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Continuous wave operation of a superlattic quantum cascade laser emitting at 2 THz

Chris Worrall, Jesse Alton, Mark Houghton, Stefano Barbieri, Harvey E. Beere, David Ritchie, and Carlo Sirtori

Open Access Open Access

Abstract

We report the operation of a 2 THz quantum cascade laser based on a GaAs/Al0.1Ga0.9As heterostructure. The laser transition is between an isolated subband and the upper state of a 14 meV wide miniband. Lasing action takes place on a high order vertical mode of a 200 μm thick double-metallic waveguide. In pulsed mode operation, with a 3.16mm long device, we report a threshold current density of 115 A/cm2 at T = 4K, with a maximum measured peak power of 50 mW. The device shows lasing action in continuous wave up to 47K, with a maximum power in excess of 15 mW at T = 4K.

©2006 Optical Society of America

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Figures (6)
Fig. 1.
Fig. 1. Band diagram of 1 period of the active region under an electric field of 1.5 kV/cm. In red are the moduli squared wavefunctions of the upper (2) and lower (1) laser state. The green shaded areas represent the superlattice minibands. Highlighted are also the lower and upper miniband ground states g and 3 (thick green lines). Injection into state 2 occurs via resonant tunnelling from level g across a 5nm thick Al0.1Ga0.9As injection barrier. Starting from this barrier, from left to right, the layer sequence in nm is: 5/12.6/4.4/12.0/3.2/12.4/3.0/13.2/2.4/14.4/2.4/14.4/1.0/11.8/1.0/14.4, with the barriers in bold. The underlined layers are n-doped at levels of 1.3 × 1016 cm-3.

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Fig. 2.
Fig. 2. (a) 1-D mode intensity profile of the first, second and third order mode for the present 200μm-thick waveguide at f = 2THz (λ = 150 μm). The computed waveguide losses and overlap factor with the active region are respectively of 1.15 cm-1 , 0.045; 2.8 cm-1 , 0.145; 2.5 cm-1, 0.13. (b) Figure of merit χ = Γ/αw for the first three modes as a function of frequency. The waveguide thickness is 200 μm (c) Figure of merit χ = Γ/αw for the 2nd order mode at f = 2THz as a function of waveguide thickness.

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Fig. 3.
Fig. 3. V/J and P/J curves for a 3.16mm long device, with high reflectivity back-facet coating (SiO/Ti/Au). The V/J curve was collected in a three-terminal configuration. The laser was operated in pulsed mode with 250ns long pulses at a repetition rate of 80kHz. An additional 7Hz, 50% duty cycle slow modulation was superimposed to match the detector response time. Insets. Laser spectra recorded with a Fourier Transform Infrared Spectrometer with a resolution of 7.5 GHz (0.25 cm-1). The device was driven in continuous wave at T = 4K.

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Fig. 4.
Fig. 4. Low bias electroluminescence spectra collected with a Fourier Transform Infrared Spectrometer at T = 4K. The device was driven in pulsed mode with a 290 Hz repetition rate and 20% duty cycle. Inset. V/J curve. The coloured circles indicate the voltage and current density at which each spectrum was recorded.

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Fig. 5.
Fig. 5. CW output power vs current density characteristics as a function of the heat sink temperature. Inset. Threshold current density vs heat sink temperature in CW and pulsed mode for the same device of Fig. 3, and for the 2.9THz QCL of Ref. [11]. The solid lines are guides the eye.

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Fig. 6.
Fig. 6. Top. J/V curves at T = 4K of the present AR (red) and of the 2.9THz AR of Ref. [11] (blue). Devices were processed in rectangular 250×185 μm2 mesa structures. Bottom. Differential resistivity derived from the J/V curves. The drawings represent schematically two AR periods separated by the injection barrier, with the black arrow indicating the main current path. The shaded regions highlight the different transport regimes: (i) at low bias electrons transport between neighbouring minibands; (ii) at intermediate biases carriers are injected in the upper state via resonant tunnelling; (iii) at high voltages the structure breaks.

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