Jintian Zhu

High-power quantum cascade lasers grown by low-pressure metal organic vapor-phase epitaxy operating in continuous wave above 400K

High-power quantum cascade lasers (QCLs) working in continuous wave (cw) above 400K are presented. The material was grown by low-pressure metal organic vapor-phase epitaxy and processed into narrow buried heterostructure lasers. A cw output power of 204mW was obtained at 300K with an 8.38μm wavelength, 3mm long and 7.5μm wide coated laser. The device operates in cw mode above 400K, which exceeds the previous maximum cw temperature operation of QCLs by approximately 60K. Preliminary reliability data obtained by accelerated aging tests indicate a remarkable robustness of the lasers.

Bowtie plasmonic quantum cascade laser antenna

We report a bowtie plasmonic quantum cascade laser antenna that can confine coherent mid-infrared radiation well below the diffraction limit. The antenna is fabricated on the facet of a mid-infrared quantum cascade laser and consists of a pair of gold fan-like segments, whose narrow ends are separated by a nanometric gap. Compared with a nano-rod antenna composed of a pair of nano-rods, the bowtie antenna efficiently suppresses the field enhancement at the outer ends of the structure, making it more suitable for spatially-resolved high-resolution chemical and biological imaging and spectroscopy. The antenna near field is characterized by an apertureless near-field scanning optical microscope; field confinement as small as 130 nm is demonstrated at a wavelength of 7.0 mum.

High-temperature continuous wave operation of strain-balanced quantum cascade lasers grown by metal organic vapor-phase epitaxy

The authors report the fabrication of high-power strained quantum cascade lasers working in continuous mode above 370K. The devices, processed in narrow buried heterostructures, were grown by low-pressure metal organic vapor-phase epitaxy. Continuous wave output power as high as 312mW at 300K was obtained at a wavelength of 5.29μm from a 3.25mm long, 7.5μm wide laser with a high-reflectivity back facet coating. The slope efficiency was in excess of 1.5W∕A and the power conversion efficiency reached almost 5%.

High temperature continuous-wave operation of 1.3- and 1.55-/spl mu/m VCSELs with InP/air-gap DBRs

We demonstrate novel electrically pumped 1.3- and 1.55-/spl mu/m vertical cavity surface emitting lasers (VCSELs) with two InP/air-gap distributed Bragg reflectors (DBRs). The active regions comprise conventional InGaAsP multiple quantum wells. A tunnel junction is placed between the active region and the top DBR to convert electrons into holes, thus minimizing the use of p-type material in the structure to reduce the free-carrier loss and achieve current confinement. The whole structure was grown in a single growth run by low-pressure metal-organic chemical vapor deposition (MOCVD). For both 1.3and 1.55-/spl mu/m emission wavelengths, air-gap DBR VCSELs exhibit room-temperature continuous wavelength (CW) threshold current density as low as 1.1 kA/cm/sup 2/, differential quantum efficiency greater than 30%, and CW operation up to 85/spl deg/C. The single-mode output power was 1.6 mW from a 1.3 /spl mu/m VCSEL with a 6.3-/spl mu/m aperture and 1.1 mW from a 1.55 /spl mu/m VCSEL with a 5.7-/spl mu/m aperture under room temperature CW operation.

Single-mode laser action in quantum cascade lasers with spiral-shaped chaotic resonators

The authors have fabricated and characterized quantum cascade lasers with spiral-shaped microresonators. The lasers operate in pulsed mode at room temperature with peak optical power greater than 20mW and in continuous wave at temperatures up to 125K. They exhibit single-mode emission in both pulsed mode and continuous wave operation, with a 30dB side-mode suppression ratio at injection currents well above threshold. Subthreshold spectral measurements indicate that the spiral cavities support whispering-gallery-like modes. Single-mode lasing occurs on one of these modes. Far-field profiles reveal enhanced directionality compared to microdisk lasers.

Near-field imaging of quantum cascade laser transverse modes.

We report near field imaging of the transverse lasing modes of quantum cascade lasers. A mid-infrared apertureless near-field scanning optical microscope was used to characterize the modes on the laser facet. A very stable mode pattern corresponding to a TM(00) mode was observed as function of increasing driving current for a narrow active region quantum cascade laser. Higher order modes were observed for devices with a larger active region width-to-wavelength ratio operated in pulsed mode close to threshold. A theoretical model is proposed to explain why specific transverse modes are preferred close to threshold. The model is in good agreement with the experimental results.

Pulsed- and continuous-mode operation at high temperature of strained quantum-cascade lasers grown by metalorganic vapor phase epitaxy

We present the pulsed operation at room temperature of different strained InGaAs∕AlInAs quantum-cascade lasers grown by low-pressure metalorganic vapor-phase epitaxy. Devices based on a bound-to-continuum transition design have threshold current densities in pulsed mode as low as 1.84kA∕cm2 at 300K. Identical lasers grown at higher rate (0.5nm∕s) also have threshold current densities lower than 2kA∕cm2 at 300K. Buried heterostructure lasers based on a double phonon resonance design were operated in continuous mode up to 280K. Overall, the performance obtained from strained quantum cascade lasers deposited by metalorganic vapor-phase epitaxy are comparable with that of similar structures grown by molecular beam epitaxy.

Time-domain upconversion measurements of group-velocity dispersion in quantum cascade lasers

A time-resolved mid-infrared upconversion technique based on sum-frequency generation was applied to measure pulse propagation in lambda approximately 5.0 mum quantum cascade lasers operated in continuous wave at 30 K. The wavelength-dependent propagation delay of femtosecond mid-infrared pulses was measured to determine the total group-velocity dispersion. The material and waveguide dispersion were calculated and their contributions to the total group-velocity dispersion were found to be relatively small and constant. The small-signal gain dispersion was estimated from a measurement of the electroluminescence spectrum without a laser cavity, and was found to be the largest component of the total GVD. A negative group-velocity dispersion of beta2 ( =d2beta/d omega2) approximately - 4.6x10-6 ps2/mum was observed at the peak emission wavelength, and good agreement was found for the measured and calculated pulse-broadening.

High temperature continuous-wave operation of 1.3-1.55 /spl mu/m VCSELs with InP/air-gap DBRs

High temperature CW operation of electrically pumped 1.3-1.55 /spl mu/m MQW VCSELs using top and bottom InP/air-gap DBRs with record low threshold current density is demonstrated.

40-gb/s EA modulators with wide temperature operation and negative chirp

Broad bandwidth external modulators are widely used in optical fiber networks to avoid the chirp associated with the direct modulation of laser diode sources. For transmission application beyond 10 Gb/s, electroabsorption modulators (EAMs) offer many advantages such as low drive voltage, low chirp characteristics, small size, and added functionality through integration with a distributed feedback laser. In this letter, we present static and dynamic characterization of an EAM demonstrating the possibility to have with the same device uncooled operation at 40 Gb/s and negative chirp at low negative bias. These features are attractive for cost reduction in short link applications and for long-haul transmission. All the measurements are performed at 40 Gb/s from room temperature up to 60/spl deg/C demonstrating a negative chirp behavior for the stand-alone modulator over the entire temperature range.