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Optics Communications
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A study on wavelength dependence and dynamic range of the quadratic response ofcommercial grade light emitting diodes
A.K. Sharma ⁎, R.K. Patidar, M. Raghuramaiah, A.S. Joshi, P.A. Naik, P.D. GuptaLaser Plasma Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, M.P., India
Experimental results of a study on the wavelength dependence and the dynamic range of the quadraticresponse of commercial grade light emitting diodes (LEDs) are reported over a broad spectral range of680 nm to 1080 nm using ~100 fs duration laser pulses from cwmode locked laser oscillator. A large dynamicrange of the quadratic response has been demonstrated in a reverse biased LED. The observed dynamic rangecompares well with that obtained using a biased photomultiplier tube with large internal gain.
Commercial semiconductor devices like photodiodes [1], laserdiodes [2], light emitting diodes [3–5], single photon counting photo-diode [6] etc. have drawn considerable interest as a substitute forquadratic intensity response of second harmonic generation (SHG)in certain non-linear crystals. The use of quadratic response in semi-conductor devices offers several advantages over the SHG techniquee.g. no phase matching, wide spectral bandwidths, no spectral filteringeffects etc, in addition to being available off-the-shelf and being quiteinexpensive. Hence such devices are used in many applications suchas ultrashort laser pulse measurements [1–6], ultra high-speed opticalcommunication systems [7], optical thresholding [8], all opticalde-multiplexing [9] etc. The quadratic photo-response to the lightintensity is obtained in devices free from impurities and defects, whenthe incident photon energy (hν) is between half to full band gap energy(Eg) of the material of the device. Thus, such devices made of differentsemiconductor materials cover a very wide spectral range of electro-magnetic spectrum for different applications. For example AlGaAs (orGaAs or GaP) based light emitting diodes or photodiodes (band gap:~650 nm) are quite commonly used as a quadratic detector over the~700 nm to ~1300 nm wavelength range [3,4,10,11]. Earlier, quadraticphoto-current in commercial semiconductor devices has been charac-terized in detail for several parameters like focal spot, pulse duration,laser polarization etc. at a given laser wavelength [1–11]. However,
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the wavelength dependence of the quadratic response over a widespectral region has not been reported for commercial semiconductordevices such as GaAs based LEDs. In the case of a GaAsP photodiode,the quadratic response has been studied over a wavelength region of720 nm to 950 nm [1] and the response was almost constant. Further,the effect of the bias voltage on the saturation of the non-linearresponse in a practical device has not been reported so far.
In this paper, we present an experimental study on the wave-length dependence of the induced quadratic current in commercialgrade LEDs over a broad spectral range of 680 nm to 1080 nm using~100 fs duration laser pulses from a cw mode locked laser oscillator.The effect of the bias voltage on the dynamic range of the inducedquadratic photo-current is also studied. A wide dynamic range ofquadratic photo-response has been demonstrated for a reverse biasedLED.
2. Experimental study and discussion
Measurements on the induced quadratic photo-currentwere carriedout on different LEDs using ~100 fs laser pulses from a cwmode-lockedTi:sapphire laser oscillator (Model: Chameleon, from Coherent Inc.)with a pulse repetition rate of 80 MHz tunable over a spectral range of680 nm to 1080 nm. Following commercial LED samples were used inthe present study: a) RS 193-4752, b) RS 590-531, c) RS 247-1842,d) RS 247-1606, e) local LED sample, and f) Huiyan 5034SCY1AC. Thewavelength dependent quadratic response and the variation of itsdynamic range with bias voltage were studied for above LED samples.The LED sample has been used to record autocorrelation signals atdifferent laserwavelengths from700 nm to 1050 nm. The experimentalresults are presented in following sections.
Fig. 2. (a) Variation of photo-response of LED RS 193-4752 with incident laser power atlaser wavelength of 700 nm, 900 nm, 1000 nm and 1050 nm; (b) Photo-response of aphotovoltaic and reverse biased LED sample at 1000 nm.
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2.1. Effect of the bias on the photo-response of a LED sample
Photo-response of LED samples has been obtained by the directexposure of femtosecond laser pulse train from a cw mode-lockedlaser oscillator on to LED junction. Emission of red light is seen fromLED when an infrared laser beam is incident on its junction understrong illumination condition. This red light emission was used tovisually check the alignment of the laser beam on to the LED junction.The effect of bias voltage and incident laser wavelength on photo-response of LED was studied by varying reverse bias voltage andlaser wavelength. The quadratic response and saturation of photo-response were estimated by varying incident laser average power.
Fig. 1 depicts the experimentally observed photo-response of fourdifferent RS series LEDs as a function of the incident laser power, at800 nm laser wavelength, for operation in photo-voltaic (no bias)and photo-conductive (i.e. reverse bias) modes. The LEDs were placeddirectly in the laser beam from the laser oscillator. Fig. 1(a), (b), (c)and (d) corresponds to LED samples RS 193-4752, RS 590-531, RS247-1842, and RS 247-1606 respectively. It may be clearly seenfrom this figure that 1) the photo-response saturated at differentlevels and 2) the onset of saturation strongly depends on the biasvoltage applied to the LEDs. For instance, the quadratic response ofthe RS 247-1606 LED (see Fig. 1(d)) showed saturation at ~1.5 V forno bias, and at ~2.5 V for 1 V reverse bias. However, no saturationwas observed for a reverse bias voltage of 60 V for incident laser(average) power up to 2 W. The lower limit on the quadratic responsecould not be observed in the present case due to limitation onmeasuring photo-current below few nA without using a lock-in ampli-fier. Thus, by using a reverse biased LED, onemay enhance the dynamicrange of a practical semiconductor device by an order of magnitude. Asmentioned earlier, the dynamic range of a practical detector is limitedby the bias voltage. This is shown in Fig. 1(d) where, in the case of RS247 1606 LED, the dynamic range of the photo-response was ~170,~200, and >103 bias voltage of 0 V, 1 V and 60 V respectively.
While a reverse biased detector has shown to provide anenhanced dynamic range, the saturation of photo response may also
Fig. 1. Variation of the photo-response with incident laser power for different LEDs. In allsquares represent the photo-conductive mode of operation of the LED. The bias voltages wThe solid triangles in Fig. 1(a) and (d) correspond to bias voltages of 15 V and 1 V respecti
be avoided by the use of a low impedance load (requiring a slightchange in electronic circuitry). For a practical quadratic detector, thereverse bias is a simple method to avoid saturation so as to enhance
the figures, the solid circles depict the photo-voltaic mode of operation and the solidere 10 V, 60 V, 30 V and 60 V corresponding to Fig. 1(a), (b), (c) and (d) respectively.vely.
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the practically achievable dynamic range of a given photo-detectorcircuit. It may also be mentioned that the noise floor of an electroniccircuit will increase with bias and may limit the achievable dynamic
Fig. 3. (a)–(h) Experimental autocorrelatio
range. The dynamic range of the photo-response is ultimately dictatedby the intrinsic property of the diodematerial as discussed later in thissection.
3303A.K. Sharma et al. / Optics Communications 285 (2012) 3300–3305
A much larger dynamic range of nearly 5 order of magnitude inphoto-current (or ~3 order of magnitude in terms of incident laserpower) was obtained in the case of LED sample RS 590-531 with areverse bias voltage of 60 V (see Fig. 1(b)). Such a large dynamicrange has been reported in the case of photo-electron counting GaAsphoto-multiplier tube (PMT) at 1550 nm laser wavelengths [12],wherein high gain of the PMT is used to enhance the output photo-current. However, there is no report of such a high dynamic rangefor any LED based photo-detector. In the case of LED samples RS 1934752 (Fig. 1(a)) and RS 247 1842 (Fig. 1(c)), one may also note thedominance of linear absorption at lower power levels with increaseof a reverse bias. This indicates that the intrinsic dynamic range ofthe device is not affected, but the saturation behavior stronglydepends on the reverse bias voltage. Comparing Fig. 1(a) with (b),one may observe that LED sample RS 193 4752 has smaller dynamicrange (~100) compared to LED sample RS 590 531, which has adynamic range of nearly 5 order of magnitude in photo-current. Thedifference is attributed to LED design and one may infer the presenceof micro cavity in case of LED RS 193 4752.
In general, for a semiconductor detector, the intensity dynamicrange is governed by the linear (or single photon) absorption coeffi-cient (α), quadratic (or two photon) absorption coefficient (β), andthe absorption length (L) of the detector [13].While the lower intensitylimit depends on the single photon absorption, the saturation intensity
Fig. 4. Variation of the photo-response with the incident laser wavelengths for
for quadratic response is governed by the higher order processes. Thecondition for dominance of quadratic photo-current is expressed as
αβ≪ Ip ≪ 1
βL; ð2Þ
where Ip is the peak laser intensity. For a detector withmicro cavity, theabove condition is modified [14,15] as
4α3fβ
≪ Ip ≪ 83f 2βL
; ð3Þ
where f is the cavity intensity enhancement factor. Therefore, the inten-
sity dynamic range is1αL
and2
fαLfor a semiconductor detector without
andwithmicro cavity respectively.Whereas the quadratic response of adetector with micro cavity is greatly enhanced, the dynamic range isreduced. Thus, for typical values of α=0.01 cm−1, β=50 cm/GW,and L=0.1 cm of AlGaAs detector, the intensity dynamic range will be~103. In presence of micro cavity with intensity enhancement factor(f) of 100, the dynamic rangewill reduce to ~20. The presence of defectsor impurities in the detector will reduce the intensity dynamic range.For a practical detector, the dynamic rangemay be limited by saturation
the different LEDs. λp denotes the peak emission wavelength for the LED.
Fig. 5. (a) Estimated two photon absorption coefficients for GaAsmaterial with a bandgapenergy of 1.9 eV; (b) Typical emission spectrum of LED sample RS 590-531 with a peakemission wavelength (λp) at 620 nm. The inset in Fig. 5(b) is the emission spectrum inlogarithmic scale.
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of photo-response and latter may be avoided by applying a reversebias voltage as illustrated in Fig. 1.
The quadratic response w. r. to incident laser power has also beenmeasured at four different laser wavelengths of 700 nm, 900 nm,1000 nm and 1050 nm, for LED sample RS 193-4752. The results arepresented in Fig. 2(a). The effect of bias voltage on LED photo-response at 1000 nm is shown in Fig. 2(b). The photo-response atdifferent laser wavelength may be seen to be similar to that given inFig. 1. The quadratic nature of the photo-response was preserved ateach laser wavelength. This was further confirmed by measuring theautocorrelation signal of ~100 fs laser pulses at eight laser wave-lengths: 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nmand 1050 nm. Typical autocorrelation signals at these laserwavelengthsare depicted in Fig. 3.
2.2. Wavelength dependence of the quadratic photo-response
The variation of the photo-response as a function of the incidencelaser wavelength at a constant incident power for different LED samplesis shown in Fig. 4. Fig. 4(a), (b), (c), (d), (e) and (f) corresponds to theLED sample RS 193-4752, RS 590-531, RS 247-1842 and RS 247-1606,local LED sample, and Huiyan 5034SCY1AC respectively. From thisfigure, it is clear that the spectral response is different for differentLED samples and is attributed to the LED design, LED material andanisotropy [11,16–18] in TPA coefficients. Fig. 4(b),(c), and (e), showsthat the two photon current increases with increasing photon energy.A plateau region is also observed (see Fig. 4(a),(b),(d) and (f)) forsome LEDs where the photo-response was nearly constant over abroad spectral region. For example, the quadratic response for RS 247-1606 LED (see Fig. 4(d)) and RS 590-531 LED (see Fig. 4(b)) is fairlyconstant over 900 nm to 1080 nmand 960 nm to 1080 nm respectively.A larger plateau region of the photo-response over 720 nm to 1080 nmis observed in the case of RS 193-4752 (see Fig. 4(a)). It may bementioned that within this plateau region, the photo-current had a20% variation (see inset of Fig. 4(a)), which closely follow the calculatedvariation of the two photon absorption (TPA) coefficient (see Fig. 5(a))over the spectral region of 750 nm to 1100 nm, for a bulk GaAs samplewith a band gap energy of 1.9 eV and using Eq. (1) presented below.
In general, the photo-response is expected to increase with thedecrease in the laser wavelength as two photon absorption in bulkGaAs based samples increases with decrease in the laser wavelength[19]. The two photon absorption coefficient (β) in a direct gap semi-conductor is given by the empirical relation [19]
β ¼K
ffiffiffiffiffiEp
qn2 ωð ÞE3g
FhνEg
!; ð1Þ
where K is material dependent constant, Ep is the photon energy, Eg isthe band gap, n(ω) is the wavelength dependent refractive index and
F xð Þ ¼ 2x−1ð Þ3=22xð Þ5 ;where x ¼ hν
Eg. For most of the materials, value of K
is ~3100. In the case of a commercial semiconductor device like aphotodiode or LED, the effective value of TPA coefficient [14,15] getsincreased in the presence of a micro cavity by a factor 3f2/8, where fis the cavity enhancement factor, defined as the ratio of the intensitywith and without micro cavity.
It may be mentioned that different commercial LEDs had slightlydifferent material composition leading to different emission spectra(λp). While the peak emission wavelength is mentioned in therespective sub-figure for each LED in Fig. 4, a typical emission spectrumfor the LED sample RS 590-531 is given in Fig. 5(b). In general, in themeasured wavelength dependence of the photo-response, the trans-mission of the epoxy on the LED tip can also affect the measurements.However, in the present case, the transmission of the epoxy on theLED tip was constant over the wavelength region of interest as
measured by a spectrophotometer. Different wavelength dependentphoto-response for different LED samples and an enhanced photo-response at shorter wavelengths (see Fig. 4) can possibly be attributedto the LED junction design and anisotropy and/or to the contributionfrom single photon absorption. A more detailed investigation is neededin this and shall be explored in future.
3. Conclusion
In conclusion, we have presented a study on photo-response ofdifferent commercial LED samples using ~100 femtosecond durationlaser pulses from a cw mode-locked laser oscillator. Autocorrelationsignals were recorded over broad spectral range from 700 nm to1050 nm. It is observed that the quadratic response of variouscommercial LEDs varies differently for different LED samples over abroad spectral range of 680 nm to 1080 nm. It has been demonstratedthat a reversed biased LED may be used to achieve a large dynamicrange of quadratic response in a given practical photo-detector. Theultimate dynamic range of quadratic response is however dictatedby the intrinsic property of the LED material and the noise floor ofan electronic circuit.
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