Patent Application: US-201314055825-A

Abstract:
disclosed are devices and methods for enhancing the performance of photoconductive switches or photomixers used to generate or receive terahertz radiation . an interlaced electrode is used to minimize carrier transit times across an absorbing semiconductor photoconductor . this electrode is designed to support a plasmonic resonance such that coupling of the optical pump signal to the absorbing photoconductor is enhanced .

Description:
nanoplasmonics involves the nanostructuring of metals to achieve an enhanced optical response and this has been used to improve the performance of detectors [ 13 ], light emitters [ 14 ], imaging systems [ 15 ], nanoantennas , [ 16 ] filters and waveguides [ 17 ]. a key property of nanoplasmonics is the capability to efficiently couple light into subwavelength structures [ 18 , 19 ]. the present disclosure describes nanoplasmonic pc switches that use a plasmonic resonance to efficiently couple light into gaps of typically 100 nm , substantially smaller than the wavelength of the optical pump signal . the small gaps enable fast carrier sweep out or transit time from a semiconducting substrate like gaas . the combination of efficient optical coupling and short transit time enables efficient thz generation or detection . in spectroscopy the disclosed methods and apparatus can be used to study different properties of materials and chemicals , such as absorption , conductivity , refractive index , etc . and for differentiating cancerous tissues from healthy tissues , as well as in imaging applications such as imaging of biological and chemical samples . a schematic of a representative thz heterodyne setup is shown in fig1 ( a ). in this setup , part of an optical pump pulse is selected using a beamsplitter ( bs ) and focused by lens l 1 onto a thz transmitter chip ( tx ). the resulting thz pulse propagates through the tx chip and is focused by a silicon lens into a test region or spectrometer . the phase of the thz pulse is varied with variations of the length of the optical excitation pulse path using a delay line ( dl ). at the receiver , the thz pulse is re - focused onto a receiver chip where the received signal is sampled by the remaining portion of the pump pulse to generate an electronic signal . the receiver is connected to a lock - in amplifier that detects the current signal as the delay ( dl ) is varied . this signal is then processed using well known techniques to yield the frequency - dependent absorption properties of the sample under test . some designs for thz receiving pc switches use a simple dipole antenna design with arms ranging in length from 10 - 100 μm . these antennas are driven by center pc gaps of width 5 - 10 μm fabricated on typically lt - gaas substrates . in accordance with one embodiment of the present invention , this center gap is replaced with a nanoplasmonic interlaced structure with gaps of typically 100 nm ( fig1 ( b ), 1 ( c )), and a period designed to resonantly enhance optical pump coupling into the spaces between the conductive fingers . to design an interlaced structure that is resonant with the incident beam , we used computer simulations ( fdtd lumerical software , mesh accuracy 1 nm × 1 nm ) and searched for the extra - ordinary optical transmission ( eot ) peak with finger and gap width variations . fig2 ( a ) shows the electric field 2d profile of a single gold finger , in a periodic structure with period of 700 nm which results in 100 nm gaps between the fingers ( fig1 ( c )). as seen in fig2 ( b ), the structure shows a peak in the substrate e - field loss at excitation wavelength of 830 nm . this peak in loss means that in the given geometry the light penetrates into substrate much more efficiently relative to other geometries . 830 nm is the center wavelength of the excitation pulse generated by a ti - sapphire femtosecond pump laser ( i . e . pulse width = 30 fs and 80 mhz repetition rate ). the electric field loss in 200 nm depth of substrate ( y axis ) is used as a criterion of transmission of each frequency through the interlaced electrode structure . to demonstrate one embodiment of this invention , we fabricated the device shown in fig1 on a semi - insulating gaas wafer ([ 100 ] orientation , 350 μm thickness , ˜ 1 . 3 × 10 8 ohm · cm resistivity and ˜ 5500 cm 2 / v . s mobility ). the carrier lifetime exceeds 200 ps for bulk gaas substrates , as confirmed by reflective pump - probe measurements . for the first step of fabrication , a closed - gap structure is fabricated by photolithography ( 95 nm au with a 5 nm cr adhesive layer ). the closed gap is then sputtered using a focused ion beam machine ( hitachi fb - 2100 focused ion beam system ). the interlaced structure is connected to gold electrodes which are connected to larger gold pads . the gold pads facilitate biasing and measurement of the signal from the structure ( fig1 ( b )). to compare the performance of the interlaced structure as a thz receiver , we also fabricated a conventional 20 μm dipole with 5 μm center gap on the same gaas substrate . additionally , we fabricated the same dipole structure on an lt - gaas substrate ( 1 μm lt - gaas grown at 250 ° c ., in - situ mbe annealing at 600 ° c . for 60 second , 0 . 5 % excess as ), as is typical in the prior art . a commercial lt - gaas pc switch ( batop pca - 800 nm ) was used as the thz transmitter . the signal is first measured with another similar commercial pc switch at the receiver side ( 10 μm dipole with 5 μm center gap with back - mounted silicon lens and substrate antireflection coating ). in order to correctly illuminate the center gap , the receiver is biased first to maximize the photocurrent measured in the thz receiving pc switch . after optical alignment , the receiver is connected to the lock - in amplifier and the silicon lens at the back of the receiver ( fig1 ( a )) is aligned for maximum signal . results for the commercial receiver are shown in fig3 ( a ). the pulse has sharp sub - picosecond features that translate into thz frequency components in the frequency domain ( fig3 ( b )). the receiver pc switch is then replaced with our pc switches and the results are measured again . for each measurement the silicon lens is aligned to obtain the maximum signal . as it is seen in fig3 ( b ) the ordinary dipole on gaas has lower bandwidth . the positive and negative peaks of the detected pulse depend on the photocarrier rise time and fall time respectively [ 19 ]. the rise time in gaas is similar to that of lt - gaas and therefore the gaas pc switch can detect the positive side of the thz pulse . however , due to long photocarrier fall time , the negative peak of the pulse is not captured completely and the results are relatively low in frequency and amplitude . the short photocarrier fall time in lt - gaas enables the pc switch to capture both positive and negative peaks and this enhances the bandwidth and sensitivity of the device as seen in fig3 . the commercial device shows better performance compared to our conventional ( not nanoplasmonic ) dipole on lt - gaas due to antireflection coating which enhances the optical coupling to its gap by roughly 30 %. the nanoplasmonic pc switch shows superior performance compared to the other devices . as seen in fig3 , the detected signal in the plasmonic pc switch is double that of commercial device on lt - gaas , one order of magnitude higher than our lt - gaas pc switch , and around 40 times more than that of ordinary 5 μm dipole on gaas . there are two mechanisms that result in the enhanced response for gaas . first , higher detection bandwidth is obtained due to short carrier sweep - out time which causes deeper photocarrier density modulations . second , there is an active region adjacent to each electrode that defines the number of photocarriers that are collected . this active region is equivalent to the photocarrier lifetime multiplied by saturation velocity . for lt - gaas with approximately 0 . 8 ps carrier lifetime and 1 . 3 × 10 7 cm / s electron saturation velocity [ 20 ] the active distance from the electrodes is around 100 nm . this implies that photocarriers that are generated further than 100 nm from the edge of the electrodes are on average more likely to recombine before reaching the electrodes . on the other hand for gaas with more than 200 ps carrier lifetime , the photocarriers are yet present long after the thz field is gone . this results in reduced bandwidth and blurring of the sharp peaks in the detected signal . transit time across the 100 nm gap between the fingers in the interlaced structure is much shorter than their original lifetime . based on gaas electron saturation velocity , the 100 nm gap size is just about the right size to artificially mimic the 0 . 8 ps carrier lifetime of the lt - gaas . nanoplasmonic resonances in the slits lead to high total optical transmission that increases the efficiency . the 100 nm gap size also increases the thz field intensity across the gap and this increases the detected current . secondary peaks appear approximately 6 ps after the main peak in the detected signal , corresponding to the roundtrip time of the thz wave internal reflections inside the 350 μm thick gaas wafer . to confirm the presence of nanoplasmonic resonances , we measured the reflection of the interlaced structure for two different optical excitation polarizations ( r ⊥ ; the reflection when the polarization is perpendicular to the slits and r ∥ ; the reflection when the e - field polarization is parallel with the slits ). we then compared the results with fdtd simulation results . based on the simulations the ratio r ∥ / r ⊥ was found to be 2 . 6 . this ratio was measured to be 2 . 1 for our fabricated interlaced structure . the measured high polarization dependent transmission indicates resonant transmission . the mismatch between the theory and measurement can be due to surface roughness that is induced in fib process . the roughness in the gold surface can induce scattering that weakens the effect [ 21 ]. a new type of photoconductive switch for use in terahertz systems has been proposed and demonstrated . the embodiment described above proves the efficacy of combining plasmonic resonant transmission of the optical pump with nano - structured electrode design . in addition to this design , numerous other embodiments can be contemplated , some of which are obvious to one skilled in the art . for example , while the demonstrated device used a substrate of gaas , a wide variety of semiconductor materials could be used . a wide variety of antenna configurations has also been demonstrated in scientific literature and in available products . these are entirely compatible with the present invention . in addition , recent activity has targeted terahertz systems in which terahertz radiation is launched directly into waveguide structures rather than being radiated from an antenna . our photoconductive switch could be used to drive such waveguide systems directly . also , our discussion above this focused on time - domain terahertz systems where short pulses are converted to terahertz radiation using photoconductive switches . our approach is also applicable to frequency - domain systems in which two continuous - wave lasers offset in frequency are mixed together on a photomixer to produce a continuous - wave terahertz signal . for the purposes of this invention , the term photomixer is synonymous with the term photoconductive switch . finally , we have described in detail one specific electrode configuration . it is understood that any electrode configuration designed to induce the plasmonic resonant enhancement while minimizing carrier transit time is within the scope of this invention . the disclosure is directed to various novel and unobvious features and combinations of features such as : 1 . use of a cost effective material ( e . g . gaas ) for sensitive thz reception and detection . 2 . plasmonic resonant electrode structures that increase transmission of an optical excitation pulse . 4 . faster response from long carrier lifetime semiconductors like gaas through use of sub - wavelength ( e . g . 100 nm ) gap sizes . the following references are cited in the disclosure . each of these references is incorporated herein by reference in its entirety . 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