Quadratic-soliton-enhanced mid-IR molecular sensing

Abstract

Optical solitons have long been of interest both from a fundamental perspective and because of their application potential. Both cubic (Kerr) and quadratic nonlinearities can lead to soliton formation, but quadratic solitons can practically benefit from stronger nonlinearity and achieve substantial wavelength conversion. However, despite their rich physics, quadratic cavity solitons have been used only for broadband frequency comb generation, especially in the mid-infrared. Here, we show that the formation dynamics of mid-infrared quadratic cavity solitons, specifically temporal simultons in optical parametric oscillators, can be effectively leveraged to enhance molecular sensing. We demonstrate significant sensitivity enhancement while circumventing constraints of traditional cavity enhancement mechanisms. We perform experiments sensing CO₂ using cavity simultons around 4 μm and achieve an enhancement of 6000. Additionally, we demonstrate large sensitivity at high concentrations of CO₂, beyond what can be achieved using an equivalent high-finesse linear cavity by orders of magnitude. Our results highlight a path for utilizing quadratic cavity nonlinear dynamics and solitons for molecular sensing beyond what can be achieved using linear methods.

Fig. 1. Enhanced sensing using quadratic cavity simultons.

a Schematic depiction of sensing in the simulton regime of a synchronously-pumped optical parametric oscillator at degeneracy. The bright soliton in the signal interacts with the sample every round trip, and the resulting competing nonlinear dynamics generate the measured signal response. b Specifically, stable simulton operation requires the simulton group advance, ΔT, to balance the round-trip group delay, ΔT_RT, and the parametric gain to balance the sample loss, α_samp, and output coupling. c Schematic representation of linear absorption sensing governed by the Beer-Lambert Law for light interacting with a sample over a path length L. d Linear methods (light blue region) face limitations in the achievable sensitivity at high sample concentrations. In contrast, active cavity sensing with quadratic cavity (orange) can achieve high sensitivities at high sample concentrations. T_rep, pump repetition period; T_circ, pulse circulation time in the cavity; ΔT, simulton group advance; T_RT, cold cavity round-trip time ΔT_RT, round-trip group delay; χ⁽²⁾, second-order susceptibility; ω, angular frequency; α_samp, sample absorption coefficient; OC, output coupling; P_in, input power; P_out, output power; L, path length; ℏ, reduced Planck’s constant.

Fig. 2. Quadratic cavity simulton enhancement mechanism.

a Theoretical behavior of near-threshold sensing, wherein the addition of sample causes an increase in threshold, resulting in a decrease in signal power at the sensing point. b The corresponding signal enhancement grows asymptotically as threshold is approached. c Experimentally measured input-output power relationships for the simulton (orange) and conventional (pink) regimes show the extremely high slope efficiency and high threshold of the simulton, suggesting its potential for near-threshold sensing with high SNR. Solid lines capture the trends through linear fits of the experimental data while the orange, dashed line shows the corresponding simulton simulation.

Fig. 3. Simulton dynamics responsible for sensing.

a Experimental power spectral densities demonstrate reduced power across the entire simulton spectrum with the addition of sample despite the relatively narrow CO₂ absorption feature. b In the far above threshold conventional regime, like other general multi-mode lasers, power in non-absorbing modes increases with the addition of sample, largely compensating the loss in the absorbing modes. c Schematic depiction of the temporal dynamics of cavity simulton formation which enable the sensing enhancement mechanism. Additional loss in the round trip limits the ability of the simulton to deplete the pump and accelerate, leading to a reduced gain for all modes at steady-state. d Simulated steady-state pulse position as a function of gas concentration (left). Comparison with the theoretical gain window (right) shows the simulton moving further towards the gain window edge as the sample concentration is increased, in accordance with (c).

Fig. 4. Sensing behaviors of quadratic cavity solitons.

a Measured output power as a function of CO₂ concentration for different number of times above threshold, N. A high sensitivity of 4.1 mW/ppm is measured near threshold, emphasized using the solid trend lines. b Simulations of the simulton response to the addition of CO₂ at various number of times above threshold exhibit good qualitative agreement with the experimental data. c Equivalent path-length enhancement calculated for neighboring points in the experiment, showing a measured enhancement as large as 6000. Solid lines show the expected asymptotic enhancement corresponding to the linear fits in (b), with dashed lines extending these fits to enable extrapolation of detector-limited enhancements for detection bandwidths of 1 MHz (x’s) and 1 Hz (stars). d Simulated change in output power as a function of CO₂ concentration with a linear fit (dashed line) showing good linearity over a dynamic range of 10⁷. e Measured sensitivity as a function of CO₂ concentration in direct comparison with linear sensing (light blue), demonstrating orders of magnitude sensitivity improvement over linear methods at high sample concentrations.