[Paper Review] Silicon beamsplitter for Fourier transform spectroscopy at far infrared frequencies

Geoffry Evans, D. C. Schmadel+2 more|ArXiv.org|Jun 28, 2007

Spectroscopy and Quantum Chemical Studies3 citations

TL;DR

This paper presents a 2 mm thick silicon wafer as a beamsplitter for Fourier transform spectroscopy in the far-infrared (0–1000 cm⁻¹), demonstrating superior throughput compared to conventional Mylar beamsplitters across most of the spectrum, especially in the 100–400 cm⁻¹ range. The Si beamsplitter achieves an average efficiency of 37%, outperforming Mylar’s 35%, and enables single-beam operation over the entire far-IR range with usable resolution down to Δν ≥ 0.3 cm⁻¹, despite narrow etalon fringes spaced at ~0.7 cm⁻¹.

ABSTRACT

We report the performance of a silicon wafer beamsplitter for use for low $Δν>0.3 cm^{-1}$ resolution Fourier transform spectroscopy at far infrared frequencies. We characterize the Si beamsplitter by comparing throughput spectra measured with it to those measured with the standard Mylar beamsplitters commonly used in that range. We find that the throughput of the silicon beamsplitter is substantially greater than that of the Mylar beamsplitters over most of the IR spectrum, and that they are comparable in some limited ranges. The 2 mm silicon beamsplitter has an etalon spacing of about 0.7 cm^{-1}, which interferes with its use for $0.1 cm^{-1}

Motivation & Objective

• To develop a single, broadband beamsplitter for far-infrared Fourier transform spectroscopy that overcomes limitations of conventional Mylar pellicles.
• To address the low throughput and spectral gaps in Mylar beamsplitters due to interference minima and absorption losses.
• To evaluate the performance of a 2 mm silicon wafer as a beamsplitter in terms of throughput, spectral resolution, and etalon effects.
• To demonstrate that silicon can replace multiple Mylar beamsplitters in a single instrument setup for full spectral coverage.

Proposed method

• Fabricated a 2 mm thick, optically polished silicon wafer from high-resistivity ( >100 Ω·cm) silicon with surface flatness ~1 μm.
• Compared the relative throughput of the Si beamsplitter against multiple Mylar beamsplitters (3 μm, 12 μm, 25 μm, 50 μm, 100 μm) using calibrated sources (Globar and Hg vapor) and bolometers (4 K and 2 K) under vacuum.
• Measured spectra at 4 cm⁻¹ resolution to assess overall throughput and etalon interference patterns.
• Used a 2 mm fluorogold filter to block radiation above 50 cm⁻¹ and reduce baseline errors in low-frequency measurements.
• Conducted high-resolution (0.1 cm⁻¹) measurements to analyze the impact of Si etalon fringes on spectral features.
• Normalized transmission data to an open-hole reference to enable direct comparison of spectral features.

Experimental results

Research questions

• RQ1 Can a single silicon wafer beamsplitter replace multiple Mylar beamsplitters in far-infrared Fourier transform spectroscopy?
• RQ2 How does the throughput of a 2 mm silicon beamsplitter compare to that of Mylar beamsplitters across the 0–1000 cm⁻¹ range?
• RQ3 What is the effect of the silicon etalon's interference fringes on spectral measurements at high resolution (e.g., 0.1 cm⁻¹)?
• RQ4 To what extent does silicon outperform Mylar in the 100–400 cm⁻¹ range, where Mylar absorption reduces efficiency?
• RQ5 What is the minimum resolvable spectral feature width when using the silicon beamsplitter, given its etalon spacing of ~0.7 cm⁻¹?

Key findings

• The silicon beamsplitter exhibits substantially higher throughput than Mylar beamsplitters across most of the 0–1000 cm⁻¹ range, particularly in the 100–400 cm⁻¹ region where Mylar absorption reduces efficiency.
• The average throughput efficiency of the silicon beamsplitter is 37%, compared to a maximum of 35% for Mylar, indicating a measurable performance advantage.
• The 2 mm silicon beamsplitter has an etalon spacing of approximately 0.7 cm⁻¹, resulting in narrow transmission minima (~0.1 cm⁻¹ wide), which limits its use only when spectral features are within ~0.1 cm⁻¹ of these minima.
• For spectral resolutions Δν ≥ 0.3 cm⁻¹, the silicon beamsplitter enables reliable spectroscopy across the entire far-IR range without requiring beamsplitter changes.
• At 0.1 cm⁻¹ resolution, the silicon beamsplitter produces spectra nearly identical to Mylar, except for sharp noise spikes near etalon minima, confirming its viability for high-resolution work when feature positions are carefully selected.
• The silicon beamsplitter shows greater throughput than the 100 μm Mylar below 10 cm⁻¹, indicating improved low-frequency response compared to Mylar.