Final Report Summary - SITELITE (Deterministic coupling between SITE-controlled, dilute nitride-based LighT Emitters and tailor-made photonic-crystal structures)
The establishment of a simple, reliable method for the deterministic coupling of nm-sized light emitters with photonic crystal (PhC) cavities is expected to propel the field of nanophotonics into a new era. Indeed, the possibility to place single quantum objects at arbitrary points of a PhC structure will allow for the realization of complex photonic circuits, integrating single and entangled-photon sources as well as PhC routers, switches, and delay lines.
The SITELiTE project aims at positioning itself at the forefront of the forthcoming nanophotonics revolution, through the exploitation of a novel method for the fabrication of site-controlled nano-emitters (quantum dots, QDs, but also individual impurity complexes) by spatially selective hydrogenation of dilute-nitride materials. This method, usually referred to as “In-plane Band Gap Engineering (BGE)”, was initially developed at the Host Institution, the G29 Laboratory at Sapienza University of Rome, and its optimization represents the first key objective of the SITELiTE project. During the Reporting Period, the efforts undertaken in this direction resulted in a generalized improvement of the properties of the QDs fabricated by In-plane BGE, eventually allowing for the observation of single-photon emission from these dots [see attached figure Fig1_PRepMC-01.tif and S. Birindelli et al., Nano Lett. 14, 1275-1280 (2014)]. In addition, a simplification of the QD fabrication process was proposed and successfully implemented. Instead of writing the pattern by performing electron-beam lithography (EBL) on a positive resist (PMMA), followed by metal deposition and lift-off, we optimized a one-step process based on a negative resist (HSQ), which yields the finished mask immediately after EBL. The effectiveness of the HSQ masks at preventing H diffusion in the sample was successfully tested, obtaining results indistinguishable from those achieved with metallic masks. This HSQ-based technique is inherently simpler and therefore less prone to fabrication errors, thereby resulting in a considerable increase of the yield of successfully processed samples.
The second goal of the SITELiTE project is represented by the design of innovative PhC cavities by means of the semi-analytic method recently devised by the Researcher, Dr. Marco Felici [M. Felici et al., Phys. Rev. B 82, 115118 (2010)]. Through the definition of a direct relationship between the target electromagnetic field distribution and the dielectric constant of the cavity supporting it, this method eliminates the need for the cumbersome, computationally demanding trial-and-error procedures that currently hinder further developments in the field of PhC cavity design. During the Reporting period, this approach was successfully applied to the design of cavities supporting modes with Gaussian envelope function and ultra-low cavity losses. At the time of the Early Termination of the Project (March 09, 2014), we were in the process of carrying out the practical realization of the first generation of PhC devices, in order to implement eventual corrections and to undertake the design of more complex structures, including systems of coupled cavities and PhC cavities with disorder-insensitive properties.
The third (and final) goal of the project consists in the fabrication of the designed PhC structures, integrated with the light emitters fabricated by In-plane BGE. These devices, realized by making use of the state-of-the-art facilities for nanofabrication available at the Institute for Photonics and Nanotechnologies (IFN, also located in Rome), are then to be characterized with the advanced optical spectroscopy techniques available in the G29 lab. When discussing matters related to this final goal, it must first of all be mentioned that all related activities have been delayed by the prolonged unavailability (June 2013-January 2014) of the micro-PL cryostat needed for the optical characterization of our samples. In spite of this delay, however, the fabrication of a first set of passive PhC cavities —to be characterized by reflectivity measurements, see [D. Englund et al., Phys. Rev. Lett. 104, 073904 (2010)]— is currently in its final stages (GaAs etching, membrane release). Within this context, it is also important to note that a series of ordered QD arrays, fully suitable for the integration with PhC cavities, have already been fabricated on a membrane sample, and are currently undergoing detailed optical spectroscopy measurements. At the end of these preliminary studies we will proceed with the deterministic integration of these QDs with our PhC cavities, and with the spectroscopic characterization of the resulting nanophotonic devices. In this respect, we would like to mention that —in spite of the Early Termination of the SITELiTE project— the scientific goals of the project will continue to be pursued by our group, thanks in large part to the Grant awarded to the Researcher (Dr. M. Felici) by the Italian Ministry of Education, Universities, and Research.
Finally, we would like to spend a few words to discuss our very interesting investigation of the effects of (spatially selective) hydrogenation on the strain properties of dilute nitrides, which might be of crucial importance for the realization of truly ground-breaking photonic structures. In particular, the undulated strain deformation that can be obtained by spatially selective hydrogenation of dilute nitrides represents an ideal environment for the observation of the so-called “Berry-phase translation effect”. According to a recent report [Y. Kohmura et al., Phys. Rev. Lett. 110, 057402 (2013)], indeed, under the right conditions X-rays can experience very large translations when propagating through a deformed medium. Within the framework of this project, we exploited the lattice expansion observed upon H irradiation of GaAsN —paired with the spatially selective hydrogenation technique described above— to realize a full set of strain-engineered patterns, which can in many ways be likened to photonic-crystal heterostructures for X-rays. Through extensive X-ray transmission experiments performed at the synchrotron light facilities in Grenoble and Trieste on a series of ordered GaAsN/GaAsN:H dot arrays, we consistently measured beam translations in excess of 100 µm, while also revealing a clear dependence of the observed translation on the dot size and spacing. Also, the role played by the periodicity of the deformation pattern was assessed by comparing periodic and randomly arranged dot arrays with equal average dot spacing. Finally, the possibility to control the linear polarization of the translated beam was introduced by having the X-rays go through a sample patterned with wires, rather than dots. These results open new, enticing prospects for the realization of tailor-made X-ray photonic structures, interesting for applications in pump-and-probe experiments and in X-ray interferometry.
• Main publications: (the list below only includes papers that had already published in peer-reviewed journals before the Early Termination of the Project).
a) M. Felici, A. Polimeni, E. Tartaglini, A. Notargiacomo, M. De Luca, R. Carron, D. Fekete, B. Dwir, A. Rudra, M. Capizzi, and E. Kapon, “Reduced temperature sensitivity of the polarization properties of hydrogenated InGaAsN V-groove quantum wires”, Appl. Phys. Lett. 101, 151114 (2012).
b) M. De Luca, A. Polimeni, M. Felici, A. Miriametro, M. Capizzi, F. Mura, S. Rubini, and F. Martelli, “Resonant depletion of photogenerated carriers in InGaAs/GaAs nanowire mats”, Appl. Phys. Lett. 102, 173102 (2013).
c) G. Pettinari, M. Felici, R. Trotta, M. Capizzi, and A. Polimeni, “Hydrogen effects in dilute III-N-V alloys: from defect engineering to nanostructuring”, J. Appl. Phys. 115, 012011 (2014).
d) Simone Birindelli, Marco Felici, Johannes S. Wildmann, Antonio Polimeni, Mario Capizzi, Annamaria Gerardino, Silvia Rubini, Faustino Martelli, Armando Rastelli, and Rinaldo Trotta, “Single photons on demand from novel site-controlled GaAsN/GaAsN:H quantum dots”, Nano Lett. 14, 1275-1280 (2014).
The SITELiTE project aims at positioning itself at the forefront of the forthcoming nanophotonics revolution, through the exploitation of a novel method for the fabrication of site-controlled nano-emitters (quantum dots, QDs, but also individual impurity complexes) by spatially selective hydrogenation of dilute-nitride materials. This method, usually referred to as “In-plane Band Gap Engineering (BGE)”, was initially developed at the Host Institution, the G29 Laboratory at Sapienza University of Rome, and its optimization represents the first key objective of the SITELiTE project. During the Reporting Period, the efforts undertaken in this direction resulted in a generalized improvement of the properties of the QDs fabricated by In-plane BGE, eventually allowing for the observation of single-photon emission from these dots [see attached figure Fig1_PRepMC-01.tif and S. Birindelli et al., Nano Lett. 14, 1275-1280 (2014)]. In addition, a simplification of the QD fabrication process was proposed and successfully implemented. Instead of writing the pattern by performing electron-beam lithography (EBL) on a positive resist (PMMA), followed by metal deposition and lift-off, we optimized a one-step process based on a negative resist (HSQ), which yields the finished mask immediately after EBL. The effectiveness of the HSQ masks at preventing H diffusion in the sample was successfully tested, obtaining results indistinguishable from those achieved with metallic masks. This HSQ-based technique is inherently simpler and therefore less prone to fabrication errors, thereby resulting in a considerable increase of the yield of successfully processed samples.
The second goal of the SITELiTE project is represented by the design of innovative PhC cavities by means of the semi-analytic method recently devised by the Researcher, Dr. Marco Felici [M. Felici et al., Phys. Rev. B 82, 115118 (2010)]. Through the definition of a direct relationship between the target electromagnetic field distribution and the dielectric constant of the cavity supporting it, this method eliminates the need for the cumbersome, computationally demanding trial-and-error procedures that currently hinder further developments in the field of PhC cavity design. During the Reporting period, this approach was successfully applied to the design of cavities supporting modes with Gaussian envelope function and ultra-low cavity losses. At the time of the Early Termination of the Project (March 09, 2014), we were in the process of carrying out the practical realization of the first generation of PhC devices, in order to implement eventual corrections and to undertake the design of more complex structures, including systems of coupled cavities and PhC cavities with disorder-insensitive properties.
The third (and final) goal of the project consists in the fabrication of the designed PhC structures, integrated with the light emitters fabricated by In-plane BGE. These devices, realized by making use of the state-of-the-art facilities for nanofabrication available at the Institute for Photonics and Nanotechnologies (IFN, also located in Rome), are then to be characterized with the advanced optical spectroscopy techniques available in the G29 lab. When discussing matters related to this final goal, it must first of all be mentioned that all related activities have been delayed by the prolonged unavailability (June 2013-January 2014) of the micro-PL cryostat needed for the optical characterization of our samples. In spite of this delay, however, the fabrication of a first set of passive PhC cavities —to be characterized by reflectivity measurements, see [D. Englund et al., Phys. Rev. Lett. 104, 073904 (2010)]— is currently in its final stages (GaAs etching, membrane release). Within this context, it is also important to note that a series of ordered QD arrays, fully suitable for the integration with PhC cavities, have already been fabricated on a membrane sample, and are currently undergoing detailed optical spectroscopy measurements. At the end of these preliminary studies we will proceed with the deterministic integration of these QDs with our PhC cavities, and with the spectroscopic characterization of the resulting nanophotonic devices. In this respect, we would like to mention that —in spite of the Early Termination of the SITELiTE project— the scientific goals of the project will continue to be pursued by our group, thanks in large part to the Grant awarded to the Researcher (Dr. M. Felici) by the Italian Ministry of Education, Universities, and Research.
Finally, we would like to spend a few words to discuss our very interesting investigation of the effects of (spatially selective) hydrogenation on the strain properties of dilute nitrides, which might be of crucial importance for the realization of truly ground-breaking photonic structures. In particular, the undulated strain deformation that can be obtained by spatially selective hydrogenation of dilute nitrides represents an ideal environment for the observation of the so-called “Berry-phase translation effect”. According to a recent report [Y. Kohmura et al., Phys. Rev. Lett. 110, 057402 (2013)], indeed, under the right conditions X-rays can experience very large translations when propagating through a deformed medium. Within the framework of this project, we exploited the lattice expansion observed upon H irradiation of GaAsN —paired with the spatially selective hydrogenation technique described above— to realize a full set of strain-engineered patterns, which can in many ways be likened to photonic-crystal heterostructures for X-rays. Through extensive X-ray transmission experiments performed at the synchrotron light facilities in Grenoble and Trieste on a series of ordered GaAsN/GaAsN:H dot arrays, we consistently measured beam translations in excess of 100 µm, while also revealing a clear dependence of the observed translation on the dot size and spacing. Also, the role played by the periodicity of the deformation pattern was assessed by comparing periodic and randomly arranged dot arrays with equal average dot spacing. Finally, the possibility to control the linear polarization of the translated beam was introduced by having the X-rays go through a sample patterned with wires, rather than dots. These results open new, enticing prospects for the realization of tailor-made X-ray photonic structures, interesting for applications in pump-and-probe experiments and in X-ray interferometry.
• Main publications: (the list below only includes papers that had already published in peer-reviewed journals before the Early Termination of the Project).
a) M. Felici, A. Polimeni, E. Tartaglini, A. Notargiacomo, M. De Luca, R. Carron, D. Fekete, B. Dwir, A. Rudra, M. Capizzi, and E. Kapon, “Reduced temperature sensitivity of the polarization properties of hydrogenated InGaAsN V-groove quantum wires”, Appl. Phys. Lett. 101, 151114 (2012).
b) M. De Luca, A. Polimeni, M. Felici, A. Miriametro, M. Capizzi, F. Mura, S. Rubini, and F. Martelli, “Resonant depletion of photogenerated carriers in InGaAs/GaAs nanowire mats”, Appl. Phys. Lett. 102, 173102 (2013).
c) G. Pettinari, M. Felici, R. Trotta, M. Capizzi, and A. Polimeni, “Hydrogen effects in dilute III-N-V alloys: from defect engineering to nanostructuring”, J. Appl. Phys. 115, 012011 (2014).
d) Simone Birindelli, Marco Felici, Johannes S. Wildmann, Antonio Polimeni, Mario Capizzi, Annamaria Gerardino, Silvia Rubini, Faustino Martelli, Armando Rastelli, and Rinaldo Trotta, “Single photons on demand from novel site-controlled GaAsN/GaAsN:H quantum dots”, Nano Lett. 14, 1275-1280 (2014).