanoparticles as an additional effect. In this way, since the ferroelectric crystal has a high number of domain walls, it is possible to have hundreds or thousands of nanoláseres that emit in several frequencies on the same crystal.
    The system described here generally employs the following elements: a medium
    30 non-linear gain based on an active material formed by a ferroelectric laser crystal that has domain walls in association with plasmonic structures; an optical resonator; an optical pumping mechanism focused on the immediate vicinity of metal nanoparticles. By immediate means the nanometric space region adjacent to the surface of the nanoparticles.


    Generally, in the order of several tens of nanometers around the nanoparticle.
    The possibility of obtaining laser at room temperature is an intrinsic characteristic of the gain medium used. It is known that efficiency decreases with increasing
    5 temperature However, in the case of solid-state lasers based on Nd3 +, Er3 +, Yb3 + ... etc, although the gain decreases somewhat with increasing temperature, the gain values at room temperature still allow laser action to be generated.
    Active material:
    The non-linear gain medium is a solid state laser crystal. The laser transition
    10 takes place between the energy levels of luminescent ions incorporated as optically active impurities in a crystalline, vitreous or ceramic matrix. In the present case, the active material is constituted by a periodically polarized ferroelectric crystal in which optically active ions are housed. The non-linearity of the crystal allows frequency conversion processes to be carried out. From
    In this way, the laser radiation itself produced by the ions in the near IR region is self-converted by the matrix crystal to simultaneously generate radiation of multiple wavelengths in the visible spectral region. In particular, frequency conversion processes are carried out at domain boundaries.
    20 Plasmonic Nanostructures:
    The nanoscale confinement of laser oscillation modes and non-linear processes is made possible by the association of the active medium with arrangements of nanometric metal structures that have been deposited on the surface of the non-linear gain medium. These nanostructures are capable of acting as 25 optical nano-antennas confining and intensifying electromagnetic radiation in space regions below the diffraction limit. The plasmonic optical nano-antennas are arranged on the surface of the domain walls in arrangements that give rise to localized surface plasmonic resonances, with wide spectral response. In this way, the generation and intensification in the
    30 nanoscale of the different types of radiation involved in the process of laser action and frequency conversion: pumping radiation (at ωp frequency), laser radiation (at ωL frequency and radiation by frequency self-conversion (at 2ωLy frequency ωp + ωL).
    Optical resonator:


    Additionally, the laser gain medium with the nanometric metal structures attached to its surface is introduced into an optical resonator that provides the necessary feedback to obtain stimulated emission under low threshold conditions. The mirrors that form the resonator cavity should
    5 be highly transparent to the pump wavelength and highly reflective to the laser wavelength.
    Optical pumping:
    The pumping is supplied to the system in confocal configuration focusing the radiation of a laser on the arrangements of plasmonic nanostructures adjacent to the medium
    10 active laser. The radiation from the pump laser is tuned to the wavelengths corresponding to the absorption bands of the active ions incorporated in the non-linear gain medium. Since very high peak powers are not required, the system has the advantage of being pumped with both pulsed light beams and continuous wave light sources.
    The threshold conditions for the generation of laser action in a nanometric space region are obtained by the simultaneous excitation of the active ions and the resonances of localized plasmons associated with the arrangements of the metal nanostructures. This enables a drastic reduction of the pumping threshold power for laser action in the regions adjacent to the
    20 plasmonic nanostructures, obtaining laser action confined in a nanometric region.
    Once the nanoscale laser action is obtained, the generation of radiation at multiple frequencies by the gain medium itself includes the participation of different non-linear wave mixing processes. Since the laser radiation involved in the frequency mixing processes is spatially confined in the vicinity of the metal nanostructures, the radiation obtained by the wave mixing is also generated in the nanoscale. On the one hand, by means of the nonlinear process known as frequency auto-bending or SFD (of the acronym in English selffrequency doubling) it is possible to obtain photons of frequency ωSFD = 2ωL from the coherent combination of two laser photons of frequency ωL . Likewise, by means of the non-linear process known as auto-sum of frequencies or SFS (of the acronym in English self-frequency sum) it is possible to obtain photons with frequency ωSFS = ωL + ωpa from the combination of laser photons (ωL) and photons of pumping radiation (ωp). Both nonlinear processes can be obtained simultaneously in the same


    gain medium providing coherent radiation at wavelengthsadditional to that obtained by stimulated emission. In this sense, the possibilityof obtaining laser radiation at ωL frequency by tuning the pumping frequency ωp inthe different absorption bands of the active laser ion (located in the visible region5 and near infrared) allows to obtain tunable radiation by means ofprocess of auto-sum of frequencies in different ranges of wavelengths. Sofor example, the generation of coherent radiation through processes of sum offrequencies using a pump in the near infrared region would providevisible radiation tunable in the blue, green and red regions of the spectrum
    10 electromagnetic.
    To complete the present detailed description, reference is made to the figures to illustrate a specific example of embodiment from the laser emission of the Nd3 + ion but it is extensible to any other optically active ion (Er3 +, Yb3 +, Tm3 +, Pr3 + ...) thus like any other ferroelectric crystal (LiTaO3, SrxBa1-x (NbO3) 2, RbTiOPO4,
    15 Ba2NaNbO15,, ...). Therefore, this example is to be understood without limitation of the scope of the invention.
    In FIG. 1 a simplified scheme of a solid-state nanollaser system according to the present invention is shown. Said scheme shows a pumping laser device 40 that emits laser radiation of frecuenciap frequency, and which focuses on a non-linear optical optical gain medium. Preferably said pumping laser 40 is tunable. The non-linear gain medium, which generates laser radiation with ωL frequency, is based on an optically active ferroelectric crystal 10 having walls of ferroelectric domains 14 and on the surface of these walls of ferroelectric domains 14, chains 12 of 25 nanoparticles are arranged metal where laser radiation is confined, at nanometric scales. The figure also shows a dichroic mirror 26 and an optical resonator 20 that includes a pair of plane-parallel mirrors 22, 24 between which said gain means is arranged. The figure also shows a microscope objective 50 coupled to the pumping laser device 40 to focus the pumping laser light on the 30 regions where the metal nanoparticles are located. By influencing the nanoparticles, laser radiation and non-linear frequency conversion processes confined to the nanoscale are generated simultaneously, so that it is possible to obtain coherent multiline radiation, with three different frequencies: a laser frequency ωL, a frequency auto-bending (SFD) ωSFD = 2ωL and, a frequency
    35 of autosuma (SFS) ωSFS = ωp + ωL


    In FIG. 2A shows a simplified scheme of the gain medium 10. The laser gain experiments were carried out with spatial resolution using confocal microscopy.
    In FIG. 2B shows a scheme of the device used. The means of profit
    5 10 depicted in FIG. 1 was placed between the plane-parallel mirrors 22 and 24 of a resonator 20 (of the Fabry-Perot type). These mirrors 22, 24 have high reflectance (R> 99%) at the generated laser wavelength (ωSFS, ωSFD, ωL) and high transmittance (T> 95%) for the wavelength of the pumping laser light (ωp ).
    The non-linear gain medium used in this example is a crystal of
    10 LiNbO3: MgO: Nd3 + periodically polarized on which linear chains of Ag nanoparticles parallel to the optical axis c of the cut glass Y were deposited as shown in FIG. 1 and FIGs. 2A, 2B. Optically active Nd3 + ions are responsible for the laser emission that takes place in the near-infrared optical region (~ 1µm) through the 4F3 / 2 (R1) → 4I11 / 2 (Y2) transition. The 12 chains of
    15 Ag nanoparticles make it possible to obtain laser action in a nanometric spatial region thanks to the confinement of optical pumping and laser gain in the immediate vicinity of plasmonic structures. Finally, the presence of ferroelectric domain walls, which act as nanometric centers for the generation of non-linear processes allows the generation of frequency conversion phenomena.
    20 in the visible region of the electromagnetic spectrum by means of the participation of the laser radiation generated in the nanoscale as fundamental radiation.
    Additionally, MgO is incorporated into the gain medium. This mitigates the characteristic photorefractive effect of LiNbO3, which can negatively affect the laser action in this crystal. The crystal of LiNbO3: MgO: Nd3 +
    25 periodically polarized, it has a structure of alternate ferroelectric domains, and is used as a functional template for the formation of metal nanoparticles forming linear chains on the surface of the walls of the mentioned ferroelectric domains of the gain medium.
    The set formed by the gain medium, (the non-linear active material with the
    30 metal nanostructures) and the resonator is mounted on the motorized platform of a confocal microscope with a spatial resolution of 0.2 µm. The laser emission, as well as the radiation corresponding to the different nonlinear wave mixing processes that are generated simultaneously, can be detected with conventional optical radiation meters.


    Mention that the effects of plasmonic nanostructures deposited on the polar surface of the LiNbO3 crystal: Nd3 + to date have always been with Z cut. According to the crystalline structure of LiNbO3 and with the electric dipole character of the Nd3 + ion transitions in this glass, with Z cut it is only possible to access the
    5 emission lines of the Nd3 + ion with σ character (electric field of the emitted light perpendicular to the c axis of the crystal).
    In contrast, the present embodiment synergistically combines the Y-section of LiNbO3 and the deposition of linear chains of metal nanoparticles parallel to the crystallographic axis c. Thanks to this, the transitions with character π of the ion are accessed
    10 Nd3 + (electric field of the emitted light parallel to the c axis of the crystal) at the same time as the optimal symmetry conditions for plasmonic intensification of transitions with that character.
    In particular, the transition 4F3 / 2 (R1) → 4I11 / 2 (Y2) allowed in π configuration is accessed. The effective gain section of said transition is 5 times higher than the corresponding transitions σ. More specifically, by having metal chains aligned parallel to the c axis, the excitation of the longitudinal mode associated with the chain, which is characterized by a broad spectral response and a strongly radiative character, spectrally overlaps with the photoluminescent transition of greater gain of the system favoring the selective and efficient intensification of the
    20 laser transition.
    It stands out, as an extraordinary result, the generation of stimulated emission in the vicinity of the chains of nanoparticles deposited on the surface Y with threshold pumping power for oscillation 5 times lower than those previously obtained in Z cut while presenting an improvement substantial operation
    25 lasers
    The reduction of the threshold favors the non-linear mechanisms of self-conversion of frequencies on the surface of the domain walls and enables the simultaneous generation of coherent radiation at different frequencies by means of different wave mixing processes (self-bending and autosumming).
    30 To obtain the optically active ferroelectric glass sheet 10 of LiNbO: MgO: Nd3 + that has an alternate ferroelectric domain structure (PPLN: MgO: Nd3 +), a growth of the PPLN: MgO: Nd3 + crystal was carried out using the modified Czochralski technique . The approximate dimensions of the sheet used were 5 x 5 x 0.8 mm, its surface perpendicular to the crystallographic axis and


    (cut Y). Silver nanoparticles were grown on the surfaces of the domain domains 14 by a previously described photochemical process. The nanoparticles were arranged forming chains 12 of several millimeters in length (2-5 mm) oriented parallel to the optical axis of the LiNbO3 crystal (c axis). Due to the
    5 periodic arrangement of the domain structures the chains 12 are distributed periodically on the surface Y of the crystal. The average size of the chain nanoparticles was 50 nm and the separation between nanoparticles was less than 5 nm.
    It was optically pumped using a laser beam directed along the Y axis.
    10 focused on the surface of the sample with a microscope objective. The assembly formed by the gain means 10 and the resonator mirrors 22, 24 was mounted on the motorized platform of a confocal microscope with a spatial resolution of 0.2 µm (not shown in the figures).
    The spectra of the laser emission, as well as those corresponding to the different
    15 non-linear processes of mixing waves generated simultaneously were collected with the same microscope objective (back-scattering geometry) for spectral and spatial analysis. Continuous pumping radiation of wavelengths in the range 813-819 nm was used.
    FIG. 3 shows the absorption spectra of the Nd3 + ion in the LiNbO3 crystal: MgO in
    20 the spectral region corresponding to the pumping used. The absorption bands shown are associated with the optical transition 4I9 / 2 → 4F5 / 2 + 2H9 / 2 of the Nd3 + ions in the π configurations (polarization of the pump beam parallel to the c axis) and σ (polarization perpendicular to the c axis) . The spectral variations between both configurations are explained taking into account the selection rules associated with
    25 the electrical dipole character of the transitions between the Stark levels and the C3 symmetry of the crystalline environment in which the Nd3 + ions are located.
    FIG. 4A shows the laser emission spectrum obtained when the system is pumped in the transition 4I9 / 2 → 4F5 / 2 + 2H9 / 2 in the vicinity of Ag nanoparticle chains with powers greater than the laser oscillation threshold (∼20 mW
    30 incident power). The laser oscillation occurs at the wavelength of 1085 nm and has a spectral width of 0.6 cm-1, at the resolution limit of the experimental system.
    In FIG. 4B the spontaneous emission spectra of the Nd3 + ion are represented for the σ / π configuration in which the electric field of the emitted light is


    perpendicular / parallel to the crystallographic axis c (z axis) of LiNbO3. As can be seen, the laser oscillation coincides with the highest gain emission line corresponding to the 4F3 / 2 (R1) → 4I11 / 2 (Y2) crystalline field transition of the Nd3 + ion in π configuration. Accordingly, the laser emission oscillates parallel to the c axis and, therefore,
    5 in parallel to the alignments of Ag's metal nanoparticles. Thisconfiguration allows the participation of longitudinal plasmonic modesassociated with the Ag 12 nanoparticle chains that show, unlikethe transverse modes, a fundamentally radiative character.
    In FIG. 4C shows the spatial distribution of the laser intensity obtained
    10 integrating the stimulated emission signal on the surface of the crystal. As can be seen, the laser emission is highly localized in the vicinity of the metal nanostructures, confirming its nanoscopic character and the threshold reduction that the regions near the Ag nanoparticle chains have with respect to the rest of the areas of the PPLN: MgO crystal : Nd3 + devoid of chains
    August 15
    In FIG. 5 the laser gain curves obtained in the vicinity of the Ag metal nanoparticles for two different polarizations of the pumping beam are shown. Parallel polarization is shown with circles. Perpendicular polarization is shown with squares. Additionally, the curve of
    20 laser gain for the volume configuration obtained for the same glass devoid of metal nanostructures. The threshold power values obtained when the nanollaser operates in Z cut are also shown to illustrate the threshold reduction obtained with this proposal.
    In order to be able to make an adequate comparison, for these experiments
    25 a pumping wavelength (λp = 811 nm) was selected for which the absorption corresponding to the transition 4I9 / 2 → 4F5 / 2 + 2H9 / 2 of the Nd3 + presents identical values in the polarizations parallel and perpendicular to the c axis ( see FIG. 3). In both cases, regardless of the polarization of the pumping, the laser radiation emitted by the system always presents polarization parallel to the c axis of the LiNbO3 and to the chains of
    30 metal nanoparticles.
    In the first place, it is worth highlighting that the values of the pumping threshold powers necessary to obtain laser oscillation are, in both cases, about 5 times lower than those obtained for the Nd3 + nanolaser that used a LiNbO3 crystal in Z cut and in that the laser oscillation generated was perpendicular to the optical axis c 35 (σ configuration). This substantial threshold improvement is due to the larger section


    Efficient stimulated emission presented by the laser transition 4F3 / 2 (R1) → 4I11 / 2 (Y2) in configuration π with respect to configuration σ (see FIG. 4B).
    On the other hand, as seen in FIG. 5, depending on the polarization state of the pumping radiation, different values of the 5 threshold power for laser oscillation are obtained. The highest threshold is obtained when the polarization of the pumping beam is perpendicular to the Ag nanoparticle chains, while for the parallel polarization the pumping threshold is reduced by approximately a factor 2. This fact can be explained taking into account the response plasmonic of Ag nanoparticle chains, whose radiative modes and whose distribution of
    10 near field are highly dependent on polarization.
    The efficiency of the laser emission, obtained through the slopes of the laser curves, is very similar for both configurations because, in both cases, the laser photons are generated in the same plasmonic mode, the first to overcome the losses and therefore able to generate profit.
    15 Finally, FIG. 5 also shows the laser gain curve for the volumetric configuration, obtained for the PPLN crystal: MgO: Nd3 + devoid of plasmonic structures. As can be seen, in this case the laser gain is clearly lower (factor 7) to the case in which the laser emission is confined in the neighborhoods of the plasmonic structures.
    20 In addition to the laser emission at ωL frequency, it is possible to simultaneously obtain wave mixing processes thanks to the presence of the domain walls (on which the nanoparticle chains are deposited) that act as centers for generating non-linear quadratic processes . In particular, the process of auto-bending (SFD) of the laser emission by which it is generated is observed
    25 radiation of frequency ωSFD = 2ωL and the self-addition process (SFS) at frequency ωSFS = ωL + ωp in which the pumping radiation (frequency ωp) is mixed with the laser radiation. Likewise, and since the laser radiation involved in the frequency mixing processes is spatially confined in the vicinity of the metal nanoparticle chains, the radiation obtained by the mixture of
    30 waves are also generated in the nanoscale, on the surfaces of the domain walls, being intensified by the chains of metal nanoparticles deposited on them.
    FIG. 6A shows a scheme of the operation of the system as a frequency autoconverter nanollaser in which the processes of light generation at frequencies


    ωL, ωSFS and ωSFD are obtained simultaneously. The frequency distribution of the generated radiations is shown in FIG. 6B. Under the particular conditions of the operating example presented, the radiation generated by the SFD and SFS frequency self-conversion processes are obtained in the region
    5 spectral of green and blue, respectively.
    In FIG. 7A shows in more detail the spectrum of the doubling signal that takes place at the wavelength of 542.5 nm, as corresponds to the second harmonic generation of the laser emission at 1085 nm. The inside box shows the spatial distribution of the radiation generated by SFD that occurs in the walls
    10 of the ferroelectric domains in the vicinity of a chain of Ag nanoparticles 12.
    FIG. 7B shows the intensity of the radiation obtained by means of the SFD process as a function of the incident power for two polarizations of the pumping beam, parallel (circles) or perpendicular (square) to the nanoparticle chains 12. In
    In both cases the intensity generated varies with the pumping power according to the quadratic character corresponding to the non-linear process of frequency bending. The difference in thresholds for detection of SFD radiation is related to the different threshold powers for laser oscillation obtained for each polarization of the pumping beam.
    On the other hand, the polarization of the radiation emitted by auto-bending of the laser radiation is parallel to the metal nanoparticle chains, regardless of the polarization of the pumping beam. This polarization is consistent with the polarization state of the laser radiation.
    Tuning:
    25 The autosum frequency process (FSS) presents an added interest since it provides radiation in the spectral region of the blue with the possibility of tuning from the coherent sum between pumping photons and laser photons.
    Additionally, since the laser action based on Nd3 + can be obtained by optical pumping at different wavelengths, the radiation generated by said
    30 process is tunable. In principle, the selection of the pumping wavelength can be carried out effectively by sweeping the spectral width of different absorption bands of the Nd3 +, which take place from the visible to the near infrared and that overlap with the plasmonic resonances associated with The metal structures.


    For example, when laser oscillation occurs at the wavelength of 1085 nm (transition 4F3 / 2 → 4I11 / 2 of the Nd3 + ion in LiNbO3), optical pumping in transitions 4I9 / 2
    → 4F7 / 2 + 4S3 / 2, and 4I9 / 2 → 4F5 / 2 + 2H9 / 2, centered around 750 and 810 nm would allow to have tunable visible radiation around 440 and 465 nm, respectively.
    5 Moreover, since the Nd3 + ion additionally exhibits laser action at 1.3 µm, from the 4F3 / 2 → 4I13 / 2 transition, the generation of coherent radiation by means of summation processes would provide tunable radiation in the blue regions and green of the electromagnetic spectrum that has never been generated in the nanoscale.
    In FIG. 8 presents an example of tuning of the SFS radiation obtained
    10 using the transition 4I9 / 2 → 4F5 / 2 + 2H9 / 2 (shown in FIG. 3) for pumping. Due to the spectral width of the corresponding absorption band in configuration π, the laser action is obtained for pumping wavelengths in the range 813-819 nm.
    Consequently, it is possible to obtain tunable radiation at different lengths of
    15 wave in the blue region of the electromagnetic spectrum (in the range 464-467 nm) by mixing pumping and laser radiation. As already mentioned, this radiation is generated in the nanoscale as a result of the joint presence of the domain walls and the confinement of the laser radiation in the immediate vicinity of the plasmonic structures. As in the case of radiation
    20 obtained by bending, the intensity of the radiation obtained by SFS varies non-linearly with the pumping power and its polarization is parallel to the Ag nanoparticle chains.

    权利要求:
    Claims (8)
    [1]
    1. Multiline solid-state nanolaser system comprising:
    - a non-linear optical gain medium configured to generate laser radiation with 5 laser frequency, ωL, comprising:
    - an optically active ferroelectric crystal (10) comprising:
    - a plurality of walls of ferroelectric domains (14), and
    - a plurality of chains (12) of metal nanoparticles disposed on the surface of the domain walls;
    10 - an optical resonator (20) configured to engage the non-linear optical gain medium and confine the generated radiation;
    - a pumping laser device (40) configured to emit a pumping laser light with a pumping frequency, ωp;
    - a microscope objective (50) coupled to the pumping laser device (40) to
    15 focus the pumping laser light on the metal nanoparticles and to generate, coherent multiline radiation with three frequencies, a laser frequency ωL, a self-bending frequency, ωSFD = 2ωL, and a self-consumption frequency, ωSFS = ωL + ωp, being said multiline radiation confined in the vicinity of the metal nanoparticles.
    [2]
    2. Solid-state nanolaser system according to claim 1, wherein the optical resonator (20) comprises facing mirrors (22,24), with at least one of the mirrors (22) configured to partially allow the passage of the laser light.
    The solid-state nanollaser system according to claim 1 or 2, wherein the pumping laser device (40) is configured to emit a pumping light with a pumping frequency, ωp, tunable.
    [4]
    4. Solid-state nano-laser system according to any one of the preceding claims 30, wherein the pumping laser device (40) is configured to emit light

    Polarized pumping laser with respect to the direction of the chains (12) of nanoparticles.
    [5]
    5. Solid-state nanolaser system according to claim 4, wherein the polarization 5 of the pumping laser light is parallel to the chains of nanoparticles (12).
    [6]
    6. Solid-state nano-laser system according to any one of the preceding claims, wherein the optically active ferroelectric crystal (10) is selected from one of the following: LiNbO3, LiTaO3, SrxBa1-x (NbO3) 2, RbTiOPO4, Ba2NaNbO15; and where the
    10 optically active ion is chosen from one of the following: Nd3 +, Er3 +, Yb3 +, Tm3 +, Pr3 +.
    [7]
    7. Solid-state nano-laser system according to any one of the preceding claims, wherein the chains (12) of metal nanoparticles are Ag.
    Solid-state nanollaser system according to claims 6 and 7, wherein the non-linear optical gain medium when comprising LiNbO3 as an optically active ferroelectric crystal also contains MgO.
    [9]
    9. Solid-state nano-laser system according to claim 8, wherein the non-linear optical gain medium is LiNbO3: MgO: Nd3 + PPLN with Y-cut.
    [10]
    10. Solid-state nanolaser system according to claim 9, the non-linear optical gain medium is obtained through the modified Czochralski crystal growth technique.

     Fig. 1 
     PPLN: MgO: Nd3 + Fig. 2A

     Fig. 3-�20�

     Fig. 4C 

     Fig. 5
    �p
    PPLN: MgO: Nd3 +

    �SFS = � p + �L �SFD = �L + �L �L
     Fig. 6B 

     Fig. 8 
    - �24�
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