Resonant Second-Order Nonlinear Optical Processes in Quantum Cascade Lasers

We demonstrate an efficient intracavity nonlinear interaction of laser modes in a specially adapted quantum cascade laser. A two-wavelength quantum cascade laser structure emitting at wavelengths of 7.1 and $9.5\mu \mathrm{m}$ included cascaded resonant optical intersubband transitions in an intracavity configuration leading to resonantly enhanced sum-frequency and second-harmonic generation at wavelengths of 4.1, 3.6, and $4.7\mu \mathrm{m}$, respectively. Laser peak optical powers of 60 and 80 mW resulted in 30 nW of sum-frequency signal and 10–15 nW of second-harmonic signal, both in good agreement with theoretical calculations.

Figure 1

(a),(b) Schematics of a portion of the conduction band diagram of the mixing SL and QC laser incorporating the optical nonlinearity, respectively. Shown also are the moduli squared of the relevant wave functions. In (a), the resonant nonlinearities result from transitions $1\to 2\to 3$. In (b), the QC-laser process is supported by levels 3, 2, and 1; the optical nonlinearities stem from resonant transitions $3\to 4\to 5$; level $g$ is the ground state of the injector region. Dotted lines in (b) indicate wave functions not directly involved in the laser or nonlinear processes. The solid arrows designate the nonlinear mixing process.

Figure 2

(a) High resolution spectra measured of a $10\mu \mathrm{m}$ wide and 1.6 mm long laser under pulsed operation at various levels of peak current as indicated. Individual longitudinal modes are broadened by thermal chirping, in particular, at high pulsed peak current levels. (b) Low resolution (1 meV) step-scan spectra of the same device measured under the same operating conditions in the short wavelength region. The spectra are offset with respect to each other, and the peaks are labeled in accordance with the main text. The small arrow indicates the spectral cutoff of one filter used in the experiment.

Figure 3

(a)–(c) Optical power measured for sum-frequency (SF) [open triangles, (b)] and second-harmonic (SH) [filled circles and squares for 7.1 (c) and 9.5 mm (a), respectively] signals versus power squared of the respective fundamental pump light. The symbols represent data, the straight lines are our calculations. (d) Light-output and voltage versus current ($L\mathrm{\text{−}}I\mathrm{\text{−}}V$) characteristic and combined short wavelength power of the laser of Fig. 2. The laser power has been split into its two wavelength components. The arrows pointing to the current axis indicate the positions where the spectra of Fig. 2 and the data points of (a)–(c) have been taken.