Both numerical and experimental results show that precise control of the density-gradient scale length can switch flexibly between the coherent-wake-emission (CWE) and relativistic-oscillating-mirror (ROM) regimes of high-order harmonic generation when a solid surface is irradiated by subrelativistic midinfrared laser pulses. The use of longer-wavelength laser sources enables the achievement of steeper wavelength-normalized plasma gradients compared to near-infrared systems. Furthermore, wavelength scaling reduces the peak intensity for a given value of normalized vector potential and makes the laser-plasma interaction more tolerant to possible prepulses and picosecond pedestals. Detailed scans of the dependence of high-order harmonic energy on the delay between the ionizing prepulse and the main pulse demonstrate a pronounced maximum in harmonic conversion efficiency in the CWE regime at delays of $1<\mathrm{\Delta }\tau <2$ ps. The density-gradient expansion velocity is derived by matching the delay-traced CWE harmonic yield to its calculated dependence on the gradient scale length. The minimum density-gradient scale length achieved is $L<\lambda /100$. For longer delays, 12 $\le \mathrm{\Delta }\tau \le$ 65 ps, which correspond to smooth plasma-density gradients $0.2\lambda \lesssim L\lesssim \lambda$, remarkable persistence of the ROM harmonics is observed.

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• High-order harmonic generation
• Multiphoton or tunneling ionization & excitation
• Chirp
• Femtosecond laser irradiation
• High-harmonic generation
• High-order harmonic spectroscopy
• Laser ablation
• Spectro-temporal characterization
• Ultrafast femtosecond pump probe