Retime-mapping terahertz vernier biosensor for boosting sensitivity based on self-reference waveguide interferometers

Abstract

The optical vernier effect serves as a potent mechanism for boosting sensitivity and accuracy in the communication band, which is a prominent hotspot in coherent detection. Extending vernier gain to the terahertz window exhibits significant appeal in next-generation wireless communication and high-resolution sensing. Here, a terahertz vernier biosensor is constructed utilizing two overlapping Mach-Zehnder interferometers within a three-channel metallic waveguide. The self-reference feature of the vernier biosensor facilitates a sensitive envelope, and the vernier gain significantly amplifies the detection sensitivity and accuracy from the superposition of slightly detuned terahertz interference spectra mapping within the time-frequency-time domain. An exalting sensitivity of 22.54 THz/RIU is demonstrated at operating frequencies near 0.9 THz and experimentally shows immense sensing performance in detection sensitivity and accuracy of biochemical sample areic mass are 10⁷ GHz/(g/mm²) and 10^-8 g/mm², respectively, presenting an enhancement of > 3000% compared to a single interferometer. Moreover, the sensor is employed to assess the amino acid oxidation characteristic curve analysis in the terahertz range, which assists in identifying specific amino acids. The validation of the vernier effect operating in the terahertz regime demonstrates the development of a rapid and label-free assistance tool for the identification of biochemical samples.

Keywords: Self-reference detection; Sensitivity enhancement; Terahertz; Vernier biosensor; Waveguide interferometer.

Graphical abstract

Fig. 1

Principle and design for THz vernier biosensor. (a) The fundamental components and (b) operational principles of THz vernier sensor. (c) Two-dimensional diagram of three-channel overlapping MZIs structure. Light blue, dark blue, and light red backgrounds represent Channels 1–3. (d) Photo of SiO₂ coated with gold films (left) and front view of THz vernier biosensor (right). (e) THz-TDS system in experiments. PPWG, parallel-plate waveguide; BOPP, biaxially-oriented polypropylene; MZI, Mach-Zehnder interferometer; WP, Wollaston prism; OAP, off-axis parabolic; PCA, photoconductive antenna; fs, femtosecond. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

The evolvement of THz vernier sensor. (a) Diagram of the time-frequency-time mapping of the transmitted signal in the Fourier domain. F¹, the first Fourier transform. F², the second Fourier transform. (b) The time domain signal of the evolvement process of the THz vernier measured in the experiments. (c) Physical models shaping the THz vernier as the waveguide channels change. The (d) modulation and (e) demodulation for the analytical interference process in transmission spectra correspond to the waveguide channels. Red, blue, and green rows represent the THz radiation from different channels. Blue dashed lines indicate the dip of the enveloped spectrum. Green and blue sticks mean the different FSRs of MZI-1, MZI-2, and vernier. Green and red dashed lines mark the OPDs of the two MZIs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Impact of Channel 1 length on transmission characteristics. (a) Schematic diagram of BOPP length change in Channel 1. (b) The time domain signals varying with the length of Channel 1. (c) The modulation spectrum in the frequency domain obtained by once Fourier transform. (d) The demodulation spectrum in the time domain was obtained through twice Fourier transforms. The red solid line represents the envelopes of the spectra. The red dashed lines indicate the time signal in Channel 1 and the relevant OPD of MZI-2 in the demodulation spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Impact of Channel 3 width on transmission characteristics. (a) Schematic diagram of width change in Channel 3. (b) The time domain signals varying with the width of Channel 3. (c) The modulation spectrum in the frequency domain obtained by once Fourier transform. (d) The demodulation spectrum in the time domain obtained through Fourier transforms twice. The blue rectangular areas represent the range of the envelope dips. The red rectangular areas indicate the contrast of the envelope dips. (e) The simulated electric field distribution at the position of the digital signs in (c). The white dotted lines mean the boundary condition of PEC. The yellow cuboid region includes the outport of the THz vernier sensor. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Biochemical sample areic mass detection with THz vernier sensor. (a) The envelope contour of THz vernier in detecting areic masses of lactose over broadband range. (b) Spectral characterization of lactose areic mass of 0–2 µg/mm². The red gradient curves represent the envelopes, while the blue curves signify interference. The red and blue dashed lines correspond to envelope dips and interference dips, respectively. (c) The frequencies and shifts of interference and envelope dips in (b). (d) The lactose detection sensitivities and accuracies of interference and envelopes correspond to (b). AM, areic mass; Lac, lactose. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Assistance in amino acid identification with THz vernier sensor. (a) Schematic diagram of online detection of amino acid oxidized status. (b) The chromogenic reaction between DTNB and sulfhydryl groups in cysteine varying with the doses of hydrogen peroxide. (c) Characteristic curves of four amino acids (cysteine, methionine, alanine, and phenylalanine) reacting with different doses of hydrogen peroxide in the THz band. (d) The normalized envelope contour of the THz vernier sensor in detecting the four amino acids reacting with different doses of hydrogen peroxide. Blue and red dots indicate the blueshift and redshift, respectively. The arrows indicate the frequencies of envelope dips. DTNB, dithio-nitrobenzene. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)