Fig. 1. Conceptual illustration of the photo-imprinted device and THz characterization of 100 nm thick GST. (a) Schematic view of photo-imprint concept. (b) Optical images of the GST layer after optical writing (left) and thermal erasing (right) processes. (c) Measured temperature-dependent transmission and corresponding conductivity of the GST film at 0.75 THz. Optical microscopy images of amorphous (left) and crystalline GST (right) are shown in the inset. (d) Measured 0.75 THz transmission of GST after the optical re-amorphization process under different pump energies (blue line) and followed the thermal recrystallization process (orange line). (e) Transmission at 0.75 THz of the same GST sample during multiple write (optical amorphization) and erase (thermal crystallization) cycles. Error bars with one standard deviation are shown by shaded areas along with data curves in (c)–(e).

Fig. 2. Photo-imprinted GST grating with reconfigurable Rayleigh anomaly. (a) Optical microscopy images of the GST grating (period is 100 μm and duty cycle is 50%) by using 100  mJ/cm2 optical pump energy. (b) Measured transmission spectra of the GST grating (period and duty cycle fixed at 100 μm and 50%, respectively) with pump energy increasing from 50 to 140  mJ/cm2. The inset shows the transmission at a Rayleigh frequency of 0.87 THz as a function of pump energy. (c) Measured (stars) Rayleigh frequencies under different grating periods with a fixed pump energy of 100  mJ/cm2 and calculated (blue line) period-dependent Rayleigh frequency.

Fig. 3. Photo-imprinted GST grating with reconfigurable beam steering performance. (a) Measured diffractive transmission spectra of a GST grating photo-imprinted by a pump power of 100  mJ/cm2 (period of 800 μm and duty cycle of 50%) for variable detection angles. The blue, green, and orange dashed lines illustrate the diffraction patterns at 20°, 30°, and 40°, respectively. The calculated frequency-dependent diffraction angle of ±1st order is depicted by the white line. The optical image of sample is shown in the inset. (b) THz signals and (c) corresponding spectra at 20°, 30°, and 40° diffraction angles. (d) Pump energy-dependent diffraction pattern of the GST grating (period and duty cycle fixed at 800 μm and 50%, respectively) at 20°, 30°, and 40°. (e) Measured and calculated relationship between THz signal frequency and diffraction angle of GST grating with different periods.

Fig. 4. Design and demonstration of a photo-imprinted GST flat lens. (a) Schematic view of the designed GST flat lens with a focus length of f. (b) Conductivity and corresponding transmission distribution of the GST lens. (c) Optical image of a photo-imprinted flat lens on a 100 nm thick GST film with a pump power of 100  mJ/cm2. Theoretical focal intensity distributions of the GST lens (diameter of 1 cm and focal length of 5 mm) designed at 0.75 THz in the (d) axial and (e) lateral planes. Measured focal intensity distributions of fabricated GST flat lenses in the (f) axial and (g) lateral cross-sections at 0.75 THz. The lens working was photo-imprinted on a 100 nm thick GST film with 100  mJ/cm2 pump energy. Extracted cross-sectional intensity distributions along the (h) z and (i) y directions.

Fig. 5. Photo-imprinted GST flat lens for variable focal lengths. Measured focal intensity distributions of the GST lens (pump energy fixed at 100  mJ/cm2) with different focal lengths in the (a), (c), (e), and (g) axial and (b), (d), (f), and (h) lateral planes at 0.75 THz. The corresponding lens samples are shown in the insets with the same scale bar of 5 mm. Extracted cross-sectional intensity distributions for GST lenses with different focal lengths along the (i) z direction and (j) y direction.

Fig. 6. Reconfigurable and multilevel modulation features of the photo-imprinted GST flat lens. Measured focal intensity distributions of the same GST sample during two write-erase cycles in the (a)–(d) axial and (e)–(h) lateral planes. The writing and erasing processes are realized by optical activation with 100  mJ/cm2 pump energy and thermal activation at 300°C for 2 min, respectively. The corresponding optical images of the sample for each step are shown in the insets of (a)–(d). (i) Measured focal intensity of the photo-imprinted GST lens as a function of pump energy with error bars of one standard deviation. The focal intensity distributions in the lateral plane corresponding to different pump powers are also shown in the top panel.

Fig. 7. Flat lens with focal length of 4 mm was optically written on a 100 nm GST layer by a single 100  mJ/cm2 pump pulse. The frequency dependent focal intensity distributions of such a GST lens along the (a) axial and (b) lateral directions were characterized using our NSTM system. (c) The THz intensity distributions along the x direction were also extracted to clearly see the FWHM of focal points for variable frequencies. Well-defined strong focusing effect with negligible sidelobes and subwavelength-scale FWHM of focal points can be observed for all the frequencies, which clearly demonstrates the broadband focusing feature of such photo-imprinted GST flat lenses.