Giuseppe Leo | Université Paris Diderot (original) (raw)

Papers by Giuseppe Leo

IEEE Photonics Journal

High-permittivity III-V semiconductor nanocavities have shown huge potential for enhanced nonline... more High-permittivity III-V semiconductor nanocavities have shown huge potential for enhanced nonlinear light-matter interactions at the nanoscale. In particular, Second Harmonic (SH) generation in AlGaAs nanoantennas can be extremely efficient; however, vertical emission is difficult to achieve, due to the zincblende χ (2) tensor and epitaxially growth on (100) substrates. Here, we demonstrate that we can shape the second harmonic radiation pattern from a single AlGaAs nanostructure by exploiting a geometrical symmetry breaking optimization approach. The optimized design allows to redirect the SH signal toward the normal direction and to increase the SH power collection efficiency by 2 orders of magnitude in a small numerical aperture of 0.1 with respect to the symmetrical counterpart structure.

We describe a technique for the simultaneous measurement of all the modal birefringences in a (ch... more We describe a technique for the simultaneous measurement of all the modal birefringences in a (chi) (2 ) optical guide through surface emitting second harmonic generation (SESHG), which we applied to multilayer AlGaAs waveguides at 1319 nm, both before and after selective ...

Technical Digest. Summaries of papers presented at the Conference on Lasers and Electro-Optics. Postconference Edition. CLEO '99. Conference on Lasers and Electro-Optics (IEEE Cat. No.99CH37013), 1999

ABSTRACT Summary form only given. There are presently two major issues in the research efforts on... more ABSTRACT Summary form only given. There are presently two major issues in the research efforts on CW parametric fluorescence in waveguide optics. First, spontaneous parametric generation is the basic element toward integrated OPO's; second, quantum cryptography seems an interesting application even at very low power levels. Parametric fluorescence and oscillation were obtained in PPLN waveguides several years ago. Nevertheless, their counterpart in semiconductor waveguides is of great interest for integrated optoelectronics. While application of QPM technique to semiconductor waveguides is still in its infancy, the recent breakthrough in oxidized-AlGaAs (ALOX) nonlinear guided optics allows us to investigate integrated parametric fluorescence through form-birefringent phase matching

Soliton-driven Photonics, 2001

Although nearly a quarter of a century has elapsed from the first theoretical prediction of quadr... more Although nearly a quarter of a century has elapsed from the first theoretical prediction of quadratic optical solitons [1], a large interest has been conveyed to the topic only during the last decade [2], partially due to a mature understanding of χ(2) cascaded processes in second harmonic generation (SHG) [3]. After the first experimental demonstration of quadratic spatial solitons (QSS’s) in KTP by Torruellas et al. [4], a considerable amount of theoretical as well as experimental work has been performed, as apparent in this Volume.

Free and Guided Optical Beams - Proceedings of the International School of Quantum Electronics, 2004

ABSTRACT This lecture is intended to provide a general introduction to quadratic nonlinear optics... more ABSTRACT This lecture is intended to provide a general introduction to quadratic nonlinear optics, along with a view on a few selected research topics of present interest. In Section 1 I will recall the quadratic effects arising from non-resonant light-matter interactions, in the absence of absorption and carriers. Specifically, I will illustrate the concept of phase matching and overview the related techniques, with emphasis on guided-wave interactions. In the last decade, novel parametric sources have been conceived and fabricated in both bulk and integrated geometries, and new phenomena and applications have been demonstrated in the domain of soliton propagation. In Section 2, on the side of novel integrated optical sources, I will focus on the recent progress towards a monolithic semiconductor OPO. In Section 3, in the framework of the phase effects associated to the cascading of chi(2), I will deal with quadratic spatial optical solitons, i.e. free-propagating optical-field configurations where diffraction is balanced by nonlinear self-focusing.

Conference Digest. 2000 Conference on Lasers and Electro-Optics Europe (Cat. No.00TH8505), 2000

ABSTRACT Summary form only given. We report on accurate measurements of layer thicknesses, modal ... more ABSTRACT Summary form only given. We report on accurate measurements of layer thicknesses, modal effective indices and guided-wave losses. The thickness in the multilayer have been measured through Kiessig fringe analysis in X-ray reflectometry. This lead to an accurate estimation of AlGaAs percentage in MBE grown GaAs-AlGaAs superlattices

Optics Express, 2012

We numerically study a passive THz source based on difference frequency generation between modes ... more We numerically study a passive THz source based on difference frequency generation between modes sustained by cylindrical AlGaAs microcavities. We show that ring-like structures are advantageous in that they provide additional degrees of freedom for tuning the nonlinear process and for maximizing the nonlinear overlap integral and conversion efficiency.

Growth, Fabrication, Devices, and Applications of Laser and Nonlinear Materials, 2001

ABSTRACT We describe a technique for the simultaneous measurement of all the modal birefringences... more ABSTRACT We describe a technique for the simultaneous measurement of all the modal birefringences in a (chi) (2) optical guide through surface emitting second harmonic generation (SESHG), which we applied to multilayer AlGaAs waveguides at 1319 nm, both before and after selective AlAs oxidation. By end-fire coupling linearly-polarized laser pulses into ridge waveguides, both forward- and back-propagating eigenpolarizations were excited due to Fresnel reflection at the output facet. Several TE-TM pairs of counterpropagating modes then interact through the quadratic nonlinearity, giving rise to interference of SESHG fields. With a single image acquisition of the SESHG far field by a CCD camera, we could evaluate the modal birefringences between all the excited TE- TM mode pairs at the fundamental frequency. This simple approach led us to estimate form-birefringence of our multilayer quadratic waveguides with the high accuracy required by optimized phase-matched interactions in parametric generators and oscillators. This technique is a valuable complement to standard m-line effective index evaluation, and a versatile one-shot tool for waveguide diagnostics.

Nonlinear Guided Waves and Their Applications, 2004

Abstract: Low-threshold spatial optical solitons are observed for the first time in buried planar... more Abstract: Low-threshold spatial optical solitons are observed for the first time in buried planar waveguides obtained by reverse-proton-exchanged periodically poled LiNbO3. ... Buried channel waveguides fabricated by reverse proton exchange (RPE) in Periodically-Poled Lithium ...

2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference (CLEO EUROPE/EQEC), 2011

Continuously tunable sources with room-temperature operation are required in the mid-infrared reg... more Continuously tunable sources with room-temperature operation are required in the mid-infrared region for applications such as spectroscopy or pollutants monitoring. In this spectral range, optical parametric oscillators (OPOs) are more versatile than laser diodes, the tunability of which is limited to a few tens of nanometers. Recently, relatively compact OPOs have started to be available, but they often remain incompatible with the necessary portability out of research laboratories. The perspective of guided-wave OPOs is promising, because of the high efficiency granted by mode confinement. In this context, GaAs is a good alternative to popular LiNbO 3 thanks to its higher χ (2) , broader transparency range, and optoelectronic integrability. We therefore focus on GaAs/AlOx form-birefringent waveguides [1] designed for type-I spontaneous parametric down conversion of a TM 00 pump at 1.06 µm into TE 00 signal-idler modes in the 1.2-4.2 µm range. Despite their encouraging 1500% W-1 cm-2 conversion efficiency (i.e. 4.5% parametric amplification in 3 mmlong samples for 30 mW of pump power) [2,3], OPO threshold has been hindered so far by the optical losses induced by AlAs oxidation process. The impact of the latter on the roughness of GaAs/AlOx interfaces can be revealed by TEM microscopy, showing an increase from 0.3 nm before oxidation to at least 0.53 nm after it. A simple, semi-analytical, Rayleigh-type model shows that the latter value of roughness accounts for scattering losses of 1.0±0.4 cm-1 for a TE 00 mode at 1.55 µm, in agreement with the measured values [4]. A deeper insight in the origin of these losses is provided by their measurement in a broad spectral range, which we have spanned with tunable lasers around 1.09 µm (Yb-YAG Koheras), 1.33 µm (Tunics-XS), 1.55 µm (Tunics-Plus) and 2.12 µm (NanoPlus DFB). Fig.1 shows the TE 00 experimental losses and the model prediction band, with roughness parameters given by TEM study of our samples. A good agreement is apparent, except at the pump wavelength, where the Fabry-Perot fringe method is less reliable, and tends to overestimate the losses, because of its sensitivity to multi-mode propagation and of an insufficient tunability of our source.

Journal of the European Optical Society: Rapid Publications, 2008

Semiconductor microcavities offer a unique way to enhance nonlinear optical processes through lig... more Semiconductor microcavities offer a unique way to enhance nonlinear optical processes through light confinement in space and time. In this article we review two different nonlinear optics semiconductor-based applications that benefit from the microcavity setting. Firstly, we discuss a difference frequency generation scheme in a GaAs microdisk. Secondly, we show how a recently demonstrated source of counter-propagating twin photons can display a sensible performance improvement when combined with a vertical cavity.

Journal of Modern Optics, 2015

ABSTRACT Integrated quantum photonics is a very active field of quantum information, communicatio... more ABSTRACT Integrated quantum photonics is a very active field of quantum information, communication, and processing. One of the main challenges to achieve massively parallel systems for complex operations is the generation, manipulation, and detection of many qubits within the same chip. Here, we present our last achievements on AlGaAs quantum photonic devices emitting nonclassical states of light at room temperature by spontaneous parametric down conversion (SPDC). The choice of this platform combines the advantages of a mature fabrication technology, a high nonlinear coefficient, a SPDC wavelength in the C-telecom band and the possibility of electrical injection.

35th International Conference on Infrared, Millimeter, and Terahertz Waves, 2010

ABSTRACT We model the coupling of THz radiation between a microdisk (MD) resonator and free-space... more ABSTRACT We model the coupling of THz radiation between a microdisk (MD) resonator and free-space. This is relevant for a new class of THz emitters based on difference-frequency mixing between two laser modes in an MD resonator. We show efficient coupling between the MD TM mode and the free-space TE mode.

Conference on Lasers and Electro-Optics 2012, 2012

Nonlinear Photonics, 2007

Gallium arsenide is an outstanding nonlinear optical material, thanks to its large second-order n... more Gallium arsenide is an outstanding nonlinear optical material, thanks to its large second-order nonlinear coefficient ( PPLN GaAs d d 33 14 5 at 1.55 m), wide transparency in the near infrared and potential integration with sources. In guided wave geometries, where strong pump ...

Optics Express, 2014

We analyze the magnitude of the radiation pressure and electrostrictive stresses exerted by light... more We analyze the magnitude of the radiation pressure and electrostrictive stresses exerted by light confined inside GaAs semiconductor WGM optomechanical disk resonators, through analytical and numerical means, and find the electrostrictive force to be of prime importance. We investigate the geometric and photoelastic optomechanical coupling resulting respectively from the deformation of the disk boundary and from the strain-induced refractive index changes in the material, for various mechanical modes of the disks. Photoelastic optomechanical coupling is shown to be a predominant coupling mechanism for certain disk dimensions and mechanical modes, leading to total coupling g om and g 0 reaching respectively 3 THz/nm and 4 MHz. Finally, we point towards ways to maximize the photoelastic coupling in GaAs disk resonators, and we provide some upper bounds for its value in various geometries.

Handbook of Optical Microcavities, 2014

Solar Energy Materials and Solar Cells, 1994

A model of light-induced defect growth in a-Si:H p-in cells is presented, based on the recombinat... more A model of light-induced defect growth in a-Si:H p-in cells is presented, based on the recombination of photo-generated carriers. Adopting a variable minority-carrier transport scheme, the defect growth exhibits a strong dependence on the electric field across the cell. At very low fields the kinetic equation is non-linear, leading to a power-law time dependence. At very high fields it becomes linear, giving rise to a simple exponential behavior. Intermediate regimes can be well apprgximated by "stretched exponential" formulas, although the kinetic equation is non-dispersive.

Procedia Computer Science, 2011

We aim at a radically new continuous-wave, electrically pumped THz emitter. Compared to existing ... more We aim at a radically new continuous-wave, electrically pumped THz emitter. Compared to existing THz sources, this source will bring together several advantages that are far from being simultaneously available in any existing source today: compactness, roomtemperature operation, output power around 1 W, custom emission frequency from 1.5 to 6.5 THz, feasibility of multi-spectral array of emitters, and the perspective of coherent detection schemes. © Selection and peer-review under responsibility of FET11 conference organizers and published by Elsevier B.V.

IEEE Photonics Journal

High-permittivity III-V semiconductor nanocavities have shown huge potential for enhanced nonline... more High-permittivity III-V semiconductor nanocavities have shown huge potential for enhanced nonlinear light-matter interactions at the nanoscale. In particular, Second Harmonic (SH) generation in AlGaAs nanoantennas can be extremely efficient; however, vertical emission is difficult to achieve, due to the zincblende χ (2) tensor and epitaxially growth on (100) substrates. Here, we demonstrate that we can shape the second harmonic radiation pattern from a single AlGaAs nanostructure by exploiting a geometrical symmetry breaking optimization approach. The optimized design allows to redirect the SH signal toward the normal direction and to increase the SH power collection efficiency by 2 orders of magnitude in a small numerical aperture of 0.1 with respect to the symmetrical counterpart structure.

We describe a technique for the simultaneous measurement of all the modal birefringences in a (ch... more We describe a technique for the simultaneous measurement of all the modal birefringences in a (chi) (2 ) optical guide through surface emitting second harmonic generation (SESHG), which we applied to multilayer AlGaAs waveguides at 1319 nm, both before and after selective ...

Technical Digest. Summaries of papers presented at the Conference on Lasers and Electro-Optics. Postconference Edition. CLEO '99. Conference on Lasers and Electro-Optics (IEEE Cat. No.99CH37013), 1999

ABSTRACT Summary form only given. There are presently two major issues in the research efforts on... more ABSTRACT Summary form only given. There are presently two major issues in the research efforts on CW parametric fluorescence in waveguide optics. First, spontaneous parametric generation is the basic element toward integrated OPO's; second, quantum cryptography seems an interesting application even at very low power levels. Parametric fluorescence and oscillation were obtained in PPLN waveguides several years ago. Nevertheless, their counterpart in semiconductor waveguides is of great interest for integrated optoelectronics. While application of QPM technique to semiconductor waveguides is still in its infancy, the recent breakthrough in oxidized-AlGaAs (ALOX) nonlinear guided optics allows us to investigate integrated parametric fluorescence through form-birefringent phase matching

Soliton-driven Photonics, 2001

Although nearly a quarter of a century has elapsed from the first theoretical prediction of quadr... more Although nearly a quarter of a century has elapsed from the first theoretical prediction of quadratic optical solitons [1], a large interest has been conveyed to the topic only during the last decade [2], partially due to a mature understanding of χ(2) cascaded processes in second harmonic generation (SHG) [3]. After the first experimental demonstration of quadratic spatial solitons (QSS’s) in KTP by Torruellas et al. [4], a considerable amount of theoretical as well as experimental work has been performed, as apparent in this Volume.

Free and Guided Optical Beams - Proceedings of the International School of Quantum Electronics, 2004

ABSTRACT This lecture is intended to provide a general introduction to quadratic nonlinear optics... more ABSTRACT This lecture is intended to provide a general introduction to quadratic nonlinear optics, along with a view on a few selected research topics of present interest. In Section 1 I will recall the quadratic effects arising from non-resonant light-matter interactions, in the absence of absorption and carriers. Specifically, I will illustrate the concept of phase matching and overview the related techniques, with emphasis on guided-wave interactions. In the last decade, novel parametric sources have been conceived and fabricated in both bulk and integrated geometries, and new phenomena and applications have been demonstrated in the domain of soliton propagation. In Section 2, on the side of novel integrated optical sources, I will focus on the recent progress towards a monolithic semiconductor OPO. In Section 3, in the framework of the phase effects associated to the cascading of chi(2), I will deal with quadratic spatial optical solitons, i.e. free-propagating optical-field configurations where diffraction is balanced by nonlinear self-focusing.

Conference Digest. 2000 Conference on Lasers and Electro-Optics Europe (Cat. No.00TH8505), 2000

ABSTRACT Summary form only given. We report on accurate measurements of layer thicknesses, modal ... more ABSTRACT Summary form only given. We report on accurate measurements of layer thicknesses, modal effective indices and guided-wave losses. The thickness in the multilayer have been measured through Kiessig fringe analysis in X-ray reflectometry. This lead to an accurate estimation of AlGaAs percentage in MBE grown GaAs-AlGaAs superlattices

Optics Express, 2012

We numerically study a passive THz source based on difference frequency generation between modes ... more We numerically study a passive THz source based on difference frequency generation between modes sustained by cylindrical AlGaAs microcavities. We show that ring-like structures are advantageous in that they provide additional degrees of freedom for tuning the nonlinear process and for maximizing the nonlinear overlap integral and conversion efficiency.

Growth, Fabrication, Devices, and Applications of Laser and Nonlinear Materials, 2001

ABSTRACT We describe a technique for the simultaneous measurement of all the modal birefringences... more ABSTRACT We describe a technique for the simultaneous measurement of all the modal birefringences in a (chi) (2) optical guide through surface emitting second harmonic generation (SESHG), which we applied to multilayer AlGaAs waveguides at 1319 nm, both before and after selective AlAs oxidation. By end-fire coupling linearly-polarized laser pulses into ridge waveguides, both forward- and back-propagating eigenpolarizations were excited due to Fresnel reflection at the output facet. Several TE-TM pairs of counterpropagating modes then interact through the quadratic nonlinearity, giving rise to interference of SESHG fields. With a single image acquisition of the SESHG far field by a CCD camera, we could evaluate the modal birefringences between all the excited TE- TM mode pairs at the fundamental frequency. This simple approach led us to estimate form-birefringence of our multilayer quadratic waveguides with the high accuracy required by optimized phase-matched interactions in parametric generators and oscillators. This technique is a valuable complement to standard m-line effective index evaluation, and a versatile one-shot tool for waveguide diagnostics.

Nonlinear Guided Waves and Their Applications, 2004

Abstract: Low-threshold spatial optical solitons are observed for the first time in buried planar... more Abstract: Low-threshold spatial optical solitons are observed for the first time in buried planar waveguides obtained by reverse-proton-exchanged periodically poled LiNbO3. ... Buried channel waveguides fabricated by reverse proton exchange (RPE) in Periodically-Poled Lithium ...

2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference (CLEO EUROPE/EQEC), 2011

Continuously tunable sources with room-temperature operation are required in the mid-infrared reg... more Continuously tunable sources with room-temperature operation are required in the mid-infrared region for applications such as spectroscopy or pollutants monitoring. In this spectral range, optical parametric oscillators (OPOs) are more versatile than laser diodes, the tunability of which is limited to a few tens of nanometers. Recently, relatively compact OPOs have started to be available, but they often remain incompatible with the necessary portability out of research laboratories. The perspective of guided-wave OPOs is promising, because of the high efficiency granted by mode confinement. In this context, GaAs is a good alternative to popular LiNbO 3 thanks to its higher χ (2) , broader transparency range, and optoelectronic integrability. We therefore focus on GaAs/AlOx form-birefringent waveguides [1] designed for type-I spontaneous parametric down conversion of a TM 00 pump at 1.06 µm into TE 00 signal-idler modes in the 1.2-4.2 µm range. Despite their encouraging 1500% W-1 cm-2 conversion efficiency (i.e. 4.5% parametric amplification in 3 mmlong samples for 30 mW of pump power) [2,3], OPO threshold has been hindered so far by the optical losses induced by AlAs oxidation process. The impact of the latter on the roughness of GaAs/AlOx interfaces can be revealed by TEM microscopy, showing an increase from 0.3 nm before oxidation to at least 0.53 nm after it. A simple, semi-analytical, Rayleigh-type model shows that the latter value of roughness accounts for scattering losses of 1.0±0.4 cm-1 for a TE 00 mode at 1.55 µm, in agreement with the measured values [4]. A deeper insight in the origin of these losses is provided by their measurement in a broad spectral range, which we have spanned with tunable lasers around 1.09 µm (Yb-YAG Koheras), 1.33 µm (Tunics-XS), 1.55 µm (Tunics-Plus) and 2.12 µm (NanoPlus DFB). Fig.1 shows the TE 00 experimental losses and the model prediction band, with roughness parameters given by TEM study of our samples. A good agreement is apparent, except at the pump wavelength, where the Fabry-Perot fringe method is less reliable, and tends to overestimate the losses, because of its sensitivity to multi-mode propagation and of an insufficient tunability of our source.

Journal of the European Optical Society: Rapid Publications, 2008

Semiconductor microcavities offer a unique way to enhance nonlinear optical processes through lig... more Semiconductor microcavities offer a unique way to enhance nonlinear optical processes through light confinement in space and time. In this article we review two different nonlinear optics semiconductor-based applications that benefit from the microcavity setting. Firstly, we discuss a difference frequency generation scheme in a GaAs microdisk. Secondly, we show how a recently demonstrated source of counter-propagating twin photons can display a sensible performance improvement when combined with a vertical cavity.

Journal of Modern Optics, 2015

ABSTRACT Integrated quantum photonics is a very active field of quantum information, communicatio... more ABSTRACT Integrated quantum photonics is a very active field of quantum information, communication, and processing. One of the main challenges to achieve massively parallel systems for complex operations is the generation, manipulation, and detection of many qubits within the same chip. Here, we present our last achievements on AlGaAs quantum photonic devices emitting nonclassical states of light at room temperature by spontaneous parametric down conversion (SPDC). The choice of this platform combines the advantages of a mature fabrication technology, a high nonlinear coefficient, a SPDC wavelength in the C-telecom band and the possibility of electrical injection.

35th International Conference on Infrared, Millimeter, and Terahertz Waves, 2010

ABSTRACT We model the coupling of THz radiation between a microdisk (MD) resonator and free-space... more ABSTRACT We model the coupling of THz radiation between a microdisk (MD) resonator and free-space. This is relevant for a new class of THz emitters based on difference-frequency mixing between two laser modes in an MD resonator. We show efficient coupling between the MD TM mode and the free-space TE mode.

Conference on Lasers and Electro-Optics 2012, 2012

Nonlinear Photonics, 2007

Gallium arsenide is an outstanding nonlinear optical material, thanks to its large second-order n... more Gallium arsenide is an outstanding nonlinear optical material, thanks to its large second-order nonlinear coefficient ( PPLN GaAs d d 33 14 5 at 1.55 m), wide transparency in the near infrared and potential integration with sources. In guided wave geometries, where strong pump ...

Optics Express, 2014

We analyze the magnitude of the radiation pressure and electrostrictive stresses exerted by light... more We analyze the magnitude of the radiation pressure and electrostrictive stresses exerted by light confined inside GaAs semiconductor WGM optomechanical disk resonators, through analytical and numerical means, and find the electrostrictive force to be of prime importance. We investigate the geometric and photoelastic optomechanical coupling resulting respectively from the deformation of the disk boundary and from the strain-induced refractive index changes in the material, for various mechanical modes of the disks. Photoelastic optomechanical coupling is shown to be a predominant coupling mechanism for certain disk dimensions and mechanical modes, leading to total coupling g om and g 0 reaching respectively 3 THz/nm and 4 MHz. Finally, we point towards ways to maximize the photoelastic coupling in GaAs disk resonators, and we provide some upper bounds for its value in various geometries.

Handbook of Optical Microcavities, 2014

Solar Energy Materials and Solar Cells, 1994

A model of light-induced defect growth in a-Si:H p-in cells is presented, based on the recombinat... more A model of light-induced defect growth in a-Si:H p-in cells is presented, based on the recombination of photo-generated carriers. Adopting a variable minority-carrier transport scheme, the defect growth exhibits a strong dependence on the electric field across the cell. At very low fields the kinetic equation is non-linear, leading to a power-law time dependence. At very high fields it becomes linear, giving rise to a simple exponential behavior. Intermediate regimes can be well apprgximated by "stretched exponential" formulas, although the kinetic equation is non-dispersive.

Procedia Computer Science, 2011

We aim at a radically new continuous-wave, electrically pumped THz emitter. Compared to existing ... more We aim at a radically new continuous-wave, electrically pumped THz emitter. Compared to existing THz sources, this source will bring together several advantages that are far from being simultaneously available in any existing source today: compactness, roomtemperature operation, output power around 1 W, custom emission frequency from 1.5 to 6.5 THz, feasibility of multi-spectral array of emitters, and the perspective of coherent detection schemes. © Selection and peer-review under responsibility of FET11 conference organizers and published by Elsevier B.V.