Persistent and Reversible All-Optical Phase Control in a Manganite Thin Film

Persistent and reversible optical phase control has been achieved in a manganite thin film through a careful choice of the composition of ${\mathrm{Pr}}_{1-x}({\mathrm{Ca}}_{1-y}{\mathrm{Sr}}_y{)}_x{\mathrm{MnO}}_3$ near a multicritical point. Pulsed laser light brings the lower temperature metallic phase out of the higher temperature charge-ordered insulator, while a cw light reverses the effect by heating. We clearly demonstrate the two competing roles played by light, heating, and excitation across the charge gap, which are important in both the application and the understanding of the physics of electron correlation.

Figure 1

(a) Temperature dependence of resistivity for PCSMO ($0.2\le y\le 0.4$) thin films without an external field. Solid and dotted lines denote the warming and cooling processes, respectively. Inset: temperature dependence of magnetization for $y=0.25$. (b) Electronic phase diagram of PCSMO. The COO transition temperature ($T_{\mathrm{CO}}$) is represented by squares. The transition temperatures from (to) ferromagnetic state ($T_{\mathrm{C}}$) are represented by circles (triangles). Those points for thin films are plotted in the filled symbols and superposed on a phase diagram of PCSMO bulk single crystals [13].

Figure 2

Temperature dependence of resistance for PCSMO ($y=0.25$) thin film. Solid and dotted lines denote the cooling and warming processes, respectively. At 77 K in the cooling process, the sample was irradiated with 100 pulses. The photon energy was 1.95 eV ($\lambda =637\mathrm{nm}$) and the pulse energy density was $I=3.82\mathrm{mJ}/{\mathrm{cm}}^2$. Insets show the resistance drop at individual pulses for various time scales. [(a) $I=45.9\mathrm{mJ}/{\mathrm{cm}}^2$, (b) $I=1.53\mathrm{mJ}/{\mathrm{cm}}^2$].

Figure 3

Threshold energy density of the laser pulse $I_{\mathrm{th}}$ as a function of photon energy $h\nu$ (closed circles) and absorption spectrum (opened circles). $I_{\mathrm{th}}$ was defined as the energy density at which resistance after irradiation with 200 pulses is one-half of its initial value. The line is a guide to the eye. The inset is the resistance after irradiation with 200 laser pulses at the photon energy $h\nu =1.95\mathrm{eV}$ ($\lambda =637\mathrm{nm}$) for various pulse energy densities. The film was reset to the same initial COO state for each energy density measurement.

Figure 4

Change of the resistance by irradiation with the pulsed laser ($h\nu =1.95\mathrm{eV}$, $I=1.53\mathrm{mJ}/{\mathrm{cm}}^2$) and the cw laser ($h\nu =2.30\mathrm{eV}$, $I=157\mathrm{W}/{\mathrm{cm}}^2$). The inset shows repeated cycling between COO and FMM states using the same photon energy of $h\nu =2.3\mathrm{eV}$ but with different peak powers, simulating an all-optical write-erase process.