Fourth-harmonic UV light generation in integrated silicon nitride microresonators

Abstract

Integrated silicon nitride (Si3N4) resonators have emerged as a leading platform for nonlinear photonics, yet generating light at wavelength in the ultraviolet (UV) has remained elusive in single-resonator systems. Here we report the first observation of fourth-harmonic generation reaching the blue and ultraviolet spectral regions in an integrated Si3N4 microring resonator. We systematically investigate the input-power dependence of the wavelength ranges supporting second-, third-, and fourth-harmonic generation, and study the input-power-dependent variation of the circulating fourth-harmonic signal in the UV. These results extend the operational bandwidth of integrated Si3N4 nonlinear photonic platforms to the lower edge of the material transparency window, enabling on-chip UV frequency conversion. Near-ultraviolet generation around 400 nm will enable on-chip excitation of defect-based quantum emitters in hexagonal boron nitride, enhance Raman spectroscopy through increased scattering cross-sections at shorter wavelengths, and support compact fluorescence-based bio-imaging platforms exploiting intrinsic cellular fluorophores.

Figures (7)

• Figure 1: Harmonic (second, third and fourth) generation in a single microresonator. (a) Schematics showing the fourth- (top), third- (middle) and second- (bottom) harmonic light generated in the cavity. The harmonic light is measured with a camera after applying bandpass filters (BPF) with transmission windows indicated in the figure. (b) With a pump wavelength of 1595 nm, we can reach the lower wavelength edge of the silicon nitride (Si$_3$N$_4$) transparency window by generating a fourth-harmonic signal. Although the intrinsic electronic bandgap of Si$_3$N$_4$ corresponds to a cut-off wavelength of approximately 243 nm ($\approx$5 eV), sub-bandgap absorption arising from band-tail states and defect-related electronic states shifts the practical optical transparency edge to around 400 nm in deposited films. (c) Image of the fourth-harmonic generation (FHG) measured with a camera with a bandpass filter at 405 nm with 10 nm bandwidth. (d) Third-harmonic generation (THG) captured through a bandpass filter at 532 nm with 10 nm bandwidth. (e) Second-harmonic image using a bandpass filter with a pass band from 790 nm to 850 nm. (f) Optical spectrum analyzer trace of the second harmonic signal.
• Figure 2: Experimental setup and resonator characterization. (a) Experimental setup with an inset camera image that shows the generation of all three harmonics simultaneously in a single Si$_3$N$_4$ microring resonator. PD: Photodiode, BPF: Bandpass filter. (b) Scanning electron microscope image of the Si$_3$N$_4$ ring resonator. (c) Transmission profile of a resonance at $1591$ nm showing its linewidth and quality factor. (d) Measured integrated dispersion as a function of wavelength for both the fundamental and higher-order spatial modes. The solid lines show the quadratic fits to the dispersion data points for each mode along with the zero dispersion line shown as gray dashes.
• Figure 3: Phase-matching regions for harmonic generation. (a-c) Wavelength ranges supporting generation of second, third, and fourth-harmonics for different on-chip input powers. The corresponding cold-cavity resonances are at (a) 1590.05 nm, (b) 1592.07 nm and (c) 1594.09 nm. The black arrow in panel (b) indicates the region supporting fourth-harmonic generation during the detuning scan at the highest input power; it is included for clarity. (d) Variation of the detuning span of the fourth-harmonic light at different input powers for a resonance near 1590 nm. (e) Transmission spectrum of several consecutive resonator modes (top row) and corresponding wavelength ranges supporting FHG (bottom row). The inset in the bottom row shows the reproducibility of FHG in another resonator of similar dimensions, despite fabrication-induced variations. FH: fourth-harmonic; TH: third-harmonic; SH: second-harmonic.
• Figure 4: Dependence of harmonic generation on input power. (a) Normalized scattered intensity of the fourth-harmonic signal as a function of input power, showing a gradual increase. The corresponding circulating power inside the resonator is indicated on the right axis. Panel (b) shows variation of the normalized scattered light intensities of different harmonics with increasing input power.
• Figure 5: Phase-matching conditions for harmonic generation. Panel (a) shows effective refractive indices as a function of wavelength showing phase-matching between the available TE modes at 1550 nm (in black color) and higher-order modes at fourth-harmonic (FH).Panel (b) depicts effective index matching between SH and FH modes. Solid and dashed curves denote TE and TM modes, respectively, with colors indicating harmonic orders (blue: FH, red: SH). Panels (c-e) show mode profiles of the pump TE$_{00}$ mode (c) and the corresponding phase-matched modes (effective refractive index of the harmonics close to that at the pump) at (d-e) FH, and (f) SH wavelengths.