Luca Sulmoni

Self-Pulsation at Zero Absorber Bias in GaN-Based Multisection Laser Diodes

We study the influence of pump current and absorber bias voltage on the pulse width and frequency of sustained pulsation in GaN-based multisection laser diodes. The observed frequencies and pulse widths range from 1.5 to 4.5 GHz and 90 to 18 ps, respectively. Negative absorber bias is found to reduce the pulsation frequency and increase the pulse width. This behavior is explained by the tuneability of absorption and charge carrier lifetime in the absorber via the applied bias voltage.

Mode locking in monolithic two-section InGaN blue-violet semiconductor lasers

Passive mode-locked pulses with repetition frequencies in the range 40 to 90 GHz were observed in blue-violet GaN-based quantum-well lasers without external cavities. The lasers had two-section geometry with built-in saturable absorber section. The individual pulses had durations as short as 3–5 ps at peak powers of around 320 mW.

Static and dynamic properties of multi-section InGaN-based laser diodes

We have studied multi-section InGaN multiple-quantum-well (MQW) laser diodes grown on c-plane freestanding GaN substrate consisting of an absorber section (AS) and an amplifier gain section. As a result of the interplay between external bias applied to the AS and the internal piezoelectric and spontaneous polarization fields inherent to c-plane InGaN MQWs, the devices exhibit non-linear non-monotonic variations of the threshold current due to the quantum-confined Stark effect that takes place in the AS MQWs. We report on how this effect tailors the lasing characteristics and lasing dynamics, leading from a steady-state cw lasing regime for an unbiased AS to self-pulsation and Q-switching regimes at high negative absorber bias.

AlGaN-based deep UV LEDs grown on sputtered and high temperature annealed AlN/sapphire

The performance characteristics of AlGaN-based deep ultraviolet light emitting diodes (UV-LEDs) grown by metalorganic vapor phase epitaxy on sputtered and high temperature annealed AlN/sapphire templates are investigated and compared with LEDs grown on epitaxially laterally overgrown (ELO) AlN/sapphire. The structural and electro-optical properties of the devices on 350 nm sputtered and high temperature annealed AlN/sapphire show similar defect densities and output power levels as LEDs grown on low defect density ELO AlN/sapphire templates. After high temperature annealing of the 350 nm sputtered AlN, the full widths at half maximum of the (0002) and (101¯2) reflections of the high resolution x-ray diffraction rocking curves decrease by one order of magnitude to 65 arc sec and 240 arc sec, respectively. The curvature of the sputtered and HTA AlN/sapphire templates after regrowth with 400 nm MOVPE AlN is with −80 km−1 much lower than the curvature of the ELO AlN/sapphire template of −160 km−1. The on-wafer...

Solitary pulse-on-demand production by optical injection locking of passively Q-switched InGaN diode laser near lasing threshold

We report on optical injection locking of Q-switched InGaN multi-section diode laser from CW tunable laser to produce solitary pulses at precise wavelength. To the best of our knowledge, this has never been done before.

Passive mode-locking in the cavity of monolithic GaN-based multi-section laser diodes

We demonstrate picosecond pulse generation in the blue-violet wavelength region by passive intra-cavity mode-locking in GaN-based ridge waveguide laser diodes with monolithically integrated absorbers. For cavity lengths of 1.2 and 0.6 mm we observe repetition frequencies of 40 and 90 GHz, and pulse lengths of 7 and 4 ps, respectively. The results are explained by an extremely short, tunneling dominated carrier life time in the saturable absorber at high negative bias. The fast depletion of the charge carriers in the absorber is investigated by bias-dependent life-time measurements in the absorber.

Determination of polarization fields in group III-nitride heterostructures by capacitance-voltage-measurements

The polarization fields in wurtzite group III-nitrides strongly influence the optical properties of InAlGaN-based light emitters, e.g., the electron and hole wave function overlap in quantum wells. In this paper, we propose a new approach to determine these fields by capacitance-voltage measurements (CVM). Sheet charges generated by a change of the microscopic polarization at heterointerfaces influence the charge distribution in PIN junctions and therefore the depletion width and the capacitance. We show that it is possible to determine the strength and direction of the internal fields by comparing the depletion widths of two PIN junctions, one influenced by internal polarization fields and one without as a reference. For comparison, we conducted coupled Poisson/carrier transport simulations on the CVM of the polarization-influenced sample. We also demonstrate the feasibility and limits of the method by determining the fields in GaN/InGaN and GaN/AlGaN double heterostructures on (0001) c-plane grown by me...

Solitary Pulse-on-Demand Production by Optical Injection Locking of Passively Q-Switched InGaN Diode Lasers Near Lasing Threshold

We report on optical injection locking of Q-switched InGaN multi-section diode laser from CW tunable laser to produce solitary pulses at precise wavelength. To the best of our knowledge, this has never been done before.

The influence of the VCSEL design on its electrical modulation properties

Here we investigate the influence of the p- and n-oxide-aperture radii in all-semiconductor GaAs-based verticalcavity surface-emitting lasers (VCSELs), designed for 980 nm, on the modulation time constant (τ). Our analysis shows that the minimum value of τ is obtained if the oxide layers on both sides of the junction have identical depths. The simulations of the number of oxide layers on both p- and n-type sides reveal that double p- and n-oxidations are the most effective in the reduction of the modulation time constant as compared to single oxide layers.

10th order laterally coupled GaN-based DFB laser diodes with V-shaped surface gratings

Single longitudinal mode operation of laterally coupled distributed feedback (DFB) laser diodes (LDs) based on GaN containing 10th-order surface Bragg gratings with V-shaped grooves is demonstrated using i-line stepper lithography and inductively coupled plasma etching. A threshold current of 82 mA, a slope efficiency of 1.7 W/A, a single peak emission at 404.5 nm with a full width at half maximum of 0.04 nm and a side mode suppression ratio of > 23 dB at an output power of about 46 mW were achieved under pulsed operation. The shift of the lasing wavelength of DFB LDs with temperature was around three times smaller than that of conventional ridge waveguide LDs.