4.1 Measurement of Thermal Conductivity
4.1.2 Experimental Design of an ASOPS-based TTR Instrument
The virtual delay line of the ASOPS instrument achieves its time-delay scanning without any mechanical stage motion. And with full-delay scans acquired at the repetition rate offset frequency, scan are collected at a very fast rate. With sufficient signal-to-noise (which is higher for TTR vs THz work), one can require all needed data potentially with only a few seconds of data acquisition time, while maintaining sub-picosecond timing resolution over>10 ns of pump-probe delay time. In scaling delay-time events to a slower lab-time frequency, experiments can now be done with low bandwidth electronics. This stands in contrast to the high- frequency modulation and lock-in recovery schemes of traditional, delay-line based TTR measurements.
The benefits of ASOPS for traditional (non-size-varied) TTR measurements have already been recognized [103], Professor Minnich was interested in expanding upon this by performing the size-dependent TTR work for finding phonon MFPs, though doing so with our ASOPS-based system. One important technical note, though, does present itself. The pump and probe beams must somehow be isolated before detection, for clean detection of the delay-dependent probe thermoreflectance signals. In the traditional TTR instrument with long delay line, as used by the Minnich group, a 2-color scheme was employed; see Figure 4.1. The beam from a single∼800nm ultrafast laser is split, and a portion nonlinearly converted to SHG as a pump beam, so that the blue pump beam can be separated easily with filters from the red, delayed probe beam. In the Blake Group ASOPS THz-TDS spectrometer, there is insufficient power for that kind of nonlinear conversion, nor is it easy to substantially change the wavelength of the lasers. As such, additional way of separating the pump and probe beams, including upon spatial separation and polarization were designed and tested; these are the focus of§4.1.2.1 and 4.1.2.2, respectively.
4.1.2.1 Pump and Probe Separation by Noncollinear Beams
A schematic of the ASOPS-TTR system is shown in Figure 4.2. The overall goal of this setup is to overlap the pump and probe beams upon the sample and having well-characterized and controlled spot sizes on the sample at that intersection point. The overlap of beams in this experimental design is achieved through the use of a pair of microscope objects oriented at an angle to each other, thus the description of this section as using noncollinear beams.
The ‘synchronization system’ noted at top is much the same as the ASOPS THz-TDS instrument as shown in Figure 2.4, with the notable difference that the ‘THz Detection & Recording’ subsystem, as highlighted in Figure 2.6, is now removed. An early version of the TTR instrument (the noncollinear version discussed in the present section) is visible and noted at bottom left in the photo of the overall ASOPS instrumentation in Figure 2.5.
The integration of the TTR instrument with the ASOPS system is actually quite straightforward; one only requires access to each of the pump and probe beams. A pair of flip mirrors were inserted into the ASOPS THz-TDS instrument, one each for the pump and probe beams. Referring to Figure 2.4 for positioning, the pump flip mirror was inserted into the pump beam line between the first (from laser) lens and the preceding beam sampler. The probe beam flip mirror was positioned just before the pair of high-reflection mirrors immediately prior to the beam input to the purge enclosure. Both flip mirrors directed their beams across the table to additional turning optics into the TTR setup. Pairs of irises were fixed in the TTR setup for easy re-alignment when the flip mirrors were used; the beams were merely ‘walked’ with a pair of input mirrors in the TTR setup.
Following the pump beam path in Figure 4.2, this beam was directed first to an optical isolator so as to prevent large back-reflections to the laser. The beam was then directed through a half-wave plate and polarizer to rotate the polarization horizontal (to pass the later polarizing beam splitter) and clean up its polarization.
The beam was then re-sized by a pair of telescopes, the first a cylindrical lens pair to adjust astigmatism, the second spherical, with one lens moveable, as a variable telescope to control the pump beam size upon the back aperture of a microscope objective, thus controlling the focused spot size on the sample. The sample was monitored with a CCD camera and tube lens positioned to receive either of un-polarized back reflected light or the illumination from a ring light placed around the microscope objective. In this overall design, the metallized, mirror-like sample reflects almost all pump light back upon the path along which it arrived.
Sending this much light back to the laser may cause instabilities in its operation, thus the laser is protected by the optical isolator at the input to the TTR setup.
Following the probe beam path, it was directed first through a pair of cylindrical lenses to correct for any beam astigmatism at this point; the lenses were generally fixed once this correction had been done in initial alignment, even between uses of the flip mirrors. The beam was then passed through a pair of spherical lenses, where one lens could be moved along the optical path, as an adjustable telescope to control the beam size into the back aperture of the microscope objective, thus controlling the probe beam spot size. Following the lenses, the probe beam was split on a non-polarizing beam splitter, directing light to both of a microscope objective for focusing on the sample, as well as to one of the balanced photodiodes (with power adjustment from a polarizer, for proper detector balancing), as a reference signal. The portion of the beam directed at the sample was collected on reflection from the metallized (i.e., mirror-like) sample surface with a pair of lenses and directed to the other photodiode of the balanced detector, as the sample signal. In collecting the probe beam at an angle with respect to the pump beam, we essentially avoid the collection of pump light along the probe path, despite the light in the two beams having the same wavelength. This spatial separation of pump
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Figure 4.2:The complete ASOPS-based TTR instrument. The pump and probe beams of the ASOPS system (optical beams, before THz emission) are redirected to the TTR system, at bottom. Note that the two beam colors are for the reader’s ease; the pulses are of the same wavelength. Separation of pump and pump in reflection from the sample is achieved via different angles of incidence and spatial filtering.
and probe beams is the major benefit of this experiment design.
In terms of the electronics and operation of the TTR instrument, an oscilloscope was connected to the balanced photodetector; sometimes the same digitizer as in the ASOPS setup (see§2.11) was used. The trigger signal for data acquisition could be acquired from the cross-correlator and PMT as in the ASOPS THz-TDS instrument, or from the sharp leading edge of the TTR signal itself. CCD camera images, for selecting a sampling spot, were acquired on a laptop computer. Samples were mounted on a metal plate and suspended in front of the pump beam microscope objective.
Upon testing this experimental apparatus, some practical challenges became apparent. The noncollinear pump-probe geometry makes it difficult to overlap the pump and probe beams at a known size. It is generally advantageous to overlap the pump and probe beam at a mutual focus; at such a beam waist the beam diameter is more tolerant to positioning errors. In taking beam size measurements with a scanning-slit beam profiler, we found the noncollinear geometry made it hard to take measurements of both beam sizes while ensuring those measurements corresponded to the same position. In this regard, a home-built scanning razor-edge beam profiler (e.g., [104–106]) would probably have helped make the measurements. However, it was found that controlling the spot position was also difficult. To overlap pump and probe, the 10X objective for the probe beam (see Figure 4.2) was moved; moving this optic, though, also changes the size of the probe beam at the overlap point with the pump beam. It was determined that a collinear pump-probe geometry had the potential to be easier to align while providing more reliable data, despite the challenge of separating spectrally overlapping pump and probe beams.
4.1.2.2 Pump and Probe Separation by Polarization
Given the issues noted above in the alignment and measurement of the pump and probe beam spots, the non- collinear TTR instrument setup in Figure 4.2 was replaced with a different design, the collinear arrangement of Figure 4.3. In moving to a collinear arrangement, we do encounter the problem that a simple spatial sep- aration of pump and probe beams is no longer possible, as it was in the noncollinear arrangement. Further, a collinear arrangement with lasers having the same wavelength precludes the use of filters, dichroic mirrors, and the like for separating the two beams. In order to prevent the pump beam from overwhelming the de- tection of the probe beam, we therefore use the polarization property of light as a basis for separating the beams.
The beginning portions of the pump and probe beam paths in the collinear setup of Figure 4.3 are much as they were in the noncollinear arrangement as was depicted in Figure 4.2. The notable exception to this statement is that the pump beam, following the optical isolator and half-wave plate, is rotated to a vertical
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Figure 4.3:The newer version of the ASOPS-based TTR instrument, now in a collinear pump-probe geom- etry. The pump and probe beams of the ASOPS system (optical beams, before THz emission) are redirected to the TTR system as before (with the ASOPS synchronization as before). The two beam colors are of the same wavelength, as in Figure 4.2. Separation of pump and pump in reflection from the sample, though, is now achieved based on cross-polarization of the two beams with respect to each other.
polarization instead of horizontal. As before, though, the concern is in setting the polarization at the beginning of the setup to achieve a desired reflection or transmission upon a later polarizing beam splitter. In the present case, the pump beam is desired to reflect not transmit, hence the 90◦ difference in polarization. In the present case, the pump beam is directed to a polarizing beam splitter directly in front of a single microscope objective. The pump beam is reflected into the objective and focused upon the sample. The reflected pump light is reflected off of the sample and, as in the noncollinear arrangement, is sent back along it’s input path and rejected by the optical isolator from continuing back to the source laser.
The probe beam polarization is also rotated differently in the collinear setup, so that it will transmit through the polarizing beam splitter. In this design, though, a prism mirror is used to direct the probe beam towards the objective, while also receiving its reflection off the sample and passing it towards the balanced detector. The prim mirror has mirrored sides with the useable mirror surface extending to a fine edge (the edge facing towards the objective in Figure 4.3. The prism mirror thus allows the incoming and outgoing probe beams to pass very close together so that they may both fit into the back aperture of the microscope objective.
However, in entering the objective along a beam path that is slightly offset from the objective’s optical axis, the probe beam is actually not quite collinear with pump beam. In some ways, this ‘collinear’ design is similar to the noncollinear design, except that the angle between the pump and probe beams is much smaller, such
that the two beams can be passed through the same microscope objective while still being separated from one another. Upon exiting the objective, the probe beam is passed through a polarizing beam splitter, and reflect by the other face of the prism mirror towards the balanced detector, with power adjustment from a polarizer for proper detector balancing. In this arrangement, the CCD camera is now placed on the probe path, where it can observe the sample under illumination from the ring light surrounding the objective or from the probe light. Overall, this design has proven much easier to operate and control than the noncollinear design. The parallel alignment of the pump and probe beams through the same objective ensures they co-focus and the small angle between the beams enables straightforward measurement of their sizes at their point of overlap.