Julien Lumeau

All-Dielectric Silicon Nanogap Antennas To Enhance the Fluorescence of Single Molecules

Plasmonic antennas have a profound impact on nanophotonics as they provide efficient means to manipulate light and enhance light-matter interactions at the nanoscale. However, the large absorption losses found in metals can severely limit the plasmonic applications in the visible spectral range. Here, we demonstrate the effectiveness of an alternative approach using all-dielectric nanoantennas based on silicon dimers to enhance the fluorescence detection of single molecules. The silicon antenna design is optimized to confine the near-field intensity in the 20 nm nanogap and reach a 270-fold fluorescence enhancement in a nanoscale volume of λ(3)/1800 with dielectric materials only. Our conclusions are assessed by combining polarization resolved optical spectroscopy of individual antennas, scanning electron microscopy, numerical simulations, fluorescence lifetime measurements, fluorescence burst analysis, and fluorescence correlation spectroscopy. This work demonstrates that all-silicon nanoantennas are a valid alternative to plasmonic devices for enhanced single molecule fluorescence sensing, with the additional key advantages of reduced nonradiative quenching, negligible heat generation, cost-efficiency, and complementary metal-oxide-semiconductor (CMOS) compatibility.

Tunable narrowband filter based on a combination of Fabry-Perot etalon and volume Bragg grating

A new type of tunable narrowband filter is proposed. This filter is a combination of a Fabry-Perot etalon, which permits the selection of a comb of discrete narrow bands, and a high-efficiency rotating volume Bragg grating recorded in photo-thermo-refractive glass, which permits tuning between the Fabry-Perot resonances. A tunable filter for fixed wavelengths in the region of 1.5 mum with a spectral width of 220 pm (FWHM), separation between channels of 800 pm, and throughput of 95% (losses <0.2 dB) is demonstrated.

Volume-chirped Bragg gratings: monolithic components for stretching and compression of ultrashort laser pulses

Abstract. An innovative type of optical component—a volume Bragg grating—has recently become available commercially and has found wide applications in optics and photonics due to its unusually fine spectral and angular filtering capability. Reflecting volume Bragg gratings, with the grating period gradually changing along the beam propagation direction (chirped Bragg gratings—CBGs) provide stretching and recompression of ultrashort laser pulses. CBGs, being monolithic, are robust devices that have a footprint three orders of magnitude smaller than that of a conventional Treacy compressor. CBGs recorded in photo-thermo-refractive glass can be used in the spectral range from 0.8 to 2.5 μm with the diffraction efficiency exceeding 90%, and provide stretching up to 1 ns and compression down to 200 fs for pulses with energies and average powers exceeding 1 mJ and 250 W, respectively, while keeping the recompressed beam quality M2<1.4, and possibly as low as 1.1. This paper discusses fundamentals of stretching and compression by CBGs, the main parameters of the gratings including the CBG effects on the laser beam quality, and currently achievable CBG specifications.

Reflection of light by composite volume holograms: Fresnel corrections and Fabry-Perot spectral filtering

Effects in composite volume Bragg gratings (VBGs) are studied theoretically and experimentally. The mathematics of reflection is formulated with a unified account of Fresnel reflections by the boundaries and of VBG reflection. We introduce the strength S of reflection by an arbitrary lossless element such that the intensity of reflection is R=tanh(2) S. We show that the ultimate maximum/minimum of reflection by a composite lossless system corresponds to addition/subtraction of relevant strengths of the sequential elements. We present a new physical interpretation of standard Fresnel reflection: strength for TE or for TM reflection is given by addition or by subtraction of two contributions. One of them is an angle-independent contribution of the impedance step, while the other is an angle-dependent contribution of the step of propagation speed. We study an assembly of two VBG mirrors with a thin immersion layer between them that constitutes a Fabry-Perot spectral filter. The transmission wavelength of the assembly depends on the phase shift between the two VBGs. Spectral resolution Deltalambda(FWHM)=25 pm at lambda=1063.4 nm is achieved with the device of small total physical thickness 2L=5.52 mm.

Ultimate efficiency of spectral beam combining by volume Bragg gratings.

Spectral beam combining (SBC) by volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass is a powerful tool for laser applications that require higher radiance than a single laser unit can achieve. The beam-combining factor (BCF) is introduced as a tool to compare various beam-combining methods and experiments. It describes the change of radiance provided by a beam-combining system but is not affected by the initial beam quality of the combined lasers. A method of optimization of VBGs providing the maximum efficiency of SBC has been described for an arbitrary number of beams. An experiment confirming the proposed modeling for a two-beam SBC system by a single VBG has demonstrated a total combined power of 301 W with a channel separation of 0.25 nm, combining efficiency of 97%, close to diffraction limited divergence with M(2)=1.18, BCF of 0.77, and spectral radiance of 770 TW/(sr·m(2)·nm), the highest to date for SBC.

Ultranarrow bandwidth moiré reflecting Bragg gratings recorded in photo-thermo-refractive glass

Over the last 15 years, a new photosensitive glass called photo-thermo-refractive (PTR) glass has been developed for the recording of volume holographic elements such as volume Bragg gratings. Main advantage of such elements is that they allow obtaining narrowband filters in both spectral and angular spaces [1]. However, the further decrease of the spectral bandwidth of volume Bragg gratings represents a technological issue and is thus limited to values between 50 and 100 pm depending on their central wavelength. For some applications such as LIDARS or the longitudinal mode selection of Nd:YAG lasers [2], bandwidth in the range of a few picometers or tens of picometers would be required. Hence, new technology must be developed in order to achieve ultra-narrowband selectivity. In this paper we present the fabrication of so-called Moiré reflecting Bragg gratings (Moiré RBG), resulting from the recording of two reflecting Bragg gratings with shifted periods (figure 1). Such structure was widely investigated in the past in germanium doped silica fibers [3]. However, due to the unavailability of bulk photosensitive materials, no experimental demonstration of Moiré RBG was performed. PTR glass allows recording of high efficiency reflecting Bragg gratings in a few millimeters glass samples and losses can be kept low below 1%. Hence, PTR glass makes an ideal candidate for the recording of Moiré RBG with very high transmission at resonance.

Strong Bragg Gratings in Highly Photosensitive Photo-Thermo-Refractive-Glass Optical Fiber

A new type of photosensitive fiber is demonstrated. Long lengths (>;100 m) of coreless optical fiber are fabricated from highly photosensitive photo-thermo-refractive glass. A minimum loss of <;2 dB/m is measured. A holographic technique using low power near-UV two-beam interference patterns is applied to record strong and robust Bragg gratings inside the fiber. The gratings show no degradation when heated up to 425 °C for several hours.

Binary volume phase masks in photo-thermo-refractive glass

Permanent binary phase masks with planar surfaces and high tolerance to laser radiation are recorded in the volume of photo-thermo-refractive glass using the contact copying technique and binary amplitude master masks. Conversion of a Gaussian beam to higher order modes is shown.

Near-IR absorption in high-purity photothermorefractive glass and holographic optical elements: measurement and application for high-energy lasers

Volume Bragg gratings (VBGs) in photothermorefractive (PTR) glass are widely used for laser beam control including high-power laser systems. Among them, spectral beam combining based on VBGs is one of the most promising. Achieving 100+ kW of combined laser beams requires the development of PTR glass and VBGs with an extremely low absorption coefficient and therefore methods of its measurement. This paper describes the calorimetric method that was developed for measuring a low absorption coefficient in PTR glass and VBGs. It is based on transmission monitoring of the intrinsic Fabry-Perot interferometer produced by the plane-parallel surfaces of the measured optical elements when heated by high-power laser radiation. An absorption coefficient at 1085 nm as low as 5×10(-5) cm(-1) is demonstrated in pristine PTR glass while an absorption coefficient as low as 1×10(-4) cm(-1) is measured in high-efficiency reflecting Bragg gratings with highest purity. The actual level of absorption in PTR glass allows laser beam control at the 10 kW level, while the 100 kW level would require active cooling and/or decreasing the absorption in PTR Bragg gratings to a value similar to that in virgin PTR glass.

Ultrashort laser pulse diffraction by transmitting volume Bragg gratings in photo-thermo-refractive glass.

We report on the characteristics of ultrashort laser pulse diffraction by transmitting volume Bragg gratings recorded in photo-thermo-refractive glass. The angular and spectral selectivity properties of these phase volume gratings are shown to obey the predictions of a modified Kogelniks coupled wave analysis that accounts for polychromatic beams. Spatio-temporal distortions such as angular dispersion and pulse front tilt that occur after diffraction by a single volume grating are studied and corrected by using a volume grating pair. The angular filtering properties of volume gratings offer potential as spatial filtering elements inside ultrafast laser cavities.