NANOSCALE CIRCUIT TO USE INCIDENT LASER RADIATION TO GENERATE AND RADIATE TERAHERTZ HARMONICS

Abstract

A nanoscale circuit has an optical antenna receiving the radiation from a mode-locked laser and it responds by transmitting selected microwave or terahertz frequencies with a separate orthogonal antenna. Only MIM diodes, low-pass filters, and a load resistor are used to generate, separate, and transmit at the harmonics of the laser pulse-repetition rate.

Claims

1. A transmitting antenna for terahertz radiation, the antenna comprising two coupled circuits, each circuit further comprising: an optical receiving antenna coupled to a MIM diode; and, a transmission antenna operatively and orthogonally coupled thereto, the two coupled circuits being coupled by a resistor between respective transmission antennas such that the optical receiving antennas of the two circuits are in line with each other and form a diopole and the transmission antennas of the two circuits are offset from one another and form a transmission dipole.

2. The transmitting antenna of claim on 3, further comprising a low pass filter located in each of the two coupled circuits, each low pass filter positioned between the MIM diode and transmission antenna of a respective coupled circuit.

3. A method of generating terahertz radiation, the method comprising: a first step of providing at least one transmitting antenna having an optical receiving antenna dipole and a radiation transmission antenna dipole; a subsequent step of irradiating the at least one transmission antenna with a mode-locked laser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic drawing of an embodiment of a nanoscale antenna within the purview of the invention.

[0013] FIG. 2 is a schematic drawing of an alternate embodiment of a nanoscale antenna within the purview of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] With reference now to the drawings, a preferred embodiment of the nanoscale antenna is herein described. It should be noted that the articles “a,” “an,” and “the,” as used in this specification, include plural referents unless the content clearly dictates otherwise.

[0015] With reference to FIG. 1, We label the two parts of the spectrum of a mode-locked laser as the “optical spectrum” at and above the principal output frequency of the laser, and the “terahertz spectrum” that is below the principal frequency of the laser.

[0016] FIG. 1 is a sketch showing all the components of the nanoscale circuit. It is not drawn to scale because the vertical dipole 140 which transmits radiation in the terahertz spectrum is much longer than the horizontal optical dipole 110 that receives the radiation from the laser. The use of two separate antennas to receive the laser radiation 110 and radiate the harmonics 140 mitigates the roll-off of the output power as the inverse square of the frequency that is caused by the cabling and instrumentation in our measurements of the microwave frequency comb using a mode-locked laser with an STM. Each optical antenna 110 is coupled to a Metal-Insulator-Metal (MIM) diode 120 and then to a Low-Pass Filter (LPF) 130. A terahertz transmission antenna 140 is coupled to the LPF 130 and oriented perpendicularly to the optical antenna 100. Two such units are connected by a load resistor 150 between opposed transmission antennas 140, which are oriented to point in opposite directions, thereby leaving the two optical antennas 110 in line with each other and create the dipole.

[0017] The non-linear current-voltage characteristics of the two MIM tunneling diodes 120 create a current at closely spaced harmonics as integer multiples of the laser pulse-repetition rate. The voltage-drop across the resistor 150, which is at the feed-point for the vertical transmission dipole 140, causes radiation at these harmonics. Note that the result of this construction is that there are two coupled circuits, one receiving the laser radiation 110 and the other transmitting the harmonics which have much lower frequencies at much lower power 140. Coupling between the two antennas is minimized by their orthogonal positioning and the considerable difference between the terahertz and optical frequencies.

[0018] MIM tunneling diodes 120 are specified in this application because of their symmetric current-voltage characteristics, excellent high-frequency response, small size, and relative ease of fabrication. Others have previously used MIM tunneling diodes in terahertz applications. For example, a tungsten-nickel diode was used in a laser frequency synthesis chain to make measurements at frequencies up to 520 THz. We have used experiments with analysis to clarify the mechanism by which MIM diodes generate radiation at frequencies exceeding 500 THz.

[0019] It is anticipated that it may not be necessary to include the low-pass filters, as is shown with the circuit 20 in FIG. 2, because of the impedance discontinuity that is presented by the resistor 250 and the terahertz antennas 240 at the optical frequencies, as well as the effect of the MIM diodes 220. Note that for both embodiments the length of the resistor 250 should be much less than the shortest wavelength for the terahertz harmonics that are of interest, so to limit the spacing of the two terahertz monopoles 240 to a small fraction of a wavelength and optimize the radiation pattern. The extremely short length of each segment with an optical monopole 110, MIM diode 120, and LPF 130, causes that part not to interfere with the flow of the terahertz harmonics through the two terahertz monopoles 140 and the resistor 150.

[0020] The dipole antenna is designed using classical antenna theory so that the impedance, radiation pattern, and other characteristics will enable it to radiate efficiently over a chosen bandwidth that corresponds to a specified set of adjacent harmonics. The extremely close spacing of 74.25 MHz between the adjacent harmonics seen in previous measurements shows that considerable information could be contained within each set. Thus, if many of these nanoscale circuits were being used simultaneously it would be possible to make simultaneous measurements from distinct groups of these devices. The two sections of the terahertz dipole would be fabricated using a suitable metal, such as gold.

[0021] The two optical monopoles 110, noted in the Figures, are essential to these nanoscale circuits. Summaries of the work on the new topic of optical antennas have been presented by others. Traditionally, in optics electromagnetic waves are redirected using tools such as lenses and mirrors that are much larger than the wavelength. By contrast, at radio wave and microwave frequencies, antennas much smaller than the wavelength are used to control the fields to direct the pattern of the radiation. An optical antenna has been defined as “a device designed to efficiently convert free-propagating optical radiation to localized energy, and vice versa.” Some work by others on resonant optical antennas including a resonant optical monopole. One exciting possibility for the present invention is to combine the function of each optical monopole with that of the corresponding metal-insulator-metal diode by fabricating the monopole on an adjacent dielectric layer. An antenna-coupled thin-film Ni—NiO—Ni diode with optical monopoles was previously fabricated and used with lasers by others in a different application where no optical mixing was done.

[0022] Present mode-locked lasers have a wide range of parameters including optical wavelengths from 0.5 to 20 μm, pulse repetition frequencies from 500 kHz to 1 THz, and pulse widths from 10 fs to 3 ps. A wide range of off-the-shelf mode-locked lasers is available, and the choice will be based on the requirements for a specific application.

Possible Applications of this Technology

[0023] Many nanoscale circuits could be distributed on a surface and the laser could be scanned over this surface to measure local phenomena with high resolution. For example, this approach could be used to determine the presence of damage to the surface. It would also be possible to develop devices for high-resolution measurements of various parameters such as the local temperature or ionizing radiation at high speeds with high spatial resolution.

[0024] Although the present invention has been described with reference to preferred embodiments, numerous modifications and variations can be made and still the result will come within the scope of the invention. No limitation with respect to the specific embodiments disclosed herein is intended or should be inferred.