Three-Dimensional Phase Modulator at Telecom Wavelength Acting

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Three-dimensional phase modulator at telecom wavelength acting as a Terahertz detector with an electro-optic bandwidth of 1.25 Terahertz Benea Chelmus Ileana Cristina, Tianqi Zhu, Francesca Fabiana Settembrini, Christopher Bonzon, Elena Mavrona, Delwin L Elder, Wolfgang Heni, Juerg Leuthold, Larry R. Dalton, and Jérôme Faist ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01407 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Three-dimensional phase modulator at telecom wavelength acting as a terahertz detector with an electro-optic bandwidth of 1.25 terahertz Ileana-Cristina Benea-Chelmus,∗,† Tianqi Zhu,† Francesca Fabiana Settembrini,† Christopher Bonzon,† Elena Mavrona,† Delwin L. Elder,‡ Wolfgang Heni,¶ Juerg Leuthold,¶ Larry R. Dalton,‡ and Jérôme Faist∗,† †ETH Zurich, Institute of Quantum Electronics, Auguste-Piccard-Hof 1, Zurich 8093, Switzerland ‡ University of Washington, Department of Chemistry, Seattle, WA 98195-1700, USA ¶ETH Zurich, Institute of Electromagnetic Fields (IEF), 8092 Zurich, Switzerland E-mail: [email protected]; [email protected] Abstract We report a thin film phase modulator employing organic nonlinear optical molecules, with an electro-optic bandwidth of 1.25 THz. The device acts as a polarization sensitive broadband Pockels medium for coherent electric field detection in a dual wavelength terahertz time-domain spectroscopy setup in the telecom band at 1550 nm. To increase the sensitivity, we combine a three-dimensional bow-tie antenna structure with strongly electro-optically active molecules JRD1 in poly(methyl methacrylate) supporting polymer. The antenna provides sub-wavelength field confinement of the terahertz wave with its waveguide gap with lateral dimensions of 2.2 µm x 5 µm x 4 µm. In the gap, the electric field is up to 150 times stronger than in a diffraction limited spacetime volume, such that an interaction length of only 4 µm suffices for the detection of

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fields below 10 V/m. This device is of promise in the growing field of quantum optics in the terahertz, single photon terahertz detection, non-linear imaging and on-chip telecommunication.

Keywords THz detection, phase modulator, non-linear organic molecules, JRD1, fast EOS, plasmonic THz detector, Pockels, χ(2) Nonlinear optical (NLO) phase modulators are devices that can effect a change of the phase of an optical wave due to χ(2) or χ(3) nonlinear mixing through the application of an electric field from a second source. They are of fundamental usage to control the polarization or the intensity of light through various configurations of the phase modulators. For example, integrating two phase modulators to form a Mach-Zehnder interferometer functions as an amplitude modulator. These devices can also produce a dynamic modulation, if a timevarying field or intensity is employed instead of constant voltage. In the optical and radiofrequency spectral range, a great variety of such devices has become available, but in other frequency ranges they lack considerably. NLO materials are very scarce in the terahertz (THz) frequency range (roughly defined from 100 GHz to 10 THz), and they are mostly based on bulk semiconductors such as zinc telluride, 1 zinc sellenide 2 and gallium arsenide, 3 or lithium niobate. 4 Few solutions are based on organic materials as well. 5,6 They have been widely used in detection and generation of THz waves. While NLO materials used for THz generation have been reported to have good efficiency also when fabricated as waveguide sources 7,8 or even as nano-wires, 9 their integration into chip-scale devices for detection (e.g. for electro-optic sampling) is so far typically limited to gallium arsenide. 10,11 Commercially available THz detectors suffer from poor performance and are not easily utilized, due to their characteristic energy scales (around 25 meV at ambient temperatures). THz detectors based on graphene 12–14 have been reported to have short response times on the 2


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order of 50 ps and operate at room temperature. THz detection based on photo-conduction in low temperature grown gallium arsenide, have been reported to have high sensitivity when integrated with large-area antenna arrays. 15 Both sensitivity 16 and detector response times are factors, that decisively determine their usability in spectroscopy, frequency combs, 17 and the newly emerging field of quantum optics in the THz. 18 The detection scheme which combines best both of these characteristics and also exhibits the advantage of a detectivity bandwidth of several decades, 19 is electro-optic sampling, 20 which employs the χ(2) effect to modulate the amplitude of a THz field onto the phase of a femtosecond optical probe. The technical challenge of integrating NLO materials into wavelength-sized structures has long slowed down the improvement in sensitivity of THz receivers, especially for probe wavelengths in the telecom band, at 1550 nm. It is therefore of general importance, that NLO materials in the THz are compatible with micro-fabrication techniques, in order to permit devices with flexible functionalities, for detection, 21,22 imaging 23 and THz telecommunication. 24

Moreover, such devices are expected to open up new avenues for time-resolved

quantum electrodynamics experiments in the THz range, as they have been pioneered so far only in the mid-infrared, 25,26 since they combine the possibility to shape the spatio-temporal dimensions of sampled quantum fields and to provide high sensitivity. On the high energy side of the electro-magnetic spectrum, in optical telecommunication, dynamic phase modulators are key elements for data transmission. Their functionality is very similar to electro-optic detectors in the THz. Typically, the data stream sent through free space is encoded on a radio-frequency (RF) carrier, and then, at the chip level, transduced onto a continuous-wave optical signal. 27,28 The transmission of the optical wave is modulated by the incoming RF wave in a dynamic fashion with Mach-Zehnder interferometers with incorporated phase modulators. Solid-state based phase modulators have long been employed in this field.By contrast, organic molecules have been successfully engineered to have very high in-device electro-optic coefficients (exceeding 200 pm/V 29 or even values close to 400 pm/V 30 ). They have the additional advantage that they can be dispensed

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80 μm

b)

c) 2.1

50 0.75:0.25 JRD1:PMMA 2.0 ng 1.9 40 1.8 nopt 1.7 30 1.6 1.5 20 1.4 1.3 10 1500 1520 1540 1560 1580 1600 wavelength (nm) 50 d)2.2 0.75:0.25 JRD1:PMMA 2.0 40

z

d Ez, top view

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D l JRD1

antenna gap: 2.2 μm x 5 μm x 4 μm

: PMMA

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0.5

1.0 1.5 Frequency (THz)

2.0

0

Figure 1: Schematics and properties of the phase modulator. a) Scanning electron micrograph pictures of the three-dimensional plasmonic antennae, with a zoom onto the antenna gap. The active non-linear polymer is in the antenna gap, where the highest field confinement is provided by the broadband bow-tie antenna. b) Finite element method calculation of the field distribution at the plasmonic antenna, demonstrating a homogenous field distribution in the gap if the antenna is excited with a THz field polarized along the z-direction. c)-d) Optical properties of the non-linear material. Upper panel: refractive index and absorption coefficient at telecom wavelengths. Lower panel: refractive index and absorption at THz frequencies as measured with THz TDS.

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from solution into any patterned nano-structured optical device. 31 The maximal continuous electro-optic bandwidth reported so far is 170 GHz, 32 which is generally electronic, as organic NLO chromophores have femtosecond response times to the oscillating electric fields. 33 Preliminary results demonstrate the punctual modulation capability of NLO polymers at 1.6 THz at 1310 nm. 34 It is therefore of high interest to demonstrate device architectures, which exhibit higher continuous electro-optic bandwidth and propose characterization techniques, which readily demonstrate the usability of the material for a significant part of the THz frequency range. In this paper, we report on a three-dimensional phase modulator with an electro-optic bandwidth of up to 1.25 THz. The device consists of a single broadband bow-tie antenna on a crystalline quartz substrate (500 µm, ST-cut). The antenna gap, which is much smaller than the free-space wavelength at THz frequencies, is filled up with optically active molecules. The antenna structure collects and enhances the electric field of incident THz light, which produces an electro-optic effect in the molecules. In this way, THz fields below 1 kV/m at the gap level can be measured. In order to determine its electro-optic bandwidth, the device acts as a passive THz electric field detector in a dual wavelength Terahertz TimeDomain Spectroscopy (THz TDS) setup. We also report the properties of this setup, which is specifically redesigned to be compatible with the highly sub-wavelength gap of the phase modulator. This preliminary device demonstrates the capabilities of organic NLO materials for the THz range, as well as the fast switching capability of this material, which could so far not be measured by state-of-the-art electronics. The plasmonic phase modulator reported in this paper is shown in figure 1 a. It consists of a gold bow-tie antenna with electrode lengths of l = 140 µm each and width D = 90 µm at the outer part, respectively d = 5 µm at the gap. The particular property of this antenna is that its thickness is substantial, roughly h = 4 µm. The antenna gap features a size of 2.2 µm x 5 µm (w x d) and is filled with the NLO molecule JRD1 35,36 embedded in poly(methyl methacrylate) (PMMA) in a ratio of 75 %wt (JRD1:PMMA). The NLO polymer

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a) Pulser

1550 nm, 80 fs

JRD1 antenna

b) THz field DC voltage IR camera

probe field

Lock-in amplifier QWP

200 MHz

PBS

BD

xyz stage PM

c)

JRD1 antenna

QWP PBS

DM SMF

dual wavelength fiber laser

1550 nm, 80 fs 775 nm, 146 fs, 90 MHz

t

THz M

translation stage

100 kHz

BD

PM

E(t)

PCA modulation

Lock-in amplifier

Figure 2: Characterization setup. a) DC measurement setup for calibration and position optimization. A DC voltage with 50 % duty cycle can be applied to the phase modulator to optimize the interaction strength between the probe and the active material. b) THz and probe field profiles at the antenna structure. The modal overlap should be maximized for efficient detection. c) Dual-wavelength setup for THz pump of the antenna detectors. Phase-locked THz pulses are generated using the frequency doubled 775 nm output of the fibre laser, and they are detected using the main pulses at 1550 nm in the antenna detector using ellipsometry at a balanced detector. Abbreviations are as follows: SMF = single mode fibre, M = mirror, DM = dichroic mirror, PM = parabolic mirror, PCA= photoconductive antenna, QWP = quarter wave plate, PBS = polarizing beam splitter, BD = balanced detector.

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has been solvent cast onto the structure after the completion of the antenna nano-structuring. Then, the nonlinearity of the material is established by electric poling. The active area of the NLO material is restricted to roughly the antenna gap, since outside this region the poling field decreases rapidly. Details on the fabrication of these devices are published elsewhere, 37 where we describe a protocol to produce high aspect ratio gold antennae by electro-plating using a resist mask. We use the phase modulator to perform electro-optic sampling in a dual wavelength THz TDS setup with probing pulses at telecom wavelength 1550 nm, where the organic material is transparent (losses α = 20 cm−1 ), see figure 1 c. In the case of electro-optic sampling, the electric field of a THz wave is measured through the birefringence it induces in the poled JRD1 material. This effects a phase delay on a femtosecond pulse, which is used to determine the acting electric field with sub-cycle resolution. We chose to detect the THz wave in a collinear, normal scheme, as shown in figure 1 a, meaning that both the THz wave (represented in grey) and the probing femtosecond beam (represented in blue) are focussed from the top perpendicularly onto the sample. The interaction length lint between the two beams is equivalent to the thickness of the antenna, h=4 µm. In figure 1 d we present the absorption and refractive index properties of this material in the THz range. We find that for a ratio of 75 %wt (JRD1:PMMA), the group index at 1550 nm (ng = 1.97) and the phase index in the THz (n = 1.9 − 1.81) match very well, insuring a good interaction length. At the same time, the phase index at 1550 nm is moderate (nopt = 1.76), and insures a high nonlinearity, since the electro-optic signal depends like ∼ n3opt . The probing beam divergence and diameter are chosen such that the beam is focussed to match the active area of the detector, of 5 µm beam diameter (see lower panel of figure 1 a). The antennae presented here have been designed to be very broadband, and to be used in THz TDS. In figure 1 b we show finite element method field simulations of our structure as performed with CST Microwave studio. The field is highly homogenous inside the gap and fringing fields exist below and above the antenna structure. The antenna reflects part of the

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THz light, and off-resonance (above 500 GHz), this value corresponds to a bulk reflection coefficient of r ∼ 0.31. Below this frequency, where the antenna provides a high field enhancement, the reflection coefficient is between r ∼ 0.3 − 0.6. Obviously, only the field polarized across the plasmonic gap, in our case, along the z-axis (Ez ) will exhibit a strong field confinement due to the boundary conditions imposed by the antenna structure. Also, the strongest non-linear coefficient of JRD1 is r33 , such that the strongest electro-optic effect

3 x 1 0

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occurs when both the THz and the probe wave are polarized along the z-axis.

A m p litu d e ( V )

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Figure 3: Frequency response of the detectors. a) Time-trace of the THz electric field as measured by the antenna detector, exhibiting polarization selectivity. Inset: SEM image of the antenna, and THz electric field polarizations as marked. kΩ denoted the k-vector of the THz wave. b) Frequency response of two distinct antennae designed to provide field enhancement around 400 GHz and 1 THz, respectively. The cut-off frequency is around 1 THz for the low frequency antenna and 1.25 THz for the high frequency antenna. The dual wavelength THz TDS setup we have developed for the characterization of our non-linear polymer detectors is shown in figure 2. It offers the convenient option to apply a pulsed DC voltage to the sample instead of an oscillating field, and was used for alignment and calibration. A femtosecond probe laser beam at 1550 nm with pulses of 80 fs duration is focussed using high numerical aperture gold mirrors (f = 76.2 mm) to a theoretical beam spot size of 5 µm Gaussian diameter (w0 = 2.5 µm). This corresponds to the lateral dimension of the gap of the plasmonic antenna. 8


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An indium gallium arsenide infrared camera is optionally mounted in the beam path to optimize the position of the antenna into the focal point of the probe beam using a highly stable xyz stage. Subsequently, we apply a DC voltage modulated with 50% duty cycle at 20 kHz to the antenna and measure the voltage-induced birefringence in a balanced detection scheme using a lock-in amplifier. In this way, we fine-tune the position of the antenna, as shown in figure 2 a. In a second step, we investigate the fast modulation capability of the antenna, by radiating it with a phase-locked THz transient. The THz source is a photoconductive antenna pumped by femtosecond pulses at 775 nm, originating from the frequency doubled port of the same main oscillator, as shown in figure 2 c. The pump at 775 nm and the probe at 1550 nm are locked, therefore the full time-domain profile of the THz field can be retrieved by scanning a delay stage. The THz mode field overlap with the active material is maximized at optimal alignment, as shown in figure 2 b. We use a dichroic element (indium tin oxide thin-film on borosilicate with 20 Ohm/sq resistivity) to combine the probe and the source beam. b)

a)

0.0025

probe polarization component: 0.0020

BD

QWP antenna LP

PBS

V=V

DC

V=0

input

0˚, fast axis

MD/ modulation depth

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20o

0.0015

r33 Γ = 63 pm/V

0.0010

0.0005

20˚ x

0.0000

z

0

1

2

3

4

5

applied field E DC (V/µm)

Figure 4: Close-up of the setup. a) Optical arrangement used for THz and DC detection with the antenna detectors (LP = linear polarizer). b) The intensity modulation depth is studied as a function of applied electric field EDC across the antenna gap, and a linear dependence is observed. The slope of the curve is utilized to retrieve the built-in effective non-linear coefficient r33 Γ of these three-dimensional antennae. r33 is the non-linear coefficient of JRD1:PMMA mixture, and Γ the overlap factor, Γ = Γ(λ0 , Ω = 0).

In figure 3 a we show the electric field time-trace as measured with the antenna shown 9


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in the inset, when the incident THz field polarization is along the longitudinal (z-direction, shown in black) direction of the antenna, and along the orthogonal polarization (shown in red). We clearly demonstrate, that efficient detection occurs for the case of the polarization which exhibits good field enhancement and has a high electro-optic coefficient r33 . The antenna is not sensitive to the orthogonal polarization. The frequency response of our detectors is investigated by performing the Fourier-transform of their respective time-traces. In figure 3 b we show the results of two different antenna geometries, that are optimized to have a good field confinement at different centre frequencies, around 400 GHz and 1 THz. We use for calibration the detected spectrum of the same THz emitter, as measured by a 1 mm thick ZnTe crystal, in combination with a probe at 775 nm, which is shown with the dashed line. In the frequency range below 1 THz, the coherence properties of a 1 mm long ZnTe can be assumed to be spectrally flat, 38 and since the shown detected spectrum is also flat in this region, the spectral features of the detected time-trace can be attributed to the detectors only. We compare the experimental results to finite element simulations of the electric field enhancement. From the measured frequency response of the antenna, the cut-off frequency is defined as the frequency where the signal starts exceeding the noise level. In the upper panel of figure 3 we present our detection results for an antenna geometry with l = 140 µm long and D = 90 µm. The dotted line shows the field enhancement, which we define as follows: FE =

Egap Eoutside

(1)

where Egap is the electric field as measured in the antenna gap and Eoutside is the electric field in the thin film of dielectric material but far away from the structure, where free space propagation is a good approximation. We find that the cut-off frequency of the detector is 1 THz. The bandwidth of the detected signal corresponds well to the spectral window, where the antenna provides field enhancement FE ≥ 30. This spectral window is narrower than the total bandwidth of the available THz field (shown by the ZnTe detection). 10


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In the lower panel of figure 3, we present a second antenna of smaller dimensions (l = 50 µm long and D = 50 µm), which was optimized for operation around 1 THz. This geometry provides high field enhancement factors for higher frequencies, with a cut-off at 1.25 THz. In this case, the field enhancement factor is FE ≥ 32. At this point, we believe that the electrooptic bandwidth is limited by the antenna and not by the non-linear material, since the coherence properties are good throughout the THz band from 0.1-3 THz and the absorption is negligible for such thin films. To calibrate the efficiency of our phase modulators, we utilize the DC measurement scheme, which provides us with an absolute value of the electric field present at the antenna gap. We derive in the supporting information section the amount of intensity difference ∆I at the balanced detector, as determined by the optical setup shown in figure 4 a.

∆I ≈

n3opt r33 Γ(λ0 , Ω = 0)lint EDC Ip 2λ0

(2)

with λ0 the free space probe wavelength, Ω the THz frequency, Ip the total probe intensity, EDC the DC field applied, nopt the optical refractive index, r33 the electro-optic coefficient of JRD1:PMMA, lint = h the interaction length between the waves and Γ(λ0 , Ω = 0) the overlap factor between the probe field mode and the DC voltage. We define the modulation depth, linear with the applied field,

MD =

n3opt r33 Γ(λ0 , Ω = 0)lint ∆I = EDC Ip 2λ0

(3)

The modulation depth (MD) is measured experimentally by varying the DC voltage applied across the antenna, and we plot it in figure 4 b. We find our overlap-nonlinearity product to be on the order of r33 Γ(λ0 , Ω = 0) = 63 pm/V . This value is reduced as compared to maximum reported performance for r33 . 35,36 Possible reasons for this fact can be the reduced concentration of JRD1 to 75 %wt (JRD1:PMMA), 11


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the overlap factor which is most probably smaller than 1, and finally variations in the poling efficiency throughout runs. The shot noise sensitivity limit for 20 seconds integration time is equivalent to a modulation depth M D = 10−7 . This determines the THz electric field limit at the gap around 220 V/m. If we account for the field enhancement coefficients we demonstrated around 30-150, the sensitivity limit is then around 2-10 V/m THz fields in free-space. This result is very encouraging, especially in the perspective, that the interaction length is only 4 µm. This makes this device to our knowledge the thinnest electro-optic sensor at 1550 nm wavelength using organic thin films. 5,6 The sensitivity of the device can be readily enhanced by employing thicker films. Up to 100 µm thickness, the absorption is limited, and coherence properties satisfactory.

Conclusions In conclusion, we have demonstrated an electro-optic THz detector employing organic nonlinear molecules operating up to 1.25 THz. The active material is constituted of a thin film of 4 µm host-cast material JRD1 in PMMA, which exhibits an built-in non-linear coefficient of 63 pm/V. The high non-linear coefficient is complemented by a three-dimensional antenna structure which enhances the THz electric field. With this combined approach, the sensitivity of the antennae to fields below 10 V/m reaches the few photon limit since the electric field of one single THz photon with a bandwidth of 1 THz at 1 THz in a diffraction-limited space– time volume is approximately 2 V/m. The phase modulator acts as a coherent electric field detector in a dual wavelength THz TDS setup. The tuning capability of the sensitivity of the device is demonstrated by changing the antennae geometry. This preliminary device demonstrates the potential of this technology for the THz frequency range, and opens up possibilities for non-linear imaging applications, single photon detection, and on-chip nonlinear optics.

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Supporting information The derivation of the electro-optic signal for the three-dimensional antenna, discussion of important simplifications, processing details.

Funding Information The authors would like to acknowledge funding from the ERC (Advanced Grant, Quantum Metamaterials in the Ultra Strong Coupling Regime) as well as Air Force Office of Scientific Research (FA9550-15-1-0319).

Acknowledgments We acknowledge the work of the mechanical workshop at ETHZ. The sample processing took place in the clean room facility of ETHZ, FIRST center.

References (1) Winnewisser, C.; Jepsen, P. U.; Schall, M.; Schyja, V.; Helm, H. Electro-optic detection of THz radiation in LiTaO3, LiNbO3 and ZnTe. APL 1997, 70, 3069–3071. (2) Holzman, J. F.; Vermeulen, F. E.; Irvine, S. E.; Elezzabi, A. Y. Free-space detection of terahertz radiation using crystalline and polycrystalline ZnSe electro-optic sensors. APL 2002, 81, 2294–2296. (3) Nagai, M.; Tanaka, K.; Ohtake, H.; Bessho, T.; Sugiura, T.; Hirosumi, T.; Yoshida, M. Generation and detection of terahertz radiation by electro-optical process in GaAs using 1.56 µ m fiber laser pulses. APL 2004, 85, 3974–3976. (4) Tani, M.; Horita, K.; Kinoshita, T.; Que, C. T.; Estacio, E.; Yamamoto, K.;

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surements for photon statistics at terahertz frequencies. Proc. SPIE 2017, 10103, 10103 – 10103–7. (38) Benea-Chelmus, I.-C.; Bonzon, C.; Maissen, C.; Scalari, G.; Beck, M.; Faist, J. Measuring intensity correlations of a THz quantum cascade laser around its threshold at sub-cycle timescales. Proc. SPIE 2016, 9747, 9747 – 9747 – 7.

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For Table of Contents Use Only Title: Three-dimensional phase modulator at telecom wavelength acting as a terahertz detector with an electro-optic bandwidth of 1.25 terahertz Authors: Ileana-Cristina Benea-Chelmus Tianqi Zhu Francesca Fabiana Settembrini Christopher Bonzon Elena Mavrona Delwin L. Elder Wolfgang Heni Juerg Leuthold Larry R. Dalton Jérôme Faist Synopsis: The paper describes a novel type of terahertz detector, employing a very simple, few-steps, fabrication technique, to detect down to few photons.

THz pulse NIR probe

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ETHz,z

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Figure 5

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Three-Dimensional Phase Modulator at Telecom Wavelength Acting

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