Highly efficient and electrically robust carbon irradiated semi-insulating GaAs based photoconductive terahertz emitters

Abstract

Abstract:We demonstrate here an efficient THz source with low electrical power consumption. We have increased the maximum THz radiation power emitted from SI-GaAs based photoconductive emitters by two orders of magnitude. By irradiating the SI-GaAs substrate with Carbon-ions up to 2 m deep, we have created lot of defects and decreased the life time of photo-excited carriers inside the substrate. Depending on the irradiation dose we find 1 to 2 orders of magnitude decrease in total current flowing in the substrate, resulting in subsequent decrease of heat dissipation in the antenna. This has resulted in increasing maximum cut-off of the applied voltage across Photo-Conductive Emitter (PCE) electrodes to operate the device without thermal breakdown from ~35 V to > 150 V for the 25 m electrode gaps. At optimum operating conditions, carbon irradiated (10 14 ions/cm 2 ) PCEs give THz pulses with power about 100 times higher in comparison to the usual PCEs on SI-GaAs and electrical to THz power conversion efficiency has improved by a factor of ~ 800.Electromagnetic radiations having frequencies in Tera-hertz (THz) range (1THz = 10 12 Hz) are not so easy to generate [1] .But due to its applications in security imaging, bio-sensing, chemical identification, material characterization etc., there is high demand of high power THz sources, particularly sources which can generate short THz pulses with broadband spectrum. Till now, photoconductive emitters (PCEs) are known to be the best sources for high power THz pulse generation. Improving the efficiency of THz pulse sources with better designs or material, is one of the major goals of ongoing research in this field. There have been several attempts to increase the THz emission from these sources by modifying the electrical and optical properties of the semiconducting substrate [2] , design of electrodes [3] and patterning the active area of PCE in between the two electrodes [4] .In THz PCEs newly photo generated charge carriers (electron-hole pairs) via laser pulse of width less than 100 fs gets accelerated under already applied electric field and this sudden jump in number of free carriers and their acceleration gives sudden rise in the current. This sudden rise in current in pico-second time domain is responsible for THz pulse emission. In the case, where the semiconductor has carrier life time of less than a pico-second like LT-GaAs, current falls down to the dark level within picoseconds as electron hole pairs recombine with each other. Such materials are useful for the generation of bipolar THz pulses. In semiconductors like SI-GaAs which has carrier life time of more than 50 ps, fall in current takes relatively much longer time. Since electric field of the emitted THz pulse , where J(t)

Nanoplasmonics enhanced terahertz sources

“…We note that there is an offset in the I-V characteristic due to photocurrent which is not present under dark conditions (see Appendix B). The observed nonlinear rise in current at higher biases is commonly found in THz devices and it comes from Ohmic heating (e.g., see [40,41]). This limits practical device operation to values below the nonlinear regime [42].…”

Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection

“…Thus, it has enabled us to apply higher bias voltages (~ 150 V, in comparison to ~ 35-40 V) on our THz sources , resulting in an increase of the emitted THz amplitude by a factor ~ 12, as reported in Ref. [8]. In fact, the efficiency of these THz sources can match the performance of LT-GaAs emitters.…”

“…In an earlier paper [8], we have reported on the advantages of carbon irradiated SI-GaAs for photoconductive THz sources compared with those made from standard SI-GaAs. Here, we present drastic improvement in the detection of THz pulses using these irradiated substrates.…”

Textured semiconductors for enhanced photoconductive terahertz emission

“…One downside of PC THz emitters is that the charge-carrier lifetime is typically much longer than the picosecond duration of the terahertz transient, which leads to residual photocurrents that cause ohmic losses and Joule heating. These effects impede the desired scaling up of the THz electric field through raising the bias voltage amplitude or excitation optical fluence [21]. (The thermal runaway onset and the excitation optical fluence have an inverse relationship [22].)…”