Optically gated spectroscopy methods (pump-probe transient absorption, fluorescence upconversion, etc.) are widely used to study fast photo-induced processes (e.g., chemical reactions, etc.). In a pump-probe setup, a laser beam is split into a pump pulse and a temporary delayed probe pulse. In general, a sample is irradiated with a laser pump pulse, which generates an excitation (or other perturbation) in the sample. Then, after an adjustable time delay and while the excited state is being relaxed, a laser probe pulse is sent to the sample. The time delay is typically controlled with an optical delay line. By analyzing the intensity of the light from the probe pulse, the transmission or reflectance of the sample is measured. This is repeated over a series of pump-probe delays to measure the photo-induced changes in in the sample over time after the periodic excitation by the pump pulse.
An optical delay line generally works by precisely controlling the position of a retroreflector. In a retroreflector, a light beam is reflected back along a vector that is parallel to but opposite the direction from the beam's source. Any change in the position of the retroreflector will affect the path length that the light beam has to travel before arriving at the sample. One can calculate how much the laser pulse is delayed in time with each change in the position of the retroreflector based on the speed of light. By monitoring the probe pulse as a function of the time delay, information can be obtained on the decay of the generated excitation or on other processes initiated by the pump pulse.
FIG. 1 illustrates an embodiment of a conventional pump-probe arrangement. As illustrated, the pump-probe device 10 includes a laser source 12 which generates a laser beam 14, and a semitransparent beam splitter 16 which splits the laser beam 14 into a pump pulse beam 18 and a probe pulse beam 20. The two beams follow different optical paths but are spatially overlapped in the sample 22. The pump pulse beam 18 is directed via a series of mirrors to the sample 22, and the incoming probe pulse beam 20a is directed to a variable (motorized) optical delay line 24. The optical delay line 24 is a reflective device which includes a retroreflector assembly 30 mounted on a motorized translation stage 28, which is moved along a track (arrow ↔). Over the course of the measurement period, the translation stage 28 and retroreflector 30 is translated along the track in order to modify the length of the beam path of the probe pulse beam 20 relative to the length of the pump pulse beam 18, thus delaying the delivery of the probe pulse beam to the sample by a set amount of time. The outgoing probe beam 20b from the retroreflector 30 is directed downstream from the delay line by mirror 32 and then to mirror 34, which directs the beam onto the sample 22. After reflection or transmission through the sample, the intensity of the probe beam 20 is monitored.
Correct alignment of the incoming probe beam 20a with the retroreflector 30 of the optical delay line 24 is a critical requirement of pump—probe measurement experiments because at any point on the delay line trajectory, pointing of the outgoing probe pulse beam 20a needs to remain the same over the course of the measurements. This is accomplished by properly aligning the incoming beam 20a before it enters the delay line 24. If the incoming beam 20a trajectory is not aligned with a proper axis of the delay stage (and retroreflector), pointing of the outgoing probe pulse beam 20b from the retroreflector will vary as the delay stage (and retroreflector) is translated along the track over the course of the measurements. This will in turn affect the spatial overlap of the pump beam 18 and the probe beam 20 in the sample and result in inaccurate data. However, achieving accurate alignment of the incoming probe beam 20a with the optical delay line is time consuming and requires special technical training.
Alignment of the incoming probe beam 20a with the delay line 24 is currently performed by manually adjusting the steering mirror 26 which directs the probe pulse beam 20 into the delay line. Upon moving the delay stage 28 (and the retroreflector 30) from one end of the track to the other end, and monitoring the position of the outgoing probe pulse beam 20b, for example, using a commercially available beam profiler or position sensitive detector 40 in combination with a beam splitter (mirror) 38, which splits the outgoing probe pulse beam 20b and directs it to the detector 40, or by monitoring the beam position on a target by eye, the angle of the steering mirror 26 is then manually adjusted based on those observations to minimize “walk-off” (i.e., drift) of the outgoing probe beam 20b by ensuring that the incoming beam 20a is aligned and enters the delay line parallel to its axis of translation 36. However, the problem with this approach is that only one mirror 26 can be steered, and a user has no control over the pointing of the outgoing probe pulse beam 20b after the delay line. Consequently, subsequent alignment of the optical set-up (e.g., at least mirrors 32 and 34 and possibly other mirrors) situated “downstream” from the delay line 24 is required after performing each alignment of the delay line.
FIG. 2 illustrates another embodiment of a conventional system 10′ that aligns a beam with a delay line by active beam stabilization. Such systems actively compensate for changes in the alignment of outgoing beam from the retroreflector in order to maintain beam pointing along a proper trajectory. In an active beam stabilization system 10′, motorized mirrors M1, M2 are controlled through a feedback loop by a closed loop controller 42, 44 connected to a corresponding position sensitive photodetector 40a, 40b. When a beam coordinate moves away from the center of the photodetector 40a, 40b, the motorized steering mirror M1, M2 is activated to compensate for the displacement and bring the beam back on the center of the detector. This is done at high speed to ensure minimum beam displacement caused by changes in the optical set-up (i.e., mirrors M1, M2, etc.). In this method, the distances between the mirror M1, M2 and the photodetector 40a, 40b are not taken into account. The beam stabilization is achieved by simply keeping the beam traveling through the same two points in space.
However, an active beam stabilization approach is unacceptable in a pump-probe experiment because once the delay line scanning starts, the incoming or outgoing beam trajectory cannot be changed. Such an “on the fly” trajectory change will unpredictably affect how much the laser beam gets delayed with changing the position of the retroreflector of the delay line.
Accordingly, there is a need for a set-up and method for aligning a probe beam in relation to a delay line in an optically gated spectroscopy measurement system that will overcome the foregoing problems and retain the initial probe beam pointing after the delay line.