Benzene (C6H6) is one of the major public health concerns. It is emitted from various natural and anthropogenic sources, like fires and volcanic emissions, petrol service stations, transportation, and the plastics industry. Here, we present our work on developing a new benzene sensor using a widely tunable difference-frequency-generation (DFG) laser emitting between 11.56 and 15 µm (667–865 cm–1). The DFG process was realized between an external-cavity quantum-cascade-laser (EC-QCL) and a CO2 gas laser in a nonlinear, orientation-patterned GaAs crystal. We obtained the absorption cross-sections of the Q-branch of the ν4 vibrational band of benzene by tuning the wavelength of the DFG laser between 14.79 and 14.93 μm (670–676 cm–1). Benzene sensing measurements were performed near 14.84 μm (673.97 cm–1) with a direct laser absorption spectroscopy scheme. The benzene concentration was varied between ppb and ppm levels. Even with a relatively short optical path-length of 23 cm, our sensor achieved a benzene detection limit of about 10 ppb.
Mid-infrared (MIR) laser sources are used in a number of applications such as remote sensing, air pollution monitoring, combustion diagnostics, and molecular spectroscopy. Here, we present our work on the development of a MIR laser source based on the difference frequency generation (DFG) process between an external-cavity quantum-cascade-laser tunable over 1750–1835 cm–1 (pump source) and a CO2 gas laser tunable over 921–1083 cm–1 (signal source). The DFG process was realized in a nonlinear, orientation-patterned GaAs crystal, and resulted in an idler spectral range between 667–865 cm–1 with a linewidth of ~2.3 MHz and an output power of up to ~31 μW. Exploiting the fine tunability of our DFG laser source, we performed high-resolution absorption measurements of ethylene (C2H4) and acetylene (C2H2).
We introduce a home-built laser-scanning nonlinear optical microscope, combining two-photon excitation fluorescence (TPEF), stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS). Narrowband pump and tunable Stokes pulses at 40-MHz are delivered by an Erbium-fiber laser source, thus greatly simplifying the excitation scheme and reducing the costs and maintenance with respect to standard bulk free-space lasers. After the sample, a dichroic beam splitter transmits the Stokes beam for SRS imaging and reflects the CARS or TPEF (at shorter wavelengths). Signal-to-noise ratio in SRS imaging is greatly enhanced (by up to 30 dB, reaching shot-noise-limited detection without the need of any electronic auto-balancing) thanks to the use of an innovative scheme that we call In-line Balanced Detection (IBD). IBD-SRS not only completely removes high-frequency laser fluctuations but also passively and automatically balances the low-frequency signal variation due to spatially varying sample transmission. We record SRS/CARS spectra in the 2800-3100 cm-1 Raman vibrational spectrum, thus providing a detailed chemical information on the sample in the C-H stretching region. We report various bioimaging applications of our instrument: the study of breast tumour cells using CARS, three-dimensional visualization of lipid distribution in HuH7 and in HepaRG hepatic cells using SRS and a combined TPEF/SRS study of plant cells. Microscopy in scattering media such as a bovine liver tissue is as well demonstrated.
We report on the measurements of spin diffusion length and lifetime in Germanium with both magneto-electro-optical
and magneto-electrical techniques. Magneto-electro-optical measurements were made by optically inject in Fe/MgO/Ge
spin-photodiodes a spin polarized population around the Γ point of the Brillouin zone of Ge at different photon energies.
The spin diffusion length is obtained by fitting by a mathematical model the photon energy dependence of the spin
signal, due to switching of the light polarization from left to right, leading to a spin diffusion length of 0.9±0.2 μm at
room temperature. Non-local four-terminals and Hanle measurements performed on Fe/MgO/Ge lateral devices, at room
temperature, instead lead to 1.2±0.2 μm. The compatibility of these values among the different measurement methods
validates the use all of all of them to determine the spin diffusion length in semiconductors. While electrical methods are
well known in semiconductor spintronics, in this work we demonstrate that the optical pumping versus photon energy is
an alternative and reliable method for the determination of the spin diffusion length whereas the band structure of the
semiconductor allows for a non-negligible optical spin orientation.
The combination of frequency combs and quantum cascade lasers is opening new opportunities in the field of precision spectroscopy in the mid-infrared. Frequency combs allow quantum cascade lasers to be referenced to a highly repeatable, precise and absolute frequency axis. Repeatability is a key feature to obtain high quality measurements of absorption profiles and thus accurate determination of spectroscopic parameters, while absolute frequency calibration makes it possible the comparison of spectroscopic data acquired in different laboratories and at different times, as well as comparison with theoretical predictions or existing databases. This work reviews some of the main results achieved at 4.3 μm by investigation of a manifold of rovibrational lines of CO2. Spectroscopic parameters such as line-centre frequencies, line intensity factors, pressure shift and pressure broadening coefficients are retrieved with an unprecedented quality from the metrological point of view.
We present a table-top source of extremely intense multi-THz transients covering the spectral region between 0.1 and
140 THz. Electric field amplitudes of up to 108 MV/cm and pulse durations as short as a single cycle are demonstrated
with our hybrid Er:fiber-Ti:sapphire laser system. All THz waveforms are electro-optically detected. This source opens
the door to a regime of non-perturbative THz nonlinearities in condensed matter. First examples range from coherent
control of excitons, via a breakdown of the power expansion of the nonlinear polarization in bulk semiconductors to twodimensional
multi-wave mixing and direct femtosecond spin control by magnetic field excitation.
The authors report on the fabrication of buried waveguides in both lithium niobate and periodically poled lithium
niobate. First a low insertion loss waveguide is fabricated in z-cut lithium niobate using femtosecond laser
waveguide inscription. To fabricate a waveguide exhibiting both low propagation and coupling losses, we used the
multiscan fabrication technique to control the size of the waveguide cross section. We measured coupling losses of
1.1 dB/facet and propagation losses as low as 0.6 dBcm-1. Optical waveguides have been also inscribed in
periodically poled lithium niobate by femtosecond laser pulses with the same multiscan technique. Second harmonic
generation experiments from a fundamental wavelength of 1567 nm demonstrate that the nonlinear optical
coefficient in the waveguides is preserved, yielding a conversion efficiency of 18% W-1.
In this paper we describe a novel approach to the fabrication of optical waveguides by focused low-repetition-rate femtosecond laser pulses. This approach overcomes the main limitation of the technique, i.e. the strong asymmetry of the waveguide profile. By using an astigmatic beam and suitably controlling both beam waist and focal position in tangential and sagittal planes, it is possible to shape the focal volume in such a way as to obtain waveguides with a circular transverse profile and of the desired size. This technique is applied to the fabrication of active waveguides in Er:Yb-doped glass substrates. The waveguides are single-mode at 1.5 mm, exhibit propagation losses of about 0.25 dB/cm and an internal gain of 1.4 dB at 1534 nm.
This paper reports the development and modelling of the reverse-proton-exchange process for the realization of high quality optical waveguides in MgO doped stoichiometric lithium tantalate, a promising nonlinear material due to its low coercive field and high damage threshold. Reverse-proton-exchange gives rise to a buried refractive index-profile providing significant advantages in terms of attenuation, insertion losses and overlap of the fields interacting through the nonlinear susceptibility. By characterizing several samples fabricated under different experimental conditions, we identified a fabrication procedure which is simpler than the conventional one used for lithium niobate: the annealing and reverse-exchange processes are indeed performed at the same temperature so that the diffusion of hydrogen ions towards the substrate during the reverse-exchange occurs in the same conditions as during the annealing. This results in very simple empirical laws relating the fabrication to the optical parameters. By such a modeling we defined fabrication parameters giving rise to a single-mode waveguide at l=1.55mm with good fiber mode-matching and high efficiency when used as a nonlinear device for telecom applications.
Graded index planar waveguides have been realized by using a nematic liquid crystal in a distorted hybrid configuration. The application of external electric fields enables to modify the orientation of the local molecular axis and hence to reshape in an easy and controlled way the refractive index profile. The waveguide, characterized for various electric field strengths by means of m-lines spectroscopy, behaves in good agreement with what expected from a simple model based on the dielectric, optical and elastic properties of the liquid crystal.
Second harmonic generation (SHG) in the Cerenkov configuration is investigated in planar proton-exchanged lithium-niobate waveguides where the use of a linear grating fabricated on top of the waveguide reproduces a 1-D photonic band-gap structure. In such conditions the conversion efficiency is strongly enhanced, due to the high density of the fundamental guided mode in correspondence of the linear grating. Optimum parameters of the linear grating are determined for different waveguides in view of the fabrication of a very compact device for highly efficient frequency doubling.
The application of Liquid Crystals (LC) to integrated optics is actively investigated because the mechanical and optical properties of these materials can be exploited to realize new schemes of opto-electronic devices. In particular the molecular alignment induced in nematic LCs by external electric fields can be very useful for mode splitting, tunable filtering and optical switching.
The aim of this work is to experimentally demonstrate the possibility of dynamically reshaping the refractive index profile of a planar waveguide, and hence the propagation characteristics of the modes, by applying a small external electric field. Graded index planar waveguides have been realized by using a nematic liquid crystal in a distorted hybrid configuration as the guiding layer. The application of external electric fields enables to modify the orientation of the local molecular axis and hence to reshape in an easy and controlled way the refractive index profile. Extensive m-lines spectroscopy measurements were performed in order to determine the anisotropic refractive index profiles for various electric field strengths. The experimental results are in good agreement with what expected from a simple model based on the dielectric, optical and elastic properties of the liquid crystal guiding layer.
We report on single mode active waveguides at 1.5 μm in Er:Yb-doped glass substrates fabricated with femtosecond laser pulses. A new spatial beam shaping approach for fabrication of waveguides with circular transverse profile is discussed, which uses an astigmatic beam and controls both beam waist and focal position in tangential and sagittal planes. The experimental results are well described by a simple nonlinear absorption model. The waveguides realized with this technique provide a significant enhancement in the whole Er-band, becoming interesting candidates for the realization of many photonic devices.
Full optical characterization of planar nonlinear waveguides is of major importance for the assessment of reliable fabrication processes of integrated all-optical devices. A high accuracy in the design of the waveguide parameters fixing the refractive index profile (typically optical depth and index change) and in the knowledge of their wavelength dependence are mandatory for the realization of nonlinear integrated optical devices that meet phase matching conditions and achieve highly efficient interactions. Conventional characterization techniques, such as m-lines spectroscopy, do not always fulfil the accuracy requirements especially in the case of very thin waveguides. Moreover additional preliminary determination of the refractive index of the substrate is often required. In our laboratory we developed different non-conventional optical characterization techniques to overcome the main problems related to nonlinear waveguides and, in particular, to proton-exchanged (PE) waveguides in lithium niobate or lithium tantalate that are the most commonly used waveguides for all-optical devices. Different methods will be presented. In most cases, both radiation and guided modes are exploited in the characterization process. The new techniques proposed allowed a full optical characterization of the ordinary and extraordinary refractive indices of the substrates and index profiles of the exchanged layers for single-mode and multimode PE waveguides and the determination of the Sellmeier dispersion curves. Experimental results on typical waveguides will be presented. A new method, based on prism coupling, for the measurement of waveguide losses will also be discussed.
The importance of LiNbO3 proton exchanged (PE) waveguides in integrated optics is well consolidated. In such waveguides, only extraordinary modes are supported, since the ordinary refractive index is decreased by proton exchange. However, the knowledge of the ordinary refractive index profile is of great importance, being related to the crystallographic phase of the exchanged layer.
This work provides a wavelength dispersion curve for both ordinary and extraordinary index changes of PE-LiNbO3 waveguides. The dispersion curve of the ordinary index change was obtained by means of a non-conventional method, developed in our laboratory, based on radiation modes, that of the extraordinary index change by standard m-lines spectroscopy.
A multimode PE waveguide was designed and fabricated specifically to obtain a very accurate determination of the index changes up to the 1.55μm spectral range. The waveguide was realized on a Z-cut LiNbO3 substrate, by a 26h proton-exchange at 247°C in benzoic acid diluted with 1% lithium benzoate, resulting in a depth of 2.75μm. The measurement of both index changes was performed at wavelengths ranging from the visible to the near infrared and the corresponding Sellmeier curves were obtained. It is worth noting that the two index changes exhibit a significantly different behaviour.
Second harmonic generation (SHG) in Cerenkov configuration is investigated under conditions for which the use of a linear grating fabricated on top of the waveguide reproduces a photonic band-gap structure. The fundamental mode of the guide, at the fundamental frequency, is tuned at the photonic band edge resonance thus experiencing a great confinement and enhancement of the electromagnetic field inside the structure. The conversion efficiency achieved in both forward and backward direction is at least one order of magnitude greater than that of a 'conventional' Cerenkov emission in a waveguide of the same length.
Vapor-phase proton-exchange has been applied to lithium tantalate for the first time, as a waveguide fabrication technique. This technique provides alpha-phase waveguides without the need for annealing. A sealed ampoule set-up has been used employing pure benzoic acid as the vapor source. Various waveguides have been realized and optically characterized by means of standard m-lines spectroscopy. The profile shape is a step plus an exponential tail toward the substrate, as that found for vapor-phase proton-exchange waveguides in lithium niobate. The total depth of the refractive index profile increases with the exchange time, following a linear diffusion model. The ordinary index change has been determined by an interferometric method, giving values that confirmed the alpha-crystallographic phase of the fabricated waveguides. The propagation losses have been measured with a new method using an isosceles coupling prism and an out-coupling objective. The values found for the different modes of the various waveguides ranged from 0.5 to 0.8 dB/cm. An aging phenomenon in the fabricate waveguides has been observed during the first month after the exchange process. The extraordinary index change decreased of 5 percent, while the optical depth increased of 2 percent. Application of this technology to periodically poled substrates for QPM devices seems feasible.
Quasi-phase-matched (QPM) LiNbO3 waveguides are at present the most efficient device for second-harmonic- generation (SHG). They require the periodic inversion of the ferroelectric domains and the fabrication of annealed- proton-exchanged (APE) waveguides. In order to work at a wavelength of interest for telecommunications, 1300nm, we realized an APE waveguide, single-mode at such wavelength, on a periodically-poled substrate with a period of 11.7 micrometers . Domain inversion was obtained by Ti-indiffusion. The sample was linearly characterized. For the nonlinear experiment, a tunable Nd:YAG-pumped OPA was used as the light source. The maximum efficiency for SHG was found around 1300nm, within a few nanometers from the expected wavelength. The ratio among the output power of the second harmonic and of the fundamental guided mode was 56 percent. Together with the SH, the third harmonic was generated as the result of a (chi) 2 cascading process. On the basis of the result obtained, the problems connected with the design and fabrication of QPM devices will be discussed, with particular attention to the tolerance sin waveguide fabrication. It is worth noting that the two wavelengths were generated simultaneously, using a single laser source. This opens the possibility of generating multicolor output in diode-pumped QPM-waveguides.
Wavelength conversion is a key function in wavelength- division multiplexing. Frequency-shifting can be obtained through cascaded second-order nonlinear processes: a pump at (omega) is coupled into the waveguide, second harmonic is generated and made to interact with a coupled signal at (omega) -(Delta) (omega) so as to obtain a converted signal at (omega) + (Delta) (omega) via difference frequency generation. For practical applications, it is essential to achieve a good control in waveguide fabrication so as to be able to design a frequency-shifting device for specific pump and signal frequencies. In this work we report frequency- shifting based on cascaded second-order nonlinear processes obtained in simple planar Ti-undiffused LiBnO3 waveguides, where phase-matching is achieved by birefringence. A Y-cut planar waveguide, 17mm long, was fabricated by diffusing a 290-angstrom-thick titanium layer for 6 hours at a temperature of 1000 degrees C. Thanks to a good modeling of the fabrication process, the waveguide behavior could be predicted directly from the fabrication parameters. A converted signal at 1.100 micrometers was obtained from a pump at 1.104 micrometers and a signal at 1.108 micrometers at a working temperature of 85 degrees C. The phenomenon was observed with a reasonable efficiency and was highly reproducible. The experimental results were in very good agreement with the expected ones.
A non conventional method is presented for the determination of the refractive-index profile parameters (index change (Delta) n and optical depth d) of single-mode second-order nonlinear planar waveguides, exploiting second harmonic generation in the Cerenkov configuration. The method is based on the dependence of the output angle of the second-harmonic Cerenkov radiation-mode on the propagation constant of the guided-mode at the fundamental frequency; this is in turn related to the refractive-index profile parameters of the waveguide and to the boundary conditions, thus in particular to the waveguide-cover refractive index nc. If two different liquids of known refractive index are placed as the waveguide cover, two different Cerenkov output-angles are obtained that can be expressed as a function of (Delta) n and d by means of two independent equations, thus allowing the determination of both waveguide parameters. It is worth noting that the waveguide parameters at the fundamental frequency, typically in the near infrared, are obtained by means of measurements performed at the second harmonic wavelength, in the visible, with evident advantages in terms of easiness of operation. The experimental results confirm the reliability of the method.
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