Ultrafast Photoresponse Research Articles

The future of photo-induced phase transition (PIPT) – How fast and slow it can be changed?

The study of photo-controled nature of materials, including their optical, magnetic, and conducting properties, is a fascinating research field. The finding of photo-induced phase transition (PIPT) has triggered the search for inorganic and organic systems with highly efficient and ultrafast photo-responses. As a result of the recent progress in quantum-beam technologies, the time-resolved study of PIPT dynamics on the femto-second time scale, which is comparable with the single-cycle of phonon vibration, has become feasible. In contrast, ultra-slow dynamics on the time scales of a few seconds to several minutes play an important role in the cooperative phenomena in complex systems. Here, we review both the ultra-fast and ultra-slow dynamics of the photo-induced cooperative effects in a typical organic CT crystal (EDO-TTF)2PF6 and a protein molecule, myoglobin (Mb). In the case of Mb, we discuss the results from the viewpoint of a unique photo-functionality, i.e., the photo-induced transportation of a small molecule in the "super-structure" of a protein molecule.

Strong Optical Nonlinearity and its Ultrafast Response Associated with Electron Ferroelectricity in an Organic Conductor

We report experimental evidence for the generation of ferroelectric polarization in an organic conductor α-[bis(ethylenedithio)tetrathiafulvalene] 2 I 3 obtained by optical second-harmonic generation. The spontaneous polarization emerges along with a metal-to-insulator transition that is driven by the Wigner crystallization of electrons. The strong optical nonlinearity and its ultrafast photoresponse demonstrated by this study exemplify the nature of the ferroelectric polarization that originates from the electron ordering.

Hg-based cuprate superconducting films patterned into structures for ultrafast photodetectors

We present results which describe procedures of the preparation and patterning of Hg,Re–Ba–Ca–Cu–O superconducting films suitable for coplanar structures usable as possible photodetectors and for microbridges. We compare structural and electrical properties of the final films from the point of their applicability for photodetectors. Our prepared Re-doped Hg-based films on LaAlO 3 substrate are continuous with a sufficient adhesion to the substrate and with a maximum zero resistance critical temperature T C0 ∼ 122 K. The X-ray diffraction of all the films confirmed the Hg-1212 phase beside the minor Hg-1223 phase and intergrowth phases in some cases. In spite of epitaxial character of the final Hg-based films, the microwave and magnetic measurements suggest a possible existence of nonstoichiometric material between the grains which we register as a presence of weak links near the transition into the superconducting state. We show some electrical properties of the prepared superconducting structures, too. The prepared coplanar structures show ultrafast photoresponse signal to incident laser pulse.

Ultrafast Photoresponse Dynamics of Current-Biased Hg-Ba-Ca-Cu-O Superconducting Microbridges

We report our time-resolved transient photoresponse studies of current-biased Hg-Ba-Ca-Cu-O superconducting microbridges excited by 100-fs-wide and 800-nm-wavelength laser pulses. Our devices consisted of 20-mum wide and 30-mum long bridges, incorporated into a 0.1-mm-wide signal line of the coplanar strip transmission line. The structures were patterned in 400-nm-thick precursor films grown on substrates by room-temperature magnetron sputtering and, subsequently, exposed to an ex-situ high-temperature mercuration process. The resulting structures consisted of a c-axis-oriented, predominantly Hg-1212 compound with the onset superconducting critical temperature Tc,on around 110 K. At low temperatures (far below Tc ), under low bias current and low optical excitation, the microbridge photoresponse signal was a positive, ~90-ps-wide (measurement- limited) pulse followed by a negative component and was related to the kinetic-inductive response. At high temperatures, or when the bias current was near the bridge critical current, we observed an additional slow resistive response due to the bolometric effect. The observed ultrafast photoresponse dynamics in our Hg-Ba-Ca-Cu-O bridges make this material very promising for optical photodetector and mixer applications.

Ultrafast spectroscopy of a photoswitchable 30-amino acid de novo synthesized peptide

The ultrafast photoresponse of small, often cyclic peptides with azobenzene units has widely been investigated during the last years. Both the photoisomerization of the optical switch as well as the different conformational states of the peptide moiety can be characterized by optical spectroscopy. Here, we investigate the fast photoisomerization dynamics of an α-helical 30mer azobenzene peptide. The peptide is based on a construct used for the assembly of di-heme-binding maquettes. The femtosecond to picosecond photodynamics for the trans to cis isomerization of the optical switch was found to occur slower upon its insertion in the peptide construct. Both isomers are sufficiently photostable to allow spectroscopic analysis of conformational states, since the thermal cis– trans relaxation occurs over a period of several hours. This approach thus offers the possibility for the de novo design of photoresponsive chromopeptides which could be instrumental in unravelling fundamental dynamic features of assembly/disassembly triggered by fast photoswitches.

Ultrafast photoresponse of metallic and semiconducting single-wall carbon nanotubes

Utilizing a transient absorption (TA) technique based on a chirp-corrected broadband probe, we have studied the ultrafast photoresponse of dispersed HiPco single-wall carbon nanotubes (SWNTs) over the range of 440--1050 nm for excitation in the range of 430--1700 nm. While both metallic and semiconducting SWNTs show transient bleaching at their ${M}_{11}$ and ${S}_{11}$ energies for excitation above these energies, metallic SWNTs uniquely exhibit a photoresponse to sub-${M}_{11}$ excitation. We observe a TA spectral response for metallic SWNTs which is consistent with thermal broadening of the ${M}_{11}$ transition bands. In contrast to metallic SWNTs, specific semiconducting SWNTs exhibit transparency for sub-${S}_{11}$ excitation, in accord with the expected zero density of states below the first interband transition. We report the observation of a long-lived $(\ensuremath{\tau}>1\phantom{\rule{0.3em}{0ex}}\mathrm{ns})$ transient bleach component for three semiconducting SWNT species.

Investigation and Application of Ultrafast Photoresponse of YBa2Cu3O7-δMicrobridges, Excited by High Fluence Optical Pulses

Investigation and Application of Ultrafast Photoresponse of YBa₂Cu₃O_7-δMicrobridges, Excited by High Fluence Optical Pulses

Ultrafast photoresponse at 1.55 μm in InGaAs with embedded semimetallic ErAs nanoparticles

We have grown epitaxial metal/semiconductor superlattice materials by molecular beam epitaxy that exhibit subpicosecond photocarrier lifetimes at 1.55 μm. The superlattice samples consist of layers of semimetallic ErAs nanoparticles embedded in a semiconducting In0.53Ga0.47As matrix. Time-resolved optical measurements are performed using a fiber-based transmission pump-probe technique with an erbium-doped-fiber mode-locked laser. Photocarrier lifetimes decrease with increasing ErAs deposition and decreasing spacing between the ErAs layers. Further reduction in the lifetime is achieved by selective beryllium doping of the superlattice; measured lifetimes ⩽0.3ps were achieved in optimized structures.

Electrical properties and ultrafast photo-response of InGaAs/InP grown by low-temperature molecular beam epitaxy with a GaAs decomposition source

Lattice-matched InGaAs on (1 0 0) InP was grown by molecular beam epitaxy with arsenic dimers (As 2) at a growth temperature ( T g) range of 250–470°C. Measurements of double-crystal X-ray diffraction reveal that Δ a ⊥/ a o of low-temperature-grown (LTG) InGaAs increases from 0.82×10 −2 ( T g=470°C) to 4.1×10 −2 ( T g=250°C) as T g decreases. The measurements also indicate that excess arsenics are easily incorporated as antisites. All grown samples show a clear crystalline structure and are strong n-type. When T g is decreased from 470°C to 250°C, then according to the Hall measurement the carrier concentration increases from 5.0×10 −16 to 2.4×10 −18 cm −3 and according to the time-resolved reflectivity measurement the carrier lifetime decreases from 14.02 to 2.33 ps. Finally, for InGaAs/InP grown at 250°C, the incorporation of Be decreases the carrier lifetime to 1.86 ps and the residual n-type carrier concentration to 5.0×10 −16 cm −3. Crystallized InGaAs/InP with LTG material properties was obtained with a smaller V/III ratio and at higher growth temperature, compared to those with As 4.

Ultrafast photoresponses of CdS nanoparticles in self-assembled films.

Ultrafast photoresponses of CdS nanoparticles in self-assembled films.

Ultra-fast photoresponses of CdS nanoparticles in Nafion films

The ultra-fast dynamics of transient absorbance bleaching and recovery of cadmium sulphide nanoparticles with sizes ranging from 2.7 to 4.9 nm in Nafion polymer films was studied upon excitation with a femtosecond laser at 400 nm. All samples, with the exception of the sample containing the smallest particles, showed maximum bleaching within the time resolution of the experiment. The bleaching then recovered with multi-exponential kinetics. Bi-exponential fits to the recovery of absorbance for samples which exhibited transient photobleaching showed that the lifetime of the short component increased as the size of the particles became larger. This behaviour was attributed to greater average distances between separated electrons and holes in the larger particles. Transient spectral studies showed that there was a change in the wavelength of maximum photobleaching with time in the cases of 4.3 and 4.9 nm CdS particles. In these cases, the peak of transient photobleaching showed a progressive red shift as the time between the pump and probe pulses was increased. There was no change in the wavelength of maximum bleaching with time for Nafion films containing 3.5 nm particles. The differences in transient spectral behaviour could be due either to their being a more polydisperse size distribution of CdS nanoparticles in films containing larger particles or to films containing larger particles having different types of trapping sites on the particle surface. It is possible that both factors are contributing to the observed transient photobleaching behaviour.

Waveguide integrated 1.55 µm photodetectorwith 45 GHz bandwidth

Evanescently coupled, waveguide integrated photodiodes were fabricated on InP. The device design is intended to provide ultrafast photoresponse together with a high responsivity. A 3 dB bandwidth of 45 GHz and 90% internal quantum efficiency is achieved at 1.55 /spl mu/m wavelength. High-speed operation with linear power response is obtained at power levels as high as 5 mW.

Ultrafast 1.55 μm all-optical switching using low-temperature-grown multiple quantum wells

Low-temperature grown surface-reflection all-optical switching has been demonstrated with ultrafast photoresponse (1.5 ps), low switching energy (2 pJ), high-contrast (13 dB), polarization independence, and wide operation wavelength range in the 1.55 μm band using low-temperature-grown Be-doped strained InGaAs/InAlAs multiple quantum wells. The combination of low-temperature growth and Be-doping contributes to the ultrafast photoresponse. Additionally, the introduction of compressive strain and a mirror with 1% reflectivity greatly enhances optical nonlinearities.

Ultrafast photoresponse in microbridges and pulse propagation in transmission lines made from high-T/sub c/ superconducting Y-Ba-Cu-O thin films

We report our femtosecond time-resolved measurements of the photoresponse of microbridges in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films, performed using an electrooptic sampling technique. Our test structures consisted of 5-/spl mu/m-wide, 7-/spl mu/m-long microbridges, incorporated in 4-mm-long coplanar waveguides, fabricated in 100-nm-thick, high-quality epitaxial YBCO films grown on LaAlO/sub 3/ substrates by laser deposition. When varying the biasing conditions between the superconducting and switched states, we observed transients of single-picosecond duration that corresponded to the nonequilibrium kinetic-inductance and the electron-heating response mechanisms, respectively. In both cases, experimental waveforms could be accurately simulated using a nonequilibrium (two-temperature) electron-heating model. From the fits, the YBCO intrinsic temporal parameters associated with the nonequilibrium conditions were extracted. The electron thermalization time was found to be 0.56 ps in the state above the material's critical temperature (T/sub c/=89 K) and 0.9/spl plusmn/0.1 ps in the superconducting state at temperatures ranging from 20 to 80 K. The electron-phonon energy relaxation time was found to be 1.1 ps. The single-picosecond pulse distortion due to propagation on a YBCO coplanar waveguide was also studied. Our results show that a YBCO microbridge can intrinsically operate as a photodetector at rates exceeding 100 Gb/s, making it useful as an optical-to-electrical transducer for optoelectronic interfaces in YBCO digital electronics. Simultaneously, YBCO mixers, based on hot-electron effects, should exhibit an intrinsic bandwidth exceeding 100 GHz.

Ultrafast photoresponse of an YBa_2Cu_3O_7−δ film to far-infrared radiation pulses

We report the observation of an ultrafast photoresponse of a high-T(c), film to far-infrared radiation pulses. The response of a sample, consisting of a current-carrying structured YBa(2)Cu(3)O(7-delta) film cooled to liquid-nitrogen temperature, was studied by use of ultrashort laser pulses from an optically pumped far-infrared laser in the frequency range from 0.7 to 7 THz. We found that the response time was limited by the time resolution, 120 ps, of our electronic registration equipment.