Method and a system for homodyne solid-state biased coherent detection of ultra-broadband terahertz pulses

Abstract

A device, a system and a method for homodyne solid-state biased coherent detection of terahertz pulses in a range between 0.1 and 11 THz, the device comprising a metallic slit between, and parallel to, two longitudinal metallic electrodes, deposited on a surface of a substrate, and covered with a layer of nonlinear material, wherein a width of the metallic slit and a thickness of the nonlinear material layer are selected in relation to a central wavelength of the THz pulses. The method comprises focusing a THz beam and a pulsed laser beam of pulse energies in a range between 10 and 100 nJ onto the metallic slit, the metallic electrodes being biased by a static DC voltage bias selected in a range between 20 V.sub.PP and 200 V.sub.PP; and retrieving a terahertz pulse waveform using the terahertz pulse repetition rate as synchronism.

Claims

1. A device for homodyne solid-state biased coherent detection of terahertz pulses in a range between 0.1 and 11 THz, the device comprising a metallic slit between, and parallel to, two longitudinal metallic electrodes, deposited on a surface of a substrate, and covered with a layer of nonlinear material, wherein a width of the metallic slit and a thickness of the nonlinear material layer are selected in relation to a central wavelength of the THz pulses, wherein the width of the metallic slit is selected in a range between 400 nanometers and 1 μm and the thickness of the layer of nonlinear material is selected in a range between 400 nanometers and 1 μm.

2. The device of claim 1, wherein the nonlinear material has a dielectric strength of at least 1 MV/cm.

3. The device of claim 1, wherein the thickness of the layer of nonlinear material is at least equal to the width of the metallic slit.

4. The device of claim 1, wherein the substrate is a quartz substrate of a thickness of 500 1 μm, the metallic electrodes are aluminum pads of transverse and longitudinal dimensions 2 mm, the width of the slit is 1 μm, and the layer of nonlinear material is a layer of silicon nitride of a thickness of 1 μm.

5. A system for homodyne solid-state biased coherent detection terahertz pulses in a range between 0.1 and 11 THz, comprising a detection device, the detection device comprising a metallic slit between, and parallel to, two longitudinal metallic electrodes deposited on a surface of a substrate, and covered with a layer of nonlinear material; a width of the metallic slit and a thickness of the nonlinear material layer being selected in relation to a central wavelength of the THz pulses; the electrodes being biased by a static DC bias voltage; a THz beam and an optical probe beam being focused onto the metallic slit of the detection device; a photomultiplier tube converting an output of the detection device into an electrical signal, and a lock-in amplifier, synchronized with the THz pulse repetition rate, acquiring said electrical signal, to reconstruct a THz pulses waveform.

6. The system of claim 5, comprising a THz source; a pulsed laser source; a focusing unit; and a DC voltage supply; said DC voltage supply applying the static DC bias voltage to the electrodes; said focusing unit focuses the THz beam emitted by said THz source and the optical probe beam emitted by said pulsed laser source onto the metallic slit of the detection device.

7. The system of claim 5, wherein the width of the metallic slit is selected in a range between 400 nanometers and 1 μm and the thickness of the layer of nonlinear material is selected in a range between 400 nanometers and 1 μm.

8. The system of claim 5, wherein the nonlinear material has a dielectric strength of at least 1 MV/cm.

9. The system of claim 5, wherein the probe beam pulses have a wavelength in a range between 0.8 μm and 2 μm, a repetition rate in a range between 10 Hz and 10 kHz, and pulse duration in a range between 35 fs and 150 fs and energies in a range between 10 and 100 nJ.

10. The system of claim 5, wherein the probe beam pulses have a wavelength of 800 nm, a repetition rate of 1 kHz, a pulse duration of 150 fs and energies in a range between 10 and 100 nJ.

11. The system of claim 5, wherein the probe beam pulses have a wavelength of 800 nm, with a repetition rate of 1 kHz, a pulse duration 150 fs and an energy of 50 nJ.

12. The system of claim 5, wherein the thickness of the layer of nonlinear material is at least the width of the metallic slit, and the bias DC voltage applied to the metallic electrodes is at most 200 V.sub.pp.

13. The system of claim 5, wherein the substrate is a quartz substrate of a thickness of 500 μm, the metallic electrodes are aluminum pads of transverse and longitudinal dimensions 2 mm, the width of the slit is 1 μm, the layer of nonlinear material is a layer of silicon nitride of a thickness of 1 μm, and the static DC bias voltage applied to the electrodes is selected in a range between 20V and 200 V.sub.PP.

14. The system of claim 5, wherein the static DC bias voltage applied to the electrodes is comprised in a range between 20 V and 200 V.sub.PP, the THz electric field THz is comprised in a range between 6 and 100 kV/cm, and the probe beam has pulse energies in a range between 10 and 100 nJ.

15. A method for homodyne solid-state biased coherent detection of terahertz pulses in a range between 0.1 and 11 THz, comprising focusing a THz beam and a pulsed laser beam of pulse energies in a range between 10 and 100 nJ onto a metallic slit provided between metallic electrodes on a surface of a substrate and covered with a layer of nonlinear material; a width of the metallic slit and a thickness of the nonlinear material layer being selected in relation to a central wavelength of the terahertz pulses, and the metallic electrodes being biased by a static DC voltage bias selected in a range between 20 V.sub.PP and 200 V.sub.PP; and retrieving a terahertz pulse waveform using the terahertz pulse repetition rate as synchronism.

16. The method of claim 15, wherein said DC bias voltage generates a DC electric field within the metallic slit, and the pulsed laser beam interacting with the DC electric field generates a local oscillator signal of a strength E.sub.DC, with E.sub.DC>> E.sub.THz where E.sub.THz is the strength of the THz electric field, the THz electric field THz being comprised in a range between 6 and 100 kV/cm.

17. The method of claim 15, wherein the probe beam pulses have a wavelength of 800 nm, a repetition rate of 1 kHz, a pulse duration of 150 fs and energies in a range between 10 and 100 nJ.

18. The method of claim 15, comprising selecting the width of the metallic slit in a range between 400 nanometers and 1 μm and the thickness of the layer of nonlinear material in a range between 400 nanometers and 1 μm.

19. The method of claim 15, comprising selecting the thickness of the layer of nonlinear material of at least the width of the metallic slit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the appended drawings:

(2) FIG. 1 is a schematic view of a solid-state biased coherent detection (SSBCD) device according to an embodiment of an aspect of the present disclosure;

(3) FIG. 2 is a schematic view of a homodyne solid-state-biased coherent detection (SSBCD) system according to an embodiment of an aspect of the present invention; and

(4) FIG. 3 shows a readout signal detected via the homodyne solid-state biased coherent detection (SSBCD) for different DC bias voltages, according to an embodiment of an aspect of the present disclosure;

(5) FIG. 4 shows FFT-evaluated spectra of the THz waveforms in FIG. 3, for different DC bias voltages, according to an embodiment of an aspect of the present disclosure; and

(6) FIG. 5 is a schematic view of an heterodyne solid-state biased coherent detection (SSBCD) system as known in the art.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(7) The present invention is illustrated in further details by the following non-limiting examples.

(8) A device for coherent detection of ultra-broadband terahertz pulses according to an embodiment of an aspect of the present disclosure is shown in FIG. 1. The detection device 10 comprises a metallic slit 20 of a width G formed on the surface of a substrate 40, between, and parallel to, longitudinal metallic electrodes 30A, 30B. A thin layer of nonlinear material 50 covers the metallic slit 20. The width G of the metallic slit and the thickness of the nonlinear material cover layer, for a given thickness of the substrate, are selected in relation to the central wavelength of the THz pulses.

(9) The nonlinear material of the cover layer 50 is selected with a dielectric strength of at least 1 MV/cm.

(10) The detection device 10 may be fabricated via standard CMOS technology. For example, a metallic slab may be formed by sputter deposition on a metallic substrate, the two metallic electrodes 30A, 30B formed by lithography and wet etching techniques, and the nonlinear material layer 50 deposited via plasma-enhanced chemical vapor deposition (PECVD) and then patterned by direct-write laser lithography and plasma etching to clear the external region of the electrodes 30A, 30B, in order to bias (V.sub.DC) the metallic slit 20 via a pair of contact wires welded thereon.

(11) A system for coherent detection of ultra-broadband terahertz pulses according to an embodiment of an aspect of the present disclosure is illustrated for example in FIG. 2.

(12) A DC (static) bias voltage V.sub.DC is independently generated (see DC supply and voltage amplifier 100) and feeds a detection device 10 according to the present disclosure.

(13) Co-propagating THz beam pulses emitted by a THz generator 80 and optical probe beam pulses emitted by a pulsed laser 60 (wavelength 800 nm, repetition rate 1 kHz, pulse duration 150 fs, energy 50 nJ) are focused onto the slit of the detection device 10 (70 and 90 are a beam splitter and a beam combiner respectively). The THz electric field strength in comprised in a range between about 6 and about 100 kV/cm.

(14) In the examples discussed herein, the pulsed laser 60 generates pulses at a wavelength of 800 nm, repetition rate 1 kHz, with pulse duration 150 fs, and energy 50 nJ. Parameters may be selected as follows: wavelength in a range between about 0.8 μm and about 2 μm, repetition rate in a range between about 10 Hz and about 10 kHz, and pulse duration in a range between about 35 fs and about 150 fs.

(15) The width G of the slit between the two electrodes on the surface of the substrate of the detection device 10 is selected in the range between 400 nanometers and 1 μm and the nonlinear layer has a sub-wavelength thickness, that is several times smaller than the central wavelength of the THz pulse (of about 300 μm), selected in the range between 400 nanometers and 1 μm, in such a way that the THz pulses propagate through the nonlinear layer on an extremely short length, comparable to a fraction of the central wavelength of the THz pulse (of about 300 μm). As a result, the terahertz-field-induced second harmonic generation process occurring within the slit is independent of constraints related to either phase-matching or lattice resonances.

(16) A thickness of the nonlinear layer 50 of at least the width G of the slit was found to prevent the occurrence of discharges induced by the high bias electric fields and spreading out of the slit towards the air above the nonlinear layer material, up to a bias voltage of 200 V.sub.PP (peak-to-peak). In the example device 10 discussed herein, the substrate 40 is a quartz substrate of a thickness of 500 μm, the metallic electrodes 30A, 30B are aluminum pads, the width G of the slit is 1 μm and the nonlinear material cover layer 50 is a thin layer of silicon nitride (SiN) of a thickness of 1 μm. The aluminum pad transverse and longitudinal dimensions were selected as r=L=2 mm, so as to completely gather the focused THz beam, which waist size is typically much smaller than 1 mm for a 10-THz-wide THz pulse spectrum.

(17) A photomultiplier tube (PMT) 110 converts the output I.sub.SH.sup.total of the detection device 10 into an electrical signal, which is then acquired by a lock-in amplifier (LIA) 120 synchronized with the THz pulse repetition rate f.sub.T (see THz chopper 82): the electrical readout of the photomultiplier tube (PMT) 110 is mixed with a reference signal synchronous to the THz pulse repetition rate f.sub.T from the THz generator 80 inside the lock-in amplifier (LIA) 120 (130 and 140 are a mixer and a low-pass filter respectively), in order to extract the heterodyne (coherent) signal. Thus, detection is carried out by using the THz pulse repetition rate f.sub.T as a synchronism for the lock-in amplifier 120, according to a homodyne scheme.

(18) In order to retrieve the THz pulse phase, that is to perform a coherent detection of the THz pulse, an electrically-driven local oscillator (LO) signal is superimposed within the detection device 10 where the interaction between the THz beam and the probe beam takes place. The metallic electrodes of the detection device 10 are biased by the static DC voltage V.sub.DC, that is, non-switching. Since the two metallic electrodes are separated by the slit 20 of the detection device 10, which is a very narrow gap that is several times smaller than the THz wavelength, significantly high bias DC electric fields E.sub.DC are generated by applying relatively low bias voltages V.sub.DC.

(19) In the above example of a slit of a width G of 1 μm, and a 1 μm-thick N cover layer of a dielectric constant of about 6.5, an applied bias voltage V.sub.DC=50 V generates a DC electric field strength of E.sub.DC=80 kV/cm. While interacting with the probe pulse, such an elevated DC field generates the local oscillator (LO) signal according to a process analogous to THz-Field-Induced Second Harmonic (TFISH) generation, yet driven by a static electric field. A local oscillator (LO) strength that overwhelms the strength of the THz-Field-Induced Second Harmonic (TFISH) signal, thus fulfilling the condition required for coherent detection (SSBCD) method (see Relation (4) hereinabove), is achieved by selecting the DC bias voltage V.sub.DC of the electrodes of the detection device 10.

(20) FIG. 3 shows readout signals detected via homodyne solid-state biased coherent detection (SSBCD) as a function of the DC bias voltage V.sub.DC, for a THz electric field strength of E.sub.THz=50 kV/cm. The THz waveform recorded via heterodyne solid-state biased coherent detection (SSBCD) is shown for comparison (V.sub.AC=100V; see FIG. 5 discussed hereinbelow). Curves are shifted along the y-axis for clarity and normalized to the peak of the curve acquired at 0 V.

(21) When the local oscillator (LO) signal is generated by an applied bias voltage V.sub.DC higher than 100 V, corresponding to a generated DC electric field strength E.sub.DC=160 kV/cm, which is about three times higher than the electric field associated with the THz pulse E.sub.THz, the phase of the THz pulse is recovered, as confirmed by the comparison with the waveform reconstructed via standard heterodyne solid-state biased coherent detection (SSBCD).

(22) FIG. 4 shows the ultra-broadband THz spectra of the detected THz pulses in FIG. 3. The Fast Fourier Transform (FFT)—evaluated spectra of the THz waveforms in FIG. 3 is shown as a function of the DC bias voltage. Curves are vertically offset for clarity and normalized to each respective maximum.

(23) There is thus demonstrated an electrically-driven homodyne THz detection method, using the THz-Field-Induced Second Harmonic (TFISH) effect occurring in the detection device. The method provides a gap-less spectral response wider than 10 THz, in terms of operating bandwidth i. e. in a range between about 0.1 and about 11 THz for an optical pulse duration of 140 fs, by applying bias voltages in the range between about 20V and 200 V.sub.PP (peak to peak) and pulse energies of the pulsed laser beam in the range between about 10 and about 100 nJ, thus combining the advantages of a solid-state ultra-broadband detection and the advantages of a homodyne detection, using commonly available and easily affordable electronics instrumentation.

(24) The present detection device, detection system and detection method are illustrated hereinabove in the case of a SiN cover layer and a 1-μm-wide metallic slit. The width of the slit of the detection device may be selected in a range between about 0.5 μm to about 1 μm, for generating a local oscillator LO signal strength higher than the strength of the THz-Field-Induced Second Harmonic (TFISH) signal so as to fulfill Relation (5), through selecting the DC bias voltage of the metallic electrodes of the detection device.

(25) In heterodyne solid-state biased coherent detection (SSBCD) method and system as known in the art and discussed in the Background section above, an electronic circuit divides the THz pulse repetition rate by two and accordingly generates an AC square wave bias voltage, phase-locked to the THz pulse train. A synchronism signal at the bias modulation frequency is mixed with the photomultiplier tube (PMT) electrical readout, inside the lock-in amplifier (LIA), in order to extract the heterodyne (coherent) signal. LPF refers to a low-pass filter (see FIG. 5).

(26) In contrast, the present system and method do not require an external electronic circuit to generate a bias voltage with modulation frequency equal to half of the THz pulse repetition rate and phase-locked with the THz pulse train. In comparison, the non-oscillating (DC) nature of the bias voltage applied to the solid-state device in the present system and method results in a significant decrease of the electrical noise contribution that would affect the recorded THz waveforms, since the electronics (voltage amplifiers) providing the DC bias voltage are not used to generate fast-switching square wave voltages. A DC bias voltage in the range between about 20 and about 200 VV.sub.PP (peak to peak) is generated using cost-effective and portable power supplies, such as compact electrical circuits based on charge pumps, which do not necessarily need a further voltage amplifier stage, thus further improving the overall noise performance.

(27) In the solid state detection device example discussed hereinabove in relation to FIG. 1, with a dielectric strength of the nonlinear cover material of about 6 MV/cm, the 1-μm-wide metallic slit can withstand static electric fields in the order of 100 kV/cm, generated by applying voltages V.sub.DC as low as 50 V to the device. This allows to achieve a very high local oscillator (LO) strength necessary to operate the homodyne mechanism.

(28) Since no plasma is generated, no distortions are introduced in the recorded THz waves shown see for example in FIGS. 3 and 4).

(29) The local oscillator (LO) signal can be quantified in terms of bias electric field strength E.sub.DC generated within the slit, which allows to straightforwardly compare the local oscillator (LO) strength with the THz electric field (E.sub.THz) and accurately adjust the DC bias voltage V.sub.DC to satisfy the condition E.sub.DC>>E.sub.THz in Relation 5 hereinabove and operate the homodyne scheme.

(30) Since the local oscillator (LO) signal is generated inside the detection device at the same time as the THz-Field-Induced Second Harmonic (TFISH) signal, there is no time delay to compensate with either laser probe pulse or THz pulse. Therefore, no additional optical components are required in the probe path.

(31) The local oscillator (LO) signal is generated according to a process very similar to the THz-Field-Induced Second Harmonic (TFISH) mechanism, using probe energies in the range between about 10 and about 100 nJ, and the readout signal is obtained with minimized background incoherent signal, thus allowing to exploit the full dynamics of the photomultiplier tube (PMT) used to acquire and to reach a high signal-to-noise-ratio (SNR).

(32) There is thus provided a device, a system and a method for homodyne solid-state biased coherent detection (SSBCD) of ultra-broadband terahertz (THz) pulses, in a range between 0.1 and 10 THz.

(33) The present method and system for detection of ultra-broadband terahertz (THz) pulses are operated at low probe energy, in the range between about 10 and about 100 nJ, and bias voltages, in the range between about 20V and 200 V.sub.PP (peak to peak), affordably generated with simple and off-of-the-shelf devices, such as common laser oscillators and battery-fed bias systems.

(34) The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

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