Sagnac Interferometer for Two-Dimensional Femtosecond Spectroscopy in the Pump-Probe Geometry Samuel D. Park, Trevor L. Courtney, Dmitry Baranov, Byungmoon Cho, and David M. Jonas 1 1 Department of Chemistry and Biochemistry, University of Colorado, 215 UCB, Boulder, Colorado 80309, USA E-mail: david.jonas@colorado.edu Abstract: An intrinsically phase-stable Sagnac interferometer is introduced for enhanced sensitivity detection in partially collinear two-dimensional spectroscopy in the short-wave IR. The sensitivity and phase accuracy of the apparatus are demonstrated on the dye IR-26. OCIS codes: (320.7150) Ultrafast spectroscopy; (300.6290) Spectroscopy, four-wave mixing. Introduction Two-dimensional (2D) Fourier transform (FT) spectra show how a nonlinear signal field, as a function of radiated frequency, depends on an excitation frequency and the excitation-radiation delay, revealing the dynamics of coupling between excitations [1]. The pump-probe geometry offers simplified phasing but the last pulse and nonlinear signal co-propagate, which can make their interference more difficult to detect [2]. The new method presented here implements a Sagnac interferometer to increase sensitivity by decreasing excessive local oscillator (LO) amplitude. A Sagnac has been used previously for optical background suppression in pump-probe spectroscopies [3]. Despite its simplicity, implementing the Sagnac for 2D experiments has presented a challenge because 2D spectra are sensitive to phase. We have recently demonstrated a Sagnac for enhanced sensitivity in detecting absorptive 2D spectra in the short-wave IR (1-2 µm wavelength) [4], and have found conditions needed to maintain the necessary π phase shift. This work presents a partially collinear 2D spectrometer with a Sagnac (Fig. 1a), which creates a nearly background-free signal and selectively detects the absorptive 2D spectrum. The 2DFT interferometer has been tested on IR-26 dye (Fig. 1b) and a new Germanium beam splitter that offers further improvements has been characterized. This demonstration of 2DFT spectroscopy in the short-wave IR enables study of low-energy electronic processes crucial to next-generation photovoltaics.                  !" #" 1.60 1.70 1.80  t [rad/fs] -1.60 -1.70 -1.80   [rad/fs] T = 0 fs 1.60 1.70 1.80  t [rad/fs] T = 100 fs Fig. 1. a) Brewster’s angle Sagnac interferometer apparatus. The dark output of the Sagnac interferometer is used, where pulse c and the reference combine to result in an attenuated local oscillator used for interference with the 2D signal. b) 2D spectra of IR26 dye with 10% contours. Red and blue denotes positive and negative peaks, respectively. The 2D spectra show a rapid loss of correlation between excitation and detection frequencies as it approaches a product line shape at T = 100 fs. Experiment Pulses from a 1 kHz Ti:Sapphire regenerative amplifier pump a single-pass, short-wave IR noncollinear optical parametric amplifier with a PPSLT crystal [5]. The tunable pulses are compressed with a deformable mirror using SHG feedback in a genetic algorithm to pulse durations of ~20 fs. After the compressor, the beam is spatially filtered with a 50-µm pinhole. All spectral IR detection uses single-mode fiber coupling to a 0.15-m Czerny-Turner spectrometer and a 1024x1 pixel InGaAs array. Pump pulse pairs are generated by an actively phase stabilized Mach-Zehnder interferometer [6] with inconel- coated beam splitters set at Brewster’s angle to prevent multiple surface reflections. The probe path is split into counter-propagating probe and reference pulse pairs upon entering a Brewster’s angle Sagnac interferometer (Fig. 1a). The pumps and probe intersect at the sample within the interferometer ~1.5 ns after the reference excites the sample. When the beams are recombined at the thin-film gold-coated beam splitter, the attenuated probe (counterclockwise-propagating beam) and reference (clockwise-propagating beam) pulses exit the dark output of the interferometer nearly out of phase with each other, destructively interfering to generate an attenuated local oscillator. The Sagnac has an odd number of mirrors and a telescope. The telescope increases the nonlinear signal and introduces an additional inversion.