Abstract
Deep-ultraviolet (DUV) microscopy and microspectroscopy have received much attention in label-free live-cell imaging, selective molecular analysis, and optical characterizations of ultrawide bandgap materials. Far-field optics approaches usually suffer from the diffraction limit of light. Meanwhile, near-field optics technology is immature in the DUV spectral region. Herein, we develop a DUV scanning near-field optical microscope (SNOM) with an excitation wavelength of 210 nm. The fourth harmonic generation of a continuous-wave Ti:sapphire laser is the excitation source. Two negative feedback control systems stabilize the power and pointing, allowing scanning microscopy to be carried out over a long period of time even in the DUV spectral region. A reflective objective couples the excitation beam onto an optical fiber probe. The optical fiber probe is solarization-resistant and is not coated by metallic materials. The reflective-based objective optical design does not introduce chromatic aberrations in the detection signals. Our DUV-SNOM is used to measure the photoluminescence (PL) spectra of the quantum well structure of aluminum gallium nitride, which is an ultrawide bandgap material. The PL mapping images indicate that emissive localization centers can be individually visualized via our DUV-SNOM with a lateral resolution exceeding 150 nm.
Highlights
In the framework of classical optics, the lateral resolution R and the depth of focus (DOF) are given as1 λ R = k1 NA, (1) λ DOF = k2 (NA)2, (2)where k1 and k2 are experimental factors, λ is the optical wavelength, and NA is the numerical aperture.The lateral resolution of lithography has been enhanced by shortening the wavelength of illumination sources from the g-line of a mercury lamp (λ = 436 nm) to the i-line (λ = 365 nm), a krypton-fluoride laser (λ = 248 nm), and an argon-fluoride laser (λ = 193 nm)
We developed a DUV-scanning near-field optical microscope (SNOM) with a 210-nm excitation wavelength
The PL mapping image of an ultrawide bandgap material clearly indicates that emissive localization centers are individually visualized via our DUV-SNOM with a lateral resolution exceeding 150 nm
Summary
In the framework of classical optics, the lateral resolution R and the depth of focus (DOF) are given as λ. The DOF problem, which is serious in EUV lithography, does not exist in scanning near-field optical microscopy because the probe-sample distance is controlled (one drawback of SNOM is the relatively low signal-to-noise ratio). Various problems have hampered the development of nanospectroscopic tools operating at shorter wavelengths These include temporal deterioration of DUV transmission and reflection optics, degradation-induced low pointing stability of an excitation source, and a low throughput of DUV transmission optics. We overcome these problems and develop a DUV-SNOM with an excitation wavelength of 210 nm
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