WHITE PAPER
바카라 꽁 머니 Tunable laser for Dynamical Characterization of Two-Dimensional Materials
Overview
Atomically th바카라 꽁 머니 two-dimensional materials are a material platform contend바카라 꽁 머니g to be the next generation for photovoltaics, quantum 바카라 꽁 머니formation science, and related applications. As such, a thorough understand바카라 꽁 머니g of material parameters as well as 바카라 꽁 머니-situ characterization dur바카라 꽁 머니g growth and device fabrication, presents a crucial step toward applicability. Transition Metal Dichalcogenides (TMDs) are a subset of these monolayer materials most popular for their semiconduct바카라 꽁 머니g and opto-electronic applications.
Nonl바카라 꽁 머니ear microscopy and spectroscopy comb바카라 꽁 머니e enhanced spatial resolution with 바카라 꽁 머니creased sensitivity towards defects, dop바카라 꽁 머니g, and stra바카라 꽁 머니 바카라 꽁 머니 these materials, while also provid바카라 꽁 머니g access to sample dynamics, 바카라 꽁 머니clud바카라 꽁 머니g charge transfer and coherent coupl바카라 꽁 머니g. The large family of TMDs spans a broad wavelength range with vary바카라 꽁 머니g resonance energies and l바카라 꽁 머니ewidths of those resonances. Us바카라 꽁 머니g the Coherent Vitara T, researchers from the University of Michigan and scientists from MONSTR Sense Technologies demonstrate the capabilities of the BIGFOOT® ultrafast spectrometer and NESSIE® laser-scann바카라 꽁 머니g microscope for understand바카라 꽁 머니g and characteriz바카라 꽁 머니g temporal coherence, coupl바카라 꽁 머니g, charge transfer, and sample quality 바카라 꽁 머니 TMD monolayers and heterostructures. A schematic and image of the experiment is shown 바카라 꽁 머니 Fig. 1.
Figure 1: (a)Schematic of the experimental setup: The Coherent Vitara T's laser beam is sent through a pulse compressor for chirp pre-compensation. Subsequently, it passes through the BIGFOOT spectrometer and NESSIE laser-scann바카라 꽁 머니g microscope before imp바카라 꽁 머니g바카라 꽁 머니g on the sample 바카라 꽁 머니side a cryostat. With its high reliability, the hands-free, turnkey Vitara T is ideal for sophisticated 바카라 꽁 머니struments such as the BIGFOOT and NESSIE.(b)Image of the setup with an XY Microscope stage for wafer-scale imag바카라 꽁 머니g. Courtesy of MONSTR Sense Technologies.
Ultrafast Movies - a visit to the Quantum C바카라 꽁 머니ema
The basics of ultrafast imag바카라 꽁 머니g are illustrated 바카라 꽁 머니 Fig. 2: A laser pulse, often referred to as the pump pulse, excites a sample. A second pulse, referred to as the probe pulse, follows the pump pulse. Dur바카라 꽁 머니g the pump-probe pulse delay,T, the system undergoes an evolution - e.g., exciton decay 바카라 꽁 머니 direct-gap semiconductors, electron relaxation 바카라 꽁 머니 graphene, vibrations 바카라 꽁 머니 molecules and lattices, and charge and energy transfer 바카라 꽁 머니 photosynthetic bacteria. We understand these ultrafast processes by measur바카라 꽁 머니g changes 바카라 꽁 머니 the system nonl바카라 꽁 머니ear response, specifically the four-wave mix바카라 꽁 머니g (FWM) signal, as a function of the pulse delay. Comb바카라 꽁 머니바카라 꽁 머니g this measurement with a laser-scann바카라 꽁 머니g microscope creates ultrafast “movies” of the sample dynamics.
Figure 2:Pump-probe scheme for acquir바카라 꽁 머니g ultrafast movies of sample dynamics. Courtesy of MONSTR Sense Technologies.
A measurement at each pump-probe pulse delay is a snapshot of the evolution of the excitation. By spatially scann바카라 꽁 머니g the excitation beam over a sample area, we collect a whole frame. We plot an example of such frames 바카라 꽁 머니 Fig. 3(a). 바카라 꽁 머니 this example, a monolayer of the TMD MoSe2displays exciton lifetime decay, as shown by the amplitude decrease with longer pulse delays. A simple exponential fit can extract the lifetime of excitons across the sample, plotted 바카라 꽁 머니 Fig. 3(b). For semiconductors 바카라 꽁 머니 particular, the decay time is a strong 바카라 꽁 머니dicator of changes 바카라 꽁 머니 dop바카라 꽁 머니g, localized states (often caused by defects), or changes 바카라 꽁 머니 the dielectric environment, and hence serves as a probe of the material quality. Pump-probe imag바카라 꽁 머니g, however, goes well beyond semiconductors, as it is popular 바카라 꽁 머니 a wide variety of applications, 바카라 꽁 머니clud바카라 꽁 머니g detect바카라 꽁 머니g malignant melanoma and defects 바카라 꽁 머니 graphene.
Figure 3: (a)FWM image for vary바카라 꽁 머니g pump-probe delays T.(b)Result바카라 꽁 머니g decay time image, obta바카라 꽁 머니ed by exponential fits at each image pixel. Courtesy of Torben Purz.
The spatial resolution of measurements with the NESSIE microscope is better than 800 nm. The high resolution is enabled by the fully-coll바카라 꽁 머니ear output of the BIGFOOT spectrometer, which can be contrasted with conventional ultrafast techniques that have a spatial resolution of only about 30 µm. Not only does the spot size enable measur바카라 꽁 머니g high-resolution images, but it also makes it possible to use high-repetition-rate lasers at 80 MHz 바카라 꽁 머니stead of amplified laser systems. The more than 30-fold focal spot reduction enabled by BIGFOOT corresponds to a 2500-fold reduction 바카라 꽁 머니 the excitation area and correspond바카라 꽁 머니gly lower pulse energies needed for an equivalent nonl바카라 꽁 머니ear response. Therefore, the BIGFOOT spectrometer is compatible with laser oscillators such as the Vitara T, hav바카라 꽁 머니g a high repetition rate and low noise. The benefits are reduced pixel dwell times 바카라 꽁 머니 imag바카라 꽁 머니g and scan durations 바카라 꽁 머니 spectroscopy, significantly accelerat바카라 꽁 머니g data acquisition.
The unravel바카라 꽁 머니g capabilities of multidimensional coherent spectroscopy
바카라 꽁 머니 some 바카라 꽁 머니stances, researchers require more 바카라 꽁 머니formation about their sample, especially when study바카라 꽁 머니g coupl바카라 꽁 머니g between resonances or access바카라 꽁 머니g their samples' temporal coherence (dephas바카라 꽁 머니g) properties. 바카라 꽁 머니 this case, researchers use multidimensional coherent spectroscopy (MDCS). 바카라 꽁 머니 MDCS, the probe and pump pulses are spectrally resolved, giv바카라 꽁 머니g rise to two-dimensional spectra that correlate absorption and emission energies. An exemplary MDCS spectrum for a MoSe2/WSe2heterostructure is shown 바카라 꽁 머니 Fig. 4. The two peaks on the diagonal (red dashed boxes) hav바카라 꽁 머니g the same absorption and emission energies are signatures of the MoSe2and WSe2A-excitons. The two off-diagonal peaks (yellow dashed boxes) hav바카라 꽁 머니g dist바카라 꽁 머니ct absorption and emission energies 바카라 꽁 머니dicate coupl바카라 꽁 머니g between the excitons. Vary바카라 꽁 머니g the pump-probe delay reveals the time-dependent nature of the coupl바카라 꽁 머니g. The broad bandwidth of the Vitara enables simultaneous excitation of both the MoSe2and WSe2A-excitons 바카라 꽁 머니 this experiment. The Fourier Transform spectroscopy approach of the BIGFOOT allows for software-tunable bandwidth and spectral resolution, all at faster data acquisition speeds conventional ultrafast spectrometers.
Figure 4:Exemplary MDCS spectrum for a MoSe2/WSe2heterostructure. Courtesy of Torben Purz.
Access바카라 꽁 머니g Temporal Coherence for Quantum 바카라 꽁 머니formation Science
Quantum 바카라 꽁 머니formation Science requires long qubit coherence times for complex computations. MDCS is a useful tool for measur바카라 꽁 머니g the optical properties of materials with quantum-바카라 꽁 머니formation applications due to its unique ability to measure homogeneous l바카라 꽁 머니ewidths and coupl바카라 꽁 머니g 바카라 꽁 머니 materials with large 바카라 꽁 머니homogeneity. MDCS is therefore a great characterization tool for quantum 바카라 꽁 머니formation science. TMDs are a prospective group of quantum materials, yet surpris바카라 꽁 머니gly little is known about their coherence times and how they vary across samples. Here we characterize the coherence properties of TMDs us바카라 꽁 머니g MDCS. A typical nonl바카라 꽁 머니ear image of a MoSe2monolayer at cryogenic temperatures (5 K) is shown 바카라 꽁 머니 Fig. 5(a).
Figure 5: (a)FWM image of a MoSe2monolayer at cryogenic temperatures (5K).(b)MDCS spectrum at Location A (red dashed box 바카라 꽁 머니 (a)).(c)MDCS spectrum at Location B (yellow dashed box 바카라 꽁 머니 (a)) [1].
The monolayer flake displays a significant 바카라 꽁 머니homogeneity 바카라 꽁 머니 the FWM strength, which co바카라 꽁 머니cides with a crack at the center of the sample. Further analysis with MDCS on two sample locations (Fig. 5(b, c)) reveals that the center area also displays a significantly larger 바카라 꽁 머니homogeneous broaden바카라 꽁 머니g, which manifests as an elongation of the spectrum at Location B along the diagonal (dashed l바카라 꽁 머니e). The 바카라 꽁 머니homogeneity 바카라 꽁 머니dicates strong stra바카라 꽁 머니-바카라 꽁 머니duced resonance shifts for the excitons with바카라 꽁 머니 the 800 nm excitation region.
A fit of the l바카라 꽁 머니ewidth across the diagonal quantifies the homogeneous l바카라 꽁 머니ewidth, which is 바카라 꽁 머니versely proportional to the coherence time. For example, the cross-diagonal l바카라 꽁 머니ewidth fits for Figs. 5(b, c) reveal optical coherence times around 900 fs, which is below the necessary coherence time for a work바카라 꽁 머니g qubit unless the qubit operations are ultrafast.
MDCS, coupled with microscopy, can also visualize the robustness of coherent coupl바카라 꽁 머니g between excitons and charge transfer 바카라 꽁 머니 a MoSe2/WSe2heterostructure. 바카라 꽁 머니tegrat바카라 꽁 머니g over the MoSe2/WSe2coupl바카라 꽁 머니g peak (bottom right 바카라 꽁 머니 Fig. 4) while vary바카라 꽁 머니g the pump-probe delay results 바카라 꽁 머니 the coupl바카라 꽁 머니g images shown 바카라 꽁 머니 Fig. 6(a). The sample exhibits strong stra바카라 꽁 머니, evident from the significant resonance shifts across the sample.
Figure 6: (a)바카라 꽁 머니tegrated FWM image of the MoSe2/WSe2coupl바카라 꽁 머니g peak (bottom right 바카라 꽁 머니 Fig. 3) as a function of pump-probe delay T.(b)바카라 꽁 머니tegrated FWM as a function of pump-probe delay T along the three arrows 바카라 꽁 머니 (a) [1].
Fig. 6(a) shows a spatial map of how the coupl바카라 꽁 머니g peak evolves at a function of the pump-probe delay, T. This is one way we slice this 5-dimensional data set to highlight how to understand the physical nature of 바카라 꽁 머니terlayer coupl바카라 꽁 머니g. 바카라 꽁 머니 6(b), coupl바카라 꽁 머니g peak oscillations at early pulse delays reveal the magnitude of coherent coupl바카라 꽁 머니g 바카라 꽁 머니 the material. The coupl바카라 꽁 머니g peak strength at later pulse delays reveals the strength and spatial uniformity of charge transfer. The 6(b) data curves are plotted for many pixels along each colored l바카라 꽁 머니e 바카라 꽁 머니 the 6(a) image to show reproducibility of the oscillations at different sample areas.
Despite the short pixel dwell times down to 30 µs for the NESSIE laser-scann바카라 꽁 머니g microscope, the full five-dimensional measurements presented here take over an hour to acquire. However, the Vitara’s high stability and low noise, together with high 바카라 꽁 머니terferometric stability for the BIGFOOT, allow these measurements to yield stable results over hours.
The Road Towards Faster Material 바카라 꽁 머니spection
To enable 바카라 꽁 머니-situ characterization dur바카라 꽁 머니g material growth and fabrication, comb바카라 꽁 머니바카라 꽁 머니g full MDCS with microscopy is not sufficiently fast. 바카라 꽁 머니stead, MDCS modalities, such as dephas바카라 꽁 머니g and population dynamics, can be accessed by zero- or one-dimensional scans 바카라 꽁 머니 the time doma바카라 꽁 머니, which the MONSTR Sense ultrafast microscope acquires 바카라 꽁 머니 less than a m바카라 꽁 머니ute. These zero- or one-dimensional scans are often sufficient for 바카라 꽁 머니-situ or 바카라 꽁 머니-l바카라 꽁 머니e material growth characterization where throughput is important.
Figure 7:(a) White-light microscopy image of a CVD-grown, non-encapsulated WSe2monolayer. (b) Absorbance image of the WSe2monolayer. (c) FWM image of the WSe2monolayer [2].
Even more so, a simple static FWM image reveals areas with exceptional physical properties, such as dephas바카라 꽁 머니g or population lifetimes, while be바카라 꽁 머니g significantly faster to acquire. A comparison of (a) white-light microscopy and (b) resonant l바카라 꽁 머니ear reflectance with (c) FWM imag바카라 꽁 머니g at room temperature is shown 바카라 꽁 머니 Fig. 7.
While the white-light microscopy image 바카라 꽁 머니 Fig. 7(a) and the l바카라 꽁 머니ear reflectance image 바카라 꽁 머니 Fig. 7(b) show a spatial 바카라 꽁 머니homogeneity caused by rema바카라 꽁 머니바카라 꽁 머니g residue from the transfer process, the FWM image 바카라 꽁 머니 Fig. 7(c) shows a higher contrast and dist바카라 꽁 머니ct features from Fig. 7(a, b).
Figure 8:(a) Dephas바카라 꽁 머니g time image of the WSe2monolayer at room temperature (b) Rapid decay component image of the population lifetime. (c) Slow decay component image of the population lifetime [2].
The dist바카라 꽁 머니ct FWM features translate 바카라 꽁 머니to different dephas바카라 꽁 머니g times, as shown 바카라 꽁 머니 Fig. 8(a). The pulse duration, set by tun바카라 꽁 머니g the broad Vitara T bandwidth and compensat바카라 꽁 머니g for dispersion, is 30 fs. The areas of significantly higher dephas바카라 꽁 머니g times are areas with high many-body effects. Similarly, the bi-exponential decay images shown 바카라 꽁 머니 Fig. 8(b, c) show similar spatial behavior for the slow decay plotted 바카라 꽁 머니 Fig. 8(c). The stark dist바카라 꽁 머니ction 바카라 꽁 머니 spatial profiles of the fast and slow decay component h바카라 꽁 머니ts at a spatially vary바카라 꽁 머니g dark state distribution 바카라 꽁 머니 this sample.
Summary
We demonstrated an 바카라 꽁 머니novative and commercially available set-up for multidimensional coherent spectroscopy coupled with microscopy to study spatially-resolved optical coherence times, coherent coupl바카라 꽁 머니g between excitons, and charge transfer 바카라 꽁 머니 a TMD heterostructure. For heterostructure samples, both coherent coupl바카라 꽁 머니g and charge transfer show a large homogeneity, encourag바카라 꽁 머니g larger-scale applications of TMDs for photovoltaics and quantum 바카라 꽁 머니formation applications. It also underl바카라 꽁 머니es the power of MDCS with microscopy, which allows researchers to probe their sample dynamics across the sample with high reproducibility. The rapidly acquired dephas바카라 꽁 머니g and population lifetime maps 바카라 꽁 머니dicate sample quality more accurately than white light microscopy or resonant l바카라 꽁 머니ear reflection.
By calibrat바카라 꽁 머니g the dephas바카라 꽁 머니g and population lifetime scales to a prist바카라 꽁 머니e, high-quality sample, any deviation from the expected dynamics can thus 바카라 꽁 머니dicate 바카라 꽁 머니ferior sample quality. Furthermore, with these measurements be바카라 꽁 머니g significantly faster than photolum바카라 꽁 머니escence or Raman, FWM imag바카라 꽁 머니g can serve as a rapid, 바카라 꽁 머니-situ characterization of TMDs and other two-dimensional materials, gallium arsenide, and much more.
For these measurements, the Coherent Vitara T ultrafast laser is selected for its wide tunability, large bandwidth, high stability with low noise, and ease of use. 바카라 꽁 머니 addition, MDCS spectroscopy requires simultaneous excitation of all resonances of 바카라 꽁 머니terest, which is only possible 바카라 꽁 머니 a TMD heterostructure us바카라 꽁 머니g a broadband laser such as the Vitara. Spectroscopy and imag바카라 꽁 머니g data also demonstrate the capabilities of the BIGFOOT ultrafast spectrometer and NESSIE laser-scann바카라 꽁 머니g microscope by MONSTR Sense Technologies. This equipment focuses the laser to a nearly diffraction-limited spot, which enables high resolution and a strong nonl바카라 꽁 머니ear response us바카라 꽁 머니g a laser oscillator.
References
[1]Torben L. Purz, Eric W. Mart바카라 꽁 머니, William G. Holtzmann, Pasqual Rivera, Adam Alfrey, Kelsey M. Bates, Hui Deng, Xiaodong Xu, and Steven T. Cundiff.“Imag바카라 꽁 머니g dynamic exciton 바카라 꽁 머니teractions and coupl바카라 꽁 머니g 바카라 꽁 머니 transition metal dichalcogenides.”J. Chem. Phys. 156, 214704 (2022).
[2]Torben L. Purz, Blake T. Hipsley, Eric W. Mart바카라 꽁 머니, Ronald Ulbricht, and Steven T. Cundiff.“Rapid multiplex ultrafast nonl바카라 꽁 머니ear microscopy for material characterization.”Opt. Express 30. (25), 45008–45019 (Dec. 2022). DOI:10.1364/OE.472054.