B. J. Stevens

Direct modulation of excited state quantum dot lasers

The use of the excited state quantum dot lasers for high speed direct modulation is proposed and demonstrated. A direct comparison of lasers utilizing the ground state and excited state from the same laser material reveals a factor of two increase in the K-factor limited bandwidth. This is attributed to an increase in the saturated gain and reduced carrier scattering time of the excited state compared to the ground state.

Epitaxially Regrown GaAs-Based Photonic Crystal Surface-Emitting Laser

A GaAs-based epitaxially regrown photonic crystal surface-emitting laser is proposed and demonstrated at room temperature. The photonic crystal band-structure is mapped through the angular dependence of subthreshold electroluminescence, allowing the photonic crystal coupling coefficients to be determined.

All semiconductor swept laser source utilizing quantum dots

An all semiconductor swept laser source demonstrating continuous sweeping over a >11nm range with a linewidth suitable for optical coherence tomography is proposed and demonstrated. The operation of this device relies upon state filling in a multiple contact laser utilizing quantum dots with strongly overlapping ground and excited states.

A Dual-Pass High Current Density Resonant Tunneling Diode for Terahertz Wave Applications

We report on a dual-pass high current density resonant tunneling diode (RTD) for terahertz wave applications. This technique reduces the overall fabrication complexity and improves the reproducibility for creating low resistance ohmic contacts. With our dual-pass technique, we demonstrate accurate control over the final device area by measuring the RTD current-voltage characteristic during the fabrication process and guiding the emitter current through the full RTD structure with a second contact electrode on the collector side. We go on to show how we may extract important information about the RTD performance using this method.

Negative differential gain due to many body effects in self-assembled quantum dot lasers

The gain spectrum of a quantum dot laser operating at 1300 nm is studied at high carrier densities, corresponding to dot occupancies of ∼8 e-h pairs per quantum dot. A reduction in peak gain with increasing carrier density is observed, attributed to the saturation of peak gain, yet the continuous increase in dephasing acting to broaden the individual quantum dot transitions.

Optimisation of Coupling between Photonic Crystal and Active Elements in an Epitaxially Regrown GaAs Based Photonic Crystal Surface Emitting Laser

The waveguide design of a GaAs based, epitaxially regrown photonic crystal surface emitting laser is discussed so as to optimise the coupling of the photonic crystal and the mode overlap with the quantum wells. Design criteria include the positioning of the quantum well and the photonic crystal layers, and the effect of varying aluminium composition in the lower cladding layer. Room-temperature, pulsed laser oscillation is demonstrated.

High Repetition Rate Ti:Sapphire Laser Mode-Locked by InP Quantum-Dot Saturable Absorber

We report the first demonstration of a Ti:sapphire laser mode-locked with a quantum-dot mode-locker (QDM) at repetition rates up to 1.77 GHz with 8-ps pulse duration and 400-mW average output power. So far, with quantum-well-based mode-lockers, a repetition rate of only up to 300 MHz has been achievable in our experiments. We show that QDM can support mode-locking in a wide range of repetition rates from 100 MHz to 1.8 GHz.

All-Semiconductor Photonic Crystal Surface-Emitting Lasers Based on Epitaxial Regrowth

The realization of all-semiconductor epitaxially regrown photonic crystal (PC) surface-emitting lasers is reported. PC coupling strengths, band structure, optimization of epitaxial regrowth, and operating characteristics are discussed. Room temperature operation allows agreement between theoretical and experimental band structure to be confirmed.

Tradeoffs in the Realization of Electrically Pumped Vertical External Cavity Surface Emitting Lasers

The design and realization of substrate emitting 980-nm electrically pumped vertical-external-cavity surface-emitting lasers (EP-VECSELs) is reported. A method to characterize the detuning of the cavity and spontaneous emission of the epitaxial material is described, and an experimental study of the effect of substrate doping on the operating characteristics of devices is presented. A reduction in optical loss and enhanced current-gain characteristics with a reduction in substrate doping from 2 × 108 to 4 × 1017 cm-3 is demonstrated. Spatial carrier distributions, evidenced by near-field profiling of devices without external feedback indicate similar current spreading behavior for the two-substrate dopings. Devices with diameter greater than 70 μm and current spreading layer thickness of 100 μm are shown to suffer from nonuniform carrier injection into the active region. Power scaling properties of the devices are investigated in both pulsed and CW operation. We realize devices with CW powers of 133 mW at 981 nm from a 150 μm device with 4 × 1017 cm-3 substrate doping at 0 °C, which is limited by nonoptimal cavity-gain peak detuning.

Electronic control of coherence in a two-dimensional array of photonic crystal surface emitting lasers

We demonstrate a semiconductor PCSEL array that uniquely combines an in-plane waveguide structure with nano-scale patterned PCSEL elements. This novel geometry allows two-dimensional electronically controllable coherent coupling of remote vertically emitting lasers. Mutual coherence of the PCSEL elements is verified through the demonstration of a two-dimensional Young’s Slits experiment. In addition to allowing the all-electronic control of the interference pattern, this type of device offers new routes to power and brightness scaling in semiconductor lasers, and opportunities for all-electronic beam steering.