Supplementary Information for
“Quadratic-soliton-enhanced mid-IR molecular
sensing”
Robert M. Gray,
1
∗
Mingchen Liu,
1
Selina Zhou,
1
Arkadev Roy,
1
Luis Ledezma,
1
and Alireza Marandi
1
†
1
Department of Electrical Engineering, California Institute of Technology,
Pasadena, CA 91125, USA
∗
Email: rmgray@caltech.edu
†
Email: marandi@caltech.edu
Abstract
This document provides supplementary information for the article titled “Quadratic-
soliton-enhanced mid-IR molecular sensing.” In Note 1, we provide details on our ex-
perimental setup and methods. In Note 2, we explain the numerical methods used for
simulating the system behavior and present additional numerical results which help to ex-
tend the results presented in the paper. In Note 3, we derive the important relationships for
understanding cavity-enhanced sensing in linear cavities, derive the
∆
α
eff
used for cal-
culating the enhancement shown for our experimental and simulated data in the main text,
and provide quantitative comparisons between our results and those that can be obtained
in linear absorption sensing. In Note 4, we extend our numerical model to include other
atmospheric gases and address the issue of selectivity. In Note 5, we introduce the basic
principles of single-mode intracavity sensing and describe in detail the important trade-offs
which must be considered in designing a sensor based on intracavity absorption in a tradi-
tional laser. In Note 6, we derive an expression for the output of an intracavity sensor based
on a continuous-wave optical parametric oscillator (OPO) operating at degeneracy and use
it to make semi-quantitative comparisons with the laser case derived in Note 5. Finally, in
Note 7, we introduce the basic theory of simulton formation in optical parametric oscilla-
tors and use it to provide a more intuitive understanding of the dynamics behind the sensing
mechanism.
1
1 Experimental Details
In this section, we explain in detail our experimental setup, which is similar to ref.
1
, as well
as our experimental procedure. Our system consists of a synchronously-pumped free-space
optical parametric oscillator (OPO) at 4.18 μm, shown in Fig. S1a. The OPO is pumped by
another PPLN-based OPO which provides pulses at 2.09 μm with a full-width, half-maximum
bandwidth of 155 nm and a 250-MHz repetition rate. The pump can provide up to 1400 mW
of average power, though the full power is not necessary for the performed sensing experiment.
Dielectric-coated mirror M1 is highly reflective for the signal at 4.18 μm and highly transmissive
for the pump at 2.09 μm, enabling coupling of the pump into the resonator. An anti-reflection
coated, plane-parallel orientation-patterned gallium phosphide (OP-GaP) crystal of length 0.5
mm with a poling period of 92.7 μm for type-0 phase matching of the pump and signal at room
temperature enables the nonlinear interaction. Concave gold mirrors M3 and M4 with a radius
of curvature of 24mm on either side of the OP-GaP crystal provide focusing and collimating
of the beam on the input and output sides of the crystal, respectively. Mirror M2 is a dielectric
mirror designed for 25% transmission of the signal around 4.18 μm, enabling outcoupling and
measurement of the generated signal. A piezoelectric actuator (PZT) on cavity mirror M1 allows
for tuning of the cavity length for entry into the simulton regime. The cavity can be locked using
the dither-and-lock protocol.
Measurement of the signal can be performed using either a photodetector (PD) or a com-
mercial Fourier-transform infrared spectrometer (FTIR) placed after the cavity. Mirror MM,
a magnetic-mounted gold mirror, allows for easy transition between the two. For performing
sensing measurements, only the photodetector (or, equivalently, a power meter) must be used,
but the FTIR allows for studying of the spectral behavior of the OPO in the presence of the
sample. The entire cavity and all measurement tools are placed inside a purging box. Purg-
2