Objective Fourier Transform Infrared spectroscopy (FTIR), renowned for its high sensitivity and resolution within the infrared spectrum, has become one of the most important spectral characterization methods in fields such as drug development, materials research, and chemical analysis. With continuous advancements in comprehensive detection technologies, FTIR is facing the demand to enhance spectral analysis capabilities while also becoming more lightweight and integrated. Graphene-based photodetectors, with their ultra-broad spectral response range, ultra-fast photo response speed, and micron-scale detector size, exhibit tremendous potential in the development of high-performance integrated FTIR spectrometers. Moreover, traditional FTIR spectrometers usually employ a reference beam to calibrate the optical path difference between the two arms of the interferometer. This reference optical path significantly increases the system's complexity and poses substantial limitations on the miniaturization of the FTIR spectrometer's optical system. The proposed FTIR spectrometer eliminates the need for a reference beam by utilizing a high-precision nano positioning stage. The development of a reference beam-free FTIR spectrometer based on graphene photodetectors is presented.

Methods Graphene devices were prepared as photodetector elements and a high-precision nano-displacement stage was used to replace the reference beam for calibrating the optical path difference in a Michelson interferometer, thereby constructing a reference beam-free FTIR spectrometer based on graphene photodetectors. The graphene was mechanically exfoliated onto fused silica, and gold electrodes were deposited using a masking technique to create the photodetector elements. The spectrometer system employed a data acquisition card with a maximum sampling rate of 50 kS/s and a high-precision nano-displacement stage with a minimum displacement of less than 1 nm, ensuring high-precision sample characterization without a reference beam. Additionally, the photocurrent scanning imaging and power dependency characterization of the graphene photodetector were performed to verify its response to light signals in the 500 nm to 8000 nm wavelength range and the linear dependence of the photoexcitation electrical signal strength on power (Fig.1).

Results and Discussions The performance of the constructed FTIR spectrometer was tested and analyzed. Initially, the optimal scanning speed and scanning distance configurations were determined. The spectrometer's spectral testing capabilities were characterized at different scanning speeds and distances. When the scanning speed was in the range of 0.01-0.10 mm/s and the scanning distance was between 1.0-8.0 mm, the developed FTIR spectrometer achieved a spectral resolution better than 5 cm⁻¹ and a signal-to-noise ratio (SNR) exceeding 40 dB. The best performance achieved a spectral resolution of 0.6 cm⁻¹ and an SNR of 40 dB (Fig.2). Further tests demonstrated the FTIR spectrometer's high accuracy under different wavelength excitations (Fig.3). To validate the material characterization performance of the developed reference beam-free FTIR spectrometer, it was used to characterize a self-prepared PDMS film. The spectrometer accurately measured absorption peaks around 2963 cm⁻¹ and 2906 cm⁻¹ (Fig.4), consistent with previously reported literature results.

Conclusions The study developed a reference beam-free FTIR spectrometer based on graphene photodetectors by utilizing a high-precision nano-displacement stage to replace the complex reference optical path and leveraging the broad spectral response range of graphene photodetectors. Experimental verification demonstrated the excellent spectral characterization performance of the spectrometer. By balancing spectral resolution, SNR, and testing time, we identified the optimal combination of scanning speed and scanning distance, achieving the best performance with a spectral resolution of 0.6 cm⁻¹ and an SNR of 40 dB. The accuracy of the reference beam-free FTIR spectrometer was confirmed under different wavelength excitations. Experimental results showed that the lightweight FTIR spectrometer design offers high resolution, superior SNR, high accuracy, and broad spectral range capabilities. This development significantly enhances the performance of Fourier-transform infrared spectroscopy systems while providing a more portable and precise solution for practical applications.