Ultrafast optical gating in a nonlinear lithium niobate microcavity

Abstract

Recent advances in optical simulation and computational techniques have renewed interest in high-finesse optical cavities for applications such as enhancing light-matter interactions, engineering complex photonic band structures, and storing quantum information. However, the extended interaction times enabled by these cavities often come at the cost of slow optical read-out protocols and limited control over system transients. To address this challenge, we demonstrate an ultrafast intra-cavity optical gating scheme in a high-finesse, second-order nonlinear microcavity incorporating a thin-film of lithium niobate. A femtosecond optical gate pulse -- tuned to the transparency region of the cavity's dielectric mirrors -- achieves instantaneous up-conversion of the intra-cavity field via sum-frequency generation. The resulting upconverted signal exits the cavity as a short pulse, providing space- and time-resolved, on-demand access to the intra-cavity state. We validate this approach by tracking the dynamics of multiple resonant modes excited in a plano-concave distributed Bragg reflector microcavity, showing close agreement with analytical models. Additionally, we demonstrate that stimulated intra-cavity difference-frequency generation can efficiently instantiate cavity modes on femtosecond timescales. This gating scheme is fully compatible with low-temperature microcavity experiments, paving the way for advanced quantum state storage, retrieval, and real-time control of light-matter interactions.

Figures (9)

• Figure 1: a. Schematic of the nonlinear microcavity structure. A $10\,\mu$m thick Mg:LiNbO$_3$ slab (TFLN) is wafer-bonded to the bottom flat DBR mirror (lower inset: micrograph of diced chip). The top curved mirror is held at a controllable distance above, defining the cavity length and transverse mode confinement (top inset: phase-contrast micrograph, featuring several curved micro-mirrors with varied radii of curvature, fabricated on an elevated SiO$_2$ mesa). A radius of curvature of $30\mu\,$m was used in our experiments. b. Transmission spectrum of a ca.$12\,\mu$m long cavity, illuminated by a broadband source polarized along the extra-ordinary axis of the TFLN slab, showing successive transverse resonances (measured mode images in insets). c. Illustration of the optical gating procedure. Cavity modes are resonantly populated by a 150 fs pump pulse. The co-propagating gating pulse (angled here is for clarity) arrives at a controlled delay time $\tau$, generating the SFG signal. d. Spectral arrangement of the experiment. The resonant cavity mode near 750 nm (red) is within the DBR stop-band. The gate pulse (1040 nm) and SFG signal (436 nm) are located in the transparency regions of the DBR.
• Figure 2: a, e. Transmission spectra of the cavity (red) when excited with a filtered pulse (gray) centered on the (a) fundamental and (e) $IG^e_{11}$ and $IG^o_{11}$ cavity modes. b, f. Snapshots of the transmitted SFG images at specific time-delays. c, g. SFG spectrograms. For clarity, the normalized intensity are corrected by a power of 0.5. d, h. Integral of the SFG spectrograms over the wavelength coordinates (black curve), and fitted decay dynamics (d) $A_1(\tau)$, (h) $A_2(\tau)$ (orange dashes).
• Figure 3: a. Transmission spectrum of the cavity (red) when excited with a broad-band resonant pulse (gray). Multiple longitudinal and transverse modes are excited. b. Snapshots of the transmitted SFG images at specific time-delays. c. SFG spectrogram. d. Fourier-transform of panel (b) along the time delay coordinate. For clarity, the normalized intensities in (c) and (d) are corrected by a power of 0.5.
• Figure 4: a. Illustration of the stimulated intra-cavity DFG generation process. The co-propagating pump and gate pulses (angled here is for clarity) reach the cavity with a controlled delay time $\tau$. Cavity modes are populated by stimulated intra-cavity DFG when the pump and gate signals overlap in space and time within the cavity. b. Spectral arrangement for intra-cavity DFG. The gate pulse (1040 nm) stimulates the DFG of resonant cavity photons near 750 nm. c Time-integrated emission spectrum of the cavity, as a function of delay between the pump and gate pulses. Top and side panels show line-cuts along the gray dashed lines, corresponding to $\tau=0$ ps and $\lambda=749.8$ nm respectively.
• Figure 5: Spectrum of the gating pulse used in the experiments, integrated using an optical spectrum analyzer (Yokagawa3670).