Rev. Sci. Instrum. 91, 073107 (2020); https://doi.org/10.1063/5.0010487 91, 073107

© 2020 Author(s).

High-resolution optical micro-spectroscopyextending from the near-infrared to thevacuum-ultravioletCite as: Rev. Sci. Instrum. 91, 073107 (2020); https://doi.org/10.1063/5.0010487Submitted: 10 April 2020 . Accepted: 22 June 2020 . Published Online: 14 July 2020

Eric Yue Ma, Lutz Waldecker, Daniel Rhodes, James Hone, Kenji Watanabe, Takashi Taniguchi, and Tony F. Heinz

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High-resolution optical micro-spectroscopyextending from the near-infraredto the vacuum-ultraviolet

Cite as: Rev. Sci. Instrum. 91, 073107 (2020); doi: 10.1063/5.0010487Submitted: 10 April 2020 • Accepted: 22 June 2020 •Published Online: 14 July 2020

Eric Yue Ma,1,2 Lutz Waldecker,1,2 Daniel Rhodes,3 James Hone,3 Kenji Watanabe,4Takashi Taniguchi,4 and Tony F. Heinz1,2,a)

AFFILIATIONS1Department of Applied Physics, Stanford University, 348 Via Pueblo Mall, Stanford, California 94305, USA2SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA3Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA4National Institute for Materials Science, Tsukuba, Ibaraki 305-004, Japan

a)Author to whom correspondence should be addressed: tony.heinz@stanford.edu

ABSTRACTOptical characterization of small samples over a wide spectral range with rapid data acquisition is essential for the analysis of many materialsystems, such as 2D van der Waals layers and their heterostructures. Here, we present the design and implementation of a tabletop micro-spectroscopy system covering the near-infrared to the vacuum-ultraviolet (1.2 eV–6.8 eV or ∼1.0 μm to 185 nm) using mostly off-the-shelfcomponents. It can measure highly reproducible local reflectance spectra with a total integration time of a few minutes and a full-width-half-maximum spot size of 2.7 by 5.6 μm. For precise positioning, the design also allows simultaneous monitoring of the measurement locationand the wide-field image of the sample. We demonstrate ultra-broadband reflectance spectra of exfoliated thin flakes of several wide-gap 2Dmaterials, including ZnPS3, hexagonal BN, and Ca(OH)2.

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I. INTRODUCTION

Recent years have witnessed the explosive development of 2Dvan der Waals (vdW) materials and their heterostructures, fromgraphene to 2D semiconductors and insulators, with exciting newphysical phenomena—including Dirac and Weyl fermion, corre-lated insulator, emerging superconductivity, Moiré exciton, andhigh-temperature exciton condensate—being discovered at a rapidpace.1–10

In revealing the intriguing optical and optoelectronic prop-erties of these 2D vdW materials, microscope-based optical spec-troscopy has proven to be an essential technique.2,6,8–11 To date,most high-quality samples are only a few micrometers in sizedue to limitations of mechanical exfoliation2,12 and require non-contact characterization due to encapsulation.13 Although micro-spectroscopy instruments based on commercial microscopes withfew-micrometer spatial resolution that cover near-infrared (IR) to

ultraviolet (UV) are readily available, the combination of sources,optics, and air operation limits the highest photon energy to 3.5 eV–4 eV,11 leaving the field of wide-gap 2D vdW materials underex-plored.

To address this need, we present the design and implemen-tation of a tabletop optical micro-spectroscopy system continu-ously covering the near-IR to the vacuum-UV (1.2 eV–6.8 eVor ∼1.0 μm to 185 nm) spectral range using mostly off-the-shelfcomponents. It is capable of measuring highly reproducible localreflectance spectra with high throughput and a full-width-half-maximum (FWHM) spatial resolution of 2.7 by 5.6 μm. Thedesign allows simultaneous monitoring of the measurement loca-tion and the wide-field image of the sample for exact positioningon complex samples. We demonstrate its performance by mea-suring the ultra-broadband reflectance spectra of exfoliated thinflakes of several wide-gap 2D materials, including ZnPS3, h-BN, andCa(OH)2.

Rev. Sci. Instrum. 91, 073107 (2020); doi: 10.1063/5.0010487 91, 073107-1

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II. DESIGN

In our design [Fig. 1(a)], the broadband output of a laser-drivenplasma source (Energetiq LDLS EQ-99X fitted with the EQ-99XFCbulb) is collimated by a UV-enhanced aluminum off-axis parabolic(OAP) mirror (Thorlabs MPD169-F01) reflected by a broadbandbeamsplitter (uncoated UV fused silica plate, Thorlabs WG41010R)and refocused onto the sample by a 40×, 0.5NA UV-enhanced alu-minum reflective objective (Thorlabs LMM-40X-UVV). Broadbandlight reflected by the sample goes through the same beamsplitterand is focused by another OAP mirror (Thorlabs MPD129-F01)onto the entrance slit of a compact grating spectrograph (HoribaMicroHR) with an open-electrode Si CCD camera (Horiba Sym-phony). A single aluminum planar mirror (Edmund Optics 48-568)is placed between the beamsplitter and the first OAP mirror foralignment. By using only reflective optics, the system is achromaticacross an extremely wide spectrum.

To achieve optical overlay of the measurement spot and wide-field image of the sample—a critical capability for small, com-plex vdW devices—a second broadband beamsplitter (ThorlabsWG41010R) is placed right before the reflective objective, whichallows a white-light imaging module to permanently image the sam-ple through the objective. The imaging module contains a visible(Vis) CMOS camera (Thorlabs DCC1645C) with a tube lens (Thor-labs AC254-150-A-ML), a white LED (Thorlabs MCWHL5) witha condenser (Thorlabs ACL2520U-A), and a pellicle beamsplitter(Thorlabs CM1-BP145B1).

The light source, microscope, imaging module, and spectrom-eter are all fixed on an optical bench. The sample is retained witha vacuum chuck on an XYZ translation stage (Newport 562-XYZ-LH) to allow relative motion with respect to the objective for

focusing and lateral movement. A 4× visible objective (Olym-pus Plan Achromat 4×/0.10) is available for sample navigation atreduced magnification.

Vacuum-UV instruments are often bulky, expensive, andrequire custom nitrogen or vacuum enclosures because vacuum-UV light is relatively inefficient to generate and manipulate, and isstrongly absorbed by air. Two major features of our design allowthe instrument to achieve tabletop ultra-broadband operation intothe vacuum-UV without needing bulky enclosures: First, the laser-driven plasma source not only is very bright (up to 100 times higherspectral radiance than a typical deuterium lamp at 6 eV) but alsohas a small source area (∼64 by 147 μm).14 We can thus achieve asufficiently small focus with very high intensity by directly imagingthe plasma onto the sample surface without spatial filtering, whichis not possible for conventional broadband sources with large areas(e.g., tungsten halogen lamp or deuterium lamp). This way, the com-bination of a bright source, relatively high collection efficiency, andreduced number of lossy optics makes possible high vacuum-UVthroughput within a compact footprint.

Second, we minimize the optical path length throughout thesystem and enclose the broadband beam path with off-the-shelf lenstubes (Thorlabs SM1 series), cage mirror mounts (Thorlabs KCB1Cand KCB1P), and cage cubes (Thorlabs C4W). A sufficiently smallleak rate is achieved by the strategic use of foam spacers (Thor-labs CPG3) and tapes. The enclosed volume is much smaller thana conventional regularly-shaped enclosure, and can be efficientlypurged by house nitrogen using off-the-shelf connectorized cageplates (Thorlabs CPPC). Purging between the reflective objectiveand the sample is achieved by a custom objective cover made fromaluminum foil that has a 1-mm-wide opening ∼0.2 mm from thesample surface when in focus [Fig. 1(a) inset]. The natural flow of

FIG. 1. Design and implementation of a tabletop ultra-broadband optical micro-spectroscopy system, including white-light imaging. (a) Detailed design of the instrument (OAP:off-axis parabola, HPFS: high purity fused silica, TL: tube lens). Inset: Zoom-in view near the sample surface. (b) Photo of the complete system. (c) Real-time capture of thewhite-light imaging system, showing the broadband light focused onto a calibration target (Thorlabs R1L3S2P). (d) Extracted horizontal and vertical intensity profiles of thefocus, showing a FWHM of 2.7 μm and 5.6 μm.

Rev. Sci. Instrum. 91, 073107 (2020); doi: 10.1063/5.0010487 91, 073107-2

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nitrogen displaces air in this small gap and maintains the nitro-gen environment above the sample surface. The additional benefitof this design is that the sample XYZ stage is in air and can beoperated freely with no wait time after moving or changing thesamples. We kept the system modular by connecting the lens tubeswith tightly-fit sleeve couplers (Thorlabs SM1CPL10), which alsoprovide easy access to inline spectrum-flattening and order-sortingfilters.

III. IMPLEMENTATION AND VALIDATIONA photo of the complete system is shown in Fig. 1(b). We first

measure the spatial resolution. Figure 1(c) shows an image capturedby the white-light imaging module, while the plasma was focusedonto a calibration target with 10-μm divisions (Thorlabs R1L3S2P).The extracted horizontal and vertical profiles show that the FWHMof the focused spot are 2.7 μm and 5.6 μm, consistent with the sourceplasma size (∼64 by 147 μm) and a system magnification of 0.033(the 40× objective has a focal length of 5 mm and the collimatingOAP mirror 152.4 mm). We note that by using only reflective optics,the system is highly achromatic, as confirmed by an excellent overlapof the focus profiles in the red, green, and blue channels in the Viscolor image, and a knife-edge scan for wavelengths below 220 nm[Fig. 2(c)]. Such spatial resolution is enough for all but the smallestvdW samples.

We validated the spectral range of the instrument by measur-ing the raw spectrum reflected off a single-side polished UV fusedsilica plate (Corning 7980), which has a relatively flat reflectancebetween 1 eV and 7 eV. The ultra-broadband spectrum in Fig. 2(a)is obtained by scaling and matching four individual spectra takenwith appropriate order-sorting filters and gratings (1800 g/mm,400 nm blaze for the two UV spectra and 300 g/mm, 500 nmblaze for the Vis and near-IR spectra). The total integration timeis ∼2 min, dominated by the vacuum-UV region due to the rapiddecrease in the count rate above ∼5.5 eV. Nonetheless, a signal-to-noise-ratio (SNR) of 10 is achieved up to ∼6.8 eV and downto ∼1.2 eV. Note that we used a Vis suppression filter (ActonFW200-W-1D) to flatten the spectrum in order to reduce strayVis light in the UV region and better utilize the dynamic range ofthe CCD.

The compact and rigid mechanical design and the stable sourcemake the measurements highly reproducible with many interveningsample exchanges. For example, Fig. 2(b) shows excellent agreementbetween two raw UV spectra of fused silica taken 3 days apart withmultiple sample exchanges in between. This capability allows quan-titative reflectance ratio measurements between different samples,instead of closely spaced spots on the same sample (as required bymost commercial-microscope-based instruments).

Next, we validate the reflectance measurement [Fig. 2(d)]. Wemeasured the reflectance ratio of a deep-UV aluminum mirror(Edmund Optics 48-568) and a deep-UV dielectric mirror (EdmundOptics 47-983) against fused silica (Corning 7980), and obtained theabsolute reflectance spectra using the reflectance of fused silica, cal-culated using Fresnel equations and the refractive index from thespec sheet.15 We used an average incident angle of 20○. The resultsshow the expected behavior, have excellent SNR and demonstratequantitative measurements up to ∼6.7 eV.

FIG. 2. Validation of the system. (a) Raw ultra-broadband reflected-light spectrumof fused silica. (b) Comparison of two UV spectra taken 3 days apart with multiplesample exchanges in between. (c) Knife-edge scan over a sharp chromium–silicainterface using wavelengths below 220 nm. (d) Reflectance spectra of two testdeep-UV mirrors, showing a favorable signal-to-noise ratio and a lack of artifactsup to ∼6.7 eV.

IV. PROOF-OF-CONCEPT WITH WIDE-GAP2D VDW MATERIALS

Finally, we demonstrate proof-of-concept ultra-wide-rangereflectance spectra of three representative wide-gap 2D vdW mate-rials, namely ZnPS3, h-BN, and Ca(OH)2 (Fig. 3). We preparedtens of μm sized flakes of tens to hundreds of nm thickness bymechanical exfoliation and deposited them on fused silica substrates.Using fused silica as a reference, we obtained the reflectance spectraof the three materials with various thicknesses. The most promi-nent common feature is the narrowing interference fringes that stopat the fundamental absorption edge, when the material becomesopaque. We can infer that the absorption edges of ZnPS3, h-BN, andCa(OH)2 are ∼4 eV, 6 eV, and >7 eV, consistent with the previous

Rev. Sci. Instrum. 91, 073107 (2020); doi: 10.1063/5.0010487 91, 073107-3

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FIG. 3. Ultra-broadband reflectance spectra of exfoliated (a) ZnPS3, (b) h-BN, and(c) Ca(OH)2 flakes. Visible-light images and flake thicknesses are noted in thefigure. The scale bars represent 10 μm lengths.

results from bulk crystals.16–18 More detailed analysis is beyond thescope of this work.

V. CONCLUSION AND OUTLOOKIn summary, we designed a tabletop optical micro-spectroscopy

system that covers the near-IR to the vacuum-UV with few-μm spa-tial resolution and measured the ultra-broadband reflectance spectraof several wide-gap 2D vdW materials.

Technically, the performance has ample space to improve withmore specialized optics, including custom ultra-broadband beam-splitters with anti-reflection back coating, mirrors with vacuum-UV-optimized aluminum coating, and custom spectral-flatteningfilters. Using such optimized optics, spatial filtering could be imple-mented to further reduce the spot size without a major loss ofintensity on the sample.

Moreover, although we mainly validated the setup with micro-reflectance measurements, the rigid design, achromatic operation,nitrogen purging, and ultra-broadband sensitivity make it suitablefor other modes of micro-spectroscopy, e.g., photo- and electro-luminance using appropriate spectral filters. This setup can also beeasily integrated with an optical cryostat with thin fused silica orMgF2 windows for low-temperature measurements.

Scientifically, the system should have immediate application instudying the optical properties of nano-materials, including wide-gap 2D vdW materials and their heterostructures in the few-tomonolayer thickness regime.

ACKNOWLEDGMENTSWe thank Ralph Page for helpful feedback on our manuscript.

This work was supported by the U.S. Department of Energy (DOE),Office of Science, Office of Basic Energy Sciences (BES), Materi-als Sciences and Engineering Division under FWP 100459. Sam-ple preparation was supported by the National Science FoundationMRSEC program through Columbia in the Center for PrecisionAssembly of Superstratic and Superatomic Solids (Grant No. DMR-1420634) and by the MEXT Element Strategy Initiative to Form CoreResearch Center, Grant No. JPMXP0112101001, and the CREST(Grant No. JPMJCR15F3), JST. L.W. acknowledges support from theAlexander von Humboldt Foundation.

DATA AVAILABILITY

The data that support the findings of this study are availablefrom the corresponding author upon reasonable request.

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