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UCSB ECE 594 - Ultrafast Photoconductivity

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Notes#12, ECE594I, Fall 2009, E.R. Brown 173 Ultrafast Photoconductivity Perhaps the biggest change in the THz landscape over the past 20 years or so has been the advent and widespread of ultrafast photoconductors for the generation of THz radiation. The most common generation technique, by far, is THz pulse generation using mode-locked lasers. Probably the second most common techniques is THz cw generation by the photomixing of two frequency-offset cw lasers. In both cases, the “ultrafast” photoconductors used must have an electron-hole recombination time t < 1 ps. While such photoconductors are rather commonplace today, their development required a challenging evolution in materials science along with advancements in mode-locked lasers, and THz printed-circuit and planar antennas technology. No comprehensive discussion of the evolution of ultrafast photoconductors can ignore the success story of GaAs, which started with the incorporation or ion-implantation of select impurities, such as Cr, and culminated in high-quality material having photocarrier lifetime in the range of 10-to-100 ps. A lasting legacy of this impressive development is, in fact, the Cr-doped semi-insulating (SI) substrate used universally for epitaxial growth of all types. For ultrafast applications, SI GaAs was superseded in the early 1990s by low-temperature-grown (LTG) GaAs. The technological breakthrough of LTG GaAs and, indeed, the mantra for this development was “defect engineering.” A key discovery was that low-temperature growth by MBE at around 200oC followed by an anneal in the range 500-600oC would create, in addition to substitutional (e.g., antisite) defects, a significant concentration of As-rich precipitates.1,2 The precipitates were associated with a large density of energy levels very near the center of the GaAs band gap where they tend to accelerate the bipolar recombination, leading to deep-subpicosecond photocarrier recombination time under cross-gap illumination just above the GaAs band-edge.3 A drawback of the LTG-GaAs material was the unavoidable creation of more shallow levels - electron and, particularly, hole traps. If influential in the recombination kinetics, such traps can impact ionize in strong bias electric fields, creating a significant increase in the lifetime.4 In the late 1990s a new approach to ultrafast GaAs was pioneered by the group of A.C. Gossard which entailed the incorporation of Er during normal growth of GaAs.Notes#12, ECE594I, Fall 2009, E.R. Brown 174 Under the right conditions, the Er was shown to form single-crystal ErAs nanoparticles embedded in nearly-defect-free GaAs. In effect, the As-precipitates in LTG GaAs had been created without all the associated defects. Almost immediately, the ErAs:GaAs material demonstrated deep-subpicosecond photocarrier recombination time.5 More recently this has lead to the development and application of record-breaking THz photomixers.6 The parallel story for InGaAs started in the early 1990s, propelled largely by the advent of erbium-doped fiber amplifiers (EDFAs) and their associated components. The development effort is summarized by the published results listed in Table I. Low-temperature growth was attempted early on and led to the achievement of ~1.0 ps lifetime at 1.55 µm in In0.53Ga0.47As/In0.52Al0.48As quantum wells7,8 and interesting ultrafast nonlinear absorption effects.9 Subpicosecond response was not reported and slowly became somewhat of a “holy grail” of 1.55-micron ultrafast field. So by the new millenium researchers were pursuing techniques other than low-temperature growth, the first successful one being old-fashioned but very careful ion implantation. The first subpicosecond results were achieved by Au+ and H+ (i.e., proton) implantation of In0.53Ga0.47As epitaxial layers.10 Shortly thereafter, lifetime down to 300 fs was reported in Fe+-implanted material.11 Naturally, Er incorporation was pursued in parallel with the ion-implantation studies and, along with Be doping for electron compensation, was able to create high-quality material with embedded ErAs nanoparticles.12 The first material had ErAs Table I. Summary of published results for ultrafast photoconductors at λ = 1.55 µm. Material Measured Lifetime at or near λ = 1.55 µm Reference LTG, Be-doped InGaAs/InAlAs quantum wells 1.5 ps [8] LTG, Be-doped InGaAs/InAlAs quantum wells Subpicosecond nonlinear absorption recovery [9] Au and H-inplanted InGaAs layers < 1 ps [10] Fe-doped InGaAs epilayers annealed between 500-600oC 300 fs [11] Be-doped 40-nm-period ErAs-InGaAs epilayer ~1 ps [14] δ-doped-Be, ErAs-InGaAs epilayer < 300 fs [16]Notes#12, ECE594I, Fall 2009, E.R. Brown 175 nanoparticle layers separated by 40 nm and displayed lifetime just 1 ps at ~800 nm pump wavelength;13 however, the lifetime in this material could only reach ~1.0 ps at 1.55 micron.14 And even though the materials was Be doped to reduce the background free electron concentration, the “dark” resistivity of the material was prohibitively low for ultrafast photoconductive applications.15 A heroic investigation involving more closely spaced ErAs layers (5 nm) and Be modulation- and delta-doping subsequently led to a significant reduction in lifetime, the shortest reported value being just under 300 fs.16 In fact, it is the latter results that motivated the next section of the present paper on the extraction of lifetimes at 1.55 micron wavelength when they are in this deep-subpicosecond regime. Mode-Locked Lasers and Photoconductive Response One of the greatest inventions in quantum electronics has been the mode-locked laser. The time-dependent pulses from mode-locked lasers are often represented by the time-dependent power, Ppump(t) = P0 sech2[a(t-t0)]. A good approximation to this function and a form much easier to evaluate by signal-processing techniques is the Gaussian, Ppump(t) = P0,pumpexp[-b(t-t0)2]. (1) In either case, if derived from mode-locked lasers having low repetition rates (frep ~100 MHz) compared to the inverse pulse width, the average power is to an excellent approximation Pave ≈ frep ⋅Upulse where ∫∞∞−= dttPUpumppulse)( . Representative curves for these two functions are plotted in Fig. 1 for single pulses having the same Upulse. Note that since Upulse = 2P0,pump/a for the sech2 function and Upulse = P0,pump(π/b)1/2 for the Gaussian,17 the two pulses in Fig. 1 have the


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