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(a) Configuration for time domain spectroscopy in a QCL. The TM polarized light pulses are focused on the facet of a QCL. The coupling efficiency is . (b) Time domain spectroscopy signals of an initial pulse propagated through the waveguide biased at 0%, 100%, and 200% of the threshold current. Shown are the transmitted pulses (solid lines), the initial pulse (dash dotted line) and calculations based on a linear time invariant material response (dashed red lines).
(a) shows the light-current (solid line) and voltage-current (dashed line) characteristics of the QCL. Light emission starts above the threshold current density of . (b) Pulse amplification after one pass (circles) and after three passes (triangles) through the resonator. The right axis is rescaled to show the material gain corresponding to the single pass data. The standard deviation is . The inset shows the spectrally resolved gain of the pulses after three passes at different bias currents.
(a) Calculation of the band structure at the design field strength of 33 and for . The barrier/well widths of each of the 35 periods in nanometers are 1.8/6.8/0.8/7.0/0.8/7.5/0.8/2.8/3.2/3.9/2.0/ / /4.6/1.2/4.8/1.1/5.0, where the barriers are in plain and the wells are in bold face. Layers that are doped to are underlined. The nominal emission frequency is . The vertical open arrow symbolizes the relaxation due to photon emission, while the sloped solid arrow shows the relaxation due to phonon emission. (b) The electrically induced gain at different bias current densities is shown in amplitude and phase. (c) Comparison of the gain center frequency (open triangles) to the emission spectra (gray solid lines) at different bias currents. We also plot the calculated stark shift (dotted line), the calculated shift due to band gap narrowing at elevated temperatures (dashed line) and the sum of both (solid line).
Effect of spectral pulse shaping after one pass (solid line), three passes (dashed line), and five passes (dash dotted line) through the cavity, measured at 110% of the threshold current density. In addition the normalized emission spectra at 150% (thin solid line) and 100% (thin dashed line) are shown. The inset shows the pulses before Fourier transformation. The parasitic reflections at 6.5 and at are caused in the beam path and cancel out in the normalization process.
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