Fig. 4: External cavity tuning. | Light: Science & Applications

Fig. 4: External cavity tuning.

From: Highly efficient surface-emitting semiconductor lasers exploiting quasi-crystalline distributed feedback photonic patterns

Fig. 4

a FTIR emission spectrum of the 2.9-mm-long Octonacci laser (W = 160 µm and L = 1.9 µm), operated at a temperature of 12 K and a 1% duty cycle, emitting multiple spectral lines labelled as: mode A (3.243 THz), mode B (3.235 THz), mode C (3.386 THz), mode D (3.396 THz), and mode E (3.440 THz). The emitted light is collected while moving a gold mirror positioned parallel to the laser top surface at a distance d, which is controlled by a piezoelectric driver. b Spectral dynamics of the emission for modes A and B at different mirror positions in the range 80 μm–230 µm. The dashed vertical lines indicate the tuning range of mode A (dashed brown lines) and mode B (dashed green lines). c Evolution of modes C, D and E as the mirror position is swept between 80 µm and 230 µm. d Contour plot of the emission spectra acquired by sweeping the mirror between 80 µm and 430 µm, highlighting the strong frequency dynamics at ~3.25 THz (modes A and B). The spectral lines at ~3.39 THz (modes C, D and E) are less influenced by the mirror movement. Each spectrum is acquired with a mirror position shift of 18 µm, and the intensity of the spectral lines is normalized, as described by the lateral colour bar. The red lines show the external cavity (comprising the top laser surface and the external mirror) frequency νcav as a function of the mirror position d. e Contour plot of the quality factor of the simulated resonating modes for the device with W = 160 µm and L = 1.9 µm in the presence of a mirror at a distance d in the 10 μm–150 µm range. When the bare external cavity frequencies (red lines) match the laser eigenmode frequencies at the appropriate mirror positions, the overall three-dimensional quality factor is strongly enhanced, as a result of the resonant coupling at 3.25 THz and 3.39 THz, in good agreement with the experimental results. A lower cut-off has been artificially set in panels d and e, so that the white areas represent the noisy spectral regions, i.e., the spectral windows without THz emission

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