Vincenzo Pusino

Correlated photon pair generation in AlGaAs nanowaveguides via spontaneous four-wave mixing

We demonstrate a source of correlated photon pairs which will have applications in future integrated quantum photonic circuits. The source utilizes spontaneous four-wave mixing (SFWM) in a dispersion-engineered nanowaveguide made of AlGaAs, which has merits of negligible two-photon absorption and low spontaneous Raman scattering (SpRS). We observe a coincidence-to-accidental (CAR) ratio up to 177, mainly limited by propagation losses. Experimental results agree well with theoretical predictions of the SFWM photon pair generation and the SpRS noise photon generation. We also study the effects from the SpRS, propagation losses, and waveguide lengths on the quality of our source.

Monolithic Integration of an Active InSb-Based Mid-Infrared Photopixel With a GaAs MESFET

Medium wavelength infrared (MWIR) detectors are of increasing importance in defense, security, commercial, and environmental applications. Enhanced integration will lead to greater resolution and lower cost focal plane arrays (FPAs). We present the monolithic fabrication of an active photopixel made in InSb on a GaAs substrate that is suitable for large-scale integration into an FPA. Pixel addressing is provided by the cointegration of a GaAs MESFET with an InSb photodiode (PD). Pixel fabrication was achieved by developing novel materials and process steps, including isolation etches, a gate recess etch, and low temperature processes, to make Ohmic contacts to both the GaAs and InSb devices. Detailed electrical and optical measurements in an FTIR demonstrated that the PD was sensitive to radiation in the range of 3-5 μm at room temperature, and that the device could be isolated from its contacts using the integrated MESFET. This heterogeneous technology creates great potential to realize a new type of monolithic FPA of addressable pixels for imaging in the MWIR range.

InSb Photodiodes for Monolithic Active Focal Plane Arrays on GaAs Substrates

We describe the development and optical characterization of a planar medium infrared (mid-IR) InSb photodetector on a GaAs substrate technology, capable of integrating MESFETs, to demonstrate a new active pixel device architecture. Our results pave the way for the development of integrated mid-IR focal plane array circuits on a single chip. Device structures with areas down to 0.0016 mm2 were investigated. By deploying a silicon nitride passivation layer, we were able to reduce leakage current in reverse bias by up to 27% to yield an improved rectifier. Extensive optical characterization was carried out in the near- and mid-IR wavelength range. Responsivities of up to 3.54 A/W and quantum efficiency values above unity were obtained in the near-IR range as a consequence of illumination above the bandgap causing impact ionization. In the mid-IR range, responsivities of up to 0.97 A/W were observed. The bandwidth of the devices proved compatible with video-rate standard sampling rates.

High-Power and Low-Noise Mode-Locking Operation of Al-Quaternary Laser Diodes

We report on the design and experimental evaluation of AlGaInAs/InP multiquantum well epistructures for mode-locked emission at 1.5 μm. We show that mode-locked lasers fabricated on an optimized three quantum well active region with a low optical confinement factor deliver pulses with increased peak power and stability over a much wider biasing range than those fabricated using a standard five quantum well design. Sonogram measurements indicate that sub-ps symmetrical pulses with a substantially reduced linear blue chirp are generated up to nearly three times the laser current threshold.

Multispectral mid-infrared light emitting diodes on a GaAs substrate

We have designed, simulated, and experimentally demonstrated four-colour mid-infrared (mid-IR) Light Emitting Diodes (LEDs) integrated monolithically into a vertical structure on a semi-insulating GaAs substrate. In order to finely control the peak wavelength of the emitted mid-IR light, quantum well (QW) structures based on AlInSb/InSb/AlInSb are employed. The completed device structure consists of three p-QW-n diodes with different well widths stacked on top of one bulk AlInSb p-i-n diode. The epitaxial layers comprising the device are designed in such a way that one contact layer is shared between two LEDs. The design of the heterostructure realising the multispectral LEDs was aided by numerical modelling, and good agreement is observed between the simulated and experimental results. Electro-Luminescence measurements, carried out at room temperature, confirm that the emission of each LED peaks at a different wavelength. Peak wavelengths of 3.40 μm, 3.50 μm, 3.95 μm, and 4.18 μm are observed in the bulk...

Passive mode-locking in semiconductor lasers with saturable absorbers bandgap shifted through quantum well intermixing

Passive mode-locking in semiconductor lasers in a Fabry–Perot configuration with a bandgap blueshift applied to the saturable absorber (SA) section has been experimentally characterized. For the first time a fully post-growth technique, quantum well intermixing, was adopted to modify the material bandgap in the SA section. The measurements showed not only an expected narrowing of the pulse width but also a significant expansion of the range of bias conditions generating a stable train of optical pulses. Moreover, the pulses from lasers with bandgap shifted absorbers presented reduced chirp and increased peak power with respect to the nonshifted case.

A new monolithic approach for mid-IR focal plane arrays

Antimonide-based photodetectors have recently been grown on a GaAs substrate by molecular beam epitaxy (MBE) and reported to have comparable performance to the devices grown on more expensive InSb and GaSb substrates. We demonstrated that GaAs, in addition to providing a cost saving substrate for antimonide-based semiconductor growth, can be used as a functional material to fabricate transistors and realize addressing circuits for the heterogeneously grown photodetectors. Based on co-integration of a GaAs MESFET with an InSb photodiode, we recently reported the first demonstration of a switchable and mid-IR sensible photo-pixel on a GaAs substrate that is suitable for large-scale integration into a focal plane array. In this work we report on the fabrication steps that we had to develop to deliver the integrated photo-pixel. Various highly controllable etch processes, both wet and dry etch based, were established for distinct material layers. Moreover, in order to avoid thermally-induced damage to the InSb detectors, a low temperature annealed Ohmic contact was used, and the processing temperature never exceeded 180 °C. Furthermore, since there is a considerable etch step (> 6 μm) that metal must straddle in order to interconnect the fabricated devices, we developed an intermediate step using polyimide to provide a smoothing section between the lower MESFET and upper photodiode regions of the device. This heterogeneous technology creates great potential to realize a new type of monolithic focal plane array of addressable pixels for imaging in the medium wavelength infrared range without the need for flip-chip bonding to a CMOS readout chip.

Signal gain from four-wave mixing in anomalous AlGaAs nanowaveguides

We experimentally demonstrate efficient four-wave mixing with a net signal gain of 4.1 dB and a conversion efficiency of 5.3 dB in low-loss AlGaAs nanowaveguides in an anomalous dispersion regime.

Multiwavelength super-structured Bragg grating laser for tunable repetition rate mode-locked operation

A multiwavelength laser based on a super-structured Bragg grating is designed and fabricated on multiquantum well AlGaInAs-InP. This laser exhibits phase locking via mutual injection of the neighboring cavities assisted by four wave mixing. We present optical and electrical characterization of its emission regimes showing a complex dynamic behavior. More specifically, this paper focuses on a pulsed regime with a quasi-continuous tunable repetition rate from 32 GHz to 49 GHz.

Polarization-entangled photon pair sources based on spontaneous four wave mixing assisted by polarization mode dispersion

Photonic-based qubits and integrated photonic circuits have enabled demonstrations of quantum information processing (QIP) that promises to transform the way in which we compute and communicate. To that end, sources of polarization-entangled photon pair states are an important enabling technology. However, such states are difficult to prepare in an integrated photonic circuit. Scalable semiconductor sources typically rely on nonlinear optical effects where polarization mode dispersion (PMD) degrades entanglement. Here, we directly generate polarization-entangled states in an AlGaAs waveguide, aided by the PMD and without any compensation steps. We perform quantum state tomography and report a raw concurrence as high as 0.91 ± 0.01 observed in a 1,100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90 ± 0.01 from another (800-nm-wide) waveguide. Our demonstration paves the way for sources that allow for the implementation of polarization-encoded protocols in large-scale quantum photonic circuits.