DOI: 10.1007/s00340-005-2090-0 Appl. Phys. B 82, 407–411 (2006) Lasers and Optics Applied Physics B a.k. sharma ✉ p.a. naik p.d. gupta Enhanced quadratic photocurrent in commercial light emitting diodes for autocorrelation measurement of ultrashort laser pulses Laser Plasma Division, Centre for Advanced Technology, Indore-452 013, India Received: 19 July 2005/Revised version: 13 October 2005 Published online: 11 January 2006 • © Springer-Verlag 2005 ABSTRACT The quadratic photocurrent in commercial light emitting diodes (LEDs) has been studied in reverse bias mode for autocorrelation measurement of ultrashort laser pulses. It is found that the photocurrent can be greatly enhanced by op- erating the LED biased just below the reverse bias breakdown threshold. The effect of aging of LEDs on laser exposure in this mode of operation is found to be similar to that for the pho- tovoltaic mode. The large internal gain in LED junction has enabled the recording of the second order autocorrelation signal of ∼ 200 fs laser pulses from 100 MHz laser oscillator with two orders of magnitude smaller average and peak power product compared to the case of the photovoltaic mode. PACS 42.65.Re; 42.50.Hz; 85.60.Jb 1 Introduction In the past decade, two-photon absorption (TPA) in semiconductor devices e.g., photo diodes [1–4], laser diodes [5, 6], light emitting diodes [7–9], single photon count- ing photodiode [10] etc. has drawn considerable interest as a substitute for quadratic intensity response of second harmonic generation (SHG) in certain non-linear crystals. The TPA in such devices has been used in many applica- tions like autocorrelation measurement of ultrashort laser pulses [1–12], ultra high-speed optical communication sys- tems [13, 14], optical thresholding [15, 16] etc. These devices, especially the semiconductor photodiodes and light emitting diodes, are commercially available off the shelf and are quite inexpensive. The use of TPA in such devices for autocor- relation measurements offers several advantages over their SHG counterparts. For example, it does not require any phase matching condition to be satisfied. Moreover, these devices are non-hygroscopic and their optical and electrical proper- ties are integrated in a single unit, which make them inherently compact, robust, maintenance-free and easy to use in any optoelectronic system. Next, for ultra-short laser pulse meas- urements, thin non-linear crystals of large area are required (to ✉ Fax: 091-731-248-8430, E-mail: aksharma@cat.ernet.in avoid group velocity dispersion in the crystal), which makes their handling difficult. Further, for ultrashort laser pulses, the spectral filtering effects [17] in non-linear crystals can dis- tort the cross-correlation or interferometric auto-correlation (IAC) signals. For instance, a cross-correlation signal of 30 fs laser pulses can be erroneous even when obtained using a BBO crystal as thin as 25 μ m, compared to that recorded using a semiconductor photodiode [18]. Laser pulse duration as short as 6 fs had been measured quite accurately by Ranka et al. [2] using a commercial photodiode. The quadratic photocurrent in semiconductor devices has been characterized in detail for the incident laser intensity dy- namic range [1–8], dependence [2] on spectral width, laser pulse width, focused spot size of the incident light, effect of ageing [7] etc. At the same time, it is desirable that these devices operate with high sensitivity so that a small opti- cal power may produce significantly large two-photon cur- rent, which exceeds the current due to any residual linear absorption. For instance, the quadratic response had been en- hanced in especially designed aperiodically poled lithium- niobate second harmonic converters [19], and through the use of excitonic enhancement of TPA in quantum confined struc- tures [20] or micro-cavities [21, 22] in semiconductor devices. However, the use of micro-cavities may not be suitable for ultrashort laser pulse characterization as the large gain is re- stricted to a limited spectral range. Moreover micro-cavities also add dispersion [22], which limits the measurement of laser pulse duration to  2 ps. Further, these especially de- signed devices will be quite expensive. It would be much beneficial if the sensitivity of commercially available devices such as LEDs or photodiodes could be increased. It may be mentioned here that the avalanche photodiodes [4] have been used in autocorrelation measurements. It will be interesting if one taps large avalanche gain of semiconductor devices by operating them in near reverse bias breakdown threshold to enhance the quadratic induced current. In fact, this has been exploited in case of optical emitters (e.g. laser diodes, and LEDs) used as linear detectors [23–25] for bi-directional communication links. For instance, the laser diode as a lin- ear detector, operated at near reverse bias breakdown voltage provided an avalanche gain in excess of 10. However, the avalanche multiplication has not been exploited in the op- tical emitters or photo detectors used as quadratic detector.