Photothermal Response in Dual-Gated Bilayer Graphene

The photovoltaic and bolometric photoresponse in gapped bilayer graphene was investigated by optical and transport measurements. A pulse coincidence technique at $1.5\mu \mathrm{m}$ was used to measure the response times as a function of temperature. The bolometric and photovoltaic response times were found to be identical implying that the photovoltaic response is also governed by hot electron thermal relaxation. Response times of $\tau \sim 100-20\mathrm{ps}$ were found for temperatures from 3–100 K. Above 10 K, the relaxation time was observed to be $\tau =25\pm 5\mathrm{ps}$, independent of temperature within noise.

Figure 1

Photovoltaic response and resistance of a dual-gated bilayer graphene. (a) Resistance and (b) photovoltaic response as a function of back gate voltage for different top gate voltages at $T=7.3\mathrm{K}$ and zero bias current. Inset in (a) shows an optical micrograph of the bilayer graphene device. Scale bar, $10\mu \mathrm{m}$. Inset in (b) shows schematic of device geometry and electric-field-effect gating.

Figure 2

Bias current dependence of the pump-probe measurements. Photoresponse from pump-probe laser pulses as a function of time delay $t_d$ at 3.2 K and laser power of 31 nW. The sample is gated to charge neutrality with $V_{\mathrm{tg}}=-30\mathrm{V}$, $V_{\mathrm{bg}}=48\mathrm{V}$. (a) Bias current $I_{dc}$ dependence of probe-induced photoresponse voltage $V_{\mathrm{pr}}(t_d)$ as a function of the probe beam delay time $t_d$. The $I_{\mathrm{dc}}=0$ curve is the photovoltaic response. (b) $r$ is the normalized bolometric response $\Delta V(I_{\mathrm{dc}})=V(I_{\mathrm{dc}})-V(0)$ and photovoltaic response $V(0)$. $r=V_{\mathrm{pr}}(t_d)/V_{\mathrm{pv}}^0$, where $V_{\mathrm{pv}}^0=V_{\mathrm{pv}}(t_d\gg \tau )$. The thermal response time $\tau$ is defined as the half-width at half-maximum of the dip. All dips have a similar time constant $\tau =0.12\pm 0.01\mathrm{ns}$.

Figure 3

Gate voltage dependence of the pump-probe measurements. Pump-probe pulse induced photovoltaic response as a function of time delay at 15 K, laser power of 58 nW, and $I_{\mathrm{dc}}=0$. The data were taken at several back gate voltages $V_{\mathrm{bg}}$ with zero top gate voltage (a) below and (b) above $V_{\mathrm{bg}}=25\mathrm{V}$ where the maximum photovoltaic response is found. The thin dashed line at $V_{\mathrm{bg}}=0$ is a guide line. All cusps have the same thermal time constant $\tau =25\pm 5\mathrm{ps}$ within error.

Figure 4

Temperature dependence of thermal response time. Thermal time constant $\tau$ of the photovoltaic response vs temperature measured by the pulse coincidence technique for several different dual-gate voltage settings.