J. Hegarty - Academia.edu (original) (raw)

Papers by J. Hegarty

IEEE Journal of Quantum Electronics, 2000

Operation of an optoelectronic crossbar switch containing a terabit-per-second free-space o... more 2005) Operation of an optoelectronic crossbar switch containing a terabit-per-second free-space optical interconnect.

Proceedings of LEOS '93, 1993

ABSTRACT Laser diode amplifiers exhibit optical bistability when biased just below lasing thresho... more ABSTRACT Laser diode amplifiers exhibit optical bistability when biased just below lasing threshold, and have applications as optical memory elements and optical switches, due to their very low switching energy (fJ) and presence of optical gain. While LDAs operated in transmission have received much attention, the reflection mode of operation has been largely neglected. In this paper we present a detailed experimental and numerical investigation of Fabry Perot LDA (FP-LDA) behavior under light injection when operated in reflection. We observe transitions from anticlockwise to clockwise hysteresis with changing bias level or incident wavelength detuning. Moreover, we also report the first experimental observation of X-shaped (or butterfly hysteresis) in this system. Our results are in very good agreement with calculation

Laser Resonators II, 1999

ABSTRACT In this paper we focus on the impact of planar microcavity structures on the spontaneous... more ABSTRACT In this paper we focus on the impact of planar microcavity structures on the spontaneous emission times and bandwidths of light emitting diodes. We compare microcavity and non- microcavity devices and show that the light extraction is ten times more efficient for the microcavity devices, while the small-signal modulation bandwidth remains unchanged. Bandwidths in excess of 1 GHz are obtained. The power- bandwidth figure of merit for the microcavity devices is thus of the order of ten times greater than for the non- microcavity devices. The role of photon-recycling on the bandwidth is investigated. It is found that no major reduction in the bandwidth occurs due to recycling for devices up to 85 micrometers in diameter.

Physical Review B, 1996

The optical-gain spectra of a 40-Å ͑Zn,Cd͒Se/ZnSe multiple quantum well has been measured at vari... more The optical-gain spectra of a 40-Å ͑Zn,Cd͒Se/ZnSe multiple quantum well has been measured at various temperatures using a variable-stripe method. By comparison with a gain calculation including many-body effects, we have shown that gain in this structure arises from an electron-hole plasma ͑EHP͒ for temperatures higher than 140 K. Excellent agreement between experiment and theory allows us to demonstrate the significance of many-body effects such as carrier dephasing and Coulomb enhancement. The breakdown of the EHP model below 140 K indicates an increasing excitonic contribution.

IEE Proceedings - Optoelectronics, 1996

Egan, A. et al: Theoretical investigations of electro-optical synchonisation of self-pulsating la... more Egan, A. et al: Theoretical investigations of electro-optical synchonisation of self-pulsating laser diodes.

Sensors and Actuators B: Chemical, 1997

Oxygen O2 sensing using high-resolution direct optical absorption with a single mode GaAs-AlGaAs ... more Oxygen O2 sensing using high-resolution direct optical absorption with a single mode GaAs-AlGaAs distributed feedback (DFB) laser diode emitting at a wavelength around 761 nm which coincides with the b1∑g+(v′ = 0) ← X3∑g−(v″ = 0) electronic transition band is reported. In particular, issues relevant to O2 sensing in the high concentration regime, not previously of concern for trace gas

Physical Review B - PHYS REV B, 1982

We have investigated the process of quenching of fluorescence from the 3P0 excited state of Pr3+ ... more We have investigated the process of quenching of fluorescence from the 3P0 excited state of Pr3+ in LaF3 due to cross relaxation among the Pr ions. The fluorescence decay of the ensemble of Pr ions after pulsed excitation is measured as a function of temperature for Pr concentration of 20 mol%. About 12% of the ions are in nonquenching sites and contribute an exponential component to the decay. The remainder of the decay originates from the bulk of the ions in quenching sites. Cross relaxation occurs directly in steps after transfer within the inhomogeneous line. Knowledge of the dynamics occurring within the inhomogeneous line derived from fluorescence line-narrowing experiments is used to analyze the cross-relaxation process from a microscopic viewpoint based on a recent model for fluorescence in the presence of traps. At 2 K the decay is consistent with an electric dipole-dipole transfer mechanism with a nearest-neighbor quenching rate of 8.9 × 104 s-1 which is independent of tem...

$Research paper thumbnail of Hot-exciton relaxation in Cd_{x}Zn_{1-x}Te/ZnTe multiple quantum wells$

Physical Review Letters, 1991

Optoelectronics, IEE …, 1994

The authors report a comparison of the self-pulsing characteristics of two types of semiconductor... more The authors report a comparison of the self-pulsing characteristics of two types of semiconductor laser. They show that the self-pulsing frequency of the DFB laser is decoupled from the resonance frequency. They discuss the various characteristics, their ...

Physica B: Condensed Matter, 1993

A series of Cd ,, ,,Zn,, ,,TeiZnTe single quantum wells of thickness 40 A, 70 A and 100 A are stu... more A series of Cd ,, ,,Zn,, ,,TeiZnTe single quantum wells of thickness 40 A, 70 A and 100 A are studied using CW and time-resolved photoluminescence techniques. The exciton-phonon interaction is found to decrease with decreasing well thickness, this result having implications for room temperature devices. Time-resolved measurements reveal photoluminescence lifetimes increasing with temperature showing that the recombination at low temperatures is predominantly radiative.

Optics Letters, 1995

Received July 3, 1995 A robust scheme for the surface-normal optical interconnection of arrays of... more Received July 3, 1995 A robust scheme for the surface-normal optical interconnection of arrays of optoelectronic devices is demonstrated. Allowing for inversion, the optical system maintains registration between input and ...

Journal of the Optical Society of America B, 1998

We have investigated the mechanism of stimulated emission in ZnCdSe-ZnSSe quantum wells through o... more We have investigated the mechanism of stimulated emission in ZnCdSe-ZnSSe quantum wells through optically pumped measurements of the gain spectrum in a variety of structures from 270 to 77 K. We also calculated the optical gain, using a model that includes many-body effects, and found excellent agreement between the calculated gain line shapes and our measurements. Under the conditions studied, which are close to those found in an operating laser diode, we conclude that the stimulated emission arises from an electronhole plasma in our samples, even down to 77 K. Although our measurements do not rule out exciton gain mechanisms at other temperatures or operating conditions, sensitive line-shape fitting does not require them in our case. However, our line-shape analysis does show that Coulomb enhancement is significant, even at room temperature.

Infrared Physics & Technology, 1997

... The peak value of the individual absorption lines in the R branch was obtained by high resolu... more ... The peak value of the individual absorption lines in the R branch was obtained by high resolution spectroscopy using current tuning of the DFB laser diode emission wavelength. This approach is not possible with the V. Weldon et al. ...

IEEE Photonics Technology Letters, 2000

Spectroscopic-based gas sensing is demonstrated using micromachined single-frequency Fabry-Perot ... more Spectroscopic-based gas sensing is demonstrated using micromachined single-frequency Fabry-Perot (FP) laser diodes emitting at 1.331 m with a side mode suppression ratio of greater than 24 dB over a significant emission wavelength tuning range. The utility of these novel mode controlled FP lasers for spectroscopic detection of gases is assessed by investigating the methane-absorption band in the 1.33 m region. By probing the narrow linewidth R2 rotational absorption line, we demonstrate a low gas detection limit of 28 ppm1m and confirm the suitability of micromachined lasers in such applications.

Electronics Letters, 1985

ABSTRACT The effects of stimulated Raman scattering between two channels in a 45 km optical fibre... more ABSTRACT The effects of stimulated Raman scattering between two channels in a 45 km optical fibre transmission system is measured at 1.5 μm as a function of channel separation and power. The penalties on received power in wavelength-division-multiplexed systems are calculated from the data.

Electronics Letters, 1996

... V. Weldon, J. O'IGorman, JJ PCrez-Camacho and J. Hegarty ... We have tar-geted and obser... more ... V. Weldon, J. O'IGorman, JJ PCrez-Camacho and J. Hegarty ... We have tar-geted and observed the complete R rotational branch of the 0, spin forbidden blC,+(v' = 0) t x'C,-(v = 0) electronic transition which is centred at 762nm, and have estimated a detection limit of 20ppm.m by ...

Applied Surface Science, 1991

The technique of resonant Rayleigh scattering is used to determine the homogeneous linewidth acro... more The technique of resonant Rayleigh scattering is used to determine the homogeneous linewidth across the inhomogeneously broadened exciton resonance in a Cd0.25Zn0.75Te/ZnTe multiple quantum well structure. An order of magnitude increase of the Rayleigh scattering signal over background is observed on tuning a narrow-band laser through the exciton resonance at low temperatures. Spectral and temporal measurements show the effect to be a true scattering process rather than luminescence. The interface and alloy fluctuations in the quantum well give rise to spatial fluctuations in the dielectric response of the system while the large exciton resonance causes strong enhancement of scattering. The homogeneous linewidth was calculated across the exciton resonance. The technique is compared with the dephasing and hole-burning techniques more commonly used in homogeneous linewidth measurements.

Applied Physics Letters, 1999

... the modes to a central pillar with a diameter ≈3 μm, with reflectivities of as high as 90 ...... more ... the modes to a central pillar with a diameter ≈3 μm, with reflectivities of as high as 90 ... kA cm using the expression where n is the carrier density per unit area, q is the ... be compared to the threshold gain calculated from laser theory by approximating quasiradial modes as confined ...