Kevin Werner

Ultra-fast bandgap photonics in mid-IR wavelengths

Ultrafast bandgap photonics in mid-infrared is an exciting area of nonlinear photonics, which shows different ultrafast damage characteristics of solids compared to that in near-infrared fields. It allows periodic surface nano-structures formation in low bandgap materials like germanium. Ultrafast mid-infrared field interaction at 2 micron wavelength with non-linear photonic crystal results in generation of high efficiency harmonic generation up to sixth harmonic.

Manifestations of photon acceleration in semiconductor metasurfaces (Conference Presentation)

The metasurface under study was engineered to enhance the local fields, which is crucial for the efficiency of the nonlinear photon conversion. In our design of the metasurface, we make use of high-Q collective resonances common to regular arrays of semiconductor particles, as verified by MIR FTIR spectroscopy. We facricate 600-nm-thick silicon rectangles situated on a sapphire substrate. In order to demonstrate power- dependent blue-shifting of THG, we focused a 200 ± 30 fs MIR laser pulses centered at λ = 3.62 μm, with a variable non-destructive fluence in the 1 < F < 6 mJ/cm2 range, onto the metasurface to achieve 5 < I < 30 GW/cm2 intensity. The advantage of operating in the MIR regime is that the refractive index change scales as ∆n(t) ∝ −N(t)λ^2, which is crucial for photon acceleration; here, N(t) is the time-dependent FC density. The concept of photon-acceleration-induced spectral shifting of the nonlinearly upconverted signal in a metasurface- based semiconductor cavity is as follows. MIR photons interact with, and get trapped by, the metasurface. As FCs are generated by four-photon absorption in silicon, the resonant frequency of the metasurface blue-shifts, and the frequency of the trapped photons follows. Accelerated MIR photons then upconvert via the standard χ(3) nonlinear process, resulting in the observed blue-shifting of the third harmonic generation (THG). As a result, the spectral peak and width of the THG light generated in the metasurface can be controlled by incident fluence. The central THG wavelength can be blue-shifted by more than 30 nm, enabling harmonics generation with center frequencies of up to ≈ 3.1ω. In contrast, the same measurements performed in unstructured silicon films yield no apparent spectral modifications to THG. A common belief is that the resonant enhancement of the THG must be accompanied by spectral narrowing; in contrast, here, due to photon acceleration, we observe spectral broadening of the resulting THG spectrum by approximately 50%. We connect the observed blue shift and broadening of the THG spectrum with the time-dependent nature of the complex eigenfrequency of the mode. Qualitative agreement is reached between the experimental data and the calculations based on a coupled mode theory with the eigenfrequency ωR(t) and damping factor γR(t) being driven by the pump pulse through free carrier generation. We find photon acceleration in semiconductor metasurfaces a promising tool for active control over the frequency of light in prospective nanophotonics devices.

All-optical control of resonant semiconductor metasurfaces for nonlinear mid-IR nanophotonics (Conference Presentation)

We designed semiconductor metasurfaces comprised of rectangular arrays of all-Si antennas exhibiting sharp transmission dips. The sizes and periodicities of the antennas were shown to control the spectral positions and linewidths of the corresponding antenna resonances. The designed resonant metasurfaces were fabricated on a 600-nm-thick silicon-on-sapphire substrate using electron beam lithography, hard mask deposition, and inductively coupled HBr plasma etching. Multiple areas with different antenna sizes were defined on the same substrate. The samples were characterized with a FTIR system under normal incidence and paraxial beam configuration. The spectra revealed transmission dips in the spectral range of 3–4 μm, with the central wavelengths corresponding to the local simulated field enhancements of Eloc = 10–15. The metasurfaces were irradiated by a train of femtosecond laser pulses from a supercontinuum-based mid-IR optical parametric amplifier (OPA) pumped by a Ti:Sapphire amplifier. The incoming 200 fs mid-IR pulses, centered at 3.6 μm, had a maximum fluence of 60 mJ/cm2. The transmitted third harmonic radiation was refocused on and detected by an InGaAs detector based short wave-IR spectrometer. For pump–probe experiments, the residual near-IR 782 nm beam was split off from the output of the OPA, sent through a delay line, and focused at the sample collinearly to the mid-IR beam. Using a home-built calibrated mid-IR spectrometer based on a blazed diffraction grating and a mid-IR camera (Electrophysics PV320), we observed a pump-dependent blue-shifting and eventual disappearance of the resonant dips, accompanied by the reduction of the third harmonic generation.

Probing the dynamics of the interaction between few-cycle laser pulses and single crystal (100) Si and GaAs near the laser-induced damage threshold

The dynamics of the laser-solid interaction with high intensity ultra-short s-polarized few-cycle pulses (FCPs) (Ephoton ~ 1.65 eV) and single crystals (100) Si and GaAs (Egap ~ 1.14 and 1.4 eV, respectivly) near the multipulse laser-induced damage threshold (LIDT) were measured using a pump-probe reflectivity technique. FCP’s with central wavelength 760 nm and FWHM duration 5 fs used as both pump and probe pulses were incident at 45°, and the reflectivity of each probe pulse was measured as the delay between the pump and probe pulses was varied with ~ 0.1 fs resolution. Near zero delay, the probe pulse reflectivity displayed oscillatory behavior relative to the unexcited reflectivity for both materials, with a period equal to the optical cycle (~2.6 fs). For Si, the crystal orientation was varied so that the field polarization was parallel to the (010) and (011) directions, and half way in between. Significantly larger zero delay oscillations were observed for the field polarization parallel to the (011) direction compared to those for the other two directions.

Single-shot femtosecond laser ablation of copper: experiment vs. simulation

Single 5 and 40 femtosecond, near IR pulses with fluences varying from 0.4 – 80 J/cm2 from a Ti:Sapphire laser was focused onto a single crystal Cu sample surface with 2.0 μm focal spot at 15 and 45 degree angle of incidence. The surface profiles after interaction were studied with an interferometric depth profiler (Wyko NT9100), and benchmarked against crater size and morphology predicted by 2D Particle-In-Cell (PIC) laser damage simulation model.