Continuous Frequency Tuning with near Constant Output Power in Coupled Y-Branched Terahertz Quantum Cascade Lasers with Photonic Lattice

Iman Kundu,Joshua R Freeman,Alexander Valavanis,Yingjun Han,Lianhe Li,Paul Dean,Mark C Rosamond,Edmund H Linfield,A Giles Davies

doi:10.1021/acsphotonics.8b00251

Iman Kundu, Joshua R Freeman + Show 7 more

Open Access

https://doi.org/10.1021/acsphotonics.8b00251

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Journal: ACS photonics	Publication Date: Jun 19, 2018
Citations: 18	License type: cc-by

Affiliation: University of Leeds

Abstract

We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-μm-wide coupler and S-bend section, which are connected to an 88-μm-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modeling and the spectral characteristics are modeled using a transfer matrix model. Continuous frequency tuning of ∼19 GHz is demonstrated while maintaining an output power of ∼4.2–4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically control...

Highlights

Terahertz-frequency quantum cascade lasers (THz QCLs)[1] are compact sources of radiation in the THz frequency band of the electromagnetic spectrum
A peak output power of 4.8 mW was measured from the coupler section facet for both QCLs, while peak output powers of 8 and 6.5 mW were recorded from the Y-branch facets of QCL1 and QCL2, respectively
The electrically controlled continuous frequency tuning range achievable in double metal (DM) THz QCLs has been increased by coupling two QCLs laterally along the length of the cavity to form a Y-branched coupler with integrated photonic lattices (PLs) structures

Summary

■ RESULTS

QCL1 was driven above threshold (at current amplitudes IQCL,1 = 0.5−1 A) and IQCL,2 = 0.5−1.5 A This continuous tuning is due to the Vernier frequency selection in the coupled device, and coincided with an increase in the output power. Increasing the heat sink temperature changes the refractive index contrast in the PL.[19] In order to increase the continuous tuning range, the total drive current supplied to the composite device was increased to ∼3 A Through these changes an emission frequency centered at ∼3.36 THz was recorded at THS = 50 K, as drive currents in both QCL ridges were varied in the range 0.5−1.6 A. The output power varies only slightly in the range ∼4.2−4.8 mW (Figure 7d, arrow) when the linear shift in emission frequency is recorded, and is due to the constant net gain in the coupled device at these drive conditions. Such a system can find application in both spectroscopy and interferometric applications, which require continuous tuning over a narrow frequency range (several GHz), and in multifrequency imaging which require discrete frequency tuning (tens of GHz)

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES