On-chip picosecond pulse detection and generation using graphene photoconductive switches

Abstract

We report on the use of graphene for room temperature on-chip detection and generation of pulsed terahertz (THz) frequency radiation, exploiting the fast carrier dynamics of light-generated hot carriers, and compare our results with conventional low-temperature-grown gallium arsenide (LT-GaAs) photoconductive (PC) switches. Coupling of picosecond-duration pulses from a biased graphene PC switch into Goubau line waveguides is also demonstrated. A Drude transport model based on the transient photoconductance of graphene is used to describe the mechanism for both detection and generation of THz radiation.

Keywords: GaAs; Goubau line; Graphene; THz detection; on-chip; photoconductive switch.

Figure 1

Schematic diagram of devices D1 and D2. (a) The setup for LT-GaAs “output” pulse detection in device D1. Gold electrodes are shown by black lines. For output pulse measurements a DC bias, V_sd, is applied to the PC switch LTA and a lock-in is connected to the PC switch LTO. The Goubau line is grounded throughout. The pump and probe beams, indicated by red circles, are focused on switches LTA and LTO, respectively. Inset: A part of a PC switch area where the blue color indicates LT-GaAs areas after etching. The dotted line indicates the edges of LT-GaAs under the metal contacts. (b) For “input” pulse measurements the probe beam and the lock-in are moved to the switch LTB, demonstrated for device D1. (c) Measurement setup for graphene detection in the device D2. Graphene is shown by a green cross, and the probe laser beam spot is focused on the spot GO1. The orange line indicates the path for the Fabry–Pérot reflections of THz pulse. Inset: Optical image of the quartz substrate with the graphene area etched in the shape of a cross covered by S1813 resist. The graphene has a length of 50 μm and width of 9 μm.

Figure 2

Characterization of graphene and LT-GaAs. (a) Raman spectra measured after excitation with a 633 nm laser for a clean quartz substrate (black), for the substrate after removal by etching of the LT-GaAs (red), and from an LT-GaAs covered region (green). No peak corresponding to LT-GaAs at 290 cm^–1 was found in the graphene region. (b) Raman spectrum of a graphene switch region (black dots) and a Lorentzian fit of the G- and 2D-peaks (red). (c) Input pulse (normalized by 2.6 nA) generated at LTA and detected at LTB (black line), together with the output pulse (normalized by 0.35 nA) detected 2 mm away at LTO (red line) measured in the device D1 with LT-GaAs PC switches. The delay between the two pulses corresponds to a pulse propagation velocity in the Goubau line of 1.65 ± 0.02 × 10⁸ ms^–1. Inset: Pulse detected by graphene at GO1 in device D2. The red line is a Lorentzian fit to the data.

Figure 3

Characterization of THz pulse detection using graphene on device D2. (a) Input pulse (normalized by 75 nA) generated at LTA and detected at LTB (black line), and the output pulse (normalized by 6.7 nA) detected at GO1 by graphene (red line) measured in the device D2 repeated from Figure 2c inset, but with the current offset removed for comparison. A reflection from the graphene–metal interface is indicated by the black arrow. Inset: Normalized output pulses detected by LT-GaAs (in device D1) and graphene (in device D2), shown by black and red lines, respectively. Normalized conductivity change obtained from the deconvolution of the two output pulses shown in green. (b) FFT spectrum for the input pulse (device D2), output pulse detected by graphene (device D2), and output pulse detected by LT-GaAs (device D1) shown by black squares, red circles, and green triangles, respectively. The dashed red line indicates the chosen level of noise for the graphene detected signal used to define the bandwidth. (c) Pulse detected by graphene as a function of incident optical power. The DC current offset at −10 ps becomes more negative with increasing illuminating laser power. Inset: Lorentzian current integral as a function of incident power is shown by black squares in the linear region and red circles in the sub-linear region. Solid lines are power law fits, ∝P^γ, where γ = 1 and 0.6 for black and red curves, respectively. DC offset photocurrent as a function of incident power is shown by green triangles, obtained from a Lorentzian fit of the peak as shown on the inset in Figure 2c.

Figure 4

THz pulse detected by graphene and generation by biased graphene. (a) Pulse detected by graphene as a function of the source–drain bias applied to the LT-GaAs PC switch at 10 mW pump and probe powers. The applied bias changes with 5 V steps between the most negative signal, measured at −30 V, to the most positive signal, measured at 30 V. A DC current offset corresponding to −1.7 nA has been subtracted from the signal for clarity. Inset: Amplitude of the pulse as a function of the PC switch bias. (b) Pulse detection on the nearest and the furthest graphene ohmic contacts indicated by GO1 (black) and GO2 (red) in Figure 1c, respectively. The position of the THz pulse detected at GO1 is taken as a reference. Inset: Band structure of graphene without applied bias near an ohmic contact. As soon as the temperature, T₁, of the illuminated graphene is larger than the temperature of graphene placed on the metal contact (yellow area), T₀ = 300 K, and p′ is larger than p, a DC photothermoelectric current flows toward the contact. (c) The pulse generated by graphene on GO1 and detected by the LT-GaAs PC switch LTA as a function of a DC bias applied to the graphene using 10 mW pump and probe powers. The setup for THz pulse generation by graphene is shown in Supporting Information. Top inset: Current amplitude as a function of the applied bias. Bottom inset: Band structure for the graphene under applied negative bias. The bias generates a negative THz pulse if the conductivity change under illumination is positive.