3248 OPTICS LETTERS / Vol. 32, No. 22 / November 15, 2007

Optical detection of terahertz radiation by usingnonlinear parametric upconversion

M. Jalal Khan,* Jerry C. Chen, and Sumanth KaushikMIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA

*Corresponding author: mjkhan@ll.mit.edu

Received July 2, 2007; revised October 2, 2007; accepted October 3, 2007;posted October 15, 2007 (Doc. ID 84559); published November 1, 2007

We describe and demonstrate sensitive room-temperature detection of terahertz (THz) radiation by nonlin-early upconverting terahertz to the near-infrared regime, relying on telecommications components. THz ra-diation at 700 GHz is mixed with pump light at 1550 nm in a bulk GaAs crystal to generate an idler waveat 1555.6 nm, which is separated and detected by using a commercial p-i-n diode. The THz detector operatesat room temperature and has an intrinsic THz-to-optical photon conversion efficiency of 0.001%. © 2007Optical Society of America

OCIS codes: 190.0190, 190.2620, 190.4410, 190.7220, 040.0040, 230.4320.

Terahertz (THz) waves possess certain unique char-acteristics that make them desirable for a variety ofsecurity and remote sensing applications, includingexplosive detection [1], vibrometry [2], and imagingbehind barriers [3]. However, commercially availableTHz component technology, particularly at high fre-quencies, is still relatively immature and for themost part suffers from low efficiencies and slow fre-quency response and requires cryogenic operation.Room-temperature THz direct detectors such as py-roelectrics and Golay cells offer poor sensitivity.Liquid-helium-cooled bolometers are much more sen-sitive but tend to be slow [4] and laborious to use be-cause of cryogenic operation. Other direct detectors,such as quantum-well detectors, electro-optic detec-tors, and photoconductive antennas are topics of on-going research [4]. In contrast, optical detectors at1.55 �m are a mature technology and offer excellentperformance with very high efficiencies, low noiseequivalent powers (NEPs), large bandwidths, androom-temperature operation. Readily available p-i-n(PIN) diodes achieve NEPs of fW/Hz1/2 sensitivity.Photon multiplier tubes and Geiger-mode avalanchephotodiodes enable detection down to the single-photon limit at room temperature, with very fast per-formance.

Our scheme to detect THz waves leverages tele-communications optical technologies to enable ultra-sensitive room-temperature detection. We employ a��2� nonlinear crystal to parametrically upconvert [5]a THz signal to an optical telecomm signal for subse-quent detection by an optical detector. A schematicrepresentation of a THz detector using this techniqueis shown in Fig. 1. ��2� nonlinear interactions are in-trinsically very fast enabling high-bandwidth detec-tors. A previous report on frequency upconversion ofTHz radiation by using GaP crystals and 1 �m radia-tion achieved a power conversion efficiency of 2�10−7%, which corresponds to a photon conversionefficiency of 2�10−9% [6]. The use of MgO:LiNbO3for upconverting THz radiation using a 1 �m pump

was reported very recently [7]; however, it was not

0146-9592/07/223248-3/$15.00 ©

obvious what conversion efficiency was obtained forthe process.

In our experiment we used (110)-cut GaAs as thenonlinear material to perform upconversion. GaAswas chosen because of its high ��2� nonlinear coeffi-cient [8] and low absorption losses at THz and opticalfrequencies [9]. Our transmission measurementsfound losses to be 0.2 and 0.065 cm−1 at terahertz�0.5–0.7 THz� and optical frequencies (near193.4 THz), respectively.

The upconversion experimental layout is shown inFig. 2. Our THz source, a backward wave oscillator(BWO), was operated at 700 GHz, where it providedabout 2.5 mW as measured by using a Thomas–Keating meter. The BWO output power and spatialmode, which is multimoded, are a strong function offrequency. The 700 GHz choice was driven by therelatively high power available. The spatial mode atthis frequency has multiple lobes. THz radiationfrom the BWO was focused by using a pair of off-axis3 in. �1 in.=2.54 cm� parabolic gold mirrors on theGaAs crystal; the crystal position was adjusted suchthat the beam waist was in the middle of the 4 mmcrystal. The 4 mm GaAs crystal length was chosen,as it provided the maximal upconversion without re-sorting to a quasi-phase-matched structure; this wasconfirmed by our conversion experiments performedon GaAs crystals of varying lengths and also agreedwith the expected coherent buildup length [5] basedon the reported indices of refraction [9].

Our high-power optical pump source was derivedfrom a 0.2 nm spectral slice of amplifed spontaneousemission (ASE) centered at 1550 nm. The 1550 nm

Fig. 1. (Color online) THz detector concept that leveragesoptical technologies to perform room-temperature, ultra-

sensitive detection.

2007 Optical Society of America

November 15, 2007 / Vol. 32, No. 22 / OPTICS LETTERS 3249

light seeded a semiconductor optical amplifier (SOA)that was driven at a 200 kHz repetition rate with a10 ns pulse width. The output of the semiconductoroptical amplifier was preamplifed and filtered to re-duce out-of-band amplified spontaneous emission; fi-nally this signal was amplified to the appropriatepower levels by using a high-power erbium-doped fi-ber amplifier (EDFA). The resulting high-power opti-cal source is very flexible and allows easy adjustmentof power, pulse width, and repetition rate. The typicaloperating pump power was about 1 W average, whichcorresponds to a 500 W peak. The optical pump ra-diation was collimated to a spot size of nearly 1 mmand then passed through two short-pass filters. Thefilters were each angle tuned to pass 1550 nm pumplight with 1 dB of attenuation per filter. The transi-tion band of the filters was about 4 nm, with an out-of-band rejection of about 25 dB per filter. This filter-ing suppressed the optical noise around 1555.6 nm,enabling easier detection of the idler. The opticalpump was copropagated with the THz waves by em-ploying a 5 mm glass prism centered at the gold mir-ror. The small size of the glass prism ensured thatthe THz beam was minimally occluded.

The overlapping THz and optical radiation nonlin-early mix in the GaAs to generate the optical idler.The THz signal is vertically linearly polarized,whereas the pump is unpolarized. The crystal orien-tation was chosen to optimize the generated idler [5].The optical idler is then separated from the pump byusing two free-space long-pass filters and a bandpassfilter. The filters were angle tuned to exclude the1550 nm pump light but to pass the 1555.6 nm idler.Pump rejection through the three filters exceeded70 dB; its measurement was limited by backgroundnoise. The idler suffered an aggregate insertion lossof about 3 dB through the filters. The THz beam is arapidly expanding beam that is not collected by the1550 nm optics; additionally, the optical detector isnot sensitive to the THz frequency range.

The idler was detected by using a New Focus 2153PIN diode with an NEP of 23 fW/Hz1/2. The THz sig-nal was chopped at 49.5 Hz. This allowed an addi-tional way to discriminate between the generatedidler and any remnant pump. The idler power is de-tected by looking at the PIN diode output on an audiospectrum analyzer; alternately, a lock-in amplifier

Fig. 2. (Color online) Schematic layout of the THz upcon-version experiment.

could have been employed. Since the idler is propor-

tional to the product of the THz signal and the opticalpump, it is produced at the chopper frequency, unlikethe unchopped pump. Therefore, any signal at thechopper frequency detected by the diode must corre-spond to idler power.

Figure 3 shows the electrical spectrum of the PINdiode signal. We see a clear spectral tone at the chop-per frequency, along with higher harmonics, that to-gether correspond to the idler. To confirm that theharmonics are indeed due to idler power, we turnedoff the BWO, and the tones at the chopper frequencydisappeared, as is evident in the figure. Moreover, wealso confirmed the presence of the idler at 1555.6 nmon an optical spectrum analyzer. The peaks at 60,120, and 180 Hz correspond to electrical line noise.The chopper slightly modulates the THz signal atabout 12.4 Hz and its higher harmonics, which re-sults in the smaller peaks in the spectrum. The noisefloor of −66 dBV2/Hz1/2 observed in the spectrumagrees well with the theoretically expected value. Byintegrating the power spectral density (PSD), mea-sured in volts2 /hertz, over the spectral peak, we canmeasure the average power in the principal harmonicof the idler, given by

PIdler =1

Rv��

−�f

+�f

PSDdf�1/2

, �1�

where Rv is the responsivity of the PIN diode in volts/watts. By calculating the power in the principal har-monic by using the above integral, we estimated thetotal average idler power to be about 220 pW for anincident pump power of nearly 100 mW and a THzpower of 2.5 mW out of the BWO or, equivalently,1.7 mW inside the uncoated GaAs crystal. The pumppower is about an order of magnitude less than ourtypical operating level of 1 W so that the generatedidler does not saturate the PIN diode. Theoretically,we can estimate the detected power by using the fol-lowing expression [8]:

Fig. 3. (Color online) New Focus 2153 PIN diode outputviewed on an audio spectrum analyzer; the upconvertedTHz signal or the optical idler shows up as a clear tone at

the chopper frequency.

3250 OPTICS LETTERS / Vol. 32, No. 22 / November 15, 2007

PIdler�L� =8�2�deff� �2L2IPumpTFiltersTFresnel

c�onTnInP�I2

��sinc��kL

2 ��2

PTHz�0�, �2�

where deff� is the effective nonlinearity, L is the lengthof the crystal, IPump is the pump intensity, and nT, nI,and nP are the refractive indices at the THz, idler,and pump frequencies, respectively. �k=kP−kI−kT isthe wave-vector mismatch. TFilters is the combined in-sertion loss of the filters, and TFresnel is the transmis-sion coefficient of the idler at an uncoated GaAs crys-tal interface. Using this expression and incorporatingthe effect of material loss in GaAs, we theoreticallyestimate the idler power to be 690 pW. This is about5 dB larger than the measured value. This differencecan stem from the spatial mode mismatch betweenthe optical pump and the THz beam. Figure 4 showsthe modes of the BWO and the optical pump as mea-sured with a Spiricon Pyrocam III camera. The BWOmode has three lobes. The optical pump beam profileis limited by the high-power EDFA, which has M2

=2 according to the manufacturer. Additionally, thereis uncertainty in the value of �k used above. Finally,the expression in Eq. (2) assumes plane waves, whichis a good assumption for the collimated pump beambut does not apply well to the focused THz beam andas a result overestimates the idler power.

For our typical optical pump operating power ofabout 1 W, we expect the generated idler power toscale linearly with the pump intensity. This was con-firmed by using a less sensitive New Focus 2033 PINdiode and a pump power of nearly 1 W. We measuredan average idler power of 2.1 nW. On the basis ofthese numbers we calculate, taking into account in-sertion losses of the filters and the Fresnel loss at thecrystal interface, a photon conversion efficiency�THz→optical Output idler photons/Input THz pho-tons to be

Fig. 4. (Color online) (a) Spatial beam profile of the BWOoperated at 700 GHz. (b) Spatial beam profile of the1550 nm optical pump.

�THz→optical =PIdler

�p�

PTHz�

fTHz

fIdler 0.001 % , �3�

where PIdler�p� is the peak idler power and PTHz is the

THz power inside the GaAs crystal. This efficiencywas obtained with an optical pump intensity of64 kW/cm2, markedly lower than in previously re-ported experiments that used 17 MW/cm2 [6] and105 MW/cm2 [7]. The conversion efficiency, thoughmodest, is better than 50 dB compared with the up-conversion experiment reported in [6].

We have shown that it is possible to detect THz ra-diation by upconverting it to an optical signal and us-ing commercially available detectors. Currently, ourconversion efficiencies are limited and can be im-proved by better matching the modal overlap be-tween the THz and the optical beams. One could alsoemploy the use of a quasi-phase-mismatched GaAscrystal instead of bulk and use optimally focusedpump beams. We are in the process of pursuing eachof these strategies. The technique of upconverting theTHz to the IR regime allows the use of mature opticaltechnology to enable sensitive, room-temperature de-tection of THz waves and offers a promising alterna-tive to existing direct detection schemes at THz fre-quencies.

This work is sponsored by the Department of theAir Force under Air Force contract FA8721-05-C-0002.

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