Near-infrared and mid-infrared semiconductor broadband light emitters

Abstract

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.

Keywords: broadband light emitters; optical coherence tomography; quantum cascade structure; quantum dot.

Figure 1

Cross-sectional transmission electron microscopic images of (a) the multiple InAs/GaAs quantum dot active layer structure and (b) the In_0.678Ga_0.322As/In_0.365Al_0.635As quantum well cascade active layer structure.

Figure 2

(a) Schematic device diagrams of the ‘J’-shaped QD and QC devices. (b) P–I characteristics of the un-doped and p-doped QD-BLEs measured at RT under CW operation. Insets: the corresponding emission spectra of the QD devices under various injection currents.

Figure 3

Scanning electron microscope (SEM) images of the MIR devices before (a) and after (b) focus ion beam milling.

Figure 4

Threshold modal gain of the ‘J’-shaped QC lasers as a function of the current density. Inset: the light–injection current density characteristics of the MIR-QC devices with different rear facet angles of 0°, 15°, 17° and 19°.

Figure 5

(a) Top-view microscopic image of the device. SEM images of the narrow facet (b) and the wide facet (c) of the broadband QC device.

Figure 6

(a) and (c) Light–current characteristics of the wide and narrow emitting facets and (b) and (d) the corresponding emission spectra from the wide and narrow emitting facets, respectively, measured under CW operation mode at different temperatures from 80 to 300 K (I=4.5 A).

Figure 7

Schematic diagram of the two simultaneous incoherent light emissions in the MIR-QC broadband light emitter.