David T. D. Childs

High-performance three-layer 1.3-/spl mu/m InAs-GaAs quantum-dot lasers with very low continuous-wave room-temperature threshold currents

The combination of high-growth-temperature GaAs spacer layers and high-reflectivity (HR)-coated facets has been utilized to obtain low threshold currents and threshold current densities for 1.3-/spl mu/m multilayer InAs-GaAs quantum-dot lasers. A very low continuous-wave (CW) room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A/cm/sup 2/ are achieved for a three-layer device with a 1-mm HR/HR cavity. For a 2-mm cavity, the CW threshold current density is as low as 17 A/cm/sup 2/ for an HR/HR device. An output power as high as 100 mW is obtained for a device with HR/cleaved facets.

1.3 µm Room Temperature Emission from InAs/GaAs Self-Assembled Quantum Dots

We have investigated the growth conditions necessary to achieve strong room temperature emission at 1.3 µm for InAs/GaAs self-assembled quantum dots (QDs) using conventional solid source molecular beam epitaxy (MBE). A relatively high substrate temperature and very low growth rate (LGR) result in long wavelength emission with a small linewidth of only 24 meV. Atomic Force Micrographs obtained from uncapped samples reveal several differences between the LGRQDs and those grown at higher growth rates. The former are larger, more uniform in size and their density is lower by a factor of about 4. LGRQDs have been incorporated in p-i-n structures and strong room temperature electroluminescence detected. The light output of the QD p-i-n diodes is found to be significantly higher than a quantum well (QW) sample at least for current densities up to 0.5 kAcm-2.

A photomodulated reflectance study of InAs/GaAs self-assembled quantum dots

Photomodulated reflectance (PR) spectra have been measured for self-assembled InAs/GaAs quantum dot(QD) structures consisting of a pair of QD layers, with a GaAs spacer either 50 or 100 A thick. The PR clearly reveals five confined-state QD transitions, at both 80 and 300 K, as well as features from the two-dimensional confining and GaAs layers. The measuredQD transition energies correlate well with photoluminescencespectra at 13 K, using high laser excitation powers to incur level filling. Annealing one of the samples produces a strong blueshift in the QD transitions.

Sensitivity Advantage of QCL Tunable-Laser Mid-Infrared Spectroscopy Over FTIR Spectroscopy

Abstract Interest in mid-infrared spectroscopy instrumentation beyond classical FTIR using a thermal light source has increased dramatically in recent years. Synchrotron, supercontinuum, and external-cavity quantum cascade laser light sources are emerging as viable alternatives to the traditional thermal black-body emitter (Globar), especially for remote interrogation of samples (“stand-off” detection) and for hyperspectral imaging at diffraction-limited spatial resolution (“microspectroscopy”). It is thus timely to rigorously consider the relative merits of these different light sources for such applications. We study the theoretical maximum achievable signal-to-noise ratio (SNR) of FTIR using synchrotron or supercontinuum light vs. that of a tunable quantum cascade laser, by reinterpreting an important result that is well known in near-infrared optical coherence tomography imaging. We rigorously show that mid-infrared spectra can be acquired up to 1000 times faster—using the same detected light intensity, the same detector noise level, and without loss of SNR—using the tunable quantum cascade laser as compared with the FTIR approach using synchrotron or supercontinuum light. We experimentally demonstrate the effect using a novel, rapidly tunable quantum cascade laser that acquires spectra at rates of up to 400 per second. We also estimate the maximum potential spectral acquisition rate of our prototype system to be 100,000 per second.

Scanning Transmission Electron Microscopy (STEM) Study of InAs/GaAs Quantum Dots

Scanning transmission electron microscopy (STEM) and energy dispersive X-ray analysis (EDX) have been used to investigate the size and composition of InAs/GaAs quantum dot (QDs). It is shown that the QD exist within the wetting layer and not on it. In QD bilayers where the dots are uncorrelated along the growth direction a comparison of the indium EDX signals from the wetting layer (WL) and a dot allow us to estimate the compositions of these regions as In0.07Ga0.93As and In0.31Ga0.69As respectively. We have used the STEM technique to investigate the effects of annealing QDs in order to modify the emission energy. EDX measurements show that the dots increase in size by a factor of 2 for the longest anneals and there is a concomitant decrease in the indium concentration resulting in blue shifts up to 300 meV and a narrowing of the linewidth to ~12 meV.

Superluminescent diode with a broadband gain based on self-assembled InAs quantum dots and segmented contacts for an optical coherence tomography light source

We report a broadband-gain superluminescent diode (SLD) based on self-assembled InAsquantum dots(QDs) for application in a high-resolution optical coherence tomography(OCT) light source. Four InAsQD layers, with sequentially shifted emission wavelengths achieved by varying the thickness of the In0.2Ga0.8As strain-reducing capping layers, were embedded in a conventional p-n heterojunction comprising GaAs and AlGaAs layers. A ridge-type waveguide with segmented contacts was formed on the grown wafer, and an as-cleaved 4-mm-long chip (QD-SLD) was prepared. The segmented contacts were effective in applying a high injection current density to the QDs and obtaining emission from excited states of the QDs, resulting in an extension of the bandwidth of the electroluminescence spectrum. In addition, gain spectra deduced with the segmented contacts indicated a broadband smooth positive gain region spanning 160 nm. Furthermore, OCTimaging with the fabricated QD-SLD was performed, and OCTimages with an axial resolution of ∼4 μm in air were obtained. These results demonstrate the effectiveness of the QD-SLD with segmented contacts as a high-resolution OCT light source.

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.

1.3 μm InAs/GaAs quantum dot led

We have determined the growth conditions which result in a narrow linewidth and room temperature emission at 1.3pm from InAs/GaAs Quantum dots (QDs). The QDs formed under these conditions are extremely uniform in size and exhibit an emission linewidth of only 25meV. Single QD layers have been incorporated into p-i-n diodes which exhibit strong electroluminescence. We have compared the efficiency of these devices with a nominally identical quantum well device. The QD based device exhibits a higher electroluminescence efficiency, especially at low current densities. At higher current densities there is a loss of efficiency due to recombination from excited states.

Near-infrared and mid-infrared semiconductor broadband light emitters

Semiconductor broadband light emitters have emerged as ideal and vital light sources for a range of biomedical sensing/imaging applications, especially for optical coherence tomography systems. Although near-infrared broadband light emitters have found increasingly wide utilization in these imaging applications, the requirement to simultaneously achieve both a high spectral bandwidth and output power is still challenging for such devices. Owing to the relatively weak amplified spontaneous emission, as a consequence of the very short non-radiative carrier lifetime of the inter-subband transitions in quantum cascade structures, it is even more challenging to obtain desirable mid-infrared broadband light emitters. There have been great efforts in the past 20 years to pursue high-efficiency broadband optical gain and very low reflectivity in waveguide structures, which are two key factors determining the performance of broadband light emitters. Here we describe the realization of a high continuous wave light power of >20 mW and broadband width of >130 nm with near-infrared broadband light emitters and the first mid-infrared broadband light emitters operating under continuous wave mode at room temperature by employing a modulation p-doped InGaAs/GaAs quantum dot active region with a ‘J’-shape ridge waveguide structure and a quantum cascade active region with a dual-end analogous monolithic integrated tapered waveguide structure, respectively. This work is of great importance to improve the performance of existing near-infrared optical coherence tomography systems and describes a major advance toward reliable and cost-effective mid-infrared imaging and sensing systems, which do not presently exist due to the lack of appropriate low-coherence mid-infrared semiconductor broadband light sources.

Coherently Coupled Photonic-Crystal Surface-Emitting Laser Array

The realization of a 1 × 2 coherently coupled photonic crystal surface emitting laser array is reported. New routes to power scaling are discussed and the electronic control of coherence is demonstrated.