PHYSICAL REVIEW A 82, 043821 (2010) High-order harmonic generation in a plasma plume of in situ laser-produced silver nanoparticles H. Singhal, 1,* R. A. Ganeev, 1,2 P. A. Naik, 1 J. A. Chakera, 1 U. Chakravarty, 1 H. S. Vora, 1 A. K. Srivastava, 1 C. Mukherjee, 1 C. P. Navathe, 1 S. K. Deb, 1 and P. D. Gupta 1 1 Raja Ramanna Centre for Advanced Technology, Indore 452 013, India 2 Institute of Electronics, 33, Dormon Yoli street, Tashkent 100125, Uzbekistan (Received 12 July 2010; published 14 October 2010) The results of the experimental study of high-order harmonic generation (HHG) from the interaction of 45-fs Ti:sapphire laser pulses with plasma plumes of Ag nanoparticles produced in situ are presented in this article. The nanoparticles were generated by the interaction of 300-ps, 20-mJ laser pulses with bulk silver targets at an intensity of ∼1 × 10 13 W/cm 2 . The spectral characteristics of the HHG from nanoparticles produced in situ are compared with the HHG from monoparticle plasma plumes and with the HHG from preformed nanoparticle-containing plasma plumes. The cutoff harmonic order generated using the in situ silver nanoparticles is at the 21st harmonic order. DOI: 10.1103/PhysRevA.82.043821 PACS number(s): 42.65.Ky, 42.79.Nv, 52.38.Mf, 78.67.Bf I. INTRODUCTION Generation of coherent extreme ultraviolet (XUV) radiation through high-order harmonic generation (HHG) is an active area of research because of its unique properties of ultrashort duration (of the order of hundreds of attoseconds) [1], good focusability (∼λ 2 )[2], and XUV wavelengths [3]. These properties make HHG an interesting high-intensity ultrashort XUV source for a wide range of practical applications. These applications include ultrafast pump-probe studies [4], mea- surement of ultrafast photorecombination cross sections [5], and soft x-ray microscopy [6]. Usually, high-order harmonics are generated by the interaction of ultrafast femtosecond laser pulses with gas jets [7], solid surfaces [8], and low-excited plasma plumes [9]. The HHG from underdense media can be understood in terms of the semiclassical “three step” model [10]. The conversion efficiency of the HHG process is rather low (∼10 −5 –10 −6 ). Therefore, increasing the conver- sion efficiency of HHG remains an active area of research. In order to increase the HHG conversion efficiency, two approaches are employed. The first one is to use different methods to control the phase characteristics of high-order harmonics during their propagation inside the medium. These methods include quasiphase matching [11], self-guiding of the laser beam [12], phase optimization by changing focusing geometry [13], etc. The second approach is based on enhancing the nonlinear response of the HHG medium. The nonlinear response of the medium can be increased using atomic or ionic resonances [14], nanostructured media [15], or through the use of gas cluster targets [16]. In the case of gases, exploiting atomic or ionic resonances is difficult since the number of gases is limited. High-order harmonics have been generated efficiently by the interaction of femtosecond laser pulses with plasma plumes containing nanoparticles [15,17–19]. It is reported that nanoparticles can be generated by the interaction of subnanosecond pulses with metal targets at an intensity of ∼10 13 W/cm 2 [20]. The plasma conditions for the generation * himanshu@rrcat.gov.in of nanoparticles contrast with the usual conditions for the preparation of optimal plasma plumes for HHG, where the plasma plumes are prepared using the low-intensity prepulse (∼5 × 10 9 –2 × 10 10 W/cm 2 ). The plasma predominantly consists of neutral and singly charged ions in order to keep the free-electron density inside the plasma plume small. However, in the case of nanoparticle-containing plasma, the adverse phase-mismatch effects of higher electron density may be compensated by the greater high-order nonlinear response of the nanoparticles. Efficient HHG can be observed from the interaction of femtosecond laser pulses with these nanoparticles. However, as these nanoparticle-containing targets get depleted very fast, the harmonic efficiency falls. To overcome this problem, we have devised a relatively simple scheme of HHG through nanoparticles produced in situ. In this report, we present experimental observations of HHG from nanoparticles produced in situ. Efficient HHG is achieved from the interaction of femtosecond laser pulses with the plume of nanoparticles generated by the interaction of picosecond pulses with solid surfaces. The presence of nanoparticles in the plasma plume was confirmed by the analysis of the deposited plasma via atomic force microscopy (AFM) and absorption spectroscopy. The spectral characteris- tics of the harmonics generated from nanoparticles produced in situ were compared with HHG from bulk targets and de- posited nanoparticle targets. It is found that the spectral charac- teristics of HHG from nanoparticles produced in situ matches that from deposited nanoparticles, which indicates that the HHG is from nanoparticles and not from the monoparticle plasma. The intensity of the harmonics is comparable to that of the harmonics generated from low-excited plasma plumes. Our studies show that HHG intensity may be improved by further refinement of the process of nanoparticle generation. II. EXPERIMENTAL ARRANGEMENT The laser used in this study was a chirped-pulse amplified Ti:sapphire laser system (Thales Lasers S. A., France) operat- ing at a 10-Hz repetition rate. A schematic of the experimental 1050-2947/2010/82(4)/043821(4) 043821-1 ©2010 The American Physical Society