Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO

Abstract

The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3–4 µm. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

High-sensitivity Frequency Comb Carrier-Envelope-Phase Metrology in Solid State High Harmonic Generation

“…HHG oriented along the centrosymmetric axis (0001, blue) yields predominately odd harmonics while the noncentrosymmetic axis (1-100, red) yields both even and odd harmonics. The peak at ∼385 nm appears to be consistent with photoluminescence on the centrosymmetric axis demonstrated in[25,26].…”

“…The impact of such polarization gating has been observed with gases, which exhibit a strong dependence on polarization [31]. However, solids such as MgO and ZnO exhibit anisotropy, and similar polarization dependent effects on harmonic generation are also expected [26,29,30]. When we measure the modulation depth (f ceo /f rep ) as a function of the 𝜆/4 plate angle, we see an increase of 23.3 dB at 45 degrees (Figure 4b).…”

“…When we measure the modulation depth (f ceo /f rep ) as a function of the 𝜆/4 plate angle, we see an increase of 23.3 dB at 45 degrees (Figure 4b). Furthermore, the shape of the curve, and singular peak of the ellipticity on the modulation depth (Figure 4b) suggests that a time-dependent elliptical profile on the driving pulse is largely maintained, despite the birefringence of the a-cut ZnO crystal [26,30]. Since the efficiency of HHG in ZnO, and most solids in general, possesses weaker sensitivity to the polarization [26,30] of the driving pulse than gas phase HHG, the shape of the modulation depth as a function of ellipticity is present but not sharp.…”

“…Furthermore, the shape of the curve, and singular peak of the ellipticity on the modulation depth (Figure 4b) suggests that a time-dependent elliptical profile on the driving pulse is largely maintained, despite the birefringence of the a-cut ZnO crystal [26,30]. Since the efficiency of HHG in ZnO, and most solids in general, possesses weaker sensitivity to the polarization [26,30] of the driving pulse than gas phase HHG, the shape of the modulation depth as a function of ellipticity is present but not sharp. Nonetheless, our measurements show that PASSAGE, and more generally polarization gating, can still be applied to solid state HHG in ZnO, yielding an increased sensitivity to the driving laser CEP.…”

“…The cut of the ZnO crystal (a-plane, [11][12][13][14][15][16][17][18][19][20] enables the crystal to be oriented such that excitation primarily occurs along either the centrosymmetric axis (0001, blue) or the noncentrosymmetric axis (1-100, red). Due to crystal symmetry, generation along the centrosymmetric axis predominantly yields odd harmonics of the fundamental 1550 nm driving laser, as well as photoluminesence at ∼385 nm [25,26]. Some even harmonics are also observed along the centrosymmetric axis, but are significantly weaker relative to the odd harmonics.…”