INTRODUCTION

Since the advent of mode-locked lasers,1 pump-probe optical measurements have been widely used to study the ultrafast dynamics of optical, electronic, and chemical processes. More recently, variations of this technique have been developed to determine the thermal and mechanical properties of thin films, interfaces, and nanostructures.2 In a typical pump-probe optical measurement, a pump optical pulse generates some type of excitation in the sample and the temporal evolution of this excitation is tracked by measuring changes in the transmitted or reflected probe optical pulse as a function of the time delay between the pump and probe.

These changes in the probe signal are typically small and considerations of dynamic range of the detector usually demand that light originating from the pump beam be rigorously excluded from the detector. In some cases—for example, when a probe beam is specularly reflected from a smooth surface—the probe beam can be adequately separated from the pump beam using different optical paths combined with orthogonal polarizations for the pump and probe. On the other hand, when sample roughness or inhomogeneities produce a significant amount of diffusely scattered light, the reflected or transmitted pump light cannot be easily separated from the probe by geometry and polarization alone. For these reasons, the use of different wavelengths of light for the pump and probe, i.e., a two-color pump-probe approach, is advantageous.

Two-color pump-probe measurements can be implemented in many ways. A common approach is frequency doubling of either the pump or the probe beam by a nonlinear crystal.3–5 Optical parametric amplifiers OPAs[fusion_builder_container hundred_percent=”yes” overflow=”visible”][fusion_builder_row][fusion_builder_column type=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”no” center_content=”no” min_height=”none”][1],6,7 optical parametric oscillators,8 and supercontinuum generation9 have been widely used to create tunable sources. For example, Katayama and Kawaguchi9 used supercontinuum generation and optical bandpass filters to produce spectrally distinct pump and probe beams. Recently, two mode-locked lasers operating at different wavelengths have been synchronized10,11 electronically. A two-color pump-probe approach provides high rejection of diffusely scattered pump light but adds significant complexity to the apparatus.

We describe a simple way to achieve high rejection of scattered pump light without the use of a nonlinear element or an additional laser cavity. Our approach is to employ sharp-edged optical filters to form spectrally distinct pump and probe beams from the high and low frequency ends of the broadband output of a Ti:sapphire laser oscillator. Since the pump and probe wavelengths are separated by only

10 nm, we refer to this approach as a “two-tint” method. In Secs. II A–II C, we describe two implementations of this idea for time-domain thermoreflectance TDTR[1]2,12–14 measurements of thermal transport properties and time-resolved incoherent anti-Stokes Raman scattering TRIARS[1].6,15–17

The two-tint method has the following technical advantages over conventional two-color methods: 1[1] No additional alignment of the optical system is needed. 2[1] The two-tint method avoids the increased fluctuations in laser power that result from frequency conversion by nonlinear optics;18 since noise in both the pump and probe beam affect the signal-to-noise SNR[1] ratio19 of a pump-probe measurement, two-color methods that adopt nonlinear optics will typically reduce the SNR. 3[1] In the two-tint method, the pump and probe powers are linear in the incident laser power and independent of repetition rate; frequency conversion by OPAs typically requires pumping by an amplified, low repetition-rate source laser. 4[1] The two-tint method avoids photodegradation by keeping both the pump and the probe in the near infrared. One of the most common ways of producing a traditional two-color measurement is to frequency double either the pump or the probe but the resulting UV photons can sometimes damage photosensitive samples.

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