Inhibition of Linear Absorption in Opaque Materials Using Phase-Locked Harmonic Generation

We theoretically predict and experimentally demonstrate inhibition of linear absorption for phase and group velocity mismatched second- and third-harmonic generation in strongly absorbing materials, GaAs, in particular, at frequencies above the absorption edge. A 100-fs pump pulse tuned to 1300 nm generates 650 and 435 nm second- and third-harmonic pulses that propagate across a $450\mathrm{\text{−}}\mu \mathrm{m}$-thick GaAs substrate without being absorbed. We attribute this to a phase-locking mechanism that causes the pump to trap the harmonics and to impress on them its dispersive properties.

Figure 1

Pump and harmonic pulses propagating through a GaAs substrate. The Lorentz parameters are ${\omega }_p=9.425$, ${\omega }_r=2.98$, and $\gamma =0$ ($n\sim 3.41$) for the pump ($\gamma =0$ effectively makes the medium transparent to the pump); ${\omega }_p=9.425$, ${\omega }_r=2.98$, and $\gamma =0.5$ ($n=3.83+i0.18$) for the SH; and ${\omega }_p=9.425$, ${\omega }_r=2.98$, and $\gamma =1.65$ ($n=4.65+i1.25$) for the TH. Only the phase-locked components survive.

Figure 2

$k$-space spectrum of the fields depicted in Fig. 1. The vertical, dashed lines show the position of the homogeneous components had absorption been neglected.

Figure 3

Bottom: Pump (right axis) and harmonic (left axes) energies inside and to the right of the substrate. Harmonic generation occurs at entry (time $\sim 150$) and exit (time $\sim 1200$) surfaces, and the energies remain constant inside the substrate. At the exit surface, some SH energy is lost, while the TH signal nearly doubles in magnitude, although opposite behavior is also possible [15] using slightly different material dispersion.

Figure 4

Experimental setup. A beam generated by an optical parametric amplifier (OPA) irradiates a GaAs substrate. After passing through the sample, a prism [equilateral dispersing prism (EDP)] separates the harmonics. The three beams are coupled to a spectrometer via a liquid light guide (LLG) and measured using a near infrared photomultiplier tube (PMT) (for the pump) or a CCD (for the SH and TH). A moveable slit assembly provides spatial filtering so that only one beam reaches the LLG.

Figure 5

Measured spectra of the pump and the transmitted harmonics. The area under each spectrum corresponds to the measured energy ($2.9\mu \mathrm{J}$ for pump, $3.7\times {10}^{-8}\mu \mathrm{J}$ for SH, and $7.3\times {10}^{-9}\mu \mathrm{J}$ for TH).