Terahertz wave generator

Disclosed is a terahertz wave generator which includes a dual mode semiconductor laser device configured to generate at least two laser lights having different wavelengths and to beat the generated laser lights; and a photo mixer formed on the same chip as the dual mode semiconductor laser device and to generate a continuous terahertz wave when excited by the beat laser light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefits, under 35 U.S.C §119, of Korean Patent Application No. 10-2010-0127117 filed Dec. 13, 2010, the entirety of which is incorporated by reference herein.

BACKGROUND

Exemplary embodiments relate to a semiconductor device, and more particularly, relate to a terahertz wave generator.

A terahertz (THz) wave may be an electromagnetic wave between a microwave and an infra ray, and may have a frequency within a range of 0.1 THz to 30 THz. The terahertz wave may have both wave-like characteristics (e.g., the dielectric penetrability of a radio wave) and the particle-like characteristics (e.g., the propagation direction of a light wave). The terahertz wave with absorption is applicable to image, spectrum, and communication fields as a new technology. It is possible to penetrate the interior of a substance or to analyze a body mechanism of a molecule motion energy level and a space signal, using the terahertz wave. Further, an ultrahigh-speed short-range wireless communication can be made more excellently using the terahertz wave as compared with a microwave and a millimeter wave.

The THz wave technology with the above-described applicability may have been limited due to a trouble of development of a light source and a detector. However, development of semiconductor and laser technologies may enable the appearance of various light sources. Hitherto developed pulse light source technologies may include a photoconductive antenna manner, an optical rectification manner, and the like. Continuous wave light sources for generating a THz wave may include a photo mixer, a hot-hole laser, a free electron laser, a quantum cascade laser, and the like.

The photo mixer may be considered as a terahertz technology with a relatively high commercialization probability as compared with other technologies. The reason may be because the photo mixer is driven at a room temperature, freely varies a frequency, and is implemented by a low-cost, small-sized system.

The photo mixer may generate or detect a THz wave using a photoconductive material whose carrier lifetime is within a range of hundreds femtoseconds (fs) to several pico-seconds (ps) and an antenna formed on the photoconductive material. The photoconductive material may be a material whose resistance varies by a carrier excited by an exciting light. The photoconductive material may play a role of a switch which enables a current to flow to an integrated antenna. The antenna and a photoconductive switch may constitute a photo mixer.

A femtosecond pulse laser may be mainly used to generate a terahertz wave. Since the intensity of light is large, the femtosecond pulse laser may generate a pulse terahertz wave having a relatively large intensity at a wide frequency region. In case of generating a continuous terahertz wave, two laser beams of different wavelengths may be beat and a resultant may be used as an excited wave. Upon generating of a continuous THz wave with a variable frequency, two continuous waves output from two distributed feedback lasers (DFBs) or a continuous light source laser may be used. If one or all (two) wavelengths are varied, a frequency of a beat signal may be varied, so that a frequency of a produced THz wave is changed. The intensity of the exciting light may be maintained largely while a wavelength is changed.

In recent, needs for portable terahertz generator/detector may increase. However, a bulky femtosecond laser generator may be used as a light source of an exciting light used at a terahertz generator/detector. Accordingly, there may be required a technology of implementing terahertz generator and detector with a low cost and a small size.

SUMMARY

One aspect of embodiments of the inventive concept is directed to provide a terahertz wave generator which comprises a dual mode semiconductor laser device configured to generate at least two laser lights having different wavelengths and to beat the generated laser lights; and a photo mixer formed on the same chip as the dual mode semiconductor laser device and to generate a continuous terahertz wave when excited by the beat laser light.

DETAILED DESCRIPTION

FIG. 1is a diagram schematically illustrating a terahertz wave generator. Referring toFIG. 1, a terahertz wave generator may generate a continuous terahertz wave and may include the first and second distributed feedback laser light sources10and20and a photo mixer30.

The first and second distributed feedback layer light sources10and20may generate laser lights having different wavelengths λ1and λ2. A light obtained by beating the laser lights may be used as an exciting light of the photo mixer30. A frequency of the exciting light may correspond to a wavelength difference of the two laser lights. At this time, the intensity of the exciting light may be maintained largely while a wavelength is changed.

At least one laser light source must be configured to tune a continuous wavelength and may necessitate a rotatable and movable stage finely operated below a micrometer and bulky optical parts such as a beam splitter, a mirror, and the like. For this reason, the at least one laser light source may occupy a space, thus becoming a high-priced equipment.

The photo mixer30may be made by forming a photoconductive material on a substrate using a thin film, forming a photo diode designed to have a fast response speed, and forming an antenna on a resultant structure to be opposite to each other.

The first and second distributed feedback laser light sources10and20may be formed of a dual mode semiconductor laser device. According to an exemplary embodiment of the inventive concept, there may be provided a dual mode semiconductor laser device which is configured to emit two laser lights having different wavelengths and to make continuous tuning. The dual mode semiconductor laser device according to an exemplary embodiment of the inventive concept may be provided as a semiconductor laser device which can be formed on the same chip as the photo mixer30.

FIG. 2is a diagram illustrating a terahertz wave generator according to an exemplary embodiment of the inventive concept. Referring toFIG. 2, a terahertz wave generator100may include a dual mode semiconductor laser device110and a photo mixer120.

The dual mode semiconductor laser device110may provide the photo mixer120with a mixed light of two different wavelengths. The dual mode semiconductor laser device110may generate two continuous wave laser lights having different wavelengths. The generated laser lights may be beat, and the beat result may be provided to the photo mixer120. A photo-mixing manner for obtaining a terahertz wave having a frequency corresponding to a beating period may be implemented with a relatively low cost and may be driven at a room temperature. At least one laser light may be tunable with respect to a continuous wave.

In particular, the dual mode semiconductor laser device110may be a device which can be integrated in a single chip with the photo mixer120. A wavelength band of 800 nm has been researched due to a physical property of a low-temperature growth gallium arsenide (GaAs). Accordingly, when formed by a device driven at the wavelength band of 800 nm, the dual mode semiconductor laser device110may have such an advantage that a tuning region is small. On the other hand, a wavelength difference between two laser lights may be about 2 nm to obtain a beating signal having a frequency of 1 THz at a band of 1500 nm. A wavelength difference between two laser lights may be reduced to about 2 nm in cased of a band of 800 nm. If a frequency-variable dual mode semiconductor laser device110driven at the band of 800 nm is fabricated, a frequency-variable region of a continuous THz wave may be widened although a tuning region is relatively small.

In general, technologies and materials needed to make a semiconductor laser at a band of 1500 nm was well developed together with development of an optical communication system. On the other hand, development on technologies and materials needed to make a semiconductor laser at a band of 800 nm may be limited due to insufficient research on the band of 800 nm. When a dual mode semiconductor laser device110of the band of 800 nm is fabricated, it is difficult to make a regrowing process due to a material such as aluminum. The low-temperature growth GaAs or the photo mixer may be sensitive to heating after growth. Accordingly, if a high-temperature process for other devices is accompanied, characteristics of such devices may be lowered.

A low-temperature growth manner using MBE (molecular beam epitaxy) cannot use regrowth-possible MOCVD (metal organic chemical vapor deposition). For this reason, there may be generated such a problem that it is impossible to regrow a low-temperature growth photo mixer120after firstly making a semiconductor laser device. In the event that a photo-conductive antenna (PCA) of the photo mixer120uses a photo diode having a fast response speed, it is advantageous to minimize a regrowth process in order to prevent characteristics of two integrated devices from being lowered. Accordingly, a simple structure of a dual mode semiconductor laser device110being integrated may be required.

The photo mixer120may be made by forming a photoconductive material on a substrate using a thin film, forming a photo diode designed to have a fast response speed, and forming antennas124and125on a resultant structure to be opposite to each other. Electrodes126and127may be further provided to bias the antennas124and125. The photo mixer120inFIG. 2may be exemplary. Various shapes of antennas may be used for the photo mixer120according to an exemplary embodiment of the inventive concept. The photo mixer120will be more fully described with reference toFIG. 5, below.

In general, the photo mixer120may be grown at a low temperature (about 200° C. to 250° C.) using the MBE (molecular beam epitaxy) or may be formed of an ion-injected semiconductor. Accordingly, it is difficult to integrate the photo mixer120with a semiconductor laser. In the event that a low-temperature growth or ion-injected semiconductor is used as a photoconductive antenna (PCA), heating after growth or ion injection may be very important. Accordingly, a regrowth process accompanied at a semiconductor laser process has no remedy but to be limited.

Development of a single-chip terahertz wave generator may be advantageous when a semiconductor laser device process does not include a regrowth process. Further, in the event that a photo diode with a fast response speed is used, it is advantageous to minimize a regrowth process in order to stably maintain properties of the dual mode semiconductor laser device110and the photo mixer120.

The dual mode semiconductor laser device110according to an exemplary embodiment of the inventive concept may be integrated on a single chip with the photo mixer120. The integrated dual mode semiconductor laser device110may be implemented by a ridge-type semiconductor to minimize a regrowth process. A metal grating or a ridge shape may be adjusted to form a distributed feedback laser. An exciting light output from the dual mode semiconductor laser device110may be provided to a photoconductive antenna (PCA) of the photo mixer120. This may accomplished by using a wave guide130from an output of a laser to the photo mixer120. The wave guide130may be implemented by an active wave guide not necessitating regrowth.

A current of a terahertz band may flow to an integrated antenna due to an exciting light incident upon the photo mixer120. As a result, the photo mixer120may emit a continuous wave of a terahertz band. The photo mixer120may be formed of a wave guide type so as to be formed by an integrated device.

FIG. 3is a diagram illustrating a dual mode semiconductor laser device inFIG. 2according to an exemplary embodiment of the inventive concept. Referring toFIG. 3, a dual mode semiconductor laser110amay be formed using a ridge-type semiconductor laser to minimize a regrowth process. The dual mode semiconductor laser110amay include the first and second distributed feedback lasers220and230and a phase tuning region240.

The first distributed feedback laser220may include the first wave guide layer222formed on a substrate210. The first wave guide layer222may be formed of an active wave guide. A passive wave guide may need a regrowth process. Accordingly, it is difficult to make a photo mixer120inFIG. 2on a single chip. To constitute the first distributed feedback laser220, the first diffraction grating224may be formed at a side of the first wave guide layer222.

The second distributed feedback laser230may include the second wave guide layer232formed on the substrate210. The second wave guide layer232may be formed of an active wave guide not needing a regrowth process like the first wave guide layer222. To constitute the second distributed feedback laser230, the second diffraction grating234may be formed at a side of the second wave guide layer232.

The phase tuning region240may be formed between the first distributed feedback laser220and the second distributed feedback laser230. The phase tuning region240may include the third wave guide layer242formed on the substrate210. The third wave guide layer242may be configured by the same active wave guide as the first and second wave guide layers222and232. The phase tuning region240may shift phases of lights generated by the first and second distributed feedback lasers220and230according to a variation of a reflective index.

Herein, the first to third wave guide layers222,232, and242may be formed using the same process. The first to third wave guide layers222,232, and242may include the same gain material, respectively. The first to third wave guide layers222,232, and242may be formed to have a reflective index larger than that of the substrate210or a clad layer (not shown).

The first and second diffraction gratings224and234may be formed at sides of the first and second wave guide layers222and232using metal grating. The first and second diffraction gratings224and234may be formed between the substrate210and a clad layer (not shown), having different reflective indexes, to have a distributed Bragg reflector (DBR) shape.

Grating periods A1and A2of the first and second diffraction gratings224and234may be depended on a wavelength of a generated laser light. The grating periods A1and A2of the first and second diffraction gratings224and234may be different or identical. The first and second diffraction gratings224and234may be formed such that an interval between gratings increases or decreases gradually. The first and second diffraction gratings224and234can be formed by an index grating formed via a change of a ridge structure of the first and second wave guide layers222and232. Locations of the first and second diffraction gratings224and234are not limited thereto.

In an exemplary embodiment, the substrate210may be formed of n-InP or n-GaAs. Further, the substrate210can be formed using n-type compound semiconductor or p-type compound semiconductor.

FIG. 4is a detailed diagram illustrating a dual mode semiconductor laser device inFIG. 2. Referring toFIG. 4, a dual mode semiconductor laser device110bmay be formed of a ridge type not needing a regrowth process. The dual mode semiconductor laser device110bmay include the first and second distributed feedback lasers320and330and a phase tuning region340.

The first distributed feedback laser320may include the first wave guide layer322formed on a substrate310. The first wave guide layer322may be formed of an active wave guide. A passive wave guide may need a regrowth process. Accordingly, it is difficult to make a photo mixer120inFIG. 2on a single chip. To constitute the first distributed feedback laser320, the first diffraction grating324may be formed at a side of the first wave guide layer322.

The second distributed feedback laser330may include the second wave guide layer332formed on the substrate310. The second wave guide layer332may be formed of an active wave guide not needing a regrowth process like the first wave guide layer322. To constitute the second distributed feedback laser330, the second diffraction grating334may be formed at a side of the second wave guide layer332.

The phase tuning region340may be formed between the first distributed feedback laser320and the second distributed feedback laser330. The phase tuning region340may include the third wave guide layer342formed on the substrate310. The third wave guide layer342may be configured by the same active wave guide as the first and second wave guide layers322and332. The phase tuning region340may shift phases of lights generated by the first and second distributed feedback lasers320and330according to a variation of a reflective index.

Herein, the first to third wave guide layers322,332, and342may be formed using the same process. The first to third wave guide layers322,332, and342may include the same gain material, respectively. The first to third wave guide layers322,332, and342may be formed to have a reflective index larger than that of the substrate310or a clad layer (not shown).

The first and second diffraction gratings324and334may be formed at sides of the first and second wave guide layers322and332using metal grating. The first and second diffraction gratings324and334may be formed between the substrate310and a clad layer (not shown), having different reflective indexes, to have a distributed Bragg reflector (DBR) shape. Grating periods A1and A2of the first and second diffraction gratings324and334may be depended on a wavelength of a generated laser light. The grating periods A1and A2of the first and second diffraction gratings324and334may be different or identical. The first and second diffraction gratings324and334may be formed such that an interval between gratings increases or decreases gradually. The first and second diffraction gratings324and334can be formed by an index grating formed via a change of a ridge structure of the first and second wave guide layers322and332. Locations of the first and second diffraction gratings324and334are not limited thereto.

In particular, the first and second micro heaters370aand370bmay be further provided on upper portions of the first and second diffraction gratings324and334. Reflective indexes of the first and second wave guide layers322and332may be changed by the Joule's heat produced by the first and second micro heaters370aand370b. Further, the reflexibility of the first and second micro heaters370aand370bmay be continuously controlled by the Joule's heat produced by the first and second micro heaters370aand370b. The first and second micro heaters370aand370bmay be formed of a metal layer deposited on an upper portion of an insulation layer360.

Electrodes380a,380b, and380cmay be formed on upper portions of the first to third wave guide layers322,332, and342. It is possible to control the reflexibility of the first and second wave guide layers322and332via the electrodes380aand380c. Further, it is possible to control the reflexibility of the third wave guide layer342via the electrode380b. A phase of a beat exciting light can be shifted by controlling the reflexibility of the third wave guide layer342via the electrode380b.

FIG. 5is a cross sectional view illustrating a photo mixer taken along a line A-A′ inFIG. 2. Referring toFIG. 5, a photo mixer120may be formed by forming a photoconductive material122or a photo diode having a fast response speed and forming antennas124and125on a resultant structure so as to be opposite to each other.

An exciting light may be a beating signal amplified or modulated by a semiconductor optical amplifier110. An electric field E may be formed at a photoconductive material122by bias voltages V and −V applied to the antennas124and125, respectively. If an exciting light is input under the bias condition, a carrier (an electron-hole pair) may be generated at the photoconductive material122by light absorption. Since accelerated by the electric field E formed at the photoconductive material122, the carrier may be shifted into an antenna instantly. A terahertz wave may be generated from the antenna by light current flowing during a carrier lifetime (about hundreds femtoseconds).

According to an exemplary embodiment of the inventive concept, it is possible to provide a terahertz wave generator which can be integrated on a single chip and does not necessitate a regrowth process. Further, it is possible to provide portable, small-sized, and reliable terahertz wave generator and detector.