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

High power, high frequency mode-locked semiconductor lasers

Integrated mode-locked laser diodes are effective sources of periodic sequences of optical pulses, which have always been of great interest for a range of spectroscopy, imaging and optical communications applications. However, some disadvantages prevent their widespread use, such as the restricted tuning of their repetition rate and their output power levels never exceeding a few mW. This thesis reports on the work done to address those limitations. Two main findings are presented, the first being the generation of ultra-high repetition rate optical signals through external injection of two continuous wave signals. This mechanism is much simpler than other techniques previously proposed to increase the repetition rate of monolithic modelocked laser, and has proved successful in generating optical signals up to quasi-THz. It is based on injection of two continuous wave signals whose spacing is an integer multiple of the pulsed cavity free spectral range and whose injection wavelengths coincide with two of the monolithic laser modes. This technique allows discrete tunability of the repetition rate with a step equal to the injected cavity free spectral range, and the injected laser has been shown to lock up to a repetition rate of 936 GHz, corresponding to 26 times that of the free-running semiconductor laser (36 GHz). The presented scheme is suitable for integration, opening the way for a successful on-chip generation of ultra-high repetition rate optical signals exploiting coupled cavity phenomena. The second main finding of this thesis regards the changes induced on the pulsed operation of monolithic passively mode-locked lasers by a blue bandgap detuning applied to their saturable absorber. The quantum well intermixing technique has been used for attaining an area-selective bandgap shift on the fabricated chip, being fully postgrowth. The lasers with a detuned absorber were found to have an extended range of gain section currents and absorber voltages in which stable mode-locking operation took place. Furthermore, a comparison of mode-locked devices fabricated on the same chip, respectively with and without a bandgap detuned absorber, showed that the emitted pulses had greater peak power and were less affected by optical chirp when the bandgap of the absorbing section was shifted. A new intermixing technique has also been developed as part of this work to address some inconsistencies of the pre-existing one; the newly introduced approach has been found to provide better spatial resolution and a more precise control of the attained bandgap shift

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

Monolithically integrated InAsSb-based nBnBn heterostructure on GaAs for infrared detection

High operating temperature i nfrared photo detectors with multi -color function that are capable of monolithic integration are of increasing importance in developing the next generation of mid -IR imag e sensors. Applications of these sensors include defense, medical diagnosis, environmental and astronomical observations. We have investigated a novel InAsSb -based nBnBn heterostructure that combines a state -of-art InAsSb nBn detector with an InAsSb/GaSb heterojuncti on detector . At room temperature, r educti on in the dark current density of more than an order of magnitude was achieved compared to previously investigated InAsSb/GaSb heterojunction dete ctors . Electrical characterization from cryogenic temperatures to roo m temperature confirmed that the nBnBn device was diffusion limited for temperature s above 150K. O ptical measurements demonstrated that the nBnBn detector was sensitive in both the SWIR and MWIR wavelength range at room temperature . The specific detectivity (D*) of the competed nBnBn devices was calculated to be 8.6 × 10 8 cm · Hz 1/2 W -1 at 300K and approximately 1.0 × 10 10 cm · Hz 1/2 W -1 when cooled down to 200K (with 0.3V reverse bias and 1550nm illumination ). In addition, all photodetector layers were grown monolithically on GaAs active layers u sing the interfacial misfit array growth mode . Our results therefore pave the way for the development of new active pixel designs for monolithically integrated mid -IR imaging arrays

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

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, especially for polarization-based protocols. 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$\pm$0.01 observed in the 1100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90$\pm$0.01 from the 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

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, 2 nm, 4 nm, and 6 nm quantum well LEDs, respectively. Under zero bias, Fourier Transform Infrared photo-response measurements indicate that these fabricated diodes can also be operated as mid-IR photodetectors with an extended cut-off wavelength up to 4.6 μm

Experimental investigation on feedback insensitivity in semiconductor ring lasers

The insensitivity to optical feedback is experimentally measured for a semiconductor ring laser (SRL) and compared to that of a Fabry–Perot laser (FPL) fabricated with the same technology and on the same material. An analysis of the optical spectra reveals that the SRL remains nearly unaffected for values of optical feedback as strong as −23  dB. Furthermore, through both optical linewidth and self-mixing measurements, we show that the tolerance to feedback in SRLs is 25–30 dB stronger than in FPLs. This property makes SRLs very interesting candidates for the development of feedback-insensitive optical sources

Ultra-narrow line width polarization-insensitive filter using a symmetry-breaking selective plasmonic metasurface

Plasmonic metasurfaces provide unprecedented control of the properties of light. By designing symmetry-breaking nanoholes in a metal sheet and engineering the optical properties of the metal using geometry, highly selective transmission and polarisation control of light is obtained. To date such plasmonic filters have exhibited broad (> 200 nm) transmission linewidths in the NIR and as such are unsuitable for applications requiring narrow passbands, e.g. multi-spectral imaging. Here we present a novel sub-wavelength elliptical and circular nanohole array in a metallic film that simultaneously exhibits high transmission efficiency, polarisation insensitivity and narrow linewidth. The experimentally obtained linewidth is 79 nm with a transmission efficiency of 44%. By examining the electric and magnetic field distributions for various incident polarisations at the transmission peak we show that the narrowband characteristics are due to a Fano resonance. Good agreement is obtained between the experimental data, simulations and analytical calculations. Our design can be modified to operate in other regions of the electromagnetic spectrum and these filters may be integrated with suitable detectors such as photodiodes and single photon avalanche diode (SPAD) arrays

Single-chip, mid-infrared array for room temperature video rate imaging

The need for energy efficiency and lower emissions from industrial plants and infrastructures is driving research into novel sensor technologies, especially those that allow observing and measuring greenhouse gases, such as CO2CO2. CO2CO2 emissions can be captured using mid-infrared imagers, but at present, these are based on hybrid technologies that need expensive manufacturing and require cooling. The high price tag prevents a wider diffusion of mid-infrared imagers and hence their use for many low-cost and large-volume applications. Here we report a monolithic III-V technology that integrates GaAs transistors with an InSb photodiode array. The monolithic material system reduces costs and provides an excellent platform for the sensor system-on-chip. We present a focal plane array imaging technology operating at room temperature in the 3–6 μm wavelength range that will address the need for identification and measurement of a range of industrially important gases.ESPRC 58833, ESPRC 6672