A review of different integration platforms for high-Q Whispering Gallery Mode bulk resonators is presented, including SOI slotted photonics crystal waveguides, suspended Si photonic crystal membranes or suspended silica waveguides. While each of these approaches allows coupling to a specific monolithic resonator, including those made in low index materials, a novel architecture, based on metamaterial engineered silicon photonics waveguides, provides unique flexibility to couple a wide range of WGM microresonators, enabling the combination of high-performance resonators with complex Si photonic circuits.
Bulk whispering gallery mode optical resonators like spheres and disks provide a wide range of remarkable optical properties and ultra-high quality factors. In this invited presentation we will show our most recent results on the use of subwavelength metamaterial engineering to couple bulk resonators and integrated Si waveguides. We experimentally achieve up to 99% light coupling efficiency for microspheres and microdisks made of silica, lithium niobate, and calcium fluoride, with diameters between 300 µm and 3.6 mm. These results open promising prospects for the implementation of a new generation of devices combining high-performance bulk resonators and complex Si photonic circuits.
High quality factor bulk resonators made in different materials have demonstrated outstanding performance in key functionalities that are very challenging to achieve in planar photonics. However, they have made no significant technological impact mainly because of their stability and scalability limitations related to the way they are connected to the outside world using prisms or tapered fibers. Here, we show several demonstrations of efficient coupling of bulk resonators to integrated waveguides using different materials like lithium niobate or polymers. Preliminary results of a universal integrated coupler that can be implemented using silicon photonics are also presented.
An optical resonator like a fiber ring (FR) or a whispering gallery mode (WGM) resonator with two couplers along its loop is referred to be in the add-drop configuration, in analogy with the add-drop multiplexer in telecom networks. Both for practical applications as well as in several fundamental studies involving high-Q resonators, this configuration is of great interest and the assessment of the intrinsic properties of the resonator and of its interaction with the coupling systems is extremely important. We developed an original method able to fully characterize high-Q resonators in an add-drop configuration. The method is based on the study of the two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We implemented the model describing the two CRD profiles from which a best fit process of the measured profiles allows deducing the key parameters. We successfully validated the model with an experiment based on a FR resonator of known characteristics. Finally, we fully characterized a high-Q, home-made, MgF2 WGM disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.
Whispering gallery mode resonators (WGMR) have attracted a great interest in the last decade. WGMR have been fabricated in different geometries, solid and hollow, spherical, toroidal, and bottled shaped. Hollow spherical WGMR or microbubble resonators (MBR) are the last arrived in the family of resonators. The approach used for their fabrication is based on surface tension driven plastic deformation on a pressurized capillary, similar to glassblowing. Using such technique we are able to fabricate large surface area and thin spherical shells with high quality factor (Q).
MBR are efficient phoxonic cavities that can sustain both optical photons and acoustic phonons. It has been demonstrated that MBR can be used to study Turing comb patterns (Kerr modulation) and Stimulated Brillouin Scattering (SBS). Radiation pressure is another mechanism that also leads to excitation of acoustic phonons with lower frequencies, in the range of hundreds of kHz to tens of MHz in the case of silica MBR. The frequency of such oscillations occurs very close to the mechanical eigenfrequencies of the cavity.
We have studied the temporal behavior of the cavity, the coexistence and the suppression of the oscillation while generating Turing comb patterns. The observed phenomenology can be explained by the geometrical characteristics of a MBR. MBRs are spheroidal WGM resonators with quite dense spectral characteristics. The total dispersion of MBR is anomalous and large, as expected for very large MBR. Thus, Kerr comb formation is allowed for all MBR used in this work.
The paper illustrates both review and original simulation results obtained via the modelling of different set-ups based on optical microresonators for applications in optical sensing, lasing and spectroscopy. Passive microbubbles and microspheres coupled via long period fiber gratings (LPGs) and tapered fibers are designed and/or constructed for sensing of biological fluids in the near infrared (NIR) wavelength range. Rare earth doped chalcogenide glass integrated microdisks are designed for active sensing in the medium infrared (MIR) wavelength range. A home-made numerical code modelling the optical coupling and the active behavior via rate equations of ion population is employed for a realistic design, by taking into account the most important active phenomena in rare earths, such as the absorption rates, the stimulated emission rates, the amplified spontaneous emission, the lifetime and branching ratios, the ion-ion energy transfers and the excited state absorption. Optical coupling is obtained by employing ridge waveguides, for micro-disks, and tapered fibers, for microspheres and microbubbles. Different dopant rare earths as Erbium (Er3+) and Praseodymium (Pr3+) are considered.
Coating of high-Q whispering gallery mode micro-resonators is typically performed in order to add the functionalities of the coating material to the unique properties of this type of resonators. Silica microspheres or microtoroids are typically used as high-Q cavity substrate on which a functional film is deposited. In order to effectively exploit the coating properties a critical step is the efficient excitation of WGMs mainly contained inside the deposited layer. We developed a simple method able to assess whether or not these modes are selectively excited. The method is based on monitoring the thermal shift of the excited resonance, which uniquely depends on the thermo-optic coefficient and on the thermal expansion coefficient of the material in which the mode is embedded. We applied this technique to the case of a SU-8 layer deposited on a silica microsphere. Main tests were performed around the wavelength of 770 nm because of potential application in biochemical sensing requiring low light absorption in aqueous environment. We show that by using integrated waveguides made with SU-8 polymer (rather than silica fiber tapers) we can fulfill the proper phase matching conditions thus exciting the fundamental WGM mainly confined in the coating. A further proof of the validity of the approach is obtained assessing the free spectral range of the excited modes which depends on the refractive index of the material in which the mode is confined.
Surface tension induced whispering gallery mode (WGM) micro-resonators can be made in glass with very high quality factor Q. In fact, low losses amorphous glassy dielectrics can be easily shaped in high-surface-quality spheroids by thermal reflow. Since the pioneering works on fused silica microspheres showing several orders of magnitude higher Qs compared to previous findings, a large number of studies have been performed in the last years on glass based microresonators. Main results include frequency conversion through non-linear effects and micro-lasers, filtering and optical switching, RF photonics and sensing. Besides spheres, alternatives shapes like micro-bottles and micro-bubbles have been implemented to improve the resonator performances depending on the application. Other glasses rather than silica have been considered in order to enhance properties like transparency windows and non-linear effects. This presentation will review the main results we obtained on micro-laser sources in erbium doped microcavities, parametric conversion in silica microspheres, and stimulated Brillouin scattering in silica microbubbles. Potentials of coated silica microspheres implemented to add the functionalities of the coating material will be also presented.
Microresonators are very suitable for sensing application and investigation of nonlinear effects, due to their enormous quality factor and small mode volume. These properties can be extended to the mid-infrared spectral range by creating microresonators from chalcogenide glasses, which are transparent in the mid-infrared and have large third-order optical nonlinearity. We present the analysis of the nonlinear effects observation in chalcogenide microspheres created by inert gas heating.
A novel optical fiber coupler to whispering gallery mode (WGM) micro-resonators, which allows frequency selective addressing of different micro-resonators along the same fiber, is proposed. The coupling unit is based on a pair of identical long period fiber gratings (LPGs) and a thick adiabatic taper (>15 μm in waist) in between, where evanescent coupling from cladding modes to WGMs takes place. This robust unit can be replicated more times along the same fiber, simply cascading LPGs with different bands. Independent addressing of two different resonators along the same fiber is demonstrated.
In order to optimize the performance of an optical microbubble resonator (OMBR) as biosensor, the chemical functionalization of its inner surface plays a key role. Here we report on a spatially selective photo – chemical procedure able to bind fluorescent biomolecules only in correspondence of the OMBR inner surface. This abruptly reduces the occurrence of an undesired specific biochemical bond event all along the microfluidic section of the device. The evidence of this method, which maintains high Q factor (> 105) for the OMBR in buffer solution, is proved by fluorescence microscopy and real time measurement of the resonance broadening.
We report efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility χ(3) interactions in resonant silica microspheres and microbubbles in the regime of normal dispersion. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS) and comb generation. Unusually strong anti-Stokes components and extraordinarily symmetric spectra have been observed. Resonant SARS and SRS corresponding to different Raman bands were also observed. The lack of correlation between stimulated anti-stokes and stokes scattering spectra indicates that the signal has to be resonant with the cavity.
We present linear and non linear optical applications of solid and hollow silica microresonators. Hollow microresonators or microbubble resonators combine the unique properties of whispering gallery mode resonators (WGMR) with the intrinsic capability of integrated microfluidics. Microbubbles were filled with water and aqueous solutions of ethanol in order to test the refractive index sensing capabilities of such resonators. We also tested the potential of microbubbles as nonlinear enhancement platform of both the filling material (Rhodamine 6G and Fluorescein) and the glass bubble.
Recently, optical micro-bubble resonators (OMBRs) have gained an increasing interest in many fields of photonics thanks to their particular properties. These hollow microstructures can be suitable for the realization of label – free optical biosensors by combining the whispering gallery mode (WGM) resonator properties with the intrinsic capability of integrated microfluidics. In fact, the WGMs are morphology-dependent modes: any change on the OMBR inner surface (due to chemical and/or biochemical binding) causes a shift of the resonance position and reduces the Q factor value of the cavity. By measuring this shift, it is possible to obtain information on the concentration of the analyte to be detected. A crucial step for the development of an OMBR-based biosensor is constituted by the functionalization of its inner surface. In this work we report on the development of a physical and chemical process able to guarantee a good homogeneity of the deposed bio-layer and, contemporary, to preserve a high quality factor Q of the cavity. The OMBR capability of working as bioassay was proved by different optical techniques, such as the real time measurement of the resonance broadening after each functionalization step and fluorescence microscopy.
A novel method based on long period fiber gratings (LPGs) for coupling light to high-Q silica whispering gallery mode
(WGM) resonators is presented. An LPG couples the fundamental mode of a fiber to higher order LP cladding modes at
selected frequencies. At an adiabatically tapered section of the fiber following the LPG we demonstrated effective
coupling of these cladding modes to WGMs both in silica microspheres and microbubbles. The taper is about one order
of magnitude thicker than standard tapers used for the same purpose. Therefore this new method offers improved
robustness for practical applications.
We report the non linear fluorescence real-time detection of labeled IgG covalently bonded to the surface of a microspherical whispering gallery mode resonator (WGMR). The immunoreagents have been immobilized onto the surface of the WGMR sensor after being activated with an epoxy silane and an orienting layer. The developed immunosensor presents great potential as a robust sensing device for fast and early detection of immunoreactions. We also tested the potential of microbubbles as nonlinear enhancement platform. The dyes used in these studies are tetramethyl rhodamine isothiocyanate and Rhodamine 6G. All measurements were performed in a modified confocal microscope.
Full exploitation of the unique properties of high quality factor micro-optical Whispering Gallery Mode (WGM) resonators requires a controllable and robust coupling of the light to the cavity, either for fundamental investigations or even more for practical applications. Fiber tapers are ideal phase-and-mode-matched couplers and are typically used for lab demonstrations in silica based micro-resonators or in low-index crystalline disks. Prism-based coupling basically adapts to any material and offers improved robustness and reliability for the implementation of devices based on larger resonators. We present the results of our studies on alternative methods based on integrated waveguides with specific reference to the coupling to lithium niobate disk resonators. We also demonstrate efficient coupling from fiber tapers to higher order azimuthal modes in coated microspheres and for third harmonic generation in silica microspheres. We finally propose a new method based on fiber gratings for improved robustness in biosensing applications.
We report on a new source able to provide probe pulses in the UV visible range and on the demonstration of its
application to hyperspectral (fluorescence lifetime) imaging measurements. The source is able to generate UV (down to
300 nm) and blue light exploiting high-order mode propagation in a microstructured fiber pumped by a Ti:Sapphire laser.
We believe that further optimization of pump wavelength, fiber length and fiber zero-dispersion wavelength could
generate light well below 300 nm using a simple and stable set-up and become a useful tool for biomedical imaging. We
demonstrated its versatility using the source for FLIM-FRET measurement a 460 nm and hyperspectral FRET-FLIM
measurements.
High quality factor whispering gallery mode microresonators are ideally suited for nonlinear optical interactions. We demonstrate x(3)-based nonlinear interactions in silica microspheres, consisting in third harmonic generation and Raman assisted TSFG in the visible. A tunable, CW multicolour emission has been quantitatively measured controlling the cavity mode dispersion by choosing suitable sized microspheres and exciting the proper modes for efficient frequency conversion.
Cell-to-cell contacts are crucial for cell differentiation. The promyogenic cell surface protein, Cdo, functions as a component of multiprotein clusters to mediate cell adhesion signaling. Connexin43, the main connexin forming gap junctions, also plays a key role in myogenesis. At least part of its effects are independent of the intercellular channel function, but the mechanisms underlying are unknown. Here, using multiple optical approaches, we provided the first evidence that Cx43 physically interacts with Cdo to form dynamic complexes during myoblast differentiation, offering clues for considering this interaction a structural basis of the channel-independent function of Cx43.
Whispering gallery mode resonators (WGMR), as silica microspheres, have been recently proposed as an efficient
tool for the realisation of optical biosensors. In this work we present a functionalization procedure based on the
DNA-aptamer sequence immobilization on WGMR, able to recognize specifically thrombin or VEGF protein,
preserving a high Q factor. The protein binding was numerically modelled and optically characterized in terms of
specificity in buffer solution or in 10% diluted human serum. The aptasensor was also chemically regenerated and
tested again, demonstrating the reusability of our system.
An ideal diagnostic device should be inexpensive, easy-to-use, rapid and reliable. Nanostructured porous silicon (PSi) satisfies these criterions including label-free optical detection and high throughput detection. Pore morphology (size, porosity) must be tailored for each specific application, and for immunosensing applications PSi morphology has been optimized for maximal pore infiltration of larger proteins as immuno gamma globlulin (IgG). Sensor degradation by high salt concentration induces a baseline drift. Different thermal oxidation procedures have been studied in order to obtain a stable sensor in the 3 hour incubation period of the immunoassay with negligible drift
We present results on the implementation of Whispering Gallery Modes (WGM) biosensors and on the demonstration of a new detection method for WGM based sensors. We first present a functionalization procedure based on the DNA-aptamer sequence immobilization on WGM resonators, able to recognize specifically thrombin protein. The protein binding was optically characterized in terms of specificity in buffer solution and in 10% diluted human serum. When performing the above measurements, we have used the typical detection scheme for WGM resonator based sensors, which relies on tracking the resonance shift − by scanning with a tunable laser − when a change of the refractive index in the region probed by the WGM takes place. In the second part of the presentation we propose a new sensing approach based instead on monitoring the position of the laser line of a fiber ring laser having a WGM microsphere in its loop. We demonstrate that the induced shift is the same for the ring laser line and for the microsphere resonance. The proposed method requires simpler and cheaper equipment and may also improve the sensor resolution because the ring laser line is narrower than the microsphere WGM resonance.
Microbubble resonators (MBRs) combine the unique properties of whispering gallery mode resonators with the intrinsic
capability of integrated microfluidics. Here an improved fabrication method of MBRs is presented, based on the heating
of a slightly pressurized capillary by a rotating arc discharge. Rotation of the electrodes ensures an homogeneous
distribution of the heat all over the capillary surface. The demonstrated MBRs have Q factors up to 107 at 773 nm.
Microbubbles were filled with water and aqueous solutions of ethanol in order to test the refractive index sensing
capabilities of such resonators, which also show a good temporal stability.
In this paper we report imaged neuronal rat cells in a confocal laser scanning microscope by simultaneous generation of
the three requested wavelengths obtained by a UV-extended supercontinuum source. This is to the best of our
knowledge that such a measure was performed using a microstructured fiber pumped by a standard Ti:Sapphire laser.
We observed efficient UV light generation when a novel pumping scheme was used. The pump wavelength is close to
the zero-dispersion wavelength of the fiber first high-order mode and offset axial pumping is used. By tuning the pump
wavelength and power level we were able to generate mW-power levels in the visible wavelength interval down and of
about hundreds of microwatt in the UV wavelength interval down to 300 nm. The pump alignment was very simple and
very stable. We believe that further optimization of pump wavelength, fiber length and fiber zero-dispersion wavelength
could generate light well below 300 nm using a simple and stable set-up. To demonstrate the potentiality of this
technique we imaged neuronal rat cells in a confocal laser scanning microscope by simultaneous generation of the three
requested wavelengths.
Coating of spherical microresonators is a very promising technique for optimizing their optical
properties. Optical coatings are constituted by glasses, polymer, and glass ceramics, passive or
activated by luminescent species, Glass ceramic activated by rare earth ions are nanocomposite
systems that exhibit specific morphologic, structural and spectroscopic properties allowing to
develop interesting new physical concepts, for instance the mechanism related to the transparency,
as well as novel photonic devices based on the enhancement of the luminescence. At the state of art
the fabrication techniques based on bottom-up and top-down approaches appear to be viable
although a specific effort is required to achieve the necessary reliability and reproducibility of the
preparation protocols. In particular, the dependence of the final product on the specific parent glass
and on the employed synthesis still remain an important task of the research in material science.
Looking to application, the enhanced spectroscopic properties typical of glass ceramic in respect to
those of the amorphous structures constitute an important point for the development of integrated
optics devices, including coating of spherical microresonators. Here we present a review regarding
spherical microresonators coated by glass and glass-ceramic film activated by Er3+ ions. Er3+ ions
appear to be embedded in a crystalline or amorphous environment and the lifetime dynamic is
influenced by the geometry and by the morphology of the system. Photoluminescence results and
morphologic properties are discussed for both amorphous and glass ceramic films.
In order to fully exploit the unique properties of micro-optical resonators with whispering gallery modes (WGMs), both
for fundamental investigations as well as for practical applications, a critical point is an efficient, controllable, and robust
coupling of the light to the cavity WGMs. We present the results of our studies on phase-matched evanescent field
couplers, with particular reference to the coupling to high-index crystalline resonators like lithium niobate disks. We
focus on couplers based on different types of waveguide configurations and include demonstration of optical coupling to
high-Q lithium niobate resonators from integrated planar waveguides as well as from angle polished waveguides. These
systems are all in guided optics architectures. We also briefly present our recent achievements in the development of
microbubble resonators fabricated from silica capillaries. We show that, as high-Q hollow 3-D cavities with intrinsic
microfluidics, these resonators represent a promising biochemical sensing platform.
Electronic Kerr effect in a polyfluorene derivative is used to reversibly switch near infrared probe beam resonantly
coupled to a hybrid polymer-silica microspherical resonator. NIR pumping at 780 nm in pulsed laser regime is used for
non-linear switching of the WGM resonances that shift as much as 2 GHz for 50 mW of average pump power, compared
to a shift of 250 MHz for the same average pump power at CW regime. The absence of temporal drift and the magnitude
of this shift confirm the Kerr nature of the switching, ruling out thermooptical effects.
A colorimetric resonant mesoporous silicon photonic crystal is used as a surface binding platform for biochemical
detection. The photonic crystal, when illuminated with white light at normal incidence, is designed to reflect a photonic
band gap (PBG) centered at a single wavelength. When molecules are attached to the internal surfaces of pores, the PBG
is shifted due to the change in refractive index of the porous silicon layers. The planar structure of the platform can be
easily integrated into a fluidic cell. We have optically verified the presence of proteins and chemicals as a colorimetric
shift in the reflectivity spectrum of mesoporous silicon photonic crystals and microcavities.
Optical sensors have a large impact in the fields of life science research, drug discovery and medical diagnostics. The
recent advances in nanotechnology and photonics have led to a new generation of nanotools, capable of probing even the
single cell: it has already been demonstrated that nanobiosensors can detect biochemical targets and proteins inside living
single cells. Here we provide a brief overview of the field of nanoprobes consisting of tapered, metal-coated optical
fibers having nanosize tips, such as those which were originally developed for use in near-field optical microscopy.
Moreover we present some preliminary results concerning the characterization of the experimental sensing system which
exploits such nanoprobes for intracellular biomedical diagnostics. The feasibility of using the Fluorescence Lifetime
Imaging Microscopy (FLIM) technique as a dynamic diagnostics tool with these nanoprobes has been demonstrated.
We report on generation of blue light exploiting high-order mode propagation in a microstructured fiber pumped by a
Ti:Sapphire close to the zero-dispersion wavelength of the first high-order mode. An new interesting regime was
observed with axial offset pump. With 230 mW of incident pump power we generated over 3 mW in the 450-510 nm
window achieving 50 μW/nm power density. In a final round of measurements we were able to show generation of a
peak at 350 nm. This complex regime has still to be fully investigated but we believe an optimized fiber design will
allow to efficiently extend the operation of Ti:Sapphire laser to UV/blue wavelength region.
Whispering gallery mode (WGM) disk resonators fabricated in single crystals can have high Q factors within their
transparency bandwidth and may have application both in fundamental and applied optics. Lithium niobate (LN)
resonators thanks to their electro-optical properties may be used in particular as tunable filters, modulators, and delay
lines. A critical step toward the actual application of these devices is the implementation of a robust and efficient
coupling system. High index prisms are typically used for this purpose. In this work we demonstrate coupling to high-Q
WGM LN disks from an integrated optical LN waveguide. The waveguides are made by proton exchange in X-cut LN.
The disks with diameters of about 5 mm and thickness of 1 mm are made from commercial Z-cut LN wafers by core
drilling a cylinder and thereafter polishing the edges into a spheroidal profile. Both resonance linewidth and cavity
photon lifetime measurements were performed to calculate the Q factor of the resonator, which is in excess of 108.
We report the non linear fluorescence real-time detection of methylboldenone, an androgenic anabolic steroid used
illegally as growth promoter based on a resonant sensing chip: a double grating waveguide structure. The limit of
detection of this synthetic steroid is two orders of magnitude lower than the Minimum Required Performance Limit
required by the World Anti-Doping Agency. The immunoreagents have been have been immobilized onto the surface of
the resonant sensor after being activated with phosphonohexanoic acid spacers. The developed immunosensor presents
great potential as a robust sensing device for fast and early detection of illegal dopants and food contaminants.
In this work we describe the characterization of high Q optical microresonators using an all fiber based system. Silica
microspheres fabricated on a fiber tip by electric arc discharge are characterized using a simple interrogation system
based on an adiabatic fiber taper coupler and on the collection of scattered radiation by a multimode fiber.
Homogeneous polymeric thin layers have been used as functionalising agents on silica microresonators in view of
immunosensing applications. We have characterised the microspheres functionalised with poly-L-lactic acid and
Eudragit® L100, as an alternative to the commonly used silanes. It is shown that after polymeric functionalization the
quality factor of the silica microspheres remains around 107, and that the Q factor is still about 3x105 after chemical
activation and covalent binding of immunogammaglobulin. This functionalising process of the microresonator
constitutes a promising step towards the achievement of a highly sensitive immunosensor.
We report on the application of a simple white light source based on the supercontinuum generation from commercial
photonic crystal fibres to confocal fluorescence microscopy and fluorescence lifetime imaging (FLIM) microscopy. The
coherent white light can be tuned by varying the wavelength and intensity of the pump, a Ti:Sapphire laser. There are
several advantages jn the use of SC sources: spatially coherent white radiation, tuning ranges of approximately 400 nm,
high brightness, a robust compact system (potentially all-fibre) and relatively low cost. Being pulsed, SC sources are
suitable for FLIM and may be used for multiple excitation.
It is well known that the efficiency and selectivity of two-photon excited fluorescence (TPEF) process can
depend on various parameters of the ultrashort pulses, such as the pulse intensity and phase, which interact with the
specimen. In order to completely understand this dependence and to obtain optimal TPEF images, techniques like
Collinear Frequency Resolved Optical Gating (CFROG) arrangement can be implemented in a microscope for complete
pulse characterisation at the sample plane. However, this adds complexity that that additional forward collecting optics is
required as well as a suitable frequency doubling crystal.
Here we report a simple way to characterize the pulses within a multiphoton microscope that do not require
forward collecting optics. This is achieved by taking advantage of the fact that backward propagating second harmonic
generation (SHG) signal can be easily generated from starch granules. Since both the fluorescence and SHG signals can
be collected using the same detection scheme the experimental arrangement is considerably simplified. Starch, being a
non- toxic and non-soluble material does not affect living cells allowing the pulse characteristics to be measured in situ,
without the need to move the sample.
We obtained real-time SHG-autocorrelations traces by using a single starch granule that was placed alongside
the living HeLa cells (GFP labeled) being imaged. Furthermore by placing a spectrometer at the output port of the
microscope, a spectrally resolved SHG autocorrelation was acquired allowing complete characterisation of the pulse to
be carried out. The temporal intensity and phase profile were retrieved using CFROG technique. Marginal analysis was
carried out to ensure that the experimental data was successfully acquired.
We demonstrate an ultra sensitive method for Two Photon Fluorescence (TPF) excitation using resonant Grating Waveguide Structures (GWS). In its basic configuration, a GWS consists of a substrate, a waveguide layer and an additional grating layer. When illuminated with laser light under resonant conditions, the GWS reflects all light and leads to very high local surface intensities. This field enhancement can be exploited for TPF spectroscopy, without the need for a highly intense, focused laser light. We present the enhanced TPF signal obtained from a 23 nM drop of tetramethylrhodamine (TMR) on the top of high-finesse resonant polymeric GWS. The resonant behaviour of the GWS was tested for normal incidence with TE polarization illumination. As expected, the transmission spectral profile has a dip at resonant wavelength. The TPF spectra of TMR molecules were observed for different excitation wavelengths. Close to resonance, TPF intensity increases and the maximum signal is obtained when the excitation wavelength coincides with the resonance wavelength of the GWS. These results clearly indicate that the huge field localization at grating surface is responsible for the TPF excitation. We obtained a detection limit down to picomolar concentration of the dye molecules, offering the possibility of a highly sensitive, compact and non-destructive tool for widespread biochemical applications.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.