Femtosecond-laser direct-write photoconductive patterns on tellurite glass

We report the formation of arbitrary photoconductive patterns made of tellurium ($\mathrm{Te}$) nanocrystals by exposing a tellurite (${\mathrm{Te}\mathrm{O}}_2$-based) glass to femtosecond laser pulses. During this process, $\mathrm{Te}/{\mathrm{Te}\mathrm{O}}_2$-glass nanocomposite interfaces with photoconductive properties form on the tellurite glass substrate. We show that these laser-written patterns exhibit a photoresponse, from the near ultraviolet (263 nm) to the visible spectrum, stable over a few months. Specifically, high responsivity (16.55 A/W) and detectivity (5.25 × ${10}^{11}$ Jones) of a single laser-written line pattern are measured for an illumination dose of 0.07 $\mathrm{mW}/{\mathrm{cm}}^2$ at 400 nm. This work illustrates a pathway for locally turning a tellurite glass into a functional photoconductor of arbitrary shape, without adding materials and using a single laser process step.

Figure 1

Schematic representations of the device preparation steps (a)–(c) and photoconductivity characterization method (d). (a) Femtosecond-laser direct-write process applied to tellurite glass: the line patterns were produced by moving the specimen under the laser beam at a prescribed velocity. (b) The fabrication of $\mathrm{Au}$ electrodes at both ends of the sample was performed by sputtering the metal through a hard mask to protect the central portion. (c) Wire bonding was used to connect the electrodes on the specimen to a standard PCB on which the glass substrate was mounted. (d) Device characterization was performed by applying a bias voltage ranging from −40 to +40 V through the PCB pins. The illumination source for photoconductivity characterization was delivered in the form of a thin elliptical spot exposing individual line patterns separately.

Figure 2

(a) Effect of the laser-writing parameters on electrical (dc) resistivity of line patterns. (b) Effect of the temperature on the resistivity of laser-written patterns with the inset image of the Arrhenius-law fitting. (c) Measured UV-VIS-NIR absorption spectra of the pristine and laser-written area (2 × 2 mm²). (d) Corresponding Tauc plots of pristine and the laser-written area, considering both indirect and direct band-gap absorption models.

Figure 3

(a) Spectral response of the device per unit of incident light power. (b) I-V curve with and without illumination with various voltage biases (−40 V to +40 V). (c) Typical temporal evolution with different optical power densities (0.07–2.1 mW/cm²). (e) Responsivity, detectivity, EQE (%), and generated photocurrent of $\mathrm{Te}/{\mathrm{Te}\mathrm{O}}_2$-glass nanocomposite structure. (e) Photocurrent rise and decay time for a line pattern. All measurements are performed under an open-air atmosphere at room temperature.

Figure 4

(a) I-V curve without and with illumination at 400 nm for various voltage biases. The measurement was carried out a month after the fabrication of the device presented in Fig. 3. (b) Typical temporal evolution of photocurrent with different optical power densities (0.07–2.1 mW/cm²) under 400-nm illumination 1 month after the fabrication of the device. (c) Responsivity, detectivity, EQE (%), and generated photocurrent of $\mathrm{Te}/{\mathrm{Te}\mathrm{O}}_2$-glass nanocomposite structure 1 month after the fabrication of the device. (d)–(f) Repetitive temporal response at 400 nm with a flux of 2.1 mW/cm² observed on the day of the fabrication, after the first month and after the second month, respectively.

Figure 5

(a) The Raman spectra of the pristine glass after irradiation with 445-nm-Raman laser for 1500 s. (b) The Raman spectra of the laser-irradiated line pattern after irradiation for 300 s. The femtosecond-laser processing parameters are 200 nJ with 0.5 mm/s at 1 MHz (corresponding to an incoming pulse fluence of 262 J/mm²). Note that Raman investigation was performed on the same day as the fabrication of the tested sample. (c) The relative intensities of $\mathrm{Te}$ [I₉₃+ I₁₂₂+ I₁₄₁+ I₁₇₀+ I₂₆₀, (%)] and pristine glass peaks [I₃₅₆+ I₄₇₀+ I₆₁₀+ I₆₇₀+ I₇₂₀+ I₇₉₀+ I₈₆₀+ I₉₂₀, (%)] at the laser-modified zone after being irradiated with Raman laser for 300 s. The optical power density of the Raman laser is 115 mW/µm².